PLETHORA transcription factors orchestrate de novo organ patterning during <i>Arabidopsis</i> lateral root outgrowth.

Abstract

Plant development is characterized by repeated initiation of meristems, regions of dividing cells that give rise to new organs. During lateral root (LR) formation, new LR meristems are specified to support the outgrowth of LRs along a new axis. The determination of the sequential events required to form this new growth axis has been hampered by redundant activities of key transcription factors. Here, we characterize the effects of three PLETHORA (PLT) transcription factors, PLT3, PLT5, and PLT7, during LR outgrowth. In <i>plt3plt5plt7</i> triple mutants, the morphology of lateral root primordia (LRP), the auxin response gradient, and the expression of meristem/tissue identity markers are impaired from the "symmetry-breaking" periclinal cell divisions during the transition between stage I and stage II, wherein cells first acquire different identities in the proximodistal and radial axes. Particularly, <i>PLT1</i>, <i>PLT2</i>, and <i>PLT4</i> genes that are typically expressed later than <i>PLT3</i>, <i>PLT5</i>, and <i>PLT7</i> during LR outgrowth are not induced in the mutant primordia, rendering "PLT-null" LRP. Reintroduction of any PLT clade member in the mutant primordia completely restores layer identities at stage II and rescues mutant defects in meristem and tissue establishment. Therefore, all <i>PLT</i> genes can activate the formative cell divisions that lead to de novo meristem establishment and tissue patterning associated with a new growth axis.