Down-Regulation of Cytokinin Receptor Gene SlHK2 Improves Plant Tolerance to Drought, Heat, and Combined Stresses in Tomato

Surviving a Double-Edged Sword: Response of Horticultural Crops to Multiple Abiotic Stressors

Climate change-induced weather events, such as extreme temperatures, prolonged drought spells, or flooding, pose an enormous risk to crop productivity. Studies on the implications of multiple stresses may vary from those on a single stress. Usually, these stresses coincide, amplifying the extent of collateral damage and contributing to significant financial losses. The breadth of investigations focusing on the response of horticultural crops to a single abiotic stress is immense. However, the tolerance mechanisms of horticultural crops to multiple abiotic stresses remain poorly understood. In this review, we described the most prevalent types of abiotic stresses that occur simultaneously and discussed them in in-depth detail regarding the physiological and molecular responses of horticultural crops. In particular, we discussed the transcriptional, posttranscriptional, and metabolic responses of horticultural crops to multiple abiotic stresses. Strategies to breed multi-stress-resilient lines have been presented. Our manuscript presents an interesting amount of proposed knowledge that could be valuable in generating resilient genotypes for multiple stressors.

Exogenous strigolactone enhanced the drought tolerance of pepper (Capsicum chinense) by mitigating oxidative damage and altering the antioxidant mechanism

KEY MESSAGE

Exogenous SL positively regulates pepper DS by altering the root morphology, photosynthetic character, antioxidant enzyme activity, stomatal behavior, and SL-related gene expression. Drought stress (DS) has always been a problem for the growth and development of crops, causing significant negative impacts on crop productivity. Strigolactone (SL) is a newly discovered class of plant hormones that are involved in plants' growth and development and environmental stresses. However, the role of SL in response to DS in pepper remains unknown. DS considerably hindered photosynthetic pigments content, damaged root architecture system, and altered antioxidant machinery. In contrast, SL application significantly restored pigment concentration modified root architecture system, and increased relative chlorophyll content (SPAD). Additionally, SL treatment reduced oxidative damage by reducing hydrogen peroxide (H2O2) (24-57%) and malondialdehyde (MDA) (79-89%) accumulation in pepper seedlings. SL-pretreated pepper seedlings showed significant improvement in antioxidant enzyme activity, proline accumulation, and soluble sugar content. Furthermore, SL-related genes (CcSMAX2, CcSMXL6, and CcSMXL3) were down-regulated under DS. These findings suggest that the foliar application of SL can alleviate the adverse effects of drought tolerance by up-regulating chlorophyll content and activating antioxidant defense mechanisms.

Genome-Wide Identification of the GhANN Gene Family and Functional Validation of GhANN11 and GhANN4 under Abiotic Stress

Annexins (ANNs) are a structurally conserved protein family present in almost all plants. In the present study, 27 GhANNs were identified in cotton and were unevenly distributed across 14 chromosomes. Transcriptome data and RT-qPCR results revealed that multiple GhANNs respond to at least two abiotic stresses. Similarly, the expression levels of GhANN4 and GhANN11 were significantly upregulated under heat, cold, and drought stress. Using virus-induced gene silencing (VIGS), functional characterization of GhANN4 and GhANN11 revealed that, compared with those of the controls, the leaf wilting of GhANN4-silenced plants was more obvious, and the activities of catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were lower under NaCl and PEG stress. Moreover, the expression of stress marker genes (GhCBL3, GhDREB2A, GhDREB2C, GhPP2C, GhRD20-2, GhCIPK6, GhNHX1, GhRD20-1, GhSOS1, GhSOS2 and GhSnRK2.6) was significantly downregulated in GhANN4-silenced plants after stress. Under cold stress, the growth of the GHANN11-silenced plants was significantly weaker than that of the control plants, and the activities of POD, SOD, and CAT were also lower. However, compared with those of the control, the elasticity and orthostatic activity of the GhANN11-silenced plants were greater; the POD, SOD, and CAT activities were higher; and the GhDREB2C, GhHSP, and GhSOS2 expression levels were greater under heat stress. These results suggest that different GhANN family members respond differently to different types of abiotic stress.

