Brassinosteroid responses in motion: dynamics of brassinosteroid hormone and receptors shape signaling and tissue establishment

Bioactive molecules such as hormones play crucial roles throughout the life cycle of organisms. Due to their profound effects, these molecules are tightly regulated at multiple levels, including biosynthesis, storage, transport, modification, and signaling. Recent technical advances in plant sciences have added dynamics of hormones and receptors as an emerging tier in such regulation, and we will use brassinosteroid (BR) phytohormones to showcase these latest developments. In this review, we first focus on the intercellular dynamics of BRs and their intermediates through plasmodesmata. Upon the completion of biosynthesis, ABCB1/19 transporters export bioactive BRs toward the extracellular space where BRs are perceived by the BRASSINOSTEROID-INSENSITIVE 1 (BRI1) receptor. BRI1 activity is tightly regulated by its redistribution within cells through membrane trafficking and the formation of nanodomains at the plasma membrane, mechanisms also influenced by environmental cues. Additionally, the relocalization of downstream signaling components enables the spatiotemporal control of the BR signaling pathway, which helps guide optimal plant development. Finally, we discuss the possible evolutionary dynamics underlying the establishment of BR signaling and perception, which may have contributed to the diversification and expansion of signaling networks in plants. Taken together, the dynamics of molecules at multiple layers shape their physiological outputs, thus impacting plant growth and development.