The WIP6 transcription factor TOO MANY LATERALS specifies vein type in C₄ and C₃ grass leaves

Grass leaves are invariantly strap shaped with an elongated distal blade and a proximal sheath that wraps around the stem. Underpinning this shape is a scaffold of leaf veins, most of which extend in parallel along the proximo-distal leaf axis. Differences between species are apparent both in the vein types that develop and in the distance between veins across the medio-lateral leaf axis. A prominent engineering goal is to increase vein density in leaves of C₃ photosynthesizing species to facilitate the introduction of the more efficient C₄ pathway. Here, we discover that the WIP6 transcription factor TOO MANY LATERALS (TML) specifies vein rank in both maize (C₄) and rice (C₃). Loss-of-function tml mutations cause large lateral veins to develop in positions normally occupied by smaller intermediate veins, and TML transcript localization in wild-type leaves is consistent with a role in suppressing lateral vein development in procambial cells that form intermediate veins. Attempts to manipulate TML function in rice were unsuccessful because transgene expression was silenced, suggesting that precise TML expression is essential for shoot viability. This finding may reflect the need to prevent the inappropriate activation of downstream targets or, given that transcriptome analysis revealed altered cytokinin and auxin signaling profiles in maize tml mutants, the need to prevent local or general hormonal imbalances. Importantly, rice tml mutants display an increased occupancy of veins in the leaf, providing a step toward an anatomical chassis for C₄ engineering. Collectively, a conserved mechanism of vein rank specification in grass leaves has been revealed.