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 are localized to cortical microtubules (CMTs) underlying the plasma membrane (PM) using a short conserved helix and where they 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 unaffected typical downstream BR signaling readouts and selected BR-regulated genes, and displayed only mild changes when probing global BR genomic responses. Consistent with the localization of MBAPs to CMTs, mbap mutants displayed a disordered CMT network and an enhanced CMT reorganization upon BR perception. Altogether, our work 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.