MEKK1/YDA-MKK4/5-MPK3/6 cascade is involved in extracellular ATP-regulated root growth of Arabidopsis thaliana.

Abstract

The mitogen-activated protein kinase (MAPK) cascades play vital roles in mediating plant cell responses to external stimuli by regulating intracellular metabolism. We have previously reported that extracellular ATP (eATP) acts as an apoplastic messenger to regulate root growth, and that two receptor-like kinases, LRK10L3 and BAK1, play essential roles in this process. However, the intracellular signaling mechanisms through which eATP influences root growth remain unclear. In this study, we found that roots of loss-of-function mutants of several MAPK cascade components exhibited significantly reduced responsiveness to eATP compared with the wild type. eATP stimulated the expression of genes encoding MAPK cascade components, and in several upstream-element null mutants, eATP-induced expression of downstream members was enhanced. Moreover, eATP increased the phosphorylation of MAPK cascade components, suggesting that a cascade comprising MEKK1/YDA-MKK4/5-MPK3/6 is involved in the response of root cells to eATP. eATP may activate this MAPK cascade via LRK10L3 and BAK1, because eATP-stimulated phosphorylation of MEKK1 and YDA was impaired in double-null mutants lacking both receptor-like kinases. eATP also induced the translocation of MKK4/5 and MPK3/6 from the cytoplasm to the nucleus, a process partially dependent on phosphorylation. In mutants lacking functional MAPKs, eATP-stimulated expression of several jasmonic acid (JA) biosynthesis-related genes was compromised. Collectively, these results demonstrate that the MAPK cascade may serve as a crucial signaling module in eATP-regulated root growth in Arabidopsis thaliana, providing insights for further investigation of eATP signaling mechanisms.