Exploitation of Drought Tolerance-Related Genes for Crop Improvement

Identification of the Gossypium hirsutum SDG Gene Family and Functional Study of GhSDG59 in Response to Drought Stress

SET-domain group histone methyltransferases (SDGs) are known to play crucial roles in plant responses to abiotic stress. However, their specific function in cotton's response to drought stress has not been well understood. This study conducted a comprehensive analysis of the SDG gene family in Gossypium hirsutum, identifying a total of 82 SDG genes. An evolutionary analysis revealed that the SDG gene family can be divided into eight subgroups. The expression analysis shows that some GhSDG genes are preferentially expressed in specific tissues, indicating their involvement in cotton growth and development. The transcription level of some GhSDG genes is induced by PEG, with GhSDG59 showing significant upregulation upon polyethylene glycol (PEG) treatment. Quantitative polymerase chain reaction (qPCR) analysis showed that the accumulation of transcripts of the GhSDG59 gene was significantly upregulated under drought stress. Further functional studies using virus-induced gene silencing (VIGS) revealed that silencing GhSDG59 reduced cotton tolerance to drought stress. Under drought conditions, the proline content, superoxide dismutase (SOD) and peroxidase (POD) enzyme activities in the GhSDG59-silenced plants were significantly lower than in the control plants, while the malondialdehyde (MDA) content was significantly higher. Transcriptome sequencing showed that silencing the GhSDG59 gene led to significant changes in the expression levels of 1156 genes. The KEGG enrichment analysis revealed that these differentially expressed genes (DEGs) were mainly enriched in the carbon metabolism and the starch and sucrose metabolism pathways. The functional annotation analysis identified known drought-responsive genes, such as ERF, CIPK, and WRKY, among these DEGs. This indicates that GhSDG59 is involved in the drought-stress response in cotton by affecting the expression of genes related to the carbon metabolism and the starch and sucrose metabolism pathways, as well as known drought-responsive genes. This analysis provides valuable information for the functional genomic study of SDGs and highlights potential beneficial genes for genetic improvement and breeding in cotton.

Expression Patterns and Molecular Mechanisms Regulating Drought Tolerance of Soybean [Glycine max (L.) Merr.] Conferred by Transcription Factor Gene GmNAC19

NAC transcription factors are commonly involved in the plant response to drought stress. A transcriptome analysis of root samples of the soybean variety 'Jiyu47' under drought stress revealed the evidently up-regulated expression of GmNAC19, consistent with the expression pattern revealed by quantitative real-time PCR analysis. The overexpression of GmNAC19 enhanced drought tolerance in Saccharomyces cerevisiae INVSc1. The seed germination percentage and root growth of transgenic Arabidopsis thaliana were improved in comparison with those of the wild type, while the transgenic soybean composite line showed improved chlorophyll content. The altered contents of physiological and biochemical indices (i.e., soluble protein, soluble sugar, proline, and malondialdehyde) related to drought stress and the activities of three antioxidant enzymes (i.e., superoxide dismutase, peroxidase, and catalase) revealed enhanced drought tolerance in both transgenic Arabidopsis and soybean. The expressions of three genes (i.e., P5CS, OAT, and P5CR) involved in proline synthesis were decreased in the transgenic soybean hairy roots, while the expression of ProDH involved in the breakdown of proline was increased. This study revealed the molecular mechanisms underlying drought tolerance enhanced by GmNAC19 via regulation of the contents of soluble protein and soluble sugar and the activities of antioxidant enzymes, providing a candidate gene for the molecular breeding of drought-tolerant crop plants.

Foliar application of strigolactones improves the desiccation tolerance, grain yield and water use efficiency in dryland wheat through modulation of non-hydraulic root signals and antioxidant defense

Non-hydraulic root signals (nHRS) are affirmed as a unique positive response to soil drying, and play a crucial role in regulating water use efficiency and yield formation in dryland wheat production. Strigolactones (SLs) can enhance plant drought adaptability. However, the question of whether strigolactones enhance grain yield and water use efficiency by regulating nHRS and antioxidant defense systems in dryland wheat remains unanswered. In this study, pot experiments were conducted to investigate the effects of strigolactones on nHRS, antioxidant defense system, and grain yield and water use efficiency in dryland wheat. The results showed that external application of SLs increased drought-induced abscisic acid (ABA) accumulation and activated an earlier trigger of nHRS at 73.4% field capacity (FC), compared to 68.5% FC in the control group (CK). This phenomenon was mechanically associated with the physiological mediation of SLs. The application of SLs significantly enhanced the activities of leaf antioxidant enzymes, reduced ROS production, and mitigated oxidative damage to lipid membrane. Additionally, root biomass, root length density, and root to shoot ratio were increased under strigolactone treatment. Furthermore, exogenous application of SLs significantly increased grain yield by 34.9% under moderate drought stress. Water use efficiency was also increased by 21.5% and 33.3% under moderate and severe drought conditions respectively, compared to the control group (CK). The results suggested that the application of strigolactones triggered earlier drought-sensing mechanism and improved the antioxidant defense ability, thus enhancing grain yield and water use efficiency in dryland wheat production.

