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

Microtubules (MTs) play a crucial role in theanisotropic deposition of cell wall material, thereby affecting the direction ofgrowth. A wide range of tip-growing cells display highly polarized cell growth, andMTs have been implicated in regulatingdirectionality and expansion. However, the molecular machinery underlying MT dynamics in tip-growing plant cells remainsunclear. Here, we show that highly dynamic MTbundles form cyclically in the polarized expansion zone of the mossPhyscomitrella patens caulonemal cells through the coalescence ofgrowing MT plus ends. Furthermore, theplant-specific kinesins (KINID1) that are is essential for the proper MT organization at cytokinesis also regulate theturnover of the tip MT bundles as well as thedirectionality and rate of cell growth. The plus ends of MTs grow toward the expansion zone, and KINID1 is necessary forthe stability of a single coherent focus of MTs in the center of the zone, whose formation coincides with theaccumulation of KINID1. We propose that KINID-dependent MT bundling is essential for the correct directionality ofgrowth as well as for promoting growth per se. Our findings indicate that twolocalized cell wall deposition processes, tip growth and cytokinesis, previouslybelieved to be functionally and evolutionarily distinct, share common andplant-specific MT regulatory components.