"The mathematical regularity of plant organ initiation, also known as phyllotaxis, has always intrigued mathematicians and botanists. Only recently was established that the plant hormone auxin has a major role in regulating organ positioning at the shoot apex of plants. Specific spatial accumulation of auxin induces organ formation and control organ spacing. The current model for phyllotaxis relies on polar auxin transport via auxin efflux facilitators of the PIN family, and postulated positive feedback loops between auxin and PIN proteins. However no regulatory mechanism and gene network have been proposed yet to explain how organ positioning is controlled in plants. The proposal aims to bridge this gap by using a unique genetic tool generated in the host laboratory, a mutant in the plant model Arabidopsis that switches from wild type spiral organ arrangement (organs arising by a divergent angle of 137.5º) to a decussate pattern (organs arising in opposite pairs). The mutant is impaired in three genes that belong to the PLETHORA (PLT) family genes previously described in roots as involved in a regulatory loop with auxin and PIN proteins to maintain an auxin gradient and root meristem function. This is the first time that a stable change in phyllotactic pattern has been observed in an Arabidopsis mutant, giving a real opportunity to understand regulatory mechanisms underlying behind this developmental process. The PLT genes impaired in this mutant are expressed throughout the shoot apical meristem, i.e. in the peripheral zone where are initiated organ primordia and in the central zone where are lying the stem cells, suggesting a role for PLT genes in establishment of phyllotactic patterns and in shoot apical meristem function. The aim of the project is to understand if PLT genes influence phyllotaxis by controlling auxin polar distribution, as in roots, or / and by regulating meristem size / organisation, the two parameters known to disturb organ positioning."