Direct non-transcriptional link between brassinosteroid perception and cortical microtubule reorientation drives hypocotyl growth

Brassinosteroids (BRs) are steroid-type phytohormones that are essential for plant growth, development and adaptation to environmental stresses. BRs promote cell wall remodelling through extensive reprogramming of gene expression, thus impacting on cell elongation and hypocotyl growth. The existence of non-genomic responses to BRs is however still unclear. Here we identified and characterized a new family of BRI1-interacting proteins identified by yeast two hybrid named 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 using a short conserved helix and where they contact with and are phosphorylated by BRI1. Combinations of mbap loss-of-function mutants showed hypersensitivity to BRs and establish MBAPs as negative regulators of BR responses. Surprisingly, mbap mutants showed unaffected typical downstream BR signaling readouts and BR target gene expression. Rather, mbap mutants displayed disordered CMT network and enhanced CMT reorganization upon BR perception. Altogether, our work shed light on the direct non-transcriptional connection between BR perception at the cell surface and CMT organization in the control of hypocotyl elongation.