High-Throughput Phenotyping Methods for Breeding Drought-Tolerant Crops

Drought is a main factor limiting crop yields. Modern agricultural technologies such as irrigation systems, ground mulching, and rainwater storage can prevent drought, but these are only temporary solutions. Understanding the physiological, biochemical, and molecular reactions of plants to drought stress is therefore urgent. The recent rapid development of genomics tools has led to an increasing interest in phenomics, i.e., the study of phenotypic plant traits. Among phenomic strategies, high-throughput phenotyping (HTP) is attracting increasing attention as a way to address the bottlenecks of genomic and phenomic studies. HTP provides researchers a non-destructive and non-invasive method yet accurate in analyzing large-scale phenotypic data. This review describes plant responses to drought stress and introduces HTP methods that can detect changes in plant phenotypes in response to drought.     

Reducing Drought Stress in Plants by Encapsulating Plant Growth-Promoting Bacteria with Polysaccharides

Drought is a major abiotic stress imposed by climate change that affects crop production and soil microbial functions. Plants respond to water deficits at the morphological, biochemical, and physiological levels, and invoke different adaptation mechanisms to tolerate drought stress. Plant growth-promoting bacteria (PGPB) can help to alleviate drought stress in plants through various strategies, including phytohormone production, the solubilization of mineral nutrients, and the production of 1-aminocyclopropane-1-carboxylate deaminase and osmolytes. However, PGPB populations and functions are influenced by adverse soil factors, such as drought. Therefore, maintaining the viability and stability of PGPB applied to arid soils requires that the PGPB have to be protected by suitable coatings. The encapsulation of PGPB is one of the newest and most efficient techniques for protecting beneficial bacteria against unfavorable soil conditions. Coatings made from polysaccharides, such as sodium alginate, chitosan, starch, cellulose, and their derivatives, can absorb and retain substantial amounts of water in the interstitial sites of their structures, thereby promoting bacterial survival and better plant growth.     

EMS Derived Wheat Mutant BIG8-1 (Triticum aestivum L.)—A New Drought Tolerant Mutant Wheat Line

Drought response in wheat is considered a highly complex process, since it is a multigenic trait; nevertheless, breeding programs are continuously searching for new wheat varieties with characteristics for drought tolerance. In a previous study, we demonstrated the effectiveness of a mutant known as RYNO3936 that could survive 14 days without water. In this study, we reveal another mutant known as BIG8-1 that can endure severe water deficit stress (21 days without water) with superior drought response characteristics. Phenotypically, the mutant plants had broader leaves, including a densely packed fibrous root architecture that was not visible in the WT parent plants. During mild (day 7) drought stress, the mutant could maintain its relative water content, chlorophyll content, maximum quantum yield of PSII (Fv/Fm) and stomatal conductance, with no phenotypic symptoms such as wilting or senescence despite a decrease in soil moisture content. It was only during moderate (day 14) and severe (day 21) water deficit stress that a decline in those variables was evident. Furthermore, the mutant plants also displayed a unique preservation of metabolic activity, which was confirmed by assessing the accumulation of free amino acids and increase of antioxidative enzymes (peroxidases and glutathione S-transferase). Proteome reshuffling was also observed, allowing slow degradation of essential proteins such as RuBisCO during water deficit stress. The LC-MS/MS data revealed a high abundance of proteins involved in energy and photosynthesis under well-watered conditions, particularly Serpin-Z2A and Z2B, SGT1 and Calnexin-like protein. However, after 21 days of water stress, the mutants expressed ABC transporter permeases and xylanase inhibitor protein, which are involved in the transport of amino acids and protecting cells, respectively. This study characterizes a new mutant BIG8-1 with drought-tolerant characteristics suited for breeding programs.     

Recent molecular advances on downstream plant responses to abiotic stress

Abiotic stresses such as extremes of temperature and pH, high salinity and drought, comprise some of the major factors causing extensive losses to crop production worldwide. Understanding how plants respond and adapt at cellular and molecular levels to continuous environmental changes is a pre-requisite for the generation of resistant or tolerant plants to abiotic stresses. In this review we aimed to present the recent advances on mechanisms of downstream plant responses to abiotic stresses and the use of stress-related genes in the development of genetically engineered crops.     

Plant Parasites under Pressure: Effects of Abiotic Stress on the Interactions between Parasitic Plants and Their Hosts

Parasitic angiosperms, comprising a diverse group of flowering plants, are partially or fully dependent on their hosts to acquire water, mineral nutrients and organic compounds. Some have detrimental effects on agriculturally important crop plants. They are also intriguing model systems to study adaptive mechanisms required for the transition from an autotrophic to a heterotrophic metabolism. No less than any other plant, parasitic plants are affected by abiotic stress factors such as drought and changes in temperature, saline soils or contamination with metals or herbicides. These effects may be attributed to the direct influence of the stress, but also to diminished host availability and suitability. Although several studies on abiotic stress response of parasitic plants are available, still little is known about how abiotic factors affect host preferences, defense mechanisms of both hosts and parasites and the effects of combinations of abiotic and biotic stress experienced by the host plants. The latter effects are of specific interest as parasitic plants pose additional pressure on contemporary agriculture in times of climate change. This review summarizes the existing literature on abiotic stress response of parasitic plants, highlighting knowledge gaps and discussing perspectives for future research and potential agricultural applications.     

