Radial askew endodermal cell divisions reveal IRK functions in division orientation

Oriented cell divisions establish plant tissue and organ patterning and produce different cell types; this is particularly true of the highly organized Arabidopsis root meristem. Mutant alleles of INFLORESCENCE AND ROOT APICES RECEPTOR KINASE (IRK) exhibit excess cell divisions in the root endodermis. IRK is a transmembrane receptor kinase that localizes to the outer polar domain of these cells, which suggests directional signal perception is necessary to repress endodermal cell division. Here, a detailed examination revealed many of the excess endodermal divisions in irk have division planes that specifically skew towards the outer lateral side, therefore we termed them radial askew divisions. Expression of an IRK truncation, lacking the kinase domain, retains polar localization and rescues these radial askew divisions, but the roots exhibit excess periclinal endodermal divisions. Using markers of cell identity, we show that the daughters of radial askew divisions transition from endodermal to cortex identity similar to those of periclinal divisions. These results extend the requirement for IRK beyond repression of cell division activity to include cell division plane positioning. Based on its polarity, we propose that IRK at the outer lateral endodermal cell face participates in division plane positioning to ensure normal root ground tissue patterning.