A wheat WRKY transcription factor TaWRKY17 enhances tolerance to salt stress in transgenic Arabidopsis and wheat plant

WRKY transcription factors are essential to plant growth, development, resistance, and the regulation of metabolic pathways. In this study, we characterized TaWRKY17, a WRKY transcription factor from wheat, which was differentially expressed in various wheat organs and was up-regulated by salt, drought, hydrogen peroxide (H2O2) and abscisic acid (ABA) treatment. To analyze TaWRKY17 function under salt stress, we obtained stable T3 generation transgenic Arabidopsis and wheat TaWRKY17 overexpression plants. TaWRKY17 overexpression in Arabidopsis and wheat caused a significant plant salt-stress tolerance enhancement. Under salt stress, superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) enzyme activities were elevated in transgenic Arabidopsis and wheat plants compared with the wild type (WT), whereas H2O2 and malondialdehyde (MDA) accumulation was reduced in the transgenic lines. Moreover, ABA/reactive oxygen species (ROS)-related, and stress-response genes were regulated in the transgenic wheat plants, increasing tolerance to salt stress. The transgenic wheat plants were highly sensitive to ABA during seed germination and early seedling growth. In addition, TaWRKY17 virus-induced gene silencing (VIGS) decreased salt tolerance. These results showed that TaWRKY17 enhances salt tolerance by regulating ABA/ROS-related, and stress-response genes and increasing anti-oxidative stress capabilities. Therefore, this gene could be a target for the genetic modification of wheat.

Abiotic Stress in Crop Production

The vast majority of agricultural land undergoes abiotic stress that can significantly reduce agricultural yields. Understanding the mechanisms of plant defenses against stresses and putting this knowledge into practice is, therefore, an integral part of sustainable agriculture. In this review, we focus on current findings in plant resistance to four cardinal abiotic stressors—drought, heat, salinity, and low temperatures. Apart from the description of the newly discovered mechanisms of signaling and resistance to abiotic stress, this review also focuses on the importance of primary and secondary metabolites, including carbohydrates, amino acids, phenolics, and phytohormones. A meta-analysis of transcriptomic studies concerning the model plant Arabidopsis demonstrates the long-observed phenomenon that abiotic stressors induce different signals and effects at the level of gene expression, but genes whose regulation is similar under most stressors can still be traced. The analysis further reveals the transcriptional modulation of Golgi-targeted proteins in response to heat stress. Our analysis also highlights several genes that are similarly regulated under all stress conditions. These genes support the central role of phytohormones in the abiotic stress response, and the importance of some of these in plant resistance has not yet been studied. Finally, this review provides information about the response to abiotic stress in major European crop plants—wheat, sugar beet, maize, potatoes, barley, sunflowers, grapes, rapeseed, tomatoes, and apples.

Developing future heat-resilient vegetable crops

Climate change seriously impacts global agriculture, with rising temperatures directly affecting the yield. Vegetables are an essential part of daily human consumption and thus have importance among all agricultural crops. The human population is increasing daily, so there is a need for alternative ways which can be helpful in maximizing the harvestable yield of vegetables. The increase in temperature directly affects the plants' biochemical and molecular processes; having a significant impact on quality and yield. Breeding for climate-resilient crops with good yields takes a long time and lots of breeding efforts. However, with the advent of new omics technologies, such as genomics, transcriptomics, proteomics, and metabolomics, the efficiency and efficacy of unearthing information on pathways associated with high-temperature stress resilience has improved in many of the vegetable crops. Besides omics, the use of genomics-assisted breeding and new breeding approaches such as gene editing and speed breeding allow creation of modern vegetable cultivars that are more resilient to high temperatures. Collectively, these approaches will shorten the time to create and release novel vegetable varieties to meet growing demands for productivity and quality. This review discusses the effects of heat stress on vegetables and highlights recent research with a focus on how omics and genome editing can produce temperature-resilient vegetables more efficiently and faster.

