Understanding abiotic stress tolerance mechanisms in soybean: a comparative evaluation of soybean response to drought and flooding stress

Impact of Water Management on Growth and Pigment Composition of Cauliflower and Broccoli

Global climate change minimizes fresh water resources used in agriculture worldwide. It causes drought stress, which has adverse effects on plants. To ensure food security, crops and vegetables capable of tolerating shortages of water over the growth period are needed. This study aimed to elucidate the morphological and biochemical responses of three colored cauliflower (Brassica oleracea var. botrytis) cultivars (Clapton, Trevi, and Di Sicilia Violetto) and one broccoli cultivar (Brassica oleracea var. italica var. Magic) to different irrigation treatments (85-100%, 65-80%, 45-60%, and 25-40% field capacity). Assessment of growth parameters revealed no significant difference among all the treatments for root fresh weight, leaf area, and floret size. Major water shortages reduced the floret and stem fresh weight of the Clapton cultivar. Additionally, under severe drought stress, only the Di Sicilia Violetto cultivar had a decrease in plant height, but no impact on the number of leaves was observed. The measurement of pigment contents in the leaves showed no significant difference in carotenoids in all the cultivars; just the chlorophyll contents decreased with moderate stress in the Di Sicilia Violetto cultivar. This research demonstrates that cauliflower and broccoli are likely drought-tolerant vegetables and common irrigation regimes may be reviewed.

Advances in Soybean Genetic Improvement

The soybean (Glycine max) is a globally important crop due to its high protein and oil content, which serves as a key resource for human and animal nutrition, as well as bioenergy production. This review assesses recent advancements in soybean genetic improvement by conducting an extensive literature analysis focusing on enhancing resistance to biotic and abiotic stresses, improving nutritional profiles, and optimizing yield. We also describe the progress in breeding techniques, including traditional approaches, marker-assisted selection, and biotechnological innovations such as genetic engineering and genome editing. The development of transgenic soybean cultivars through Agrobacterium-mediated transformation and biolistic methods aims to introduce traits such as herbicide resistance, pest tolerance, and improved oil composition. However, challenges remain, particularly with respect to genotype recalcitrance to transformation, plant regeneration, and regulatory hurdles. In addition, we examined how wild soybean germplasm and polyploidy contribute to expanding genetic diversity as well as the influence of epigenetic processes and microbiome on stress tolerance. These genetic innovations are crucial for addressing the increasing global demand for soybeans, while mitigating the effects of climate change and environmental stressors. The integration of molecular breeding strategies with sustainable agricultural practices offers a pathway for developing more resilient and productive soybean varieties, thereby contributing to global food security and agricultural sustainability.

Fibrillin gene family and its role in plant growth, development, and abiotic stress

Fibrillins (FBNs), highly conserved plastid lipid-associated proteins (PAPs), play a crucial role in plant physiology. These proteins, encoded by nuclear genes, are prevalent in the plastoglobules (PGs) of chloroplasts. FBNs are indispensable for maintaining plastid stability, promoting plant growth and development, and enhancing stress responses. The conserved PAP domain of FBNs was found across a wide range of photosynthetic organisms, from plants and cyanobacteria. FBN families are classified into 12 distinct groups/clades, with the 12th group uniquely present in algal-fungal symbiosis. This mini review delves into the structural attributes, phylogenetic classification, genomic features, protein-protein interactions, and functional roles of FBNs in plants, with a special focus on their effectiveness in mitigating abiotic stresses, particularly drought stress.

Water volumes and mulches affect plant growth, leaf nutrient status and orchard soil mineral content of sweet orange cv. Mosambi

Day-by-day increasing irrigation water scarcity requires the application of water-saving irrigation techniques to sustain agriculture production. A two-year field investigation was conducted during 2018 to 2020 to determine the effects of various mulches and irrigation volumes on the growth, leaf chemicals and soil properties of one-year-old sweet oranges (Citrus sinensis) cv. Mosambi. The study included three irrigation schedules, viz.100% ETc (I1), 80% ETc (I2), and 60% ETc (I3), and five different mulches were used, viz. without mulch, white polythene, coriander straw, dry grass and black polythene mulches, replicated thrice. Results demonstrated that drip irrigation with 100% ETc and mulching with black polythene mulch significantly increase the plant growth attributes like height of the plant (28.64%) (30.31%), rootstock girth (36.61%) (37.90%), plant canopy spread (E-W and N-S) (EW- 63.82%, NS- 63.87%) (EW- 67.56%, NS- 67.90%) and leaf area (2.4%) (2.34%). Furthermore, plant leaf chlorophyll content (2.41 mg g-1) (2.41 mg g-1) and leaf mineral content such as N (2.39%) (2.40%), P (0.16%) (0.165%), K (1.57%) (1.59%), Ca (47.34 g kg-1) (47.80 g kg-1), Mg (4.54 g kg-1) (4.57 g kg-1), Fe (120.51 g kg-1) (123.15 g kg-1) and Zn (39.00 g kg-1) (37.84 g kg-1) were noted to be significantly (p ≤ 0.05) higher in plants that received 100% (were ETc (I1) and mulching with black polythene mulch (M1) treatment. Taken together, the results suggested that treatments I1 and M1 have the potential to maximize plant growth, leaf chemicals and soil nutrients of sweet orange (Citrus sinensis) cv. Mosambi plants.

Differential Drought Responses of Soybean Genotypes in Relation to Photosynthesis and Growth-Yield Attributes

Water scarcity leads to significant ecological challenges for global farming production. Sustainable agriculture depends on developing strategies to overcome the impacts of drought on important crops, including soybean. In this present study, seven promising soybean genotypes were evaluated for their drought tolerance potential by exposing them to water deficit conditions. The control group was maintained at 100% field capacity (FC), while the drought-treated group was maintained at 50% FC on a volume/weight basis. This treatment was applied at the second trifoliate leaf stage and continued until maturity. Our results demonstrated that water shortage exerted negative impacts on soybean phenotypic traits, physiological and biochemical mechanisms, and yield output in comparison with normal conditions. Our results showed that genotype G00001 exhibited the highest leaf area plant-1 (483.70 cm2), photosynthetic attributes like stomatal conductance (gs) (0.15 mol H2O m-2 s-1) and photosynthetic rate (Pn) (13.73 μmol CO2 m-2 s-1), and xylem exudation rate (0.25 g h-1) under drought conditions. The G00001 genotype showed greater leaf greenness by preserving photosynthetic pigments (total chlorophylls (Chls) and carotenoids; 4.23 and 7.34 mg g-1 FW, respectively) in response to drought conditions. Soybean plants accumulated high levels of stress indicators like proline and malondialdehyde when subjected to drought stress. However, genotype G00001 displayed lower levels of proline (4.49 μg g-1 FW) and malondialdehyde (3.70 μmol g-1 FW), indicating that this genotype suffered from less oxidative stress induced by drought stress compared to the other investigated soybean genotypes. Eventually, the G00001 genotype had a greater yield in terms of seeds pod-1 (SP) (1.90) and 100-seed weight (HSW) (14.60 g) under drought conditions. On the other hand, BD2333 exhibited the largest decrease in plant height (37.10%), pod number plant-1 (85.90%), SP (56.20%), HSW (54.20%), gs (90.50%), Pn (71.00%), transpiration rate (59.40%), relative water content (34.40%), Chl a (79.50%), total Chls (72.70%), and carotenoids (56.70%), along with the maximum increase in water saturation deficit (290.40%) and malondialdehyde content (280.30%) under drought compared to control conditions, indicating its higher sensitivity to drought stress. Our findings suggest that G00001 is a promising candidate to consider for field trials and further evaluation of its molecular signature may help breeding other elite cultivars to develop drought-tolerant, high-yielding soybean varieties.

CRISPR-Cas9-mediated editing of GmARM improves resistance to multiple stresses in soybean

The growth and development of soybean plants can be affected by both abiotic and biotic stressors, such as saline-alkali stress and Phytophthora root rot. In this study, we identified a stress-related gene-GmARM-whose promoter contained several hormone-response and stress-regulatory elements, including ABRE, TCA element, STRE, and MBS. qRT-PCR analysis showed that the expression of GmARM was the highest in seeds at 55 days after flowering. Furthermore, this gene was upregulated after exposure to saline-alkali stress and Phytophthora root rot infection at the seedling stage. Thus, we generated GmARM mutants using the CRISPR-Cas9 system to understand the role of this gene in stress response. T3 plants showed significantly improved salt tolerance, alkali resistance, and disease resistance, with a significantly higher survival rate than the wildtype plants. Moreover, mutations in GmARM affected the expression of related stress-resistance genes, indicating that GmARM mutants achieved multiple stress tolerance. Therefore, this study provides a foundation for further exploration of the genes involved in resistance to multiple stresses in soybean that can be used for breeding multiple stress-resistance soybean varieties.

