Kinesins Have a Dual Function in Organizing Microtubules during Both Tip Growth and Cytokinesis in Physcomitrella patens

Microtubules (MTs) play a crucial role in the anisotropic deposition of cell wall material, thereby affecting the direction of growth. A wide range of tip-growing cells display highly polarized cell growth, and MTs have been implicated in regulating directionality and expansion. However, the molecular machinery underlying MT dynamics in tip-growing plant cells remains unclear. Here, we show that highly dynamic MT bundles form cyclically in the polarized expansion zone of the moss Physcomitrella patens caulonemal cells through the coalescence of growing MT plus ends. Furthermore, the plant-specific kinesins (KINID1) that are is essential for the proper MT organization at cytokinesis also regulate the turnover of the tip MT bundles as well as the directionality and rate of cell growth. The plus ends of MTs grow toward the expansion zone, and KINID1 is necessary for the stability of a single coherent focus of MTs in the center of the zone, whose formation coincides with the accumulation of KINID1. We propose that KINID-dependent MT bundling is essential for the correct directionality of growth as well as for promoting growth per se. Our findings indicate that two localized cell wall deposition processes, tip growth and cytokinesis, previously believed to be functionally and evolutionarily distinct, share common and plant-specific MT regulatory components.