Identification of three LRR-RKs involved in perception of root meristem growth factor in Arabidopsis

A peptide hormone, root meristem growth factor (RGF), regulates root meristem development through the PLETHORA (PLT) stem cell transcription factor pathway, but it remains to be uncovered how extracellular RGF signals are transduced to the nucleus. Here we identified, using a combination of a custom-made receptor kinase (RK) expression library and exhaustive photoaffinity labeling, three leucine-rich repeat RKs (LRR-RKs) that directly interact with RGF peptides in Arabidopsis. These three LRR-RKs, which we named RGFR1, RGFR2, and RGFR3, are expressed in root tissues including the proximal meristem, the elongation zone, and the differentiation zone. The triple rgfr mutant was insensitive to externally applied RGF peptide and displayed a short root phenotype accompanied by a considerable decrease in meristematic cell number. In addition, PLT1 and PLT2 protein gradients, observed as a gradual gradient decreasing toward the elongation zone from the stem cell area in wild type, steeply declined at the root tip in the triple mutant. Because RGF peptides have been shown to create a diffusion-based concentration gradient extending from the stem cell area, our results strongly suggest that RGFRs mediate the transformation of an RGF peptide gradient into a PLT protein gradient in the proximal meristem, thereby acting as key regulators of root meristem development.