Integrating cortical microtubules into plant gravity signaling: dynamics, tubulin phosphorylation, and growth regulation

Abstract

Plant gravity responses depend on coordinated signaling and growth-control mechanisms that convert directional and mechanical stimuli into differential cell expansion. Cortical microtubules are central to the conversion: they pattern cellulose synthase trajectories, set cell-wall anisotropy, and are remodeled in altered-gravity and tubulin-mutant systems. Yet the cytoskeletal regulatory pathway remains only loosely connected to the canonical, auxin-centered picture of how plants perceive and respond to gravity. Physiological, genetic, and phosphoproteomic evidence linking microtubules to gravitropism and gravity resistance, obtained under gravistimulation by reorientation at 1 g, hypergravity, and real or simulated microgravity, is organized into an explicit, layered framework. Three regulatory layers are proposed to act on cortical microtubule organization on distinct time scales: a slow transcriptional layer that adjusts the tubulin and regulator pool, an intermediate regulator-mediated layer of nucleation and severing, and a fast post-translational layer that modifies tubulin directly. Within the framework, phosphorylation of α-tubulin at threonine 349 (Thr349) by PROPYZAMIDE-HYPERSENSITIVE 1 (PHS1) is the best-characterized candidate for a molecular link between stress signaling and control of microtubule polymerization. Whole-seedling spaceflight phosphoproteomic data show reduced Thr349 phosphorylation across four α-tubulin isoforms (TUA1, TUA3, TUA4, and TUA6) and highlight Thr349 phosphorylation as a candidate post-translational regulator of cortical microtubule responses under spaceflight conditions. Future studies should test whether PHS1 controls Thr349 phosphorylation during gravity responses and identify the activating signal and opposing phosphatase. Resolving links with γ-tubulin-dependent nucleation, katanin-mediated severing, auxin transport, and cell-wall remodeling will explain how cortical microtubules shape gravity-directed growth.