Involvement of KATANIN1, a microtubule-severing enzyme, in hypergravity-induced modification of growth anisotropy in Arabidopsis hypocotyls

The mechanism by which gravity affects the growth anisotropy of plant organs is an important issue for the cultivation of plants under microgravity conditions in space. This study aimed to examine the roles of KATANIN1 (KTN1), a microtubule-severing enzyme, in the modification of the direction of cortical microtubules (CMTs) and growth anisotropy in Arabidopsis hypocotyls induced by gravity using hypergravity conditions that can be created on Earth. The KTN1 mutants (ktn1^P393S, ktn1^R465Q, and ktn1^S342F) exhibited shorter and thicker hypocotyls than the wild type (ecotype Columbia-0). Hypergravity at 300 g modified growth anisotropy in wild-type hypocotyls; hypergravity inhibited elongation but stimulated lateral growth. In contrast, hypergravity-induced modification of growth anisotropy was suppressed in hypocotyls of the three mutants. The wild type had an abundance of CMTs in transverse orientation (between 0° and 30°) under 1 g conditions and a tendency toward increased CMTs in longitudinal orientation under hypergravity conditions (between 60° and 90°). However, hypergravity-induced reorientation was not observed in hypocotyls of ktn1 mutants. The transcript level of the KTN1 gene in wild-type hypocotyls increased within 1 h of onset of hypergravity treatment and promptly decreased to the same level as the 1 g control. These findings suggest that the reorientation of CMTs is mediated by KTN1, which is regulated by transient expression upregulation, which is responsible for the modification of growth anisotropy induced by hypergravity in Arabidopsis hypocotyls.