Enhancement of osmotic stress tolerance in soybean seed germination by bacterial bioactive extracts

Soybean (Glycine max (L.) Merr.) is important to the global food industry; however, its productivity is affected by abiotic stresses such as osmosis, flooding, heat, and cold. Here, we evaluated the bioactive extracts of two biostimulant bacterial strains, Bacillus butanolivorans KJ40 and B. siamensis H30-3, for their ability to convey tolerance to osmotic stress in soybean seeds during germination. Soybean seeds were dip-treated in extracts of KJ40 (KJ40E) or H30-3 (H30-3E) and incubated with either 0% or 20% polyethylene glycol 6000 (PEG), simulating drought-induced osmotic stress. We measured malondialdehyde content as a marker for lipid peroxidation, as well as the activity of antioxidant enzymes, including catalase, glutathione peroxidase, and glutathione reductase, together with changes in sugars content. We also monitored the expression of genes involved in the gibberellic acid (GA)-biosynthesis pathway, and abscisic acid (ABA) signaling. Following osmotic stress in the extract-treated seeds, malondialdehyde content decreased, while antioxidant enzyme activity increased. Similarly, the expression of GA-synthesis genes, including GmGA2ox1 and GmGA3 were upregulated in KJ40E-dipped seeds at 12 or 6 h after treatment, respectively. The ABA signaling genes GmABI4 and GmDREB1 were upregulated in H30-3E- and KJ40E-treated seeds at 0 and 12 h after treatment under osmotic stress; however, GmABI5, GmABI4, and GmDREB1 levels were also elevated in the dip-treated seeds in baseline conditions. The GA/ABA ratio increased only in KJ40E-treated seeds undergoing osmotic stress, while glucose content significantly decreased in H30-3E-treated seeds at 24 h after treatment. Collectively, our findings indicated that dip-treatment of soybean seeds in KJ40E and H30-3E can enhance the seeds' resistance to osmotic stress during germination, and ameliorate cellular damage caused by secondary oxidative stress. This seed treatment can be used agriculturally to promote germination under drought stress and lead to increase crop yield and quality.

Physiological and transcriptional regulation in Taxodium hybrid 'Zhongshanshan' leaves in acclimation to prolonged partial submergence

MAIN CONCLUSION

Taxodium 703 leaves activate fermentation, amino acids metabolism and ROS detoxification, and reduce TCA cycle and ABA biosynthesis in acclimation to prolonged partial submergence stress. Taxodium hybrid 'Zhongshanshan 703' (T. mucronatum × T. distichum; Taxodium 703) is a highly flooding-tolerant woody plant. To investigate the physiological and transcriptional regulatory mechanisms underlying its leaves in acclimation to long-term flooding, we exposed cuttings of Taxodium 703 to either non-flooding (control) or partial submergence for 2 months. The leaf tissues above (AL) and below (BL) flooding-water were separately harvested. Partial submergence decreased concentrations of chlorophyll (a + b) and dehydroascorbate (DHA) and lactate dehydrogenase (LDH) activity in AL, and reduced biomass, concentrations of succinic acid, fumaric acid and malic acid, and transcript levels of genes involved in tricarboxylic acid (TCA) cycle in BL. Under partial submergence, concentrations of starch, malondialdehyde and abscisic acid (ABA) decreased, and also mRNA levels of nine-cis-epoxycarotenoid dioxygenases that are involved in ABA biosynthesis in AL and BL of Taxodium 703. Partial submergence increased O2- content in AL, and improved concentrations of pyruvate and soluble sugars and activities of LDH and peroxidase in BL. In addition, partial submergence increased concentrations of ethanol, lactate, alanine, γ-aminobutyric acid, total amino acids and ascorbic acid (ASA), and ASA/DHA, activities of alcohol dehydrogenases (ADH) and ascorbate peroxidase, as well as transcript levels of ADH1A, ADH1B and genes involved in alanine biosynthesis and starch degradation in AL and BL of Taxodium 703. Overall, these results suggest that Taxodium 703 leaves activate fermentation, amino acids metabolism and reactive oxygen species detoxification, and maintain a steady supply of sugars, and reduce TCA cycle and ABA biosynthesis in acclimation to prolonged partial submergence stress.

Multi-Omics Pipeline and Omics-Integration Approach to Decipher Plant's Abiotic Stress Tolerance Responses

The present day's ongoing global warming and climate change adversely affect plants through imposing environmental (abiotic) stresses and disease pressure. The major abiotic factors such as drought, heat, cold, salinity, etc., hamper a plant's innate growth and development, resulting in reduced yield and quality, with the possibility of undesired traits. In the 21st century, the advent of high-throughput sequencing tools, state-of-the-art biotechnological techniques and bioinformatic analyzing pipelines led to the easy characterization of plant traits for abiotic stress response and tolerance mechanisms by applying the 'omics' toolbox. Panomics pipeline including genomics, transcriptomics, proteomics, metabolomics, epigenomics, proteogenomics, interactomics, ionomics, phenomics, etc., have become very handy nowadays. This is important to produce climate-smart future crops with a proper understanding of the molecular mechanisms of abiotic stress responses by the plant's genes, transcripts, proteins, epigenome, cellular metabolic circuits and resultant phenotype. Instead of mono-omics, two or more (hence 'multi-omics') integrated-omics approaches can decipher the plant's abiotic stress tolerance response very well. Multi-omics-characterized plants can be used as potent genetic resources to incorporate into the future breeding program. For the practical utility of crop improvement, multi-omics approaches for particular abiotic stress tolerance can be combined with genome-assisted breeding (GAB) by being pyramided with improved crop yield, food quality and associated agronomic traits and can open a new era of omics-assisted breeding. Thus, multi-omics pipelines together are able to decipher molecular processes, biomarkers, targets for genetic engineering, regulatory networks and precision agriculture solutions for a crop's variable abiotic stress tolerance to ensure food security under changing environmental circumstances.

Physiological mechanism of melatonin attenuating to osmotic stress tolerance in soybean seedlings

Drought is one of the most significant abiotic stress threatening to crop production worldwide. Soybean is a major legume crop with immense economic significance, but its production is highly dependent on optimum rainfall or abundant irrigation. As the global climate changes, it is more important to find solutions to make plants more resilient to drought. The prime aimed of the study is to investigate the effect of melatonin on drought tolerance in soybean and its potential mechanisms. Soybean seedlings were treated with 20% polyethylene glycol 6000 (PEG 6000) and subjected to osmotic stress (14 days) with or without 100 μM melatonin treatment. Our results revealed that melatonin supplementation significantly mitigated PEG-induced growth retardation and increased water absorption ability. Foliar application of melatonin also increased gas exchange and the chlorophyll fluorescence attributes by the mitigation of the osmotic-induced reduction of the reaction activity of photosystems I and II, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), electron transport activity, and photosynthetic efficiency. In addition, PEG-induced elevated production of reactive oxygen species (ROS) and malondialdehyde (MDA) content were significantly reversed by melatonin treatment. Equally important, melatonin boosted the antioxidant activities of soybean plants. Moreover, osmotic stress substantially increased abscisic acid (ABA) accumulation in roots and leaves, while melatonin-received plant leaves accumulated less ABA but roots content higher ABA. Similarly, melatonin significantly suppressed ABA biosynthesis and signaling gene expression in soybean exposed to drought stress. Furthermore, osmotic stress significantly suppressed plasmalemma (GmPIPs) and tonoplast aquaporin (GmTIPs) genes expression, and their transcript abundance was up-regulated by melatonin co-addition. Taken together, our results indicated that melatonin potentially improves drought tolerance of soybean through the regulation of ABA and aquaporin gene expression, increasing photosynthetic efficiency as well as enhancing water uptake efficiency.

Beyond transcription factors: more regulatory layers affecting soybean gene expression under abiotic stress

Abiotic stresses such as nutritional imbalance, salt, light intensity, and high and low temperatures negatively affect plant growth and development. Through the course of evolution, plants developed multiple mechanisms to cope with environmental variations, such as physiological, morphological, and molecular adaptations. Epigenetic regulation, transcription factor activity, and post-transcriptional regulation operated by RNA molecules are mechanisms associated with gene expression regulation under stress. Epigenetic regulation, including histone and DNA covalent modifications, triggers chromatin remodeling and changes the accessibility of transcription machinery leading to alterations in gene activity and plant homeostasis responses. Soybean is a legume widely produced and whose productivity is deeply affected by abiotic stresses. Many studies explored how soybean faces stress to identify key elements and improve productivity through breeding and genetic engineering. This review summarizes recent progress in soybean gene expression regulation through epigenetic modifications and circRNAs pathways, and points out the knowledge gaps that are important to study by the scientific community. It focuses on epigenetic factors participating in soybean abiotic stress responses, and chromatin modifications in response to stressful environments and draws attention to the regulatory potential of circular RNA in post-transcriptional processing.

Genetic control of tolerance to drought stress in soybean

BACKGROUND

Drought stress limits the production of soybean [Glycine max (L.) Merr.], which is the most grown high-value legume crop worldwide. Breeding for drought tolerance is a difficult endeavor and understanding the genetic basis of drought tolerance in soybean is therefore crucial for harnessing the genomic regions involved in the tolerance mechanisms. A genome-wide association study (GWAS) analysis was applied in a soybean germplasm collection (the EUCLEG collection) of 359 accessions relevant for breeding in Europe, to identify genomic regions and candidate genes involved in the response to short duration and long duration drought stress (SDS and LDS respectively) in soybean.