Transcriptomic and biochemical analyses of drought response mechanism in mung bean (Vignaradiata (L.) Wilczek) leaves

Drought is a major factor that limiting mung bean development. To clarify the molecular mechanism of mung bean in response to drought stress, 2 mung bean groups were established, the experimental group (drought-treated) and the control group (normal water management). With prominent difference of 2 groups in stomatal conductance, relative water content and phenotype, leaf samples were collected at 4 stages, and the physiological index of MDA, POD, chlorophyll, and soluble proteins were estimated. RNA-seq was used to obtain high quality data of samples, and differentially expressed genes were identified by DESeq2. With GO and KEGG analysis, DEGs were enriched into different classifications and pathways. WGCNA was used to detect the relationship between physiological traits and genes, and qPCR was performed to confirm the accuracy of the data. We obtained 169.49 Gb of clean data from 24 samples, and the Q30 of each date all exceeded 94%. In total, 8963 DEGs were identified at 4 stages between the control and treated samples, and the DEGs were involved in most biological processes. 1270 TFs screened from DEGs were clustered into 158 TF families, such as AP2, RLK-Pelle-DLSVA, and NAC TF families. Genes related to physiological traits were closely related to plant hormone signaling, carotenoid biosynthesis, chlorophyll metabolism, and protein processing. This paper provides a large amount of data for drought research in mung bean.

Melatonin in Micro-Tom Tomato: Improved Drought Tolerance via the Regulation of the Photosynthetic Apparatus, Membrane Stability, Osmoprotectants, and Root System

Environmental variations caused by global climate change significantly affect plant yield and productivity. Because water scarcity is one of the most significant risks to agriculture's future, improving the performance of plants to cope with water stress is critical. Our research scrutinized the impact of melatonin application on the photosynthetic machinery, photosynthetic physiology, root system, osmoprotectant accumulation, and oxidative stress in tomato plants during drought. The results showed that melatonin-treated tomato plants had remarkably higher water levels, gas exchange activities, root system morphological parameters (average diameter, root activity, root forks, projected area, root crossings, root volume, root surface area, root length, root tips, and root numbers), osmoprotectant (proline, trehalose, fructose, sucrose, and GB) accumulation, and transcript levels of the photosynthetic genes SlPsb28, SlPetF, SlPsbP, SlPsbQ, SlPetE, and SlPsbW. In addition, melatonin effectively maintained the plants' photosynthetic physiology. Moreover, melatonin treatment maintained the soluble protein content and antioxidant capacity during drought. Melatonin application also resulted in membrane stability, evidenced by less electrolyte leakage and lower H₂O₂, MDA, and O₂^- levels in the drought-stress environment. Additionally, melatonin application enhanced the antioxidant defense enzymes and antioxidant-stress-resistance-related gene (SlCAT1, SlAPX, SlGR, SlDHAR, SlPOD, and SOD) transcript levels in plants. These outcomes imply that the impacts of melatonin treatment on improving drought resistance could be ascribed to the mitigation of photosynthetic function inhibition, the enhancement of the water status, and the alleviation of oxidative stress in tomato plants. Our study findings reveal new and incredible aspects of the response of melatonin-treated tomato plants to drought stress and provide a list of candidate targets for increasing plant tolerance to the drought-stress environment.

Ectopic Expression of CsSUN in Tomato Results in Elongated Fruit Shape via Regulation of Longitudinal Cell Division

