Regulator of the microtubular cytoskeleton (PubMed:22427339). Microtubule-associated protein essential for the functional association of cellulase synthase (CESA) complexes (CSCs) and cortical microtubules (PubMed:22294619, PubMed:22190487, PubMed:22751327, PubMed:23623553, PubMed:24368796). Promotes dynamics of CSCs in the plasma membrane (PubMed:21150290, PubMed:20616083, PubMed:22294619, PubMed:22190487, PubMed:22751327, PubMed:23623553, PubMed:24368796). Regulates primary cell wall biosynthesis and cellulose microfibrils organization (PubMed:21150290, PubMed:20616083, PubMed:23623553). Required for the regulation of root cell elongation/expansion (PubMed:7743935, PubMed:21150290, PubMed:20616083, PubMed:22294619, PubMed:22427339). Necessary for the formation of ovules, pollen cell wall morphogenesis and pollen tube development (PubMed:22294619). Involved in anther dehiscence, via dehydration-induced microtubule depolymerization and reorganization. May play a role in early gynoecial development (PubMed:22427339). Target of the cellulase synthase (CESA) complexes (CSCs) trafficking inhibitor CESTRIN, which reduces cellulose content and alters anisotropic growth of hypocotyls; CESTRIN treatment inhibits the dynamics of CSCs. Associates with cellulase synthase (CESA) complexes (PubMed:20616083, PubMed:22751327). Interacts with CESA1, CESA3 and CESA6 (PubMed:20616083, PubMed:24368796). Binds to cortical microtubules (PubMed:22190487, PubMed:22427339, PubMed:22751327). Undergoes dynamic changes in response to dehydration (PubMed:22427339). Colocalizes with cellulase synthase (CESA) complexes (CSCs) in a dynamic way, both at the plasma membrane and in post-Golgi compartments (PubMed:24368796, PubMed:20616083, PubMed:22294619, PubMed:22190487). Present in distinct punctae at the cell cortex, called microtubule-associated cellulose synthase compartments, that move with constant velocities of 200 to 300 nm/min (PubMed:22294619). Present with cortical microtubule depolymerizing ends (PubMed:22294619, PubMed:22190487). Expressed in seedlings, roots, stems, leaves, flowers and pollen (PubMed:20616083, PubMed:22427339). Highly expressed in floral tissues (PubMed:22427339). Strongly expressed in the stigma and pollen grains. Present at lower levels in sepals, filaments, and anther wall. Absent from the ovary. Altered microtubule-dependent dynamics of cellulase synthase (CESA) complexes (CSCs) (PubMed:22190487, PubMed:21150290, PubMed:20616083, PubMed:22294619, PubMed:22751327, PubMed:23623553, PubMed:24368796). Reduced cellulose content, with abnormal organization of cellulose microfibrils (PubMed:20616083, PubMed:23623553). Anisotropic growth defect; abnormal cell expansion, such as reduced cell elongation but increased cell volume, leading to radially swollen epidermal cells in both primary root and dark-grown hypocotyls, as well as dwarfism (PubMed:7743935, PubMed:21150290, PubMed:20616083, PubMed:22294619, PubMed:22190487, PubMed:22427339, PubMed:24368796). Organ and cell twisting leading to alterated phyllotaxis, such as subtle right-handed torsion of the inflorescence stems and hypocotyls, and associated with altered microtubule organization and uncoupled cellulose deposition from cortical microtubules (PubMed:22294619, PubMed:23623553). Reduced fertility due to smaller gynoecia with fewer ovules, heterostyly, the inability of anthers to release pollen, and pollen defects displaying irregular or collapsed cell wall morphologies as well as altered pollen tube development (PubMed:22427339, PubMed:21150290, PubMed:20616083, PubMed:22294619). Defective anther dehiscence. Decreased number of ovules per gynoecium. Hypersensitive to the microtubule-disrupting drug oryzalin. Altered depolymerization of cortical microtubules in response to dehydration. Enhanced sensitivity to exogenous calcium leading to root growth inhibition (PubMed:22427339). The csi1 csi3 double mutants shows an enhanced cell expansion defect compared to csi1 as well as an additive reduction of cellulase synthase (CESA) complexes (CSCs) velocities (PubMed:24368796). Truncated N-terminus.