Anisotropic growth is a hallmark of plant cell growth, which relies on mechanical anisotropic property of the cell wall, in particular, the ordered arrangement of cellulose microfibrils. Cortical microtubules are long known to influence the direction in which cellulose micorfibrils are deposited. However, the mechanism by which microtubules guide cellulose deposition is poorly understood. We use recently identified cellulose synthase interactive proteins to decipher the unknown mechanism. Our lab discovered that cellulose synthase interactive protein 1 (CSI1) links cellulose synthase complex (CSC) at the plasma membrane to the cortical microtubules. With this molecular link in our hand, we now begin to unravel the mystery of regulation of cellulose synthesis, which is critical to the growth morphogenesis of plant cells.