A kinesin motor complex localizes microtubule-severing proteins to direct gravitropism in Physcomitrium patens

Gravitropism enables plants to adjust growth direction and architecture in response to gravity. The classical Cholodny-Went theory posits that asymmetric auxin redistribution drives differential growth during gravitropism in seed plants, but the nature of the equivalent asymmetric process in moss protonemata is still unclear. Here, through forward genetic screening, we identified GTRD, which encodes a coiled-coil protein whose loss reverses the direction of protonemal gravitropism. We further demonstrated that GTRD forms a complex with the kinesin-14 motor GTRC, and that GTRC drives the minus-end-directed movement of GTRD along microtubules. This GTRC-GTRD complex transports a microtubule anchoring module MSD1-WDR8 that positions katanin. Under gravistimulation, katanin-dependent microtubule severing facilitates the asymmetric formation of a microtubule focus on the upper flank of the tip cell, where it spatially constrains the apical actin cluster. This cytoskeletal reorganization guides the upward growth of the protonemata. Together, the GTRC-GTRD motor complex spatially organizes katanin-mediated microtubule remodeling to establish cytoskeletal polarity, which serves as a key asymmetric cue that directs gravitropic growth in protonemal tip cells.