In higher plants, reorientation of cortical microtubule arrays has been postulated to be of importance for modifying cell growth to adapt to environmental conditions. However, the process of microtubule reorientation is largely unknown. Recent genetic and live cell imaging studies of microtubule dynamics shed light on the regulatory mechanisms of microtubule molecular nucleation and severing apparatuses, which are required for array reorientation in response to blue light signaling. Branching nucleation from -tubulin complexes creates a small population of discordant microtubules that are acted on by KATANIN-mediated severing in two ways. KATANIN releases microtubules from nucleation sites and rapidly amplifies discordant microtubules by severing at microtubule crossovers. In this review, I focus on the molecular details of these two enzymes, which enable microtubule array transition.