Fruit shape, an important agronomic trait of cucumber (Cucumis sativus L.), is tightly controlled by a series of genes such as CsSUN, a homologue of SlSUN that is responsible for the tomato (Solanum lycopersicum) fruit shape via the modulation of cell division. However, the direct genetic evidence about the CsSUN-mediated regulation of fruit shape is still scarce, limiting our mechanistic understanding of the biological functions of CsSUN. Here, we introduced CsSUN into the round-fruited tomato inbred line ‘SN1′ (wild type, WT) via the Agrobacterium tumefaciens-mediated method. The high and constitutive expression of CsSUN was revealed by real-time PCR in all the tested tissues of the transgenic plants, especially in the fruits and ovaries. Phenotypic analyses showed that the ectopic expression of CsSUN increased fruit length while it decreased fruit diameter, thus leading to the enhanced fruit shape index in the transgenic tomato lines relative to the WT. Additionally, the reduction in the seed size and seed-setting rate and the stimulation of seed germination were observed in the CsSUN-expressed tomato. A histological survey demonstrated that the elongated fruits were mainly derived from the significant increasing of the longitudinal cell number, which compensated for the negative effects of decreased cell area in the central columellae. These observations are different from action mode of SlSUN, thus shedding new insights into the SUN-mediated regulation of fruit shape.

Rhizofiltration of combined arsenic-fluoride or lead-fluoride polluted water using common aquatic plants and use of the 'clean' water for alleviating combined xenobiotic toxicity in a sensitive rice variety

Groundwater co-contamination with toxic pollutants like arsenic-fluoride or lead-fluoride is a serious threat for safe rice cultivation, since major stretches of land, involved in cultivation of this staple food crop are presently experiencing severe endemic pollution from these xenobiotic combinations. Preliminary investigations established that the combined pollutants together exerted more phytotoxicity in the widely cultivated indica rice variety Khitish, compared with that exerted by the individual contaminants. Thus, an ecologically sustainable and economically viable phytoremediative strategy was designed where three aquatic plants, viz., Azolla (water fern), Pistia (water lettuce) and Eichhornia (water hyacinth) (commonly located across the co-polluted regions) were tested for their ability to rhizofiltrate the water samples that had been polluted with arsenic-fluoride or lead-fluoride. Water lettuce exhibited the highest ability to 'clean' both arsenic-fluoride and lead-fluoride polluted water due to its capacity of efficient phytoextraction and phytostabilization. Irrigation of Khitish seedlings with this de-polluted water appreciably reduced malondialdehyde formation, electrolyte leakage and irreversible protein carbonylation due to suppression in NADPH oxidase activity and reactive oxygen species production, compared with those in sets grown with non-treated, arsenic-fluoride or lead-fluoride contaminated water. Oxidative injuries, cytotoxic methylglyoxal synthesis and inhibition of biomass growth were ameliorated, and chlorophyll synthesis and Hill activity were increased due to reduced bioaccumulation of xenobiotics, along with the improved uptake of vital micronutrients like iron, copper and nickel. Overall, the current investigation illustrated a cheap, farmer-friendly blueprint which could be easily promulgated to ensure safe rice cultivation even across territories that are severely co-polluted with the mixed contaminants.

Auxin/Cytokinin Antagonistic Control of the Shoot/Root Growth Ratio and Its Relevance for Adaptation to Drought and Nutrient Deficiency Stresses

The hormones auxin and cytokinin regulate numerous aspects of plant development and often act as an antagonistic hormone pair. One of the more striking examples of the auxin/cytokinin antagonism involves regulation of the shoot/root growth ratio in which cytokinin promotes shoot and inhibits root growth, whereas auxin does the opposite. Control of the shoot/root growth ratio is essential for the survival of terrestrial plants because it allows growth adaptations to water and mineral nutrient availability in the soil. Because a decrease in shoot growth combined with an increase in root growth leads to survival under drought stress and nutrient limiting conditions, it was not surprising to find that auxin promotes, while cytokinin reduces, drought stress tolerance and nutrient uptake. Recent data show that drought stress and nutrient availability also alter the cytokinin and auxin signaling and biosynthesis pathways and that this stress-induced regulation affects cytokinin and auxin in the opposite manner. These antagonistic effects of cytokinin and auxin suggested that each hormone directly and negatively regulates biosynthesis or signaling of the other. However, a growing body of evidence supports unidirectional regulation, with auxin emerging as the primary regulatory component. This master regulatory role of auxin may not come as a surprise when viewed from an evolutionary perspective.