Abstract：

C; and C, grasses directly and indirectly provide the vast majority of calories to the human diet, yet our understanding of themolecular mechanisms driving photosynthetic productivity in grasses is largely unexplored. Ground meristem cells divide toform mesophyll or vascular initial cels early in leaf development in C, and C, grasses.Here we dehine a genetic circuit composedof SHORT ROOT (SHR),INDETERMINATE DOMAIN (IDD), and PIN-FORMED (PIN) family members that specifes vascularidentify and ground cell proliferation in leaves of both C; and C grasses. Ectopic expression and loss-of-function mutant stud'jes of SHR paralogs in the C plant 0ryza sativa (rice) and the C, plant Setaria viridis (green millet) revealed the roles of thesegenes in both minor vein formation and ground cell diferentiation. Genetic and in vitro studies further suggested that SHRregulates this process through its interactions with lDD12 and 13. We also revealed direct interactions of these lDD proteinswith a putative regulatory element within the auxin transporter gene PlN5c. Collectively, these fndings indicate that a SHR-IDD regulatory circuit mediates auxin transport by negatively regulating PIN expression to modulate minor vein patterning inthe grasses.

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