Regulator of the microtubular cytoskeleton (By similarity). Microtubule-associated protein involved in the association of cellulase synthase (CESA) complexes (CSCs) and cortical microtubules. Promotes dynamics of CSCs in the plasma membrane in both microtubules-dependent and microtubules-independent manners. Regulates primary cell wall biosynthesis and cellulose microfibrils organization (PubMed:24368796). Associates with cellulase synthase (CESA) complexes (PubMed:24368796). Binds to cortical microtubules (By similarity). Interacts with CESA3 and CESA6 (PubMed:24368796). Colocalizes with cellulase synthase (CESA) complexes (CSCs) in a CSI1-dependent dynamic way. Present with cortical microtubules. Expressed in dark-grown hypocotyls, leaves (confined to vasculature and trichomes), stamen, pollen, developing siliques, and roots. Restricted in meristematic tissue of the shoot and root. Present in distinct punctae at the cell cortex, called microtubule-associated cellulose synthase compartments, that move with constant velocities of 10 to 3000 nm/min. No visible phenotype. 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. Truncated N-terminus.