MICROTUBULE ORGANIZATION 1 Regulates Structure and Function of Microtubule Arrays during Mitosis and Cytokinesis in the Arabidopsis Root

Abstract

MICROTUBULE ORGANIZATION 1 (MOR1) is a plant member of the highly conserved MAP215/Dis1 family of microtubuleassociated proteins. Prior studies with the temperature-sensitive mor1 mutants of Arabidopsis (Arabidopsis thaliana), which harbor single amino acid substitutions in an N-terminal HEAT repeat, proved that MOR1 regulates cortical microtubule organization and function. Here we demonstrate by use of live cell imaging and immunolabeling that the mor1-1 mutation generates specific defects in the microtubule arrays of dividing vegetative cells. Unlike the universal cortical microtubule disorganization in elongating mor1-1 cells, disruption of mitotic and cytokinetic microtubule arrays was not detected in all dividing cells. Nevertheless, quantitative analysis identified distinct defects in preprophase bands (PPBs), spindles, and phragmoplasts. In nearly one-half of dividing cells at the restrictive temperature of 30°C, PPBs were not detected prior to spindle formation, and those that did form were often disrupted. mor1-1 spindles and phragmoplasts were short and abnormally organized and persisted for longer times than in wild-type cells. The reduced length of these arrays predicts that the component microtubule lengths are also reduced, suggesting that microtubule length is a critical determinant of spindle and phragmoplast structure, orientation, and function. Microtubule organizational defects led to aberrant chromosomal arrangements, misaligned or incomplete cell plates, and multinucleate cells. Antiserum raised against an N-terminal MOR1 sequence labeled the full length of microtubules in interphase arrays, PPBs, spindles, and phragmoplasts. Continued immunolabeling of the disorganized and short microtubules of mor1-1 at the restrictive temperature demonstrated that the mutant mor1-1 L174F protein loses function without dissociating from microtubules, providing important insight into the mechanism by which MOR1 may regulate microtubule length.

Tangled localization at the cortical division site of plant cells occurs by several mechanisms

“…For example, Adiantum capilus-veneris cells lacking PPBs show late phragmoplast misorientation and oblique cell plate insertion (Mineyuki et al, 1991). In Arabidopsis thaliana (McClinton and Sung, 1997;Camilleri et al, 2002;Kawamura et al, 2006;Azimzadeh et al, 2008) and Zea mays , mutants that fail to make PPBs display cells with random planes of division. Cultured A. thaliana (Chan et al, 2005) and tobacco cells (Vanstraelen et al, 2006) lacking PPBs also exhibit random division planes.…”

Seed coat mucilage cells ofArabidopsis thalianaas a model for plant cell wall research

“…To test this hypothesis, MSCs were examined in the mor1-1 mutant, whose microtubules lose organization when grown at high temperatures. 34,35 Although release of mucilage from seed coats was partially disrupted, the secretion of mucilage did not appear to be significantly affected in mor1-1 MSCs. Thus, the role of microtubules lining the mucilage pocket of MSCs remains enigmatic.…”

Update: Plant Cortical Microtubule Arrays

“…CLASP mutants show an increased sensitivity to the microtubule-disrupting drug oryzalin, implicating CLASP as a microtubule-stabilizing protein (Ambrose et al, 2007). Interestingly, CLASP exhibits structural similarity to MICROTUBULE ORGANIZER1 (MOR1), a homolog of the conserved XMAP215 family, which promotes overall microtubule stability, increases microtubule plus end growth and shortening rates, and suppresses the frequency of plus end pausing in vivo (Kawamura et al, 2006;Kawamura and Wasteneys, 2008). CLASP and MOR1 are both single-gene families containing multiple HEAT repeats and an N-terminal Tumor Overexpressed Gene (TOG) domain, with four additional TOG domains present in MOR1 (Whittington et al, 2001;Ambrose et al, 2007).…”