The Growth and Recovery of Soybean During Drought

Soybean is an important legume crop throughout the world and widely grown as a source of vegetable oil, protein, carbohydrates, macronutrient and minerals for human consumption. Recently, soy foods and its bioactive compounds received significant attention because of their human health benefits. Studies have shown that global demand for soybean production will be doubled in 2050 due to population growth, diet shifts, and increased biofuel consumption. The United States is the leading soybean-producing country, followed by Brazil, Argentina, and China (USDA-FAS 2016).

Stress, Parameters, Physiology:

Like other crops, soybean is normally exposed to both the biotic and abiotic stresses that play a major role in controlling soybean seed yield, quality, and its chemical composition. Among many of these abiotic factors, drought caused by insufficient rainfall and/or altered precipitation patterns cause severe reduction in soybean productivity in rain-fed areas since it is the most drought-sensitive plant among legume species. Drought can decrease the yield upto 50% besides hampering the nutraceutical compounds in soybean seeds. Drought tolerance is the ability of plants to survive, grow, and reproduce under water-deficit conditions. In crops, drought tolerance means not only the ability of plants to survive under water-deficit conditions, but also, more importantly, the production of a higher yield during periods of stress than a non-tolerant crop or cultivar.

During drought, the whole life cycle of soybean is affected whereas the extent of damage, recovery capability and productivity change are directly associated with the developmental stage, duration of stress and intensity. Plant has its own adaptive mechanism for resistance to drought stress (drought escape, drought avoidance and drought tolerance) and a wide range of studies have been performed by scientists in different parts of the world to understand it. The escape mechanism refers to the induction of shorter life cycles in response to drought stress, and it usually leads to a large reduction in seed production. The avoidance mechanism is characterized by the maintenance of a favourable water status under drought either by enhancing the water uptake capacity of roots, by limiting water loss from leaves through reduced stomatal conductance, or by reducing leaf absorption of radiation by leaf rolling or folding. Drought tolerance is accomplished through various physiological changes. Tolerance responses also involve the accumulation of osmoprotectants like proline to avoid water loss. However, there are multiple events and metabolic cross-talks triggered by drought, such as hormone regulation, sugar synthesis and redox signals. Some morphological changes like development of more root hairs and rolling of leaves are also observed.

Studies showed that drought plays a major role in reducing plant growth parameters and also negatively interferes with plants’ physiology by causing losses in CO2 accumulation which results low photosynthetic rate or the photochemical efficiency. Similarly, reactive oxygen species (ROS) is produced during drought-stressed which acts as a messenger to activate defense mechanisms that damages important macromolecules and cell structure. On the other hand, stomatal conductance is highly impacted due to accumulation of ABA in high amount.

The yield and quality losses during reproductive stage is highly susceptible than the early vegetative stage. During the reproductive stage, water deficit accelerated leaf senescence, shortened the period of seed-filling, and reduced the seed number and seed size by the plant. Of all the reproductive processes, the stage of anther and pollen developments is the most sensitive to drought. Drought during Grain filling stage can reduce yield till 42%. In order to screen the Drought tolerance genotype a wide range of parameters like leaf area, water use efficiency, root and shoot biomass, root architecture, osmotic adjustment, pod number per plant, Presence of dense leaf pubescence, carbohydrate storage and remobilization and grain weight are measured.

Breeding,

The contributions of scientists and plant breeders are enormous during the 20th century and looking for appropriate strategies to enhance legume productivity by identifying drought-tolerant genotypes and incorporating practices for efficient water management. Breeding for any desired trait requires a significant amount of genetic variation at intra-specific, inter-specific or inter-generic levels. Great efforts and substantial progress have been made through innovative research findings and rapid development of many novel techniques and methodologies in drought-resistance breeding. Drought tolerance is complex and is controlled by a number of genes throughout the genome, each with minor effects and interacting with the environment. Various genomic approaches have been used to dissect genetic control of drought stress tolerance. Quantitative trait loci (QTL) identification using molecular markers is one way to dissect the traits associated with drought tolerance.
Through marker-assisted breeding (MAB) it is now possible to examine the usefulness of thousands of genomic regions of a crop germplasm under water limited regimes, which was, in fact, previously not possible.

Genetic Engineering:

Transgenic approach is being pursued actively throughout the world to improve traits including tolerance to biotic and abiotic stresses in a number of crops. It involves many genes with additive effects, so the prospects of improving drought tolerance in crops seem not to be very bright. But the completion of soybean genomic sequence has helped to understand the structure and function of soybean genes. Similarly rapid advance in recombinant-DNA technology with precise efficient gene-transfer protocols and advances in research of drought tolerance mechanisms have helped to improve the drought tolerance soybean trait by using the dehydration-responsive candidate genes. These genes include both functional genes, such as those encoding various metabolites important for drought tolerance, or regulatory genes, such as transcription factors (TFs) or two-component systems (TCS) encoding genes regulating drought-responsive cascades. These have been shown to function in plant stress response and adaptation. In response to drought, a wide range of studies have been performed on the role of cysteine protease enzymes, Salicyclic acid (SA), NAC, ABA, Proteases, Coronatine, and responsive elements binding members (DREB). Drought inducible promoter also has been identified and characterized for future uses.

