Structure and function of the Arabidopsis ABC transporter ABCB19 in brassinosteroid export

Abstract

Brassinosteroids are steroidal phytohormones that regulate plant development and physiology, including adaptation to environmental stresses. Brassinosteroids are synthesized in the cell interior but bind receptors at the cell surface, necessitating a yet to be identified export mechanism. Here, we show that a member of the ATP-binding cassette (ABC) transporter superfamily, ABCB19, functions as a brassinosteroid exporter. We present its structure in both the substrate-unbound and the brassinosteroid-bound states. Bioactive brassinosteroids are potent activators of ABCB19 ATP hydrolysis activity, and transport assays showed that ABCB19 transports brassinosteroids. In Arabidopsis thaliana , ABCB19 and its close homolog, ABCB1, positively regulate brassinosteroid responses. Our results uncover an elusive export mechanism for bioactive brassinosteroids that is tightly coordinated with brassinosteroid signaling.

HSP90 provides plasticity to plant development by gradually stabilizing plasma membrane presence of ABCB-type auxin transporters

“…Analysis of cell sizes exposed that in twd1 roots, meristematic division zone cells are indeed smaller than in Wt (Extended Data Fig. 8a-b, f-g), resembling abcb1 abcb19 roots 55 . However, HSP90 inhibition by GDA resulted in altered cell-size distributions in abcb and twd1 mutant root meristems, whereas Wt, pin2 and aux1 meristems were practically unaffected (Extended Data Fig.…”

“…An answer to this might come from the nding that brassinosteroid transport and signaling, also known to contribute to cell elongation, seems to be controlled as well by HSP90. Very recently, ABCB19 was demonstrated to export beside IAA also brassinosteroids 55 , while TWD1 was shown to be essential for PM stabilization 56 and tra cking 57,58 of the brassinosteroid receptor, BRASSINOSTEROID INSENSITIVE1. Our results thus pinpoint towards a conjunctional auxinbrassinosteroid crosstalk that is controlled by HSP90 and that eventually enforces developmental plasticity.…”