RESULTS

The phenotypic response to drought was stronger in the long duration drought (LDS) than in the short duration drought (SDS) experiment. Over the four traits considered (canopy wilting, leaf senescence, maximum absolute growth rate and maximum plant height) the variation was in the range of 8.4-25.2% in the SDS, and 14.7-29.7% in the LDS experiments. The GWAS analysis identified a total of 17 and 22 significant marker-trait associations for four traits in the SDS and LDS experiments, respectively. In the genomic regions delimited by these markers we identified a total of 12 and 16 genes with putative functions that are of particular relevance for drought stress responses including stomatal movement, root formation, photosynthesis, ABA signaling, cellular protection and cellular repair mechanisms. Some of these genomic regions co-localized with previously known QTLs for drought tolerance traits including water use efficiency, chlorophyll content and photosynthesis.

CONCLUSION

Our results indicate that the mechanism of slow wilting in the SDS might be associated with the characteristics of the root system, whereas in the LDS, slow wilting could be due to low stomatal conductance and transpiration rates enabling a high WUE. Drought-induced leaf senescence was found to be associated to ABA and ROS responses. The QTLs related to WUE contributed to growth rate and canopy height maintenance under drought stress. Co-localization of several previously known QTLs for multiple agronomic traits with the SNPs identified in this study, highlights the importance of the identified genomic regions for the improvement of agronomic performance in addition to drought tolerance in the EUCLEG collection.

The intervention of classical and molecular breeding approaches to enhance flooding stress tolerance in soybean - An review

Abiotic stresses and climate changes cause severe loss of yield and quality of crops and reduce the production area worldwide. Flooding stress curtails soybean growth, yield, and quality and ultimately threatens the global food supply chain. Flooding tolerance is a multigenic trait. Tremendous research in molecular breeding explored the potential genomic regions governing flood tolerance in soybean. The most robust way to develop flooding tolerance in soybean is by using molecular methods, including quantitative trait loci (QTL) mapping, identification of transcriptomes, transcription factor analysis, CRISPR/Cas9, and to some extent, genome-wide association studies (GWAS), and multi-omics techniques. These powerful molecular tools have deepened our knowledge about the molecular mechanism of flooding stress tolerance. Besides all this, using conventional breeding methods (hybridization, introduction, and backcrossing) and other agronomic practices is also helpful in combating the rising flooding threats to the soybean crop. The current review aims to summarize recent advancements in breeding flood-tolerant soybean, mainly by using molecular and conventional tools and their prospects. This updated picture will be a treasure trove for future researchers to comprehend the foundation of flooding tolerance in soybean and cover the given research gaps to develop tolerant soybean cultivars able to sustain growth under extreme climatic changes.

Responses of the growth, photosynthetic characteristics, endogenous hormones and antioxidant activity of Carpinus betulus L. seedlings to different light intensities

Light is an important ecological factor that affects plant growth, survival and distribution. Carpinus betulus L. is native to central Europe and is used as an ornamental plant with strong adaptability. It is an important tree species for landscaping and timber use. What's more, the antioxidant- and anticancer-related properties of C. betulus leaf extracts are remarkable, that make it a possible raw material for medicine. Light intensity is an important environmental factor affecting the growth and physicochemical changes of C. betulus, but the mechanism of its effect on this species still remains unknown. In this study, the growth, photosynthetic characteristics, endogenous hormones and antioxidant activity responses of C. betulus seedlings to four light intensity gradients (T0: normal light; T1: 75%; T2: 50%; T3: 25% of normal light) were studied after 60 days of shading treatments. The results showed a significant effect of low light intensity on the values of the growth and physiological parameters of C. betulus. The low light intensity caused the inhibition of plant biomass accumulation and the degradation of photosynthetic capacity and stomatal behavior and aggravated the cell membrane lipid peroxidation. However, the plant height growth, leaf area, specific leaf area, photosynthetic pigment content, and contents of GA₃ and ABA of C. betulus increased with decreasing light intensity. We found that C. betulus can tolerate mild (T1) and moderate (T2) shading stress by developing photoprotective mechanisms and maintaining relatively high concentrations of organic osmolytes and high antioxidant enzyme activities (superoxide dismutase, peroxidase, catalase and ascorbate peroxidase), but the ability of C. betulus to synthesize osmotic substances and enzymatic antioxidants was impaired under severe shading conditions (T3). Our results suggest that C. betulus can make effective use of low light resources by adjusting its morphology, material distribution, photosynthetic rate and antioxidant enzyme system in suitable low-light environments (50%~75% of normal light); however, light intensity levels that are too low (25% of normal light) will have adverse effects on plant growth. The results of this study provide not only new insights into the response mechanisms of C. betulus to light intensity but also a scientific basis for the cultivation and application of C. betulus in China.

Modulation of GmFAD3 expression alters abiotic stress responses in soybean

KEY MESSAGE

This study focused on enhancing resilience of soybean crops to drought and salinity stresses by overexpression of GmFAD3A gene, which plays an important role in modulating membrane fluidity and ultimately influence plants response to various abiotic stresses. Fatty acid desaturases (FADs) are a class of enzymes that mediate desaturation of fatty acids by introducing double bonds. They play an important role in modulating membrane fluidity in response to various abiotic stresses. However, a comprehensive analysis of GmFAD3 in drought and salinity stress tolerance in soybean is lacking. We used bean pod mottle virus (BPMV)-based vector for achieving rapid and efficient overexpression as well as silencing of Omega-3 Fatty Acid Desaturase gene from Glycine max (GmFAD3) to assess the functional role of GmFAD3 in abiotic stress responses in soybean. Higher levels of recombinant BPMV-GmFAD3A transcripts were detected in overexpressing soybean plants. Overexpression of GmFAD3A in soybean resulted in increased levels of jasmonic acid and higher expression of GmWRKY54 as compared to mock-inoculated, vector-infected and FAD3-silenced soybean plants under drought and salinity stress conditions. The GmFAD3A-overexpressing plants showed higher levels of chlorophyll content, efficient photosystem-II, relative water content, transpiration rate, stomatal conductance, proline content and also cooler canopy under drought and salinity stress conditions as compared to mock-inoculated, vector-infected and FAD3-silenced soybean plants. Results from the current study revealed that GmFAD3A-overexpressing soybean plants exhibited tolerance to drought and salinity stresses. However, soybean plants silenced for GmFAD3 were vulnerable to drought and salinity stresses.

Physiological Comparison of Wheat and Maize Seedlings Responses to Water Stresses

The aim of the study was to investigate specific responses of spring wheat (C3 photosynthesis) and maize (C4 photosynthesis) to drought and flooding stress. Analyses of water content, gas exchange intensity, photosynthetic apparatus activity, chlorophyll content, plant height and biological membrane integrity were performed on the 10th day of drought and flooding in both species at the third leaf stage. A specific response of wheat under both drought and flooding conditions involved an increase in ETo/RC ratio, describing electron transport flux converted into a single reaction center in PSII. Correlations between electrolyte leakage and the probability of electron transport beyond the plastoquinone QA, and the amount of energy used for the electron transport were also found. A specific response of maize during flooding was the increase of stomatal conductance. Additionally, a significant correlation between PN/Ci and relative water content was exhibited. Furthermore, the parameters differentiating the studied species only under stressful conditions were rendered. The application of such parameters can be widely used, e.g., for studying the reaction of a potential cultivars to drought and flooding. Providing such information to potential farmers can help better select cultivars for their environmental conditions.

Fibrillin2 in chloroplast plastoglobules participates in photoprotection and jasmonate-induced senescence

Fibrillins (FBNs) are the major structural proteins of plastoglobules (PGs) in chloroplasts. PGs are associated with defense against abiotic and biotic stresses, as well as lipid storage. Although FBN2 is abundant in PGs, its independent function under abiotic stress has not yet been identified. In this study, the targeting of FBN2 to PGs was clearly demonstrated using an FBN2-YFP fusion protein. FBN2 showed higher expression in green photosynthetic tissues and was upregulated at the transcriptional level under high-light stress. The photosynthetic capacity of fbn2 knockout mutants generated using CRISPR/Cas9 technology decreased rapidly compared with that of wild-type (WT) plants under high-light stress. In addition to the photoprotective function of FBN2, fbn2 mutants had lower levels of plastoquinone-9 and plastochromanol-8. The fbn2 mutants were highly sensitive to methyl jasmonate (MeJA) and exhibited root growth inhibition and a pale-green phenotype due to reduced chlorophyll content. Consistently, upon MeJA treatment, the fbn2 mutants showed faster leaf senescence and more rapid chlorophyll degradation with decreased photosynthetic ability compared with the WT plants. The results of this study suggest that FBN2 is involved in protection against high-light stress and acts as an inhibitor of jasmonate-induced senescence in Arabidopsis (Arabidopsis thaliana).

The Role of Phytohormones in Plant Response to Flooding

Climatic variations influence the morphological, physiological, biological, and biochemical states of plants. Plant responses to abiotic stress include biochemical adjustments, regulation of proteins, molecular mechanisms, and alteration of post-translational modifications, as well as signal transduction. Among the various abiotic stresses, flooding stress adversely affects the growth of plants, including various economically important crops. Biochemical and biological techniques, including proteomic techniques, provide a thorough understanding of the molecular mechanisms during flooding conditions. In particular, plants can cope with flooding conditions by embracing an orchestrated set of morphological adaptations and physiological adjustments that are regulated by an elaborate hormonal signaling network. With the help of these findings, the main objective is to identify plant responses to flooding and utilize that information for the development of flood-tolerant plants. This review provides an insight into the role of phytohormones in plant response mechanisms to flooding stress, as well as different mitigation strategies that can be successfully administered to improve plant growth during stress exposure. Ultimately, this review will expedite marker-assisted genetic enhancement studies in crops for developing high-yield lines or varieties with flood tolerance.

