Stable ARMADILLO REPEAT KINESIN 2 in light inhibits hypocotyl elongation and facilitates light-induced cortical microtubule reorientation in Arabidopsis.

Hypocotyls undergo different morphogenesis in light and dark conditions, with cortical microtubules being reoriented in response to light to coordinate cell growth status. Kinesins are microtubule-based motor proteins that are mostly responsible for transporting organelles and vesicles, although some can also regulate microtubule organization; however, it is currently not known whether they are involved in microtubule reorientation and hypocotyl elongation. In this study, we found that ARMADILLO REPEAT KINESIN 2 (ARK2) negatively regulated the hypocotyl elongation of Arabidopsis. The hypocotyl cells of plants with the ark2 null allele were longer than those of the wild type and had relatively more transversely arranged cortical microtubules. In addition, ARK2 co-localized with cortical microtubules and facilitated the light-induced reorientation of the cortical microtubule arrays. Interestingly, the ARK2 protein is stable in the light and degraded through the 26S proteasome pathway in the dark. Furthermore, we determined that ARK2 could interact with the E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), which contributed to down-regulation of ARK2 in darkness that might benefit hypocotyl growth in the dark.