Arabidopsis CAP1 interacts with myosin XI-K to regulate root hair tip growth

Root hair serves as a classical single-cell model system for studying polarized cellular growth. Our prior work established Calcium-Associated Protein Kinase 1 (CAP1) as an essential regulator of Arabidopsis root hair tip growth, with cap1-1 mutants display stunted and aberrant root hair. Phosphoproteomic analysis identified ‌myosin XI-Kas a putative substrate of CAP1, we here decipher the molecular mechanism underlying CAP1-XI-K interaction and the biological role of XI-K in CAP1-mediated tip growth. Through integrated approaches including yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BiFC), pull-down assays, and kinase activity profiling, we demonstrate that CAP1 specifically interacts with the cargo-binding domain (CBD) of XI-K and phosphorylates it to strengthen their physical association in vitro. Phosphorylation at Ser¹¹⁶⁸ within XI-K's CBD enhances its binding affinity for CAP1 in vitro, and phenotypic analyses confirme XI-K Ser¹¹⁶⁸ phosphorylation plays an indispensable role in sustaining tip growth. Transgenic XI-K and phosphomimetic XI-K^S1168D substantially restored root hair apical growth of cap1-1 mutants, while non-phosphorylatable XI-K^S1168A showed little rescue capacity. Moreover, complementation assays in xik-3 mutants confirmed the critical function of Ser¹¹⁶⁸ phosphorylation in tip growth, with XI-K^S1168A failing to restore wild-type resemble morphology. We demonstrate that XI-K functions as a key CAP1 effector in root hair morphogenesis, where Ser¹¹⁶⁸ phosphorylation strengthens their interaction in vitro and sustains polarized growth.