Lateral Inhibition by a Peptide Hormone-Receptor Cascade during Arabidopsis Lateral Root Founder Cell Formation

In plants, the position of lateral roots (LRs) depends on initiation sites induced by auxin. The domain of high auxin response responsible for LR initiation stretches over several cells, but only a pair of pericycle cells (LR founder cells) will develop into LRs. In this work, we identified a signaling cascade controlling LR formation through lateral inhibition. It comprises a peptide hormone TARGET OF LBD SIXTEEN 2 (TOLS2), its receptor RLK7, and a downstream transcription factor PUCHI. TOLS2 is expressed at the LR founder cells and inhibits LR initiation. Time-lapse imaging of auxin-responsive DR5:LUCIFERASE reporter expression revealed that occasionally two pairs of LR founder cells are specified in close proximity even in wild-type and that one of them exists only transiently and disappears in an RLK7-dependent manner. We propose that the selection of LR founder cells by the peptide hormone-receptor cascade ensures proper LR spacing. •Lateral root founder cells (LRFCs) express TOLS2 peptide, inhibiting nearby LRFCs•The receptor RLK7 and transcription factor PUCHI make up the TOLS2 signaling pathway•DR5, an LRFC marker, is occasionally activated in proximity to pre-existing LRFCs•TOLS2-RLK7-PUCHI inhibits LRFC identity in proximal cells, ensuring proper LR spacing Lateral roots form at regulated intervals. Toyokura and Goh et al. identify a peptide hormone, TOLS2, expressed in lateral root founder cells in Arabidopsis thaliana. TOLS2 and its receptor, RLK7, inhibit the formation of closely spaced lateral root founder cells.