Abiotic stresses such as low or high temperature, drought, floods, high salinity, heavy metals, and ultraviolet radiation and chemical fertilizers affect the morphology and physiology of plants are all detrimental to their growth and development, resulting in significant crop yield losses around the world. To relieve the pressure of environmental changes and meet the demand of population growth, it is becoming increasingly important to equip crops with multi-stress tolerance.
Many plants have developed various generalized defenses against abiotic stresses, which include the cuticle on the outside of the plant, as well as unsaturated fatty acids, reactive species scavengers, molecular chaperones, and compatible solutes within cells. A dynamic regulatory network involving upstream signaling molecules in the stress response includes upstream signaling molecules like stress hormones, reactive oxygen species, gasotransmitters, polyamines, phytochromes, and calcium, as well as downstream gene regulation factors like transcription factors in the stress response. Recent research suggests that epigenetic differences can also play a role in heritable variations in economically significant traits. The regulation of stress-related genes has been linked to epigenetic modifications that are reversible and have been linked to both gene inactivation and activation. Epigenetics is the study of heritable variations in gene expression caused by changes in DNA and its related chromatin proteins without affecting the respective nucleotide sequences. Various abiotic and biotic factors affect DNA methylation and histone modifications, resulting in plant adaptation. DNA methylation is currently the most well-understood epigenetic process. Figure 1: Drought stressed wheat (Photo: Alfonso Cortés/CIMMYT)
Understanding Drought Tolerance:
One of the most prevalent abiotic stresses that affect plant growth and development is drought. It persists to be a crucial problem to both researchers and breeders. Studies show that by 2025, about 1.8 billion people will face absolute water shortage and 65% of the world’s population will live under water-stressed conditions. Drought avoidance and dehydration tolerance are two different types of tolerance. Root depth, fair use of available water by plants, and improvements in plant lifestyle to use rainfall are all factors in drought avoidance. The capacity of plants to partially dehydrate and expand again as rain continues is known as dehydration tolerance.
Both dicotyledonous and monocotyledonous plants share common stress tolerance transcription factors and adopt various molecular mechanisms like signal transduction cascade and activation/regulation of transcription, production of late-embryogenesis abundant proteins to protect functional proteins and chaperone proteins, osmolyte accumulation, chemical antioxidant induction of ascorbic acid and glutathione, etc. Physiological studies show a significant decrease in transpiration that causes gradual heat loss to increase leaf temperature. This results in increased CO2 concentrations and photosynthesis that affect plant growth and improves water use efficiency. Figure 2: Effects of Drought Stress on Wheat (Can. J. Microbiol. 2019)
Drought Tolerance in Wheat:
Along with the common molecular tactics adopted by most of the plans, drought may also result in pollen sterility, grain depletion, abscisic acid synthesis genes in the anthers and its accumulation in spikes of drought-susceptible wheat genotypes. An increase in Reactive Oxygen Species has been observed in which oxidative balance of the cell changes. A rise in the generation of ROS promotes abscisic acid production to regulate the antioxidant genes expressions.
miRNA Based Regulation:
The expression of plant hormones, TFs, and other developmental/stress signaling pathways is regulated by microRNA or miRNA-mediated gene silencing. Several genes, as well as their targets, have been studied using genome-wide expression studies in response to drought. Some miRNA species have been established as conserved entities in drought-mediated regulatory mechanisms across plant species. It is interesting to note that certain drought-responsive elements have been found in the promoter region of miRNA genes that express differently under drought conditions.
The miRNAs that are directly involved in stress response are miR167, miR168, miR393, and miR394. miR167 has been found to target auxin response factors ARF6 and ARF8 and miR168 is said to target ARGONAUTE 1 (AGO1) mRNA. miR393 and miR394 target the mRNAs of F-box proteins that play pivotal roles in abiotic stress tolerance in various plants including wheat.
Some of the well established miRNAs involved in drought tolerance in wheat are:
● miR398- involved in the regulation of ROS production under drought stress.
● miR1432- is excessively expressed to target calcium-binding EF which activates signal transduction pathways
● miR171 targets GRAS TF and is significant in abiotic stress responses
A significant study shows that under drought stress, miRNAs are differentially expressed in different tissues. The miRNA microarray method is useful for analyzing the expression patterns of miRNAs and characterizing their functional roles against drought stress. Taking a contrast between regulations in leaves and roots, the following observations were made:
Table 1. Comparison of miRNA regulations in leaves and roots (12)
From the above report, it is clear that miRNAs play an important role in controlling drought resistance pathways and can also be exploited to enhance wheat strains for such abiotic stressors. However, in practice, the occurrence of many abiotic stresses at the same time, rather than a single stress state, is often the most lethal to crops. Research shows that plants' responses to a combination of two distinct abiotic stresses are special and cannot be extrapolated directly from the responses of plants to either of the individual stresses. Future research programmes need to aim at developing transgenic crops and plants with enhanced tolerance to naturally occurring environmental conditions and focus on tolerance to a combination of different stress conditions.
Department of Biochemistry and Biotechnology
St. Xavier's College, Ahmedabad
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