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.     

Regulation of Plant Responses to Salt Stress

Salt stress is a major environmental stress that affects plant growth and development. Plants are sessile and thus have to develop suitable mechanisms to adapt to high-salt environments. Salt stress increases the intracellular osmotic pressure and can cause the accumulation of sodium to toxic levels. Thus, in response to salt stress signals, plants adapt via various mechanisms, including regulating ion homeostasis, activating the osmotic stress pathway, mediating plant hormone signaling, and regulating cytoskeleton dynamics and the cell wall composition. Unraveling the mechanisms underlying these physiological and biochemical responses to salt stress could provide valuable strategies to improve agricultural crop yields. In this review, we summarize recent developments in our understanding of the regulation of plant salt stress.     

Strategies to Apply Water-Deficit Stress: Similarities and Disparities at the Whole Plant Metabolism Level in Medicago truncatula

Water-deficit stresses such as drought and salinity are the most important factors limiting crop productivity. Hence, understanding the plant responses to these stresses is key for the improvement of their tolerance and yield. In this study M. truncatula plants were subjected to 250 mM NaCl as well as reduced irrigation (No-W) and 250 g/L polyethylene glycol (PEG)-6000 to induce salinity and drought stress, respectively, provoking a drop to −1.7 MPa in leaf water potential. The whole plant physiology and metabolism was explored by characterizing the stress responses at root, phloem sap and leaf organ level. PEG treatment led to some typical responses of plants to drought stress, but in addition to PEG uptake, an important impairment of nutrient uptake and a different regulation of carbon metabolism could be observed compared to No-W plants. No-W plants showed an important redistribution of antioxidants and assimilates to the root tissue, with a distinctive increase in root proline degradation and alkaline invertase activity. On the contrary, salinity provoked an increase in leaf starch and isocitrate dehydrogenase activity, suggesting key roles in the plant response to this stress. Overall, results suggest higher protection of salt-stressed shoots and non-irrigated roots through different mechanisms, including the regulation of proline and carbon metabolism, while discarding PEG as safe mimicker of drought. This raises the need to understand the effect at the whole plant level of the different strategies employed to apply water-deficit stress.     

OsGRP3 Enhances Drought Resistance by Altering Phenylpropanoid Biosynthesis Pathway in Rice (Oryza sativa L.)

As a sessile organism, rice often faces various kinds of abiotic stresses, such as drought stress. Drought stress seriously harms plant growth and damages crop yield every year. Therefore, it is urgent to elucidate the mechanisms of drought resistance in rice. In this study, we identified a glycine-rich RNA-binding protein, OsGRP3, in rice. Evolutionary analysis showed that it was closely related to OsGR-RBP4, which was involved in various abiotic stresses. The expression of OsGRP3 was shown to be induced by several abiotic stress treatments and phytohormone treatments. Then, the drought tolerance tests of transgenic plants confirmed that OsGRP3 enhanced drought resistance in rice. Meanwhile, the yeast two-hybrid assay, bimolecular luminescence complementation assay and bimolecular fluorescence complementation assay demonstrated that OsGRP3 bound with itself may affect the RNA chaperone function. Subsequently, the RNA-seq analysis, physiological experiments and histochemical staining showed that OsGRP3 influenced the phenylpropanoid biosynthesis pathway and further modulated lignin accumulation. Herein, our findings suggested that OsGRP3 enhanced drought resistance in rice by altering the phenylpropanoid biosynthesis pathway and further increasing lignin accumulation.     

Identification of CDPK Gene Family in Solanum habrochaites and Its Function Analysis under Stress

Tomato is an important vegetable crop. In the process of tomato production, it will encounter abiotic stress, such as low temperature, drought, and high salt, and biotic stress, such as pathogen infection, which will seriously affect the yield of tomato. Calcium-dependent protein kinase (CDPK) is a class of major calcium signal receptor which has an important regulatory effect on the perception and decoding of calcium signals. CDPK plays a key role in many aspects of plant growth, such as the elongation of pollen tubes, plant growth, and response to biotic and abiotic stress. While some studies have concentrated on Arabidopsis and pepper, Solanum habrochaites is a wild species relative of cultivated tomato and there is no report on CDPK in Solanum habrochaites to date. Using tomato genomic data, this study identified 33 members of the CDPK gene family. Evolutionary analysis divides family members into four Asian groups, of which the CDPK family members have 11 gene replication pairs. Subcellular location analysis showed that most proteins were predicted to be located in the cytoplasm, and less protein existed on the cell membrane. Not all CDPK family members have a transmembrane domain. Cis regulatory elements relating to light, hormones, and drought stress are overrepresented in the promoter region of the CDPK genes in Solanum habrochaites. The expression levels of each gene under biotic stress and abiotic stress were quantified by qRT-PCR. The results showed that members of the CDPK family in Solanum habrochaites respond to different biotic and abiotic stresses. Among them, the expression of ShCDPK6 and ShCDPK26 genes change significantly. ShCDPK6 and ShCDPK26 genes were silenced using VIGS (virus-induced gene silencing), and the silenced plants illustrated reduced stress resistance to Botrytis cinerea, cold, and drought stress. The results of this study will provide a basis for the in-depth study of the CDPK gene family in Solanum habrochaites, laying the foundation for further analysis of the function of the gene family.