Deletion of MP/ARF5 domains III and IV reveals a requirement for Aux/IAA regulation in Arabidopsis leaf vascular patterning.

Combinatorial interactions of AUXIN RESPONSE FACTORs (ARFs) and auxin/indole acetic acid (Aux/IAA) proteins through their common domains III and IV regulate auxin responses, but insight into the functions of individual proteins is still limited. As a new tool to explore this regulatory network, we generated a gain-of-function ARF genotype by eliminating domains III and IV from the functionally well-characterized ARF MONOPTEROS(MP)/ARF5. This truncated version of MP, termed MPΔ, conferred complementing MP activity, but also displayed a number of semi-dominant traits affecting auxin signaling and organ patterning. In MPΔ, the expression levels of many auxin-inducible genes, as well as rooting properties and vascular tissue abundance, were enhanced. Lateral organs were narrow, pointed and filled with parallel veins. This effect was epistatic over the vascular hypotrophy imposed by certain Aux/IAA mutations. Further, in MPΔ leaves, failure to turn off the procambium-selecting gene PIN1 led to the early establishment of oversized central procambial domains and very limited subsequent lateral growth of the leaf lamina. We conclude that MPΔ can selectively uncouple a single ARF from regulation by Aux/IAA proteins and can be used as a genetic tool to probe auxin pathways and explore leaf development.