Bacteria:

Plant-associated bacteria (PAB) are used as biocontrol agents against plant pathogens and some strains may offer additional benefits to the plants by providing tolerant to abiotic stresses and promoting growth. Plant growth-promoting rizhobacteria (PGPR) is a useful practice for enhance plant growth under drought through direct and indirect mechanisms. Nitrogen fixation, phosphorus solublization, production of siderophores, organic acids and plant growth-promoting compounds are major functions that result root hair development and lateral root formation, helping to improve water and nutrient uptake. PGPR can also regulate the main phytohormones such as gibberellins, auxins, cytokinins, ABA and ethylene. Recent research has also provided evidence that plant growth-promoting rhizobacterium (PGPR) improve plant adaptation to drought by stimulating lateral root formation and increasing shoot growth with stimulation partly caused by bacterium-produced volatile organic compounds. So improving the soybean-rhizobia symbiosis might also contribute to better drought tolerance.

Root

In crop plants, deeper rooting genotypes have shown improved efficiencies in nitrogen and water uptake when impacted by abiotic stress factors. However, a shallow root system with enhanced lateral rooting is beneficial for phosphorus uptake. Soybean has an allorhizic root system consisting of a primary root (tap root) and lateral (basal) roots. It can survive drought stress if there is a robust and deep root system at the early vegetative growth stage. Decreased root lengths and dry biomass accumulation have been reported in many soybean accessions under drought conditions. Drought not only changes root architecture (root depth, root branching density, and root angle) but also partitioning of root to shoot biomass with an increase in root mass.

However, identification of soybean cultivars with improved root architecture characteristics still remains challenging. Classic root phenotyping approaches including analysis of soil cores and applying standard excavation techniques to determine root traits are still the methods of choice. Future more accurate non-destructive methods under development are transparent tubes (mini-rhizotrons), to measure with a camera various root characteristics around the outside walls of the tubes, or in situ tomographic measurements of the root system with X-rays.

Among many factors that are strongly associated with drought tolerance in legumes, architecture of roots is one of the most promising traits for drought escape and could be used positively in drought tolerance breeding programs. This aims to improve drought-resistance, enabling the plant to mine water efficiently from deeper soil layer under catastrophic dry environments and could be introduced or manipulated by a single gene. In soybean, experiments suggested that roots and root nodules are indispensable sensors of drought tolerance, and the feedback of these crucial organs on drought tolerance is the key feature. Direct screening of roots and nodule traits in the field along with identification of genes, proteins and metabolites will be necessary in order to gain a comprehensive and thoughtful understanding of regulation of root architecture.

Metabolmics

The complex molecular mechanisms, signaling perception, integrated responses, and molecular cross talk activated in response to different abiotic stress are not well-understood in soybean. In this regard, several efforts have been made to elucidate the molecular mechanisms. Endogenous hormones are the most important regulators throughout the plant’s life cycle, and they play roles in transducing stress signals when plants are under stress. Through antagonistic regulations among endogenous hormones, these hormones regulate the plant physio-biochemical metabolism, growth and development to promote plant adaptation to a stressful environment.
In response to drought stress, various biochemical reactions are triggered in plants to accumulate many kinds of solutes, such as sugars, amino acids, glycine betaine and polyamines, to help plants cope with drought-associated osmotic stresses. Increase in concentration of these osmo-protectants under drought stress is the consequence of induction of their biosynthetic pathways, indicating that genes encoding the biosynthetic enzymes play an essential role in pro-biosynthetic pathways, indicating that genes encoding the biosynthetic enzymes play an essential role in protecting plants from drought. Accumulation of trehalose the biosynthetic enzymes play an essential role in protecting plants from drought.

Many important cellular pathways in plants, such as signal transduction and environmental stress adapta-tion, are under regulation of gene expression at tran-scriptional level regulation and signal transduction involved in drought responses. Plant hormones are involved in every aspect of plant development as well as in the reaction of plants to abiotic and biotic stress [12,13]. The overlap in hormone-regulated pathways and interactions indicate a complex network of extensive cross talk between the different hormone signaling pathways during adaptation to drought.

Seed Priming:

Melatonin which is a well-known agent of having multiple roles in animals is found Coating seeds with melatonin significantly promoted soybean growth as judged from leaf size and plant height. This enhancement was also observed in soybean production and their fatty acid content. Melatonin increased pod number and seed number. Melatonin also improved soybean tolerance drought stresses.