AtMAP70-5, a divergent member of the MAP70 family of microtubule-associated proteins, is required for anisotropic cell growth in Arabidopsis

AtMAP70-5 is the most divergent of a recently described multigene family of plant-specific microtubule-associated proteins (MAPs). It is significantly smaller than other members and has several isoform-specific sequence features. To confirm that this protein still functions as a MAP we show that it directly binds microtubules in vitro and decorates microtubules in vivo. When added to tubulin polymerization assays, AtMAP70-5 increases the length distribution profile of microtubules indicating that it stabilizes microtubule dynamics. The overexpressed fusion protein perturbs cell polarity in cell suspensions by inducing extra poles for growth. Similarly, in Arabidopsis plants the overexpression of AtMAP70-5 causes epidermal cells to swell; it also stunts growth and induces right-handed organ twisting. RNAi-mediated downregulation of AtMAP70-5 results in reduced inflorescence stem length and diameter and individual cells are inhibited in their capacity for expansion. These observations suggest that the control over AtMAP70-5 expression levels is important in order to maintain axial polarity and to ensure regular extension of plant organs.