The CaM1-Associated CCaMK-MKK1/6 cascade positively affects the lateral root growth through auxin signaling under salt stress in rice.

Abstract

CCaMKs and MAPKKs are two kinases that regulate salt stress response in plants. It remains unclear, however, how they cooperatively affect the lateral root growth under salt stress. Here, two conserved phosphorylation sites (S102 and T118) of OsCaM1 were identified, and affected the capability of binding to Ca 2+ \xa0in vitro and kinase activity of OsCCaMK in vivo. OsCCaMK specifically interacted with OsMKK1/6 in a Ca 2+/CaM-dependent manner. The in vitro kinase and in vivo dual-luciferase assays revealed that OsCCaMK phosphorylated OsMKK6 while OsMKK1 phosphorylated OsCCaMK. Overexpression and antisense-RNA repression expression of OsCaM1-1 and CRISPR/Cas9-mediated gene edition mutations of OsMKK1, OsMKK6 and OsMKK1/6 proved that OsCaM1-1, OsMKK1 and OsMKK6 enhanced the auxin content in roots and lateral root growth under salt stress. Consistently, OsCaM1-1, OsMKK1 and OsMKK6 regulated the transcript levels of the genes of this cascade, salt stress-related and lateral root growth-related auxin signaling under salt stress in rice roots. These findings demonstrate that the OsCaM1-associated OsCCaMK-OsMKK1/6 cascade plays a critical role in recruiting auxin signaling in rice roots. These results also provide new insight into the regulatory mechanism of the CaM-mediated phosphorylation relay cascade to auxin signalling in lateral root growth under salt stress in plants.