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Abstract
Myosin motor proteins are thought to carry out important functions in the establishment and maintenance of cell polarity by moving cellular components such as organelles, vesicles, or protein complexes along the actin cytoskeleton. In Arabidopsis thaliana, disruption of the myosin XIK gene leads to reduced elongation of the highly polar root hairs, suggesting that the encoded motor protein is involved in this cell growth. Detailed live-cell observations in this study revealed that xik root hairs elongated more slowly and stopped growth sooner than those in wild type. Overall cellular organization including the actin cytoskeleton appeared normal, but actin filament dynamics were reduced in the mutant. Accumulation of RabA4b-containing vesicles, on the other hand, was not significantly different from wild type. A functional YFP-XIK fusion protein that could complement the mutant phenotype accumulated at the tip of growing root hairs in an actin-dependent manner. The distribution of YFP-XIK at the tip, however, did not match that of the ER or several tip-enriched markers including CFP-RabA4b. We conclude that the myosin XIK is required for normal actin dynamics and plays a role in the subapical region of growing root hairs to facilitate optimal growth.
Highlights
Myosin motor proteins couple the hydrolysis of ATP to the active movement of organelles or macromolecular assemblies along the actin cytoskeleton and are responsible for cytoplasmic streaming in plant cells [1]
An analogous process occurs in plant cells that expand at a single point on their surface, i.e. in tip-growing pollen tubes and root hairs
A subset of these vesicles, that likely function as secretory vesicles [7], can be labeled with the small GTPase YFP-RabA4b [8]. Since accumulation of these vesicles during growth depends on the actin cytoskeleton [8], it is likely that myosin motors may play a role in their movement to the root hair tip
Summary
Myosin motor proteins couple the hydrolysis of ATP to the active movement of organelles or macromolecular assemblies along the actin cytoskeleton and are responsible for cytoplasmic streaming in plant cells [1]. Tip growth of root hairs is regulated by a complex selforganizing feedback mechanism that involves numerous signaling molecules such as calcium ions, reactive oxygen species, membrane lipids and small G-proteins [3,4,5] This results in a dynamic actin cytoskeleton which directs the steady state accumulation of a large number of small vesicles in the apical dome where secretion occurs [6]. A subset of these vesicles, that likely function as secretory vesicles [7], can be labeled with the small GTPase YFP-RabA4b [8] Since accumulation of these vesicles during growth depends on the actin cytoskeleton [8], it is likely that myosin motors may play a role in their movement to the root hair tip. This conjecture is supported by the recent finding that mutants in two class XI myosins were found to affect the length of root hairs [9,10], supporting a role for these motors in this process
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