Together with RGI1, RGI3, 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:27229312, PubMed:27229311, 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 (PubMed:27001831, PubMed:27229311). Interacts with UBP13 (PubMed:29339500). Specific to root meristems, especially in lateral root meristems (LRM). Present in the whole roots with a predominant expression in the proximal meristem, including the elongation zone, and a gradual decreases toward the differentiation zone. Phosphorylated and ubiquitinated upon interaction with RGF1, thus leading to activation a subsequent degradation (By similarity). Stabilized by UBP12 and UBP13-mediated deubiquitination (By similarity). 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.