A SHR-IDD-PIN regulatory network mediates minor vein differentiation in rice

C3 and C4 grasses directly and indirectly provide the vast majority of calories to the human diet, yet our understanding of the molecular mechanisms driving photosynthetic productivity in grasses is largely unexplored. Here we define a genetic circuit comprised of SHR, IDD and PIN family members that specify vascular identify and ground cell proliferation in leaves of both C3 and C4 grasses. Ectopic expression and loss-of-function mutant studies of SHORT ROOT (SHR) paralogs in C3 Oryza sativa (rice) and C4 Setaria viridis (green millet) revealed a role in both minor vein formation and ground cell differentiation. Genetic and in vitro studies further suggest that SHR regulates this process through its interaction with Indeterminate Domain (IDD) IDD 12 and 13. We further show a direct interaction of these IDD proteins with a putative regulatory element within the auxin transporter PIN5c gene. Collectively, these studies indicated that a SHR-IDD regulatory circuit mediates auxin flow through the negative regulation of PIN protein expression to modulate minor vein patterning in the grasses.