TOR coordinates plant root growth via brassinosteroid-mediated regulation of the microtubule-associated protein CLASP in Arabidopsis

The evolutionarily conserved kinase Target of Rapamycin (TOR) enables eukaryotic organisms to match growth to energy and nutrient availability. In the plant root apical meristem, cell proliferation is dictated by the supply of sucrose from photosynthetic organs, and TOR plays a pivotal role in modulating meristem activity in response to changing energy levels. The progression of cell division requires continual remodeling of microtubule arrays, and the microtubule rescue factor CLIP-Associated Protein (CLASP) is crucial for sustaining cell proliferation. In its absence microtubule arrays lose complexity and cell production plummets. In this report we explore the relationship between TOR signalling and CLASP in the model system Arabidopsis thaliana, and show that TOR, by activating brassinosteroid (BR) signalling, downregulates CLASP expression, leading to increased microtubule dynamics associated with cell cycle progression. We further show that TOR activity enhances the motility of the CLASP-associated retromer subunit sorting nexin 1, which in turn stabilizes the auxin efflux carrier PIN2 at the plasma membrane. Thus, in addition to moderating CLASPs function as a microtubule stabilizer to promote cell division, TOR supports CLASP in sustaining PIN2 levels to control auxin flow, which is a key determinant of the switch from cell division to differentiation. Finally, we found that cell division is suppressed both by continuous chemical inhibition of TOR, which increases CLASP levels, or continuous activation of the brassinosteroid pathway with epi-brassinolide, which decreases CLASP expression. This finding suggests that TOR-mediated oscillations in CLASP levels could be a key for timing cell cycle progression. Significance StatementHow plants translate metabolic energy into physical development requires precise molecular timing. This study reveals that the energy-sensing TOR kinase dictates root growth by tuning the microtubule-stabilizing protein CLASP. We show that TOR activates brassinosteroid signaling to downregulate CLASP, which accelerates microtubule dynamics to drive the cell cycle. Simultaneously, TOR utilizes CLASP to stabilize PIN2 auxin transporters, controlling the switch from cell division to differentiation. Because pushing CLASP levels too high or too low arrests development, our findings suggest a novel metabolic-hormonal pathway by which oscillations in CLASP-dependent cellular architecture set the pace of plant growth.