Plants continuously generate various kinds of organs such as roots, leaves and flowers. The growth morphology of each organ is achieved by the directional growth of plant cells. Prior to cell growth, an intracellular cytoskeleton called a microtubule aligns perpendicularly to the growth axis to determine growth direction of plant cells (Fig. 1). Various proteins have been shown to regulate the dynamic behavior of microtubules, but the mechanism of microtubule alignment remains unresolved.