Q&A: what are brassinosteroids and how do they act in plants?

Abstract

Brassinosteroids (BRs) are a class of polyhydroxylated steroidal phytohormones in plants with similar structures to animals' steroid hormones. Brassinosteroids regulate a wide range of physiological processes including plant growth, development and immunity. Brassinosteroid signalling and its integration with other signalling pathways have been investigated thoroughly at the molecular level.

Fig. 1

The chemical structure of brassinolide (BL) with the steroid rings labelled as A, B, C and D. The parts within the dashed lines can be substituted by different groups, as described in the text

Fig. 2

Brassinolide induces BRI1 heterodimerization with SERKs. a Overall structure of BL bound by BRI1LRR (green) shown as a structure graphic. BL (pink) is shown as a stick diagram. The island domain of BRI1 is shown in grey. ‘N’ and ‘C’ represent the N-terminal and C-terminal side of BRI1LRR, respectively. b Crystal structure of BRI1LRR-BL-BAK1LRR complex. BAK1LRR is shown in pale green

Fig. 3

The brassinosteroid signal transduction pathway in the absence (a) and presence (b) of brassinosteroid (BR) in Arabidopsis. Brassinosteroid binding induces BRI1 heterodimerization with its co-receptor BAK1, resulting in activation of BRI1, phosphorylation of BKI1 by BRI1 and dissociation of BKI1 from BRI1. These events lead to full activation of BRI1, which phosphorylates BSK1 and CDG1. Once activated, BSK1 and CDG1 promote the activation of the phosphatase BSU1, thus dephosphorylating and inactivating BIN2. The inactivated BIN2 allows the presence of the dephosphorylated form of BZR1/2, which can move into the nucleus and regulate transcription of many target genes. The phosphorylated BZR1/2 are retained in the cytoplasm by 14-3-3. Arrows and short lines represent promotion and inhibition, respectively. The small circles containing the letter ‘P’ indicate phosphorylation