The mysterious role of fibrillin in plastid metabolism: current advances in understanding

Fibrillins (FBNs) are a family of genes in cyanobacteria, algae, and plants. The proteins they encode possess a lipid-binding motif, exist in various types of plastids, and are associated with lipid bodies called plastoglobules, implicating them in lipid metabolism. FBNs present in the thylakoid and stroma are involved in the storage, transport, and synthesis of lipid molecules for photoprotective functions against high-light stress. In this review, the diversity of subplastid locations in the evolution of FBNs, regulation of FBNs expression by various stresses, and the role of FBNs in plastid lipid metabolism are comprehensively summarized and directions for future research are discussed.

Intraspecific Variation for Leaf Physiological and Root Morphological Adaptation to Drought Stress in Alfalfa (Medicago sativa L.)

Drought stress reduces crop biomass yield and the profitability of rainfed agricultural systems. Evaluation of populations or accessions adapted to diverse geographical and agro-climatic environments sheds light on beneficial plant responses to enhance and optimize yield in resource-limited environments. This study used the morphological and physiological characteristics of leaves and roots from two different alfalfa subspecies during progressive drought stress imposed on controlled and field conditions. Two different soils (Experiments 1 and 2) imposed water stress at different stress intensities and crop stages in the controlled environment. Algorithm-based image analysis of leaves and root systems revealed key morphological and physiological traits associated with biomass yield under stress. The Medicago sativa subspecies (ssp.) sativa population, PI478573, had smaller leaves and maintained higher chlorophyll content (CC), leaf water potential, and osmotic potential under water stress. In contrast, M. sativa ssp. varia, PI502521, had larger leaves, a robust root system, and more biomass yield. In the field study, an unmanned aerial vehicle survey revealed PI502521 to have a higher normalized difference vegetation index (vegetation cover and plant health characteristics) throughout the cropping season, whereas PI478573 values were low during the hot summer and yielded low biomass in both irrigated and rainfed treatments. RhizoVision Explorer image analysis of excavated roots revealed a smaller diameter and a narrow root angle as target traits to increase alfalfa biomass yield irrespective of water availability. Root architectural traits such as network area, solidity, volume, surface area, and maximum radius exhibited significant variation at the genotype level only under limited water availability. Different drought-adaptive strategies identified across subspecies populations will benefit the plant under varying levels of water limitation and facilitate the development of alfalfa cultivars suitable across a broad range of growing conditions. The alleles from both subspecies will enable the development of drought-tolerant alfalfa with enhanced productivity under limited water availability.

Seed Germination Behavior, Growth, Physiology and Antioxidant Metabolism of Four Contrasting Cultivars under Combined Drought and Salinity in Soybean

Drought and salinity stresses are persistent threat to field crops and are frequently mentioned as major constraints on worldwide agricultural productivity. Moreover, their severity and frequency are predicted to rise in the near future. Therefore, in the present study we investigated the mechanisms underlying plant responses to drought (5, 10 and 15% polyethylene glycol, PEG-6000), salinity (50, 100, and 150 mM NaCl), and their combination, particularly at the seed germination stage, in terms of photosynthesis and antioxidant activity, in four soybean cultivars, viz., PI408105A (PI5A), PI567731 (PI31), PI567690 (PI90), and PI416937 (PI37). Results showed that seed germination was enhanced by 10% PEG and decreased by 15% PEG treatments compared to the control, while seed germination was drastically decreased under all levels of NaCl treatment. Furthermore, combined drought and salinity treatment reduced plant height and root length, shoot and root total weights, and relative water content compared with that of control. However, the reductions were not similar among the varieties, and definite growth retardations were observed in cultivar PI5A under drought and in PI37 under salinity. In addition, all treatments resulted in substantially reduced contents of chlorophyll pigment, anthocyanin, and chlorophyll fluorescence; and increased lipid peroxidation, electrolyte leakage, and non-photochemical quenching in all varieties of soybean as compared to the control plants. However, proline, amino acids, sugars, and secondary metabolites were increased with the drought and salinity stresses alone. Moreover, the reactive oxygen species accumulation was accompanied by improved enzymatic antioxidant activity, such as that of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase. However, the enhancement was most noticeable in PI31 and PI90 under both treatments. In conclusion, the cultivar PI31 has efficient drought and salinity stress tolerance mechanisms, as illustrated by its superior photosynthesis, osmolyte accumulation, antioxidative enzyme activity, and secondary metabolite regulation, compared to the other cultivars, when stressed.

Differential Physio-Biochemical and Metabolic Responses of Peanut (Arachis hypogaea L.) under Multiple Abiotic Stress Conditions

The frequency and severity of extreme climatic conditions such as drought, salinity, cold, and heat are increasing due to climate change. Moreover, in the field, plants are affected by multiple abiotic stresses simultaneously or sequentially. Thus, it is imperative to compare the effects of stress combinations on crop plants relative to individual stresses. This study investigated the differential regulation of physio-biochemical and metabolomics parameters in peanut (Arachis hypogaea L.) under individual (salt, drought, cold, and heat) and combined stress treatments using multivariate correlation analysis. The results showed that combined heat, salt, and drought stress compounds the stress effect of individual stresses. Combined stresses that included heat had the highest electrolyte leakage and lowest relative water content. Lipid peroxidation and chlorophyll contents did not significantly change under combined stresses. Biochemical parameters, such as free amino acids, polyphenol, starch, and sugars, significantly changed under combined stresses compared to individual stresses. Free amino acids increased under combined stresses that included heat; starch, sugars, and polyphenols increased under combined stresses that included drought; proline concentration increased under combined stresses that included salt. Metabolomics data that were obtained under different individual and combined stresses can be used to identify molecular phenotypes that are involved in the acclimation response of plants under changing abiotic stress conditions. Peanut metabolomics identified 160 metabolites, including amino acids, sugars, sugar alcohols, organic acids, fatty acids, sugar acids, and other organic compounds. Pathway enrichment analysis revealed that abiotic stresses significantly affected amino acid, amino sugar, and sugar metabolism. The stress treatments affected the metabolites that were associated with the tricarboxylic acid (TCA) and urea cycles and associated amino acid biosynthesis pathway intermediates. Principal component analysis (PCA), partial least squares-discriminant analysis (PLS-DA), and heatmap analysis identified potential marker metabolites (pinitol, malic acid, and xylopyranose) that were associated with abiotic stress combinations, which could be used in breeding efforts to develop peanut cultivars that are resilient to climate change. The study will also facilitate researchers to explore different stress indicators to identify resistant cultivars for future crop improvement programs.

Direct Infusion Metabolomics of the Photosystem and Chlorophyll Related Metabolites within a Drought Tolerant Plant Introduction of Glycine max

Drought is the most prolific form of abiotic stress that legumes and cereal plants alike can endure, and the planting of an improper cultivar at the beginning of a season can cause unexpected losses up to fifty percent under water deficient conditions. Herein, a plant introduction (PI) of an exotic cultivar of soybean (Glycine max), PI 567731, which demonstrates a slow wilting (SW) canopy phenotype in maturity group III, was profiled under drought conditions in field trials in Missouri against a drought susceptible check cultivar, Pana. Metabolomic profiling was carried out on samples of leaves from each of these cultivars at V5 and R2 growth stages both while irrigated and while under drought stress for three weeks. PI 567731 was observed to have differential phytochemical content, and enhanced levels of chlorophyll (Chl) a/b and pheophytin (Pheo) were profiled by direct infusion electrospray Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Indicating drought induced changes of the photosystem and photosynthetic capabilities alongside water preservation strategies are important within the SW phenotype drought response. Subsequent multivariate analysis was able to form predictive models, encompassing the variance of growth and drought stress of the cultivar. Moreover, the existence of unique Chl-related metabolites (CRM) (m/z > 900) were confirmed through tandem mass spectrometry. The resultant coordination of fatty acids to the core of the porphyrin ring was observed and played an unknown role in the proliferation of the photosynthesis. However, the relative ratio of the most abundant CRM is undisturbed by drought stress in PI 567731, in contrast to the drought susceptible cultivar. These results provide key insights into drought related metabolic mechanisms.

