SummaryThe proper organization of cortical microtubule arrays is essential for anisotropic growth in plants but how distinct array patterns are formed is not understood. Here, we report a relationship between microtubule dynamics and array organization using transgenic plants expressing modified tubulins. When green fluorescent protein (GFP) or a hemaglutinin epitope tag was fused to the N-terminus of tubulins and expressed in Arabidopsis plants, these tubulins were incorporated into microtubules along with endogenous tubulins. Plants expressing the modified b-tubulins were phenotypically normal and possessed transversely oriented cortical arrays in the epidermal cells of the root elongation zone; however, the expression of modified a-tubulins caused right-handed helical growth, increased trichome branching, and a shallow left-handed (Sform) helical array organization. In cells expressing the modified a-tubulins, microtubule dynamicity was suppressed and polymerization was promoted, and GFP-EB1 (End Binding 1) labeled larger regions of the microtubule end more frequently, when compared with control cells. We propose that the N-terminal appendage introduced into a-tubulin inhibits GTP hydrolysis, thus producing polymerization-prone microtubules with an extended GTP cap. Consistent with this interpretation, plants expressing an a-tubulin mutated in the GTPase-activating domain exhibited similar microtubule properties, with regard to dynamics and the localization of GFP-EB1, and showed right-handed helical growth.