Arabidopsis MICROTUBULE-ASSOCIATED PROTEIN18 functions in directional cell growth by destabilizing cortical microtubules

Microtubule-associated proteins (MAPs) play important roles in the regulation of microtubule function in cells. We describe Arabidopsis thaliana MAP18, which binds to microtubules and inhibits tubulin polymerization in vitro and colocalizes along cortical microtubules as patches of dot-like structures. MAP18 is expressed mostly in the expanding cells. Cells over-expressing MAP18 in Arabidopsis exhibit various growth phenotypes with loss of polarity. Cortical microtubule arrays were significantly altered in cells either overexpressing MAP18 or where it had been downregulated by RNA interference (RNAi). The cortical microtubules were more sensitive to treatment with microtubule-disrupting drugs when MAP18 was overexpressed, but more resistant when MAP18 was eliminated in cells expressing MAP18 RNAi. Our study demonstrated that MAP18 may play a role in regulating directional cell growth and cortical microtubule organization by destabilizing microtubules.