The anisotropy1 D604N mutation in the Arabidopsis cellulose synthase1 catalytic domain reduces cell wall crystallinity and the velocity of cellulose synthase complexes.

Multiple cellulose synthase (CesA) subunits assemble into plasma membrane complexes responsible for cellulose production. In the Arabidopsis (Arabidopsis thaliana) model system, we identified a novel D604N missense mutation, designated anisotropy1 (any1), in the essential primary cell wall CesA1. Most previously identified CesA1 mutants show severe constitutive or conditional phenotypes such as embryo lethality or arrest of cellulose production but any1 plants are viable and produce seeds, thus permitting the study of CesA1 function.

Cortical microtubules optimize cell-wall crystallinity to drive unidirectional growth in Arabidopsis.

The shape of plants depends on cellulose, a biopolymer that self-assembles into crystalline, inextensible microfibrils (CMFs) upon synthesis at the plasma membrane by multi-enzyme cellulose synthase complexes (CSCs). CSCs are displaced in directions predicted by underlying parallel arrays of cortical microtubules, but CMFs remain transverse in cells that have lost the ability to expand unidirectionally as a result of disrupted microtubules. These conflicting findings suggest that microtubules are important for some physico-chemical property of cellulose that maintains wall integrity.

Hypersensitivity to cytoskeletal antagonists demonstrates microtubule-microfilament cross-talk in the control of root elongation in Arabidopsis thaliana.

Elongation of diffusely expanding plant cells is thought to be mainly under the control of cortical microtubules. Drug treatments that disrupt actin microfilaments, however, can reduce elongation and induce radial swelling. To understand how microfilaments assist growth anisotropy, we explored their functional interactions with microtubules by measuring how microtubule disruption affects the sensitivity of cells to microfilament-targeted drugs.

MICROTUBULE ORGANIZATION 1 regulates structure and function of microtubule arrays during mitosis and cytokinesis in the Arabidopsis root.

MICROTUBULE ORGANIZATION 1 (MOR1) is a plant member of the highly conserved MAP215/Dis1 family of microtubule-associated 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.

COBRA, an Arabidopsis extracellular glycosyl-phosphatidyl inositol-anchored protein, specifically controls highly anisotropic expansion through its involvement in cellulose microfibril orientation.

The orientation of cell expansion is a process at the heart of plant morphogenesis. Cellulose microfibrils are the primary anisotropic material in the cell wall and thus are likely to be the main determinant of the orientation of cell expansion. COBRA (COB) has been identified previously as a potential regulator of cellulose biogenesis.

Cellulose microfibril alignment recovers from DCB-induced disruption despite microtubule disorganization.

Cellulose microfibril deposition patterns define the direction of plant cell expansion. To better understand how microfibril alignment is controlled, we examined microfibril orientation during cortical microtubule disruption using the temperature-sensitive mutant of Arabidopsis thaliana, mor1-1. In a previous study, it was shown that at restrictive temperature for mor1-1, cortical microtubules lose transverse orientation and cells lose growth anisotropy without any change in the parallel arrangement of cellulose microfibrils.

Mutation or drug-dependent microtubule disruption causes radial swelling without altering parallel cellulose microfibril deposition in Arabidopsis root cells.

As critical determinants of growth anisotropy in plants, cortical microtubules are thought to constrain the movement of cellulose synthase complexes and thus align newly deposited cellulose microfibrils. We tested this cellulose synthase constraint model using the temperature-sensitive mor1-1 mutant of Arabidopsis. Contrary to predictions, the disruption of cortical microtubules in mor1-1 root epidermal cells led to left-handed root twisting and radial swelling but did not alter the transverse orientation of cellulose microfibrils.