Together with RGI1, RGI2, RGI3 and RGI4, acts as receptor of RGF1, a peptide hormone that maintains the postembryonic root stem cell niche by regulating the expression levels and patterns of the transcription factor PLETHORA (PLT) (PubMed:27229312, PubMed:27229311). Links RGF1 signal with its downstream components (PubMed:27229311). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Binds to RGF1; this interaction triggers the formation of heterodimers with SERK1. Expressed in roots and hypocotyls. Present in the whole roots. Phosphorylated and ubiquitinated upon interaction with RGF1, thus leading to activation a subsequent degradation. Autophosphorylated. Smaller root meristem size and fewer root meristematic cortex cells, associated with shorter roots and a slighty reduced sensitivity to RGF1 (PubMed:27229311). Quintuple mutants rgi1 rgi2 rgi3 rgi4 rgi5 display a consistent short primary root phenotype with a small size of meristem associated with a total insensitivity to RGF1 and undetectable levels of PLT1 and PLT2 (PubMed:27229312). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.