Phosphorylation of MAP18 by MPK6 orchestrates microtubule reorganization in Arabidopsis during adaptation to salt stress

Plants adapt to salt stress by modulating growth, but the signaling mechanisms remain unclear. Cortical microtubules exhibit biphasic reorganization (disassembly followed by reassembly) under salinity. However, the underlying molecular regulatory mechanisms are poorly understood. Here, we identify that Arabidopsis microtubule-associated protein 18 (MAP18), a microtubule-destabilizing factor, mediates salt-induced microtubule disassembly. During prolonged salt stress, MAP18 phosphorylation increases progressively, coinciding with microtubule reassembly and suppressed root growth. We further demonstrate that mitogen-activated protein kinase 6 (MPK6) directly phosphorylates MAP18, thereby attenuating its microtubule-destabilizing activity. MPK6 kinase activity rises in a duration-dependent manner under sustained salt stress, paralleling MAP18 phosphorylation. We propose a dual-phase temporal regulatory mechanism under salt stress. Initially, MAP18 contributes to salt-induced microtubule disassembly; as stress persists, escalated MPK6 activity phosphorylates MAP18 to suppress its destabilizing function, enabling microtubule reassembly. Our findings reveal a kinase-mediated temporal control mechanism linking microtubule dynamics to root growth plasticity under salt stress.