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 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-RGI1 and nuclear PLTs, however, were not identified. Here, we report that the mitogen-activated protein (MAP) kinase kinase 4 (MKK4) and MAP kinase 3 (MPK3) were coimmunoprecipitated with RGI1-FLAG after Arabidopsis seedlings were treated with RGF1. Genetic and biochemical assays confirmed that MKK4 and MKK5, and their downstream targets MPK3 and MPK6, are essential RGI-dependent regulators of root meristem development. In addition, we found that the MKK4/MKK5-MPK3/MPK6 module functions downstream of YDA, a MAPKKK. Our results demonstrate that RGF1-RGI1 regulate the expression of PLT1/PLT2 via a YDA-MKK4/MKK5-MPK3/MPK6 signaling cascade.

An MAP Kinase Cascade Downstream of RGF/GLV Peptides and Their RGI Receptors Regulates Root Development

“…These data altogether indicate that MPK3 and 6 also act downstream of RGF/GLV-RGI pathway in the RAM. In agreement with this, MPK3 and 6 were phosphorylated after treatment with RGF1p, which was blocked in the rgi12345 mutant (Lu et al, 2020;Shao et al, 2020). MPK3/6 phosphorylation peaked after 10-15 minutes of RGF1p addition and decreased after 30-60 minutes, thus showing similar phosphorylation kinetics as previously obtained for GLV6p (Fernandez et al, 2020).…”

“…The authors of both studies associated the mpk3mpk6 phenotype with reduced PLT1 and 2 transcript and protein levels. In addition, induced MPK3SR and MPK6SR roots showed decreased sensitivity to the RGF1p-related increase in RAM size (Lu et al, 2020;Shao et al, 2020). These data altogether indicate that MPK3 and 6 also act downstream of RGF/GLV-RGI pathway in the RAM.…”

“…MPK6, and to a lesser extent its close homologue MPK3, were phosphorylated in response to the addition of the GLV6p. Similar to glv6glv10 mutants, mpk6 mutants showed increased lateral root initiation, altogether indicating that MPK6, and likely also MPK3, act as downstream targets of the RGF/GLV-RGI pathway to inhibit asymmetric cell division during lateral root initiation (Fernandez et al, 2020 (Lu et al, 2020).…”

RGI-GOLVEN signalling promotes FLS2 abundance to regulate plant immunity

“…Another scenario for the control of FLS2 levels via the RGI3-GLV pathway may be the regulation by RGI downstream factors, such as MAPKs or RITF1 (Fernandez et al, 2020;Lu et al, 2020;Shao et al, 2020;Yamada et al, 2019). Also, GLV-controlled plasma membrane protein stabilization is not a general effect as other plasma-membrane bound receptors and proteins do not change their abundance in response to GLV treatment (Whitford et al, 2012).…”

“…MPK6, together with MPK3 also acts downstream of GLV11 perception by RGI1-5 during the control of meristematic activity. Here, MPK3 and MPK6 are activated by the MAPKKK YODA and the MAPKK MKK4 and MKK5 (Lu et al, 2020;Shao et al, 2020).…”