RGF1-RGI1, a peptide-receptor complex, regulates Arabidopsis root meristem development via a MAPK signaling cascade.

Abstract Root growth is maintained by the continuous division of cells in the apical meristem. ROOT MERISTEM GROWTH FACTOR 1 (RGF1) is a critical peptide hormone regulating root stem cell niche maintenance. Previous studies from our lab and others discovered that five closely related leucine-rich repeat receptor-like protein kinases (LRR-RLKs), named RGF1 INSENSITIVES (RGIs) or RGF1 RECEPTORS (RGFRs), are able to perceive the RGF1 signal and redundantly control root stem cell niche maintenance. RGF1 regulates root meristem activity mainly via two downstream transcription factors, PLETHORA 1 (PLT1) and PLT2. Regulatory proteins connecting cell surface RGF1-RGIs and nuclear PLTs, however, were not identified. Here we report that a mitogen-activated protein kinase kinase 4 (MKK4) and MAP kinase 3 (MPK3) were co-immunoprecipitated with RGI1-FLAG after Arabidopsis seedlings were treated with RGF1. Genetic and biochemical assays confirmed that MKK4 and MKK5, and their downstream MPK3 and MPK6, are the essential regulators of root meristem development in a RGIs-dependent manner. In addition, we found that the MKK4/MKK5-MPK3/MPK6 is the downstream module of YDA, a MAPKKK. Our results therefore demonstrate that RGF1-RGIs regulates the expression of PLT1/PLT2 via an YDA-MKK4/MKK5-MPK3/MPK6 signaling cascade.