Co-inoculation of rhizobacteria promotes growth, yield, and nutrient contents in soybean and improves soil enzymes and nutrients under drought conditions

Drought stress is the major abiotic factor limiting crop production. Co-inoculating crops with nitrogen fixing bacteria and plant growth-promoting rhizobacteria (PGPR) improves plant growth and increases drought tolerance in arid or semiarid areas. Soybean is a major source of high-quality protein and oil for humans. It is susceptible to drought stress conditions. The co-inoculation of drought-stressed soybean with nodulating rhizobia and root-colonizing, PGPR improves the root and the shoot growth, formation of nodules, and nitrogen fixation capacity in soybean. The present study was aimed to observe if the co-inoculation of soybean (Glycine max L. (Merr.) nodulating with Bradyrhizobium japonicum USDA110 and PGPR Pseudomonas putida NUU8 can enhance drought tolerance, nodulation, plant growth, and nutrient uptake under drought conditions. The results of the study showed that co-inoculation with B. japonicum USDA110 and P. putida NUU8 gave more benefits in nodulation and growth of soybean compared to plants inoculated with B. japonicum USDA110 alone and uninoculated control. Under drought conditions, co-inoculation of B. japonicum USDA 110 and P. putida NUU8 significantly enhanced the root length by 56%, shoot length by 33%, root dry weight by 47%, shoot dry weight by 48%, and nodule number 17% compared to the control under drought-stressed. Co-inoculation with B. japonicum, USDA 110 and P. putida NUU8 significantly enhanced plant and soil nutrients and soil enzymes compared to control under normal and drought stress conditions. The synergistic use of B. japonicum USDA110 and P. putida NUU8 improves plant growth and nodulation of soybean under drought stress conditions. The results suggested that these strains could be used to formulate a consortium of biofertilizers for sustainable production of soybean under drought-stressed field conditions.

Association mapping of drought tolerance and agronomic traits in rice (Oryza sativa L.) landraces

BACKGROUND

Asian cultivars were predominantly represented in global rice panel selected for sequencing and to identify novel alleles for drought tolerance. Diverse genetic resources adapted to Indian subcontinent were not represented much in spite harboring useful alleles that could improve agronomic traits, stress resilience and productivity. These rice accessions are valuable genetic resource in developing rice varieties suited to different rice ecosystem that experiences varying drought stress level, and at different crop stages. A core collection of rice germplasm adapted to Southwestern Indian peninsular genotyped using SSR markers and characterized by contrasting water regimes to associate genomic regions for physiological, root traits and yield related traits. Genotyping-By-Sequencing of selected accessions within the diverse panel revealed haplotype variation in genic content within genomic regions mapped for physiological, morphological and root traits.

RESULTS

Diverse rice panel (99 accessions) were evaluated in field and measurements on plant physiological, root traits and yield related traits were made over five different seasons experiencing varying drought stress intensity at different crop stages. Traits like chlorophyll stability index, leaf rolling, days to 50% flowering, chlorophyll content, root volume and root biomass were identified as best predictors of grain yield under stress. Association mapping revealed genetic variation among accessions and revealed 14 genomic targets associated with different physiological, root and plant production traits. Certain accessions were found to have beneficial allele to improve traits, plant height, root length and spikelet fertility, that contribute to the grain yield under stress. Genomic characterization of eleven accessions revealed haplotype variation within key genomic targets on chromosomes 1, 4, 6 and 11 for potential use as molecular markers to combine drought avoidance and tolerance traits. Genes mined within the genomic QTL intervals identified were prioritized based on tissue specific expression level in publicly available rice transcriptome data.

CONCLUSION

The genetic and genomic resources identified will enable combining traits with agronomic value to optimize yield under stress and hasten trait introgression into elite cultivars. Alleles associated with plant height, specific leaf area, root length from PTB8 and spikelet fertility and grain weight from PTB26 can be harnessed in future rice breeding program.

Leaf photosynthesis and senescence in heated and droughted field-grown soybean with contrasting seed protein concentration

Soybean plants under heat and drought generate a multiplicity of responses in photosynthesis and senescence, impairing growth, yield, and seed quality. The goal of this study was to analyze and quantify independent and combined effects of heat and drought during seed filling on photosynthesis and senescence, and its subsequent effects on the filling duration in soybean genotypes contrasting on seed protein. Two field experiments were conducted using high and low seed protein genotypes. During seed filling plants were exposed to four treatments: control (ambient temperature and soil water content near field capacity), heat stress (HS, episodes above 32 °C 6 h d-1) during 15-d, drought stress (DS, soil water content ≤ 25% of field capacity) during the entire seed filling, and HS × DS. We found non-genotypic variation in leaf photosynthesis in both experiments. Irrigated HS, did no alter photosynthesis and senescence. Drought, regardless of heat, reduced photosynthesis, carbohydrate production and affected membranes integrity, leading to premature leaf senescence and shortening the filling duration. The magnitude of responses was similar between drought alone and stresses combined, indicating a dominant role of drought over heat. The seed filling duration was not shorter in high protein compared to low protein genotype, nor was senescence pattern altered across treatments. These results indicated that the higher seed protein content exhibited by some genotypes are not necessarily associated with an earlier onset of senescence and shortening of the filling period as suggested by previous studies analyzing genotypes differing in protein concentration.

Effect of uniconazole to soybean seed priming treatment under drought stress at VC stage

Studying the mechanism of drought stress in soybean root at vegetative cotyledon (VC) stage by soaking seeds with uniconazole revealed new insights into soybean stress physiology. Therefore, a completely random pot experiments with different time gradients for water cut-off (24, 48, 72, 96 and 120 h, respectively) were carried out with uniconazole (0.4 mgL-1) with respect to morphological, microscopic, ultramicroscopic, physiological, and molecular studies on varieties Hefeng55 (H50, drought tolerant variety) and Kenfeng16 (K16, drought susceptible variety). Results revealed that uniconazole effectively alleviated the inhibition on root growth caused by drought stress, increased the number of root tips, significantly reduced lignification of vessels, alleviated the damage of mitochondria and nucleus caused by drought stress, further strengthened osmotic adjustment system and antioxidant system, especially when the soil moisture content was less than 14%, broke expression restriction of IAA due to drought stress, and inhibited GA3 generation; finally, we found that high-intensity drought stress significantly increased the expression levels of GmNAC003, GmNAC004, GmNAC015, GmNAC020, GmHK07, GmRR01, GmRR02 and GmRR16 genes relating to drought tolerance, while uniconazole had a significant inhibitory effect on GmNAC003, GmNAC004, GmNAC015, GmNAC020, GmRR01, GmRR02 and GmRR16 genes. Our results provided a reference for the mechanism of drought resistance in legume and the effect of uniconazole on alleviating drought stress.

Improved response of triploid citrus varieties to water deficit is related to anatomical and cytological properties

Polyploidy plays a major role in citrus plant breeding to improve the adaptation of polyploid rootstocks as well as scions to adverse conditions and to enhance agronomic characteristics. In Citrus breeding programs, triploidy could be a useful tool to react to environmental issues and consumer demands because the produced fruits are seedless. In this study, we compared the physiological, biochemical, morphological, and ultrastructural responses to water deficit of triploid and diploid citrus varieties obtained from 'Fortune' mandarin and 'Ellendale' tangor hybridization. One diploid clementine tree was included and used as a reference. All studied scions were grafted on C-35 citrange rootstock. Triploidy decreased stomatal density and increased stomata size. The number of chloroplasts increased in 3x varieties. These cytological properties may explain the greater photosynthetic capacity (P_net, g_s, F_v/F_m) and enhanced water-holding capacity (RWC, proline). In addition, reduced degradation of ultrastructural organelles (chloroplasts and mitochondria) and thylakoids accompanied by less photosynthetic activity and low oxidative damages were found in 3x varieties. Triploid varieties, especially T40-3x, had a better ability to limit water loss and dissipate excess energy (NPQ) to protect photosystems. Higher starch reserves in 3x varieties suggest a better carbon and energy supply and increases in plastoglobuli size suggest less oxidative damage (H₂O₂, MDA), especially in T40-3x, and preservation of photosynthetic apparatus. Taken together, our results suggest that desirable cytological and ultrastructural traits induced by triploidy improve water stress response and could be a useful stress marker during environmental constraints.

Photosynthetic resistance and resilience under drought, flooding and rewatering in maize plants

Abnormally altered precipitation patterns induced by climate change have profound global effects on crop production. However, the plant functional responses to various precipitation regimes remain unclear. Here, greenhouse and field experiments were conducted to determine how maize plant functional traits respond to drought, flooding and rewatering. Drought and flooding hampered photosynthetic capacity, particularly when severe and/or prolonged. Most photosynthetic traits recovered after rewatering, with few compensatory responses. Rewatering often elicited high photosynthetic resilience in plants exposed to severe drought at the end of plant development, with the response strongly depending on the drought severity/duration. The associations of chlorophyll concentrations with photosynthetically functional activities were stronger during post-tasseling than pre-tasseling, implying an involvement of leaf age/senescence in responses to episodic drought and subsequent rewatering. Coordinated changes in chlorophyll content, gas exchange, fluorescence parameters (PSII quantum efficiency and photochemical/non-photochemical radiative energy dissipation) possibly contributed to the enhanced drought resistance and resilience and suggested a possible regulative trade-off. These findings provide fundamental insights into how plants regulate their functional traits to deal with sporadic alterations in precipitation. Breeding and management of plants with high resistance and resilience traits could help crop production under future climate change.

