Alternative Strategies for Multi-Stress Tolerance and Yield Improvement in Millets

Abstract

Millets are important cereal crops cultivated in arid and semiarid regions of the world, particularly Africa and southeast Asia. Climate change has triggered multiple abiotic stresses in plants that are the main causes of crop loss worldwide, reducing average yield for most crops by more than 50%. Although millets are tolerant to most abiotic stresses including drought and high temperatures, further improvement is needed to make them more resilient to unprecedented effects of climate change and associated environmental stresses. Incorporation of stress tolerance traits in millets will improve their productivity in marginal environments and will help in overcoming future food shortage due to climate change. Recently, approaches such as application of plant growth-promoting rhizobacteria (PGPRs) have been used to improve growth and development, as well as stress tolerance of crops. Moreover, with the advance of next-generation sequencing technology, genome editing, using the clustered regularly interspaced short palindromic repeats (CRISPR/Cas9) system are increasingly used to develop stress tolerant varieties in different crops. In this paper, the innate ability of millets to tolerate abiotic stresses and alternative approaches to boost stress resistance were thoroughly reviewed. Moreover, several stress-resistant genes were identified in related monocots such as rice (Oryza sativa), wheat (Triticum aestivum), and maize (Zea mays), and other related species for which orthologs in millets could be manipulated by CRISPR/Cas9 and related genome-editing techniques to improve stress resilience and productivity. These cutting-edge alternative strategies are expected to bring this group of orphan crops at the forefront of scientific research for their potential contribution to global food security.

Keywords: CRISPR/Cas9; abiotic stress; genome editing; millets; rhizobacteria.

Figure 1

Potential application of PGPRs in abiotic stress mitigation and crop yield improvement in millets. PGPRs can be isolated from plants harboring the bacteria. The PGPRs are then cultured in a laboratory and applied to the soil of plant-growing substrate.

Figure 2

Illustration of CRISPR/Cas9 application in genome editing for yield improvement and stress mitigation. For CRISPR/Cas9 gene editing, a guide RNA (gRNA) is designed from a gene of interest, such as those listed in Table 2, and inserted into a binary vector. A bacterial CAS9 Nick-ase enzyme (Cas9) protein is also inserted into a binary vector. The gRNA and Cas9 expression cassettes are then used for Agrobacterium-mediate transformation for trait improvement. The fox-tail millet plant is used as a new model plant for CRISPR/Cas9 editing [241].