PLETHORAs shape Arabidopsis phyllotaxis through modulation of patterning robustness and accelerated inflorescence development.

Phyllotaxis is the arrangement of lateral organs on a stem axis, which in Arabidopsis follows a spiral pattern. We previously described that loss of three PLETHORA (PLT) transcription factors shifts the phyllotactic spiral to novel metastable patterns, but the mechanism behind these shifts remained unclear. In this study, we aimed to fill this knowledge gap by performing a detailed analysis of phyllotaxis in plt rosettes, inflorescences, and meristems. We supplement our quantitative measurements with transcriptomic profiling of plt3 plt7 meristems, genome-wide in vitro binding assays, and an EMS enhancer screen in plt3 plt5 plt7. Contrary to earlier beliefs, primordium positioning at the inflorescence meristem is only subtly noisier in plt3 plt5 plt7, which we attribute to loss of PLT-controlled regulation of auxin and cytokinin networks. However, the meristematic patterning defects alone cannot explain the large deviations from the spiral pattern in mature mutant inflorescences. Instead, accelerated inflorescence development of plt3 plt5 plt7 generates larger patterning deviations through the joint action of meristem chirality and stem torsion. We demonstrate the importance of the relationship between chirality, torsion, and internode length in tissue-twisting mutants. We conclude that shoot meristematic PLTs provide robustness to primordium patterning and phyllotaxis through the integration of developmental processes during bolting.