Millets genetic engineering: the progress made and prospects for the future

Millets comprise a highly variable small-seeded group of Poaceae members that can grow in extreme environmental conditions of drought, high temperature and low soil fertility hence, recognized as climate-resilient. Among millets, the phylogenetic closeness of Setaria with other agronomically important grasses like maize, sugarcane, and sorghum helped in its adoption as a translational model plant. Established efficient gene transfer methodology is a prerequisite for embracing plant species as models. However, genetic engineering of some of the economically important millets has been started in the 1990s, but inadequate progress made this group lag behind other members of Poaceae as rice, maize and wheat. Genetic transformation in millets has generally been achieved by a physical method of microprojectile bombardment, recently Agrobacterium-mediated gene transfer technique has also established in some of the millets but with very few reports. The central hindrance in millet transformation is its recalcitrant nature to regeneration through tissue culture techniques. Optimization of highly efficient regeneration procedure for each millet species is thus, necessary to establish advanced transformation system for them. The possibility of alternative transformation approaches is also discussed. The establishment of robust gene transfer methods whether it's conventional in-vitro tissue culture dependent or in-planta are important for functional validation studies and would enable development of crop improvement strategies. This review presents the progress made on millet genetic transformation, discussing the major challenges that need to be overcome and future opportunities of transgenic techniques in various millets.