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

Abstract

ORCID IDs: 0000-0002-3996-0549 (Y.H.); 0000-0001-6027-1919 (J.H.D.) 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.

Interplay between Ions, the Cytoskeleton, and Cell Wall Properties during Tip Growth

“…In support of this hypothesis, Hiwatashi et al (2014) discovered that two microtubule-based motors, KINESIN FOR IN-TERDIGITATED MICROTUBULES1a (KIND1a) and KIND1b, regulate microtubule organization and protonemal growth. The kind1a,b knockout plants exhibit protonemata that do not grow straight but still have tipward-growing cytoplasmic microtubules.…”

“…In moss protonemata, there are also two populations of microtubules: cytoplasmic and cortical. In the apical cell, the polarity of cytoplasmic microtubules between the nucleus and the cell apex is tipward, with an apical focus of microtubule plus ends (Hiwatashi et al, 2014). In contrast, the cortical microtubule array consists of shorter microtubules that lack polarity (Burkart et al, 2015;Nakaoka et al, 2015).…”

Cytoplasmic Nucleation and Atypical Branching Nucleation Generate Endoplasmic Microtubules inPhyscomitrella patens

“…In this regard, the branching nucleation in this system might be a variation of cytoplasmic nucleation; cytoplasmic-like nucleation occasionally occurs on MTs, rather than in the cytoplasm. Longitudinal alignment of endoplasmic MTs might occur after MT nucleation, for example, through MT transport ( Figure 2B) or cross-linking (Doonan et al, 1985;Hiwatashi et al, 2014). g-Tubulin was detected at the majority of the branching sites.…”

“…In P. patens, germinated spores develop into a tissue called "protonemata," in which two types of cells, chloronemata and caulonemata, have been identified. Immunofluorescence microscopy and live imaging of MTs have shown that these cells do not form conventional cortical MT arrays but rather form "endoplasmic MTs" that run throughout the cytoplasm (Doonan et al, 1985;Hiwatashi et al, 2014). Endoplasmic MTs play a critical role in protonemal cell growth (Doonan et al, 1985(Doonan et al, , 1988, but how they are generated is unknown.…”

The Microtubule Plus-End Tracking Protein ARMADILLO-REPEAT KINESIN1 Promotes Microtubule Catastrophe in Arabidopsis

“…The KINID1a and KINID1b kinesins in these cells are responsible for the bundling of microtubule plus ends. Loss of both these kinesins leads to reduced microtubule bundling and aberrant tip growth (Hiwatashi et al, 2014) The ARMADILLO-REPEAT KINESIN1 (ARK1), originally named MORPHOGENESIS OF ROOT HAIR2 (MRH2) (Jones et al, 2006), is thought to participate in microtubule dynamics. Because ARK1 is an uncharacterized kinesin belonging to an ungrouped plantspecific family of kinesins, its function cannot be predicted based on phylogeny (Reddy and Day, 2011;Zhu and Dixit, 2012).…”