Genetic Polymorphism in the Amaranthaceae Species in the Context of Stress Tolerance

The adaptive potential and biochemical properties of the Amaranthaceae species make them promising for introduction into agriculture and markets, particularly in arid conditions. Molecular genetic polymorphism analysis is the most powerful tool for studying plant resources; therefore, the current study aimed to investigate the polymorphisms of allelic variations in the ARF and SOD gene families, as well as the genetic diversity of six Amaranthaceae species, using retrotransposon-based fingerprinting with the multi-locus EPIC-PCR profiling approach. Additionally, the iPBS PCR amplification was employed for genome profiling, revealing variations in genetic diversity among the studied Amaranthaceae samples. The observed genetic diversity in Amaranthaceae species contributes to their enhanced tolerance to adverse environmental conditions. The knowledge about the genetic diversity of genes crucial in plant development and stress resistance can be useful for the genetic improvement of cultivated Amaranthaceae species.

Monitoring Drought Tolerance in Oil Palm: Choline Monooxygenase as a Novel Molecular Marker

Water scarcity negatively impacts oil palm production, necessitating the development of drought-tolerant varieties. This study aimed to develop molecular markers for oil palm breeding programs focused on drought tolerance. Genes associated with drought tolerance were selected, and single nucleotide polymorphism (SNP)-based markers were developed. Genomic DNA was successfully extracted from 17 oil palm varieties, and 20 primers out of 44 were effectively amplified. Screening with single-strand conformation polymorphism (SSCP) revealed an informative SNP marker from the choline monooxygenase (CMO) gene, exhibiting CC, CT, and TT genotypes. Notably, the oil palm variety La Mé showed the CT genotype, while Surat Thani 2 (Deli × La Mé) exhibited the CT and CC genotypes in a 1:1 ratio. Gene expression analysis confirmed the association of the CMO gene with drought tolerance in commercial oil palm varieties. The full-length CMO gene was 1308 bp long and shared sequence similarities with other plant species. However, amino acid sequence variations were observed compared with existing databases. These findings highlight the potential utility of the CMO marker for drought tolerance selection, specifically within the La Mé parent of oil palm Surat Thani 2 varieties, and strongly confirm the La Mé S5 population and Surat Thani 2 as drought-tolerant varieties.

DIW1 encoding a clade I PP2C phosphatase negatively regulates drought tolerance by de-phosphorylating TaSnRK1.1 in wheat

Drought seriously impacts wheat production (Triticum aestivum L.), while the exploitation and utilization of genes for drought tolerance are insufficient. Leaf wilting is a direct reflection of drought tolerance in plants. Clade A PP2Cs are abscisic acid (ABA) co-receptors playing vital roles in the ABA signaling pathway, regulating drought response. However, the roles of other clade PP2Cs in drought tolerance, especially in wheat, remain largely unknown. Here, we identified a gain-of-function drought-induced wilting 1 (DIW1) gene from the wheat Aikang 58 mutant library by map-based cloning, which encodes a clade I protein phosphatase 2C (TaPP2C158) with enhanced protein phosphatase activity. Phenotypic analysis of overexpression and CRISPR/Cas9 mutant lines demonstrated that DIW1/TaPP2C158 is a negative regulator responsible for drought resistance. We found that TaPP2C158 directly interacts with TaSnRK1.1 and de-phosphorylates it, thus inactivating the TaSnRK1.1-TaAREB3 pathway. TaPP2C158 protein phosphatase activity is negatively correlated with ABA signaling. Association analysis suggested that C-terminal variation of TaPP2C158 changing protein phosphatase activity is highly correlated with the canopy temperature, and seedling survival rate under drought stress. Our data suggest that the favorable allele with lower phosphatase activity of TaPP2C158 has been positively selected in Chinese breeding history. This work benefits us in understanding the molecular mechanism of wheat drought tolerance, and provides elite genetic resources and molecular markers for improving wheat drought tolerance.

Plant Tolerance to Drought Stress with Emphasis on Wheat

Environmental stresses, such as drought, have negative effects on crop yield. Drought is a stress whose impact tends to increase in some critical regions. However, the worldwide population is continuously increasing and climate change may affect its food supply in the upcoming years. Therefore, there is an ongoing effort to understand the molecular processes that may contribute to improving drought tolerance of strategic crops. These investigations should contribute to delivering drought-tolerant cultivars by selective breeding. For this reason, it is worthwhile to review regularly the literature concerning the molecular mechanisms and technologies that could facilitate gene pyramiding for drought tolerance. This review summarizes achievements obtained using QTL mapping, genomics, synteny, epigenetics, and transgenics for the selective breeding of drought-tolerant wheat cultivars. Synthetic apomixis combined with the msh1 mutation opens the way to induce and stabilize epigenomes in crops, which offers the potential of accelerating selective breeding for drought tolerance in arid and semi-arid regions.

