Together with RGI1, RGI2, RGI4 and RGI5, acts as receptor of RGF peptides (e.g. RGF1, GLV5/CLEL1/RGF2, GLV7/CLEL3/RGF3, GLV3/RGF4, GLV10/CLEL7/RGF5 and RGF10/CLELN), peptide hormones which maintain the postembryonic root stem cell niche by regulating the expression levels and patterns of the transcription factor PLETHORA (PLT, e.g. PLT1 and PLT2) (PubMed:27229311, PubMed:27229312, PubMed:27001831). Links RGF peptides signal with their downstream components (PubMed:27229311, PubMed:27001831). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Binds to RGF peptides such as RGF1, GLV5/CLEL1/RGF2, GLV7/CLEL3/RGF3, GLV3/RGF4, GLV10/CLEL7/RGF5 and RGF10/CLELN; these interactions trigger the formation of heterodimers with SERK1, SERK2 or BAK1/SERK3 via LRR regions. Expressed in roots. In roots, detected in the more basal region of the elongation zone and the differentiation zone, mainly restricted to columella, transition zone and root stem cell niche. 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). The triple mutant missing RGI1, RGI2 and RGI3 is insensitive to externally applied RGF peptides (e.g. RGF1 and RGF2) and has short roots characterized by a strong decrease in meristematic cell number and declined levels of PLT1 and PLT2 at the root tip (PubMed:27001831). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.