Antioxidant Enzymatic Activity and Osmotic Adjustment as Components of the Drought Tolerance Mechanism in Carex duriuscula

Drought stress is a major environmental constraint for plant growth. Climate-change-driven increases in ambient temperatures resulted in reduced or unevenly distributed rainfalls, leading to increased soil drought. Carex duriuscula C. A. Mey is a typical drought-tolerant sedge, but few reports have examined the mechanisms conferring its tolerant traits. In the present study, the drought responses of C. duriuscula were assessed by quantifying activity of antioxidant enzymes in its leaf and root tissues and evaluating the relative contribution of organic and inorganic osmolyte in plant osmotic adjustment, linking it with the patterns of the ion acquisition by roots. Two levels of stress-mild (MD) and severe (SD) drought treatments-were used, followed by re-watering. Drought stress caused reduction in a relative water content and chlorophyll content of leaves; this was accompanied by an increase in the hydrogen peroxide (H2O2) and superoxide (O2-) contents in leaves and roots. Under MD stress, the activities of catalase (CAT), peroxidase (POD), and glutathione peroxidase (GPX) increased in leaves, whereas, in roots, only CAT and POD activities increased. SD stress led to an increase in the activities of CAT, POD, superoxide dismutase (SOD), and GPX in both tissues. The levels of proline, soluble sugars, and soluble proteins in the leaves also increased. Under both MD and SD stress conditions, C. duriuscula increased K+, Na+, and Cl- uptake by plant roots, which resulted in an increased K+, Na+, and Cl- concentrations in leaves and roots. This reliance on inorganic osmolytes enables a cost-efficient osmotic adjustment in C. duriuscula. Overall, this study revealed that C. duriuscula was able to survive arid environments due to an efficient operation of its ROS-scavenging systems and osmotic adjustment mechanisms.

Transcriptomics of Biostimulation of Plants Under Abiotic Stress

Plant biostimulants are compounds, living microorganisms, or their constituent parts that alter plant development programs. The impact of biostimulants is manifested in several ways: via morphological, physiological, biochemical, epigenomic, proteomic, and transcriptomic changes. For each of these, a response and alteration occur, and these alterations in turn improve metabolic and adaptive performance in the environment. Many studies have been conducted on the effects of different biotic and abiotic stimulants on plants, including many crop species. However, as far as we know, there are no reviews available that describe the impact of biostimulants for a specific field such as transcriptomics, which is the objective of this review. For the commercial registration process of products for agricultural use, it is necessary to distinguish the specific impact of biostimulants from that of other legal categories of products used in agriculture, such as fertilizers and plant hormones. For the chemical or biological classification of biostimulants, the classification is seen as a complex issue, given the great diversity of compounds and organisms that cause biostimulation. However, with an approach focused on the impact on a particular field such as transcriptomics, it is perhaps possible to obtain a criterion that allows biostimulants to be grouped considering their effects on living systems, as well as the overlap of the impact on metabolism, physiology, and morphology occurring between fertilizers, hormones, and biostimulants.

Prioritization and Evaluation of Flooding Tolerance Genes in Soybean [Glycine max (L.) Merr.]

Soybean [Glycine max (L.) Merr.] is one of the most important legume crops abundant in edible protein and oil in the world. In recent years there has been increasingly more drastic weather caused by climate change, with flooding, drought, and unevenly distributed rainfall gradually increasing in terms of the frequency and intensity worldwide. Severe flooding has caused extensive losses to soybean production and there is an urgent need to breed strong soybean seeds with high flooding tolerance. The present study demonstrates bioinformatics big data mining and integration, meta-analysis, gene mapping, gene prioritization, and systems biology for identifying prioritized genes of flooding tolerance in soybean. A total of 83 flooding tolerance genes (FTgenes), according to the appropriate cut-off point, were prioritized from 36,705 test genes collected from multidimensional genomic features linking to soybean flooding tolerance. Several validation results using independent samples from SoyNet, genome-wide association study, SoyBase, GO database, and transcriptome databases all exhibited excellent agreement, suggesting these 83 FTgenes were significantly superior to others. These results provide valuable information and contribution to research on the varieties selection of soybean.

Photosynthetic performance of quinoa (Chenopodium quinoa Willd.) after exposure to a gradual drought stress followed by a recovery period

Drought is an abiotic scourge, one of the major environmental stress factors that adversely affect plant growth and photosynthesis machinery through a disruption of cell organelles, arrangement thylakoid membranes and the electron transport chain. Herein, we probed the effect of drought stress on photosynthetic performance of Chenopodium quinoa Willd. Beforehand, plants were subjected to water deficit (as 15% Field Capacity, FC) for one (D-1W) or two weeks (D-2W), and were then re-watered at 95% FC for 2 weeks. Light and electron microscopy analysis of leaves showed no apparent changes in mesophyll cell organization and chloroplast ultrastructure after one week of drought stress, while a swelling of thylakoids and starch accumulation were observed after the prolonged drought (D-2W). The latter induced a decrease in both PSI and PSII quantum yields which was accompanied by an increase in F₀ (minimum fluorescence) and a decline in F_m (maximum fluorescence). Drought stress influenced the fluorescence transients, where the major changes at the OJIP prompt FI level were detected in the OJ and IP phases. Prolonged drought induced a decrease in chl a fluorescence at IP phase which was readjusted and established back after re-watering and even more an increase was observed after 2 weeks of recovery. The maximum quantum yield of primary photochemistry (φ_Po) was unaffected by the different drought stress regimes. Drought induced an increase in the ABS/RC and DI₀/RC ratios which was concurrent to a stable φ_Po (maximum quantum yield of PSII primary photochemistry). A substantial decrease in PI_(ABS) was detected especially, during severe drought stress (D-2W) suggesting a drop in the PSII efficiency and the level of electron transport through the plastoquinone pool (PQ pool) towards oxidized PSI RCs (P₇₀₀⁺). The immunoblot analysis of the main PSII proteins revealed considerable changes in the D1, D2, CP47, OEC, PsbQ and LHCII proteins under drought. These changes depend on the stress duration and recovery period. The main message of this investigation is the elevated recovery capacities of PSII and PSI photochemical activities after re-watering.

Towards Developing Drought-smart Soybeans

Drought is one of the significant abiotic stresses threatening crop production worldwide. Soybean is a major legume crop with immense economic significance, but its production is highly dependent on optimum rainfall or abundant irrigation. Also, in dry periods, it may require supplemental irrigation for drought-susceptible soybean varieties. The effects of drought stress on soybean including osmotic adjustments, growth morphology and yield loss have been well studied. In addition, drought-resistant soybean cultivars have been investigated for revealing the mechanisms of tolerance and survival. Advanced high-throughput technologies have yielded remarkable phenotypic and genetic information for producing drought-tolerant soybean cultivars, either through molecular breeding or transgenic approaches. Further, transcriptomics and functional genomics have led to the characterisation of new genes or gene families controlling drought response. Interestingly, genetically modified drought-smart soybeans are just beginning to be released for field applications cultivation. In this review, we focus on breeding and genetic engineering approaches that have successfully led to the development of drought-tolerant soybeans for commercial use.

Growth and respiratory metabolic adaptation strategies of riparian plant Distylium chinense to submergence by the field study and controlled experiments

Submergence tolerance is crucial when thinking in promising species for restoration of ecosystems prone to suffer extreme flooding events. In this study, two-year-old seedlings of Distylium chinense were subjected to one field study and five controlled experiments: unsubmerged and watered daily as controls (CK) and completely submerged for 30, 60, 90 and 120 days, respectively followed by a 60-day recovery period to test the submergence tolerance. The results showed that the survival decreased with the increasing flooding duration. Different submergence duration treatments affected dry mass accumulation and carbohydrate content of roots, stems and leaves. Flooding stress affected the activities of pyruvate decarboxylase (PDC), ethanol dehydrogenase (ADH) and lactic dehydrogenase (LDH) enzymes, which indicated the roots and leaves adapt to long-term flooding by reinforcing their anaerobic respiration and activities of ADH were higher than those of LDH for roots and leaves with stronger alcoholic fermentation mainly. After de-submergence, the recovery patterns of carbohydrate were coincided with those of dry mass accumulation of the roots, stems and leaves. A significant regression equation analysis showed root starch content and dry mass accumulation were the major factors affecting the seedling survival. And D. chinense accumulated substantial amounts of carbohydrate before submergence and invested more in roots and stems than in leaves, which enhances long-term survival under submergence. Carbohydrate storage is a key functional trait that can explain high survival under submergence. D. chinense may have adopted a suite of growth and respiratory metabolic adaptation strategies to survive long-term submergence.

Potential of Hyperspectral and Thermal Proximal Sensing for Estimating Growth Performance and Yield of Soybean Exposed to Different Drip Irrigation Regimes Under Arid Conditions

Proximal hyperspectral sensing tools could complement and perhaps replace destructive traditional methods for accurate estimation and monitoring of various morpho-physiological plant indicators. In this study, we assessed the potential of thermal imaging (TI) criteria and spectral reflectance indices (SRIs) to monitor different vegetative growth traits (biomass fresh weight, biomass dry weight, and canopy water mass) and seed yield (SY) of soybean exposed to 100%, 75%, and 50% of estimated crop evapotranspiration (ETc). These different plant traits were evaluated and related to TI criteria and SRIs at the beginning bloom (R1) and full seed (R6) growth stages. Results showed that all plant traits, TI criteria, and SRIs presented significant variations (p < 0.05) among irrigation regimes at both growth stages. The performance of TI criteria and SRIs for assessment of vegetative growth traits and SY fluctuated when relationships were analyzed for each irrigation regime or growth stage separately or when the data of both conditions were combined together. TI criteria and SRIs exhibited a moderate to strong relationship with vegetative growth traits when data from different irrigation regimes were pooled together at each growth stage or vice versa. The R6 and R1 growth stages are suitable for assessing SY under full (100% ETc) and severe (50% ETc) irrigation regimes, respectively, using SRIs. The overall results indicate that the usefulness of the TI and SRIs for assessment of growth, yield, and water status of soybean under arid conditions is limited to the growth stage, the irrigation level, and the combination between them.

