Morpho-Physiological, Biochemical and Molecular Adaptation of Millets to Abiotic Stresses: A Review

Abiotic stresses such as drought, heat, cold, nutrient deficiency, excess salt and hazardous metals can hamper plantgrowth and development. Intensive agriculture of only a few major staple food crops that are sensitive and intolerant to environmental stresses has led to an agrarian crisis. On the other hand, nutritionally rich, gluten free and stress tolerant plants like millets are neglected and underutilized. Millets sustain about one-third of the world's population and show exceptional tolerance to various abiotic and biotic stresses. Millets are C4 plants that are adapted to marginal and dry lands of arid and semi-arid regions, and survive low rainfall and poor soils. Abiotic stresses significantly affect plant growth which ultimately results in reduced crop yields. However, various adaptation mechanisms have evolved in millets to withstand different stresses. This review aims at exploring various of these morphophysiological, biochemical and molecular aspects of mechanisms in millets. Morphological adaptations include short life span, smallplant height and leaf area, dense root system, adjusted flowering time, increased root and decreased shoot lengths, high tillering, and leaf folding. A high accumulation of various osmoprotectants (proline, soluble sugars, proteins) improves hyperosmolarity and enhances the activity of antioxidant enzymes (e.g., Ascorbate peroxidase, Superoxide dismutase, Catalase, Peroxidase) providing defense against oxidative damage. Physiologically, plants show low photosynthetic and stomatal conductance rates, and root respiration which help them to escape from water stress. Molecular adaptations include the upregulation of stress-related transcriptional factors, signalling genes, ion transporters, secondary metabolite pathways, receptor kinases, phytohormone biosynthesis and antioxidative enzymes. Lack of genetic resources hampers improvement of millets. However, several identified and characterized genes for stress tolerance can be exploited for further development of millet resilience. This will provide them with an extra characteristic plant resistance to withstand environmental pressures, besides their excellent nutritional value over the conventional staple crops like rice, wheat and maize.