Sterile line Dexiang074A enhances drought tolerance in hybrid rice

Heterosis has been widely used in rice breeding, especially in improving rice yield. But it has rarely been studied in rice abiotic stress, including the drought tolerance, which is becoming one of the most important threaten in decreasing rice yield. Therefore, it is essential to studying the mechanism underlying heterosis in improving drought tolerance of rice breeding. In this study, Dexiang074B (074B) and Dexiang074A (074A) served as maintainer lines and sterile lines. Mianhui146 (R146), Chenghui727 (R727), LuhuiH103 (RH103), Dehui8258 (R8258), Huazhen (HZ), Dehui938 (R938), Dehui4923 (R4923), and R1391 served as restorer lines. The progeny were Dexiangyou (D146), Deyou4727 (D4727), Dexiang 4103 (D4103), Deyou8258 (D8258), Deyou Huazhen (DH), Deyou 4938 (D4938), Deyou 4923 (D4923), and Deyou 1391 (D1391). The restorer line and hybrid offspring were subjected to drought stress at the flowering stage. The results showed that Fv/Fm values were abnormal and oxidoreductase activity and MDA content were increased. However, the performance of hybrid progeny was significantly better than their respective restorer lines. Although the yield of hybrid progeny and restorer lines decreased simultaneously, the yield in hybrid offspring is significantly lower than the respective restorer line. Total soluble sugar content was consistent with the yield result, so we found that 074A can enhance drought tolerance in hybrid rice.

H2S-mediated balance regulation of stomatal and non-stomatal factors responding to drought stress in Chinese cabbage

Increased evidence has shown that hydrogen sulfide (H₂S), a novel gasotransmitter, could enhance drought resistance in plants by inducing stomatal closure, with concurrent enhancement of photosynthetic efficiency, but little is known about the mechanism behind this contradictory phenomenon. This study examined the regulating mechanism of H₂S in response to drought stress from stomatal and non-stomatal factors in Chinese cabbage. The results showed that exogenous H₂S could increase the accumulation of photosynthetic pigments and alleviate the damage caused by drought stress. It also regulated the expression in transcriptional level and the activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (BrRuBisCO) under drought stress. The large subunit of BrRuBisCO was found to be modified by S-sulfhydration, which might be the reason for its increased enzyme activity. The fluxes of Cl^-, K⁺, and H⁺ in the guard cells were detected by non-invasive micro-test techniques while under drought stress. The results indicated that H₂S signaling induced a transmembrane Cl^- and H⁺ efflux and inhibited K⁺ influx, and the Cl^- channel was the main responders for H₂S-regulated stomatal movement. In conclusion, H₂S signal not only activated the ion channel proteins located in the guard cell membrane to induce stomatal closure, but also regulated the transcriptional expression and the activity of RuBisCO, a non-stomatal factor to enhance the photosynthetic efficiency of leaves. There is therefore a beneficial balance between the regulation of H₂S signaling on stomatal factors and non-stomatal factors due to drought stress, which needs to be better understood to apply it practically to increase crop yields.

Transcription Factor ZmNAC20 Improves Drought Resistance by Promoting Stomatal Closure and Activating Expression of Stress-Responsive Genes in Maize

Drought is a major environmental threat that limits crop growth, development, and productivity worldwide. Improving drought resistance with genetic engineering methods is necessary to tackle global climate change. It is well known that NAC (NAM, ATAF and CUC) transcription factors play a critical role in coping with drought stress in plants. In this study, we identified an NAC transcription factor ZmNAC20, which regulates drought stress response in maize. ZmNAC20 expression was rapidly upregulated by drought and abscisic acid (ABA). Under drought conditions, the ZmNAC20-overexpressing plants had higher relative water content and survival rate than the wild-type maize inbred B104, suggesting that overexpression of ZmNAC20 improved drought resistance in maize. The detached leaves of ZmNAC20-overexpressing plants lost less water than those of wild-type B104 after dehydration. Overexpression of ZmNAC20 promoted stomatal closure in response to ABA. ZmNAC20 was localized in the nucleus and regulated the expression of many genes involved in drought stress response using RNA-Seq analysis. The study indicated that ZmNAC20 improved drought resistance by promoting stomatal closure and activating the expression of stress-responsible genes in maize. Our findings provide a valuable gene and new clues on improving crop drought resistance.