The bZIP Transcription Factor GmbZIP15 Negatively Regulates Salt- and Drought-Stress Responses in Soybean

Soybean (Glycine max), as an important oilseed crop, is constantly threatened by abiotic stress, including that caused by salinity and drought. bZIP transcription factors (TFs) are one of the largest TF families and have been shown to be associated with various environmental-stress tolerances among species; however, their function in abiotic-stress response in soybean remains poorly understood. Here, we characterized the roles of soybean transcription factor GmbZIP15 in response to abiotic stresses. The transcript level of GmbZIP15 was suppressed under salt- and drought-stress conditions. Overexpression of GmbZIP15 in soybean resulted in hypersensitivity to abiotic stress compared with wild-type (WT) plants, which was associated with lower transcript levels of stress-responsive genes involved in both abscisic acid (ABA)-dependent and ABA-independent pathways, defective stomatal aperture regulation, and reduced antioxidant enzyme activities. Furthermore, plants expressing a functional repressor form of GmbZIP15 exhibited drought-stress resistance similar to WT. RNA-seq and qRT-PCR analyses revealed that GmbZIP15 positively regulates GmSAHH1 expression and negatively regulates GmWRKY12 and GmABF1 expression in response to abiotic stress. Overall, these data indicate that GmbZIP15 functions as a negative regulator in response to salt and drought stresses.

Maintenance of photosynthetic capacity in flooded tomato plants with reduced ethylene sensitivity

Ethylene is considered one of the most important plant hormones orchestrating plant responses to flooding stress. However, ethylene may induce deleterious effects on plants, especially when produced at high rates in response to stress. In this paper, we explored the effect of attenuated ethylene sensitivity in the Never ripe (Nr) mutant on leaf photosynthetic capacity of flooded tomato plants. We found out that reduced ethylene perception in Nr plants was associated with a more efficient photochemical and non-photochemical radiative energy dissipation capability in response to flooding. The data correlated with the retention of chlorophyll and carotenoids content in flooded Nr leaves. Moreover, leaf area and specific leaf area were higher in Nr, indicating that ethylene would exert a negative role in leaf growth and expansion under flooded conditions. Although stomatal conductance was hampered in flooded Nr plants, carboxylation activity was not affected by flooding in the mutant, suggesting that ethylene is responsible for inducing non-stomatal limitations to photosynthetic CO₂ uptake. Upregulation of several cysteine protease genes and high protease activity led to Rubisco protein loss in response to ethylene under flooding. Reduction of Rubisco content would, at least in part, account for the reduction of its carboxylation efficiency in response to ethylene in flooded plants. Therefore, besides its role as a trigger of many adaptive responses, perception of ethylene entails limitations in light and dark photosynthetic reactions by speeding up the senescence process that leads to a progressive disassembly of the photosynthetic machinery in leaves of flooded tomato plants.

Drought stress induces biosynthesis of flavonoids in leaves and saikosaponins in roots of Bupleurum chinense DC

Drought stress affects vegetative and reproductive growth processes and synthesis of secondary metabolites in plants. We assessed relevant indicators of vegetative and reproductive growth in Bupleurum chinense DC. during drought stress. Samples were collected on days 4, 8, 12, 20, and 24 of a drought treatment according to drought stress severity in order to elucidate potential effects on synthesis of flavonoids in leaves and saikosaponins in roots of B. chinense. The results showed that B. chinense can adapt to drought stress mainly by increasing concentrations of osmoregulatory substances (soluble protein and proline) and increasing activity of protective enzymes (superoxide dismutase and catalase), as observed on days 12 and 20 of the treatment. Secondary metabolite concentrations in B. chinense roots and leaves showed significant differences-drought stress increased saikosaponin concentrations in roots by 9.85% and 6.41% during vegetative and reproductive growth, respectively, on day 20, and saikosaponin concentrations in roots were higher during vegetative growth than during reproductive growth. In leaves, large amounts of antioxidants were consumed owing to drought stress, which decreased leaf rutin concentrations by 38.79% and 30.11% during vegetative and reproductive growth, respectively, as observed on day 20; overall, leaf rutin concentrations were lower during vegetative growth than during reproductive growth. Changes in soil water content are known to affect synthesis of secondary metabolites in medicinal plants by altering gene transcription, and affected genes may synergistically respond to soil water changes and alter concentrations of flavonoid in leaves and of saikosaponin in roots. The gene F3H down-regulates flavonoid production in leaves. Squalene epoxidase and β-amyrin synthase genes may be key genes regulating saikosaponin accumulation, and changes in their expression corresponded to accumulation of saikosaponins. Our results provide insights in B. chinense adaptation to drought stress through physiological changes and regulation of secondary metabolite production in different plant tissues.

Biochar addition alleviate the negative effects of drought and salinity stress on soybean productivity and water use efficiency

BACKGROUND

Environmental stress is a crucial factor restricting plant growth as well as crop productivity, thus influencing the agricultural sustainability. Biochar addition is proposed as an effective management to improve crop performance. However, there were few studies focused on the effect of biochar addition on crop growth and productivity under interactive effect of abiotic stress (e.g., drought and salinity). This study was conducted with a pot experiment to investigate the interaction effects of drought and salinity stress on soybean yield, leaf gaseous exchange and water use efficiency (WUE) under biochar addition.

RESULTS

Drought and salinity stress significantly depressed soybean phenology (e.g. flowering time) and all the leaf gas exchange parameters, but had inconsistent effects on soybean root growth and WUE at leaf and yield levels. Salinity stress significantly decreased photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration rate by 20.7, 26.3, 10.5 and 27.2%, respectively. Lower biomass production and grain yield were probably due to the restrained photosynthesis under drought and salinity stress. Biochar addition significantly enhanced soybean grain yield by 3.1-14.8%. Drought stress and biochar addition significantly increased WUE-yield by 27.5 and 15.6%, respectively, while salinity stress significantly decreased WUE-yield by 24.2%. Drought and salinity stress showed some negative interactions on soybean productivity and leaf gaseous exchange. But biochar addition alleviate the negative effects on soybean productivity and water use efficiency under drought and salinity stress.

CONCLUSIONS

The results of the present study indicated that drought and salinity stress could significantly depress soybean growth and productivity. There exist interactive effects of drought and salinity stress on soybean productivity and water use efficiency, while we could employ biochar to alleviate the negative effects. We should consider the interactive effects of different abiotic restriction factors on crop growth thus for sustainable agriculture in the future.

Overexpression of GmNFYA5 confers drought tolerance to transgenic Arabidopsis and soybean plants

BACKGROUND

Crop productivity is challenged by abiotic stresses, among which drought stress is the most common. NF-Y genes, especially NF-YA genes, regulate tolerance to abiotic stress.

RESULTS

Soybean NF-Y gene GmNFYA5 was identified to have the highest transcript level among all 21 NF-YA genes in soybean (Glycine max L.) under drought stress. Drought-induced transcript of GmNFYA5 was suppressed by the ABA synthesis inhibitor naproxen (NAP). GmNFYA5 transcript was detected in various tissues at vegetative and reproductive growth stages with higher levels in roots and leaves than in other tissues, which was consist with the GmNFYA5 promoter: GUS fusion assay. Overexpression of GmNFYA5 in transgenic Arabidopsis plants caused enhanced drought tolerance in seedlings by decreasing stomatal aperture and water loss from leaves. Overexpression and suppression of GmNFYA5 in soybean resulted in increased and decreased drought tolerance, respectively, relative to plants with an empty vector (EV). Transcript levels of ABA-dependent genes (ABI2, ABI3, NCED3, LEA3, RD29A, P5CS1, GmWRKY46, GmNCED2 and GmbZIP1) and ABA-independent genes (DREB1A, DREB2A, DREB2B, GmDREB1, GmDREB2 and GmDREB3) in transgenic plants overexpressing GmNFYA5 were higher than those of wild-type plants under drought stress; suppression of GmNFYA5 transcript produced opposite results. GmNFYA5 probably regulated the transcript abundance of GmDREB2 and GmbZIP1 by binding to the promoters in vivo.

CONCLUSIONS

Our results suggested that overexpression of GmNFYA5 improved drought tolerance in soybean via both ABA-dependent and ABA-independent pathways.

Simulated seawater flooding reduces oilseed rape growth, yield and progeny performance

BACKGROUND AND AIMS

Of the many threats to global food security, one of the most pressing is the increased incidence of extreme weather events. In addition to extreme rainfall, a combination of global sea level rise and storm surge is likely to result in frequent episodes of seawater flooding in arable systems along low-lying coasts. Our aim was to elucidate the effects of simulated seawater and freshwater flooding on the survival, growth and reproductive potential of four cultivars of the important seed crop, Brassica napus [canola, or oilseed rape (OSR)].

METHODS

Established plants were exposed to 24 or 96 h freshwater or seawater root zone immersion (with a no immersion 'control'). Initial post-treatment performance over 7 weeks was quantified using dry weight biomass. A second group of plants, cultivated until maturity, were used to quantify reproductive yield (siliqua and seed number, and seed size) and subsequent progeny performance (germination and seedling growth).

