Minor millets as model system to study C<sub>4</sub> photosynthesis -A review

Abstract

C<sub>4</sub> photosynthesis is the primary mode of carbon capture and drives productivity in several major food crops and bioenergy grasses. Gains in productivity associated with C<sub>4</sub> photosynthesis include improved water and nitrogen use efficiencies. Within grasses rice and brachypodium are used as model species. Since these two crops are using C<sub>3</sub> photosynthesis for their growth and development, it cannot be used as model for to study C<sub>4</sub> photosynthesis. In order to characterize the evolutionary innovations and to provide genomic insight into crop improvement for the many important crop species, a new genomic and genetic model species is required. Minor millets have small diploid genomes, shorter life cycles, self pollination and prolific seed production. Due to these characteristics it gains importance over major C<sub>4</sub> species which lack all of these traits. Within Minor millets, <italic>Setaria italica</italic> and <italic>Setaria viridis</italic> are used as model systems since these crops fulfils all the traits responsible to be a model species. Importantly, <italic>Setaria</italic> species uses NADP-Malic enzyme subtype C<sub>4</sub> photosynthetic system to fix carbon and therefore is a potential powerful model system for dissecting C<sub>4</sub> photosynthesis. C<sub>4</sub> grasses have a shorter distance between longitudinal veins in the leaves than C<sub>3</sub> grasses. The C<sub>4</sub> grasses have denser transverse and small longitudinal veins than the C<sub>3</sub> grasses. It indicates that C<sub>4</sub> grasses have a structurally superior photosynthate translocation and water distribution system by developing denser networks of small longitudinal and transverse veins. <italic>Setaria</italic> has high vein density and kranz anatomy that helps to concentrate CO<sub>2</sub> in the bundle sheath cells. This minimizes photorespiration thereby prevents the loss of energy.