Arabidopsis microtubule-BRI1-associated proteins negatively regulate hypocotyl elongation by controlling brassinosteroid-dependent cortical microtubule reorientation.

Brassinosteroids (BRs) are steroid-type phytohormones that are essential for plant growth, development, and adaptation to environmental stresses. BRs are known to control microtubule (MT) orientation, which is pivotal for directional growth, but the underlying molecular mechanisms are still largely unknown. Here, we identified and characterized a new family of BRI1-interacting proteins named MICROTUBULE-BRI1-ASSOCIATED PROTEINS (MBAPs). We confirmed, using several complementary approaches, that MBAPs are genuine BRI1 partners in plant cells. We demonstrated that MBAPs localize to cortical microtubules (CMTs) underlying the plasma membrane using a short conserved helix, where they make contact with BRI1. Combinations of mbap loss-of-function mutants showed hypersensitivity to BRs and established MBAPs as negative regulators of BR responses. mbap mutants showed no changes in typical downstream BR signaling readouts and selected BR-regulated genes, and they displayed only mild changes when global BR genomic responses were probed. Consistent with the localization of MBAPs to CMTs, mbap mutants displayed a disordered CMT network and enhanced CMT reorganization upon BR perception. Together, our results shed light on the role of MBAPs as a bridge between BR perception at the cell surface and CMT organization in the control of hypocotyl elongation.