RESULTS

Oilseed rape growth and reproductive responses were unaffected by freshwater, but seawater negatively affected growth and siliqua number for all cultivars, and seed mass for two ('Agatha' and 'Cubic'). In addition to impacts on crop yield, the growth of seedlings cultivated from seed collected from maternal plants subjected to seawater immersion was also reduced.

CONCLUSIONS

Our results demonstrate the potential impact of seawater inundation on coastal cropping systems; although OSR may survive acute saline flooding, there are longer term impacts on growth and yield for some cultivars. The threat may necessitate changes in land-use practice and/or the development of salt-tolerant cultivars to maintain economically viable yields. In addition, by evidencing a hitherto unknown effect on reproductive performance (i.e. reduced seed yield) and subsequent seedling growth, our study highlights an important potential impact of coastal flooding on plant community dynamics for (semi-) natural habitats.

Modeling Flood-Induced Stress in Soybeans

Despite the detrimental impact that excess moisture can have on soybean (Glycine max [L.] Merr) yields, most of today's crop models do not capture soybean's dynamic responses to waterlogged conditions. In light of this, we synthesized literature data and used the APSIM software to enhance the modeling capacity to simulate plant growth, development, and N fixation response to flooding. Literature data included greenhouse and field experiments from across the U.S. that investigated the impact of flood timing and duration on soybean. Five datasets were used for model parameterization of new functions and three datasets were used for testing. Improvements in prediction accuracy were quantified by comparing model performance before and after the implementation of new stage-dependent excess water functions for phenology, photosynthesis and N-fixation. The relative root mean square error (RRMSE) for yield predictions improved by 26% and the RRMSE predictions of biomass improved by 40%. Extensive model testing found that the improved model accurately simulates plant responses to flooding including how these responses change with flood timing and duration. When used to project soybean response to future climate scenarios, the model showed that intense rain events had a greater negative effect on yield than a 25% increase in rainfall distributed over 1 or 3 month(s). These developments advance our ability to understand, predict and, thereby, mitigate yield loss as increases in climatic volatility lead to more frequent and intense flooding events in the future.

Comprehensive Genome-Wide Association Analysis Reveals the Genetic Basis of Root System Architecture in Soybean

Increasing the understanding genetic basis of the variability in root system architecture (RSA) is essential to improve resource-use efficiency in agriculture systems and to develop climate-resilient crop cultivars. Roots being underground, their direct observation and detailed characterization are challenging. Here, were characterized twelve RSA-related traits in a panel of 137 early maturing soybean lines (Canadian soybean core collection) using rhizoboxes and two-dimensional imaging. Significant phenotypic variation (P < 0.001) was observed among these lines for different RSA-related traits. This panel was genotyped with 2.18 million genome-wide single-nucleotide polymorphisms (SNPs) using a combination of genotyping-by-sequencing and whole-genome sequencing. A total of 10 quantitative trait locus (QTL) regions were detected for root total length and primary root diameter through a comprehensive genome-wide association study. These QTL regions explained from 15 to 25% of the phenotypic variation and contained two putative candidate genes with homology to genes previously reported to play a role in RSA in other species. These genes can serve to accelerate future efforts aimed to dissect genetic architecture of RSA and breed more resilient varieties.

Understanding the combining ability for physiological traits in soybean

Photosynthetic efficiency has become the target of several breeding programs since the positive correlation between photosynthetic rate and yield in soybean suggests that the improvement of photosynthetic efficiency may be a promising target for new yield gains. However, studies on combining ability of soybean genotypes for physiological traits are still scarce in the literature. The objective of this study was to estimate the combining ability of soybean genotypes based on F2 generation aiming to identify superior parents and segregating populations for physiological traits. Twenty-eight F2 populations resulting from partial diallel crossings between eleven lines were evaluated in two crop seasons for the physiological traits: photosynthesis, stomatal conductance, internal CO2 concentration, and transpiration. General combining ability (GCA) of the parents and specific combining ability (SCA) of the F2 populations were estimated. Our findings reveal the predominance of additive effects in controlling the traits. The genotype TMG 7062 IPRO is the most promising parent for programs aiming at photosynthetic efficiency. We have also identified other promising parents and proposed cross-breeding with higher potential for obtaining superior lines for photosynthetic efficiency.

Research Progress and Perspective on Drought Stress in Legumes: A Review

Climate change, food shortage, water scarcity, and population growth are some of the threatening challenges being faced in today's world. Drought stress (DS) poses a constant challenge for agricultural crops and has been considered a severe constraint for global agricultural productivity; its intensity and severity are predicted to increase in the near future. Legumes demonstrate high sensitivity to DS, especially at vegetative and reproductive stages. They are mostly grown in the dry areas and are moderately drought tolerant, but severe DS leads to remarkable production losses. The most prominent effects of DS are reduced germination, stunted growth, serious damage to the photosynthetic apparatus, decrease in net photosynthesis, and a reduction in nutrient uptake. To curb the catastrophic effect of DS in legumes, it is imperative to understand its effects, mechanisms, and the agronomic and genetic basis of drought for sustainable management. This review highlights the impact of DS on legumes, mechanisms, and proposes appropriate management approaches to alleviate the severity of water stress. In our discussion, we outline the influence of water stress on physiological aspects (such as germination, photosynthesis, water and nutrient uptake), growth parameters and yield. Additionally, mechanisms, various management strategies, for instance, agronomic practices (planting time and geometry, nutrient management), plant growth-promoting Rhizobacteria and arbuscular mycorrhizal fungal inoculation, quantitative trait loci (QTLs), functional genomics and advanced strategies (CRISPR-Cas9) are also critically discussed. We propose that the integration of several approaches such as agronomic and biotechnological strategies as well as advanced genome editing tools is needed to develop drought-tolerant legume cultivars.

Evaluation of Hyperspectral Reflectance Parameters to Assess the Leaf Water Content in Soybean

Nondestructive assessment of water content and water stress in plants is an important component in the rational use of crop irrigation management in precision agriculture. Spectral measurements of light reflectance in the UV/VIS/NIR region (350–1075 nm) from individual leaves were acquired under a rapid dehydration protocol for validation of the remote sensing water content assessment in soybean plants. Four gravimetrical approaches of leaf water content assessment were used: relative water content (RWC), foliar water content as percent of total fresh mass (FWCt), foliar water content as percent of dry mass (FWCd), and equivalent water thickness (EWT). Leaf desiccation resulted in changes in optical properties with increasing relative reflectance at wavelengths between 580 and 700 nm. The highest positive correlations were observed for the relations between the photochemical reflectance index (PRI) and EWT (rP = 0.860). Data analysis revealed that the specific water absorption band at 970 nm showed relatively weaker sensitivity to water content parameters. The prediction of leaf water content parameters from PRI measurements was better with RMSEs of 12.4% (rP = 0.786), 9.1% (rP = 0.736), and 0.002 (rP = 0.860) for RWC, FWCt, and EWT (p < 0.001), respectively. The results may contribute to more efficient crop water management and confirmed that EWT has a statistically closer relationship with reflectance indices than other monitored water parameters.

Understanding genetic control of root system architecture in soybean: Insights into the genetic basis of lateral root number

Developing crops with better root systems is a promising strategy to ensure productivity in both optimum and stress environments. Root system architectural traits in 397 soybean accessions were characterized and a high-density single nucleotide polymorphisms (SNPs)-based genome-wide association study was performed to identify the underlying genes associated with root structure. SNPs associated with root architectural traits specific to landraces and elite germplasm pools were detected. Four loci were detected in landraces for lateral root number (LRN) and distribution of root thickness in diameter Class I with a major locus on chromosome 16. This major loci was detected in the coding region of unknown protein, and subsequent analyses demonstrated that root traits are affected with mutated haplotypes of the gene. In elite germplasm pool, 3 significant SNPs in alanine-glyoxalate aminotransferase, Leucine-Rich Repeat receptor/No apical meristem, and unknown functional genes were found to govern multiple traits including root surface area and volume. However, no major loci were detected for LRN in elite germplasm. Nucleotide diversity analysis found evidence of selective sweeps around the landraces LRN gene. Soybean accessions with minor and mutated allelic variants of LRN gene were found to perform better in both water-limited and optimal field conditions.

Improving Flooding Tolerance of Crop Plants

A major problem of climate change is the increasing duration and frequency of heavy rainfall events. This leads to soil flooding that negatively affects plant growth, eventually leading to death of plants if the flooding persists for several days. Most crop plants are very sensitive to flooding, and dramatic yield losses occur due to flooding each year. This review summarizes recent progress and approaches to enhance crop resistance to flooding. Most experiments have been done on maize, barley, and soybean. Work on other crops such as wheat and rape has only started. The most promising traits that might enhance crop flooding tolerance are anatomical adaptations such as aerenchyma formation, the formation of a barrier against radial oxygen loss, and the growth of adventitious roots. Metabolic adaptations might be able to improve waterlogging tolerance as well, but more studies are needed in this direction. Reasonable approaches for future studies are quantitative trait locus (QTL) analyses or genome-wide association (GWA) studies in combination with specific tolerance traits that can be easily assessed. The usage of flooding-tolerant relatives or ancestral cultivars of the crop of interest in these experiments might enhance the chances of finding useful tolerance traits to be used in breeding.