Mechanisms and Applications of Bacterial Inoculants in Plant Drought Stress Tolerance

Agricultural systems are highly affected by climatic factors such as temperature, rain, humidity, wind, and solar radiation, so the climate and its changes are major risk factors for agricultural activities. A small portion of the agricultural areas of Brazil is irrigated, while the vast majority directly depends on the natural variations of the rains. The increase in temperatures due to climate change will lead to increased water consumption by farmers and a reduction in water availability, putting production capacity at risk. Drought is a limiting environmental factor for plant growth and one of the natural phenomena that most affects agricultural productivity. The response of plants to water stress is complex and involves coordination between gene expression and its integration with hormones. Studies suggest that bacteria have mechanisms to mitigate the effects of water stress and promote more significant growth in these plant species. The underlined mechanism involves root-to-shoot phenotypic changes in growth rate, architecture, hydraulic conductivity, water conservation, plant cell protection, and damage restoration through integrating phytohormones modulation, stress-induced enzymatic apparatus, and metabolites. Thus, this review aims to demonstrate how plant growth-promoting bacteria could mitigate negative responses in plants exposed to water stress and provide examples of technological conversion applied to agroecosystems.

Conventional and Omics Approaches for Understanding the Abiotic Stress Response in Cereal Crops-An Updated Overview

Cereals have evolved various tolerance mechanisms to cope with abiotic stress. Understanding the abiotic stress response mechanism of cereal crops at the molecular level offers a path to high-yielding and stress-tolerant cultivars to sustain food and nutritional security. In this regard, enormous progress has been made in the omics field in the areas of genomics, transcriptomics, and proteomics. Omics approaches generate a massive amount of data, and adequate advancements in computational tools have been achieved for effective analysis. The combination of integrated omics and bioinformatics approaches has been recognized as vital to generating insights into genome-wide stress-regulation mechanisms. In this review, we have described the self-driven drought, heat, and salt stress-responsive mechanisms that are highlighted by the integration of stress-manipulating components, including transcription factors, co-expressed genes, proteins, etc. This review also provides a comprehensive catalog of available online omics resources for cereal crops and their effective utilization. Thus, the details provided in the review will enable us to choose the appropriate tools and techniques to reduce the negative impacts and limit the failures in the intensive crop improvement study.

The E3 Ligase GmPUB21 Negatively Regulates Drought and Salinity Stress Response in Soybean

E3-ubiquitin ligases are known to confer abiotic stress responses in plants. In the present study, GmPUB21, a novel U-box E3-ubiquitin ligase-encoding gene, was isolated from soybean and functionally characterized. The expression of GmPUB21, which possesses E3-ubiquitin ligase activity, was found to be significantly up-regulated by drought, salinity, and ABA treatments. The fusion protein GmPUB21-GFP was localized in the cytoplasm, nucleus, and plasma membrane. Transgenic lines of the Nicotiana benthamiana over-expressing GmPUB21 showed more sensitive to osmotic, salinity stress and ABA in seed germination and inhibited mannitol/NaCl-mediated stomatal closure. Moreover, higher reactive oxygen species accumulation was observed in GmPUB21 overexpressing plants after drought and salinity treatment than in wild-type (WT) plants. Contrarily, silencing of GmPUB21 in soybean plants significantly enhanced the tolerance to drought and salinity stresses. Collectively, our results revealed that GmPUB21 negatively regulates the drought and salinity tolerance by increasing the stomatal density and aperture via the ABA signaling pathway. These findings improved our understanding of the role of GmPUB21 under drought and salinity stresses in soybean.

Comparative Metabolic Study of Two Contrasting Chinese Cabbage Genotypes under Mild and Severe Drought Stress

Chinese cabbage (Brassica rapa L. ssp. pekinensis) is an important leafy vegetable crop cultivated worldwide. Drought is one of the most important limiting factors for the growth, production and quality of Chinese cabbage due to its weak drought tolerance. In order to deepen the understanding of drought stress response in Chinese cabbage, metabolomics studies were conducted in drought−tolerant (DT) and drought−susceptible (DS) genotypes of Chinese cabbage under water deficit−simulated mild and severe drought stress conditions. A total of 777 metabolites were detected, wherein 90 of them were proposed as the drought−responsive metabolites in Chinese cabbage, with abscisic acid (ABA), serine, choline alfoscerate, and sphingosine as potential representative drought stress biomarkers. We also found that drought−tolerant and drought−susceptible genotypes showed differential metabolic accumulation patterns with contrasting drought response mechanisms. Notably, constitutively high levels of ABA and glutathione were detected in drought−tolerant genotype in all tested and control conditions. In addition, proline, sucrose, γ−aminobutyric acid, and glutathione were also found to be highly correlated to drought tolerance. This study is the first metabolomic study on how Chinese cabbage responds to drought stress, and could provide insights on how to develop and cultivate new drought−resistant varieties.