PIN-FORMED 1 regulates cell fate at the periphery of the shoot apical meristem

The process of organ positioning has been addressed, using the pin-formed 1 (pin1) mutant as a tool. PIN1 is a transmembrane protein involved in auxin transport in Arabidopsis, Loss of function severely affects organ initiati on, and pin1 mutants are characterised by an inflorescence meristem that do es not initiate any flowers, resulting in the formation of a naked inflores cence stem. This phenotype, combined with the proposed role of PIN1 in horm one transport, makes the mutant an ideal tool to study organ formation and phyllotaxis, and here we present a detailed analysis of the molecular modif ications at the shoot apex caused by the mutation, We show that meristem st ructure and function are not severely affected in the mutant. Major alterat ions, however, are observed at the periphery of the pin1 meristem, where or gan initiation should occur. Although two very early markers of organ initi ation, LEAFY and AINTEGUMENTA, are expressed at the periphery of the mutant meristem, the cells are not recruited into distinct primordia, Instead a r ing-like domain expressing those primordium specific genes is observed arou nd the meristem, This ring-like domain also expresses a boundary marker, CU P-SHAPED COTYLEDON 2, involved in organ separation, showing that the zone a t the meristem periphery has a hybrid identity. This implies that PIN1 is n ot only involved in organ outgrowth, but that it is also necessary for orga n separation and positioning. A model is presented in which PIN1 and the lo cal distribution of auxin control phyllotaxis.