BRInging light to hormone receptor activation

Abstract

Brassinosteroid (BR), a plant hormone of growing interest and importance, is involved in a range of developmental processes, such as stem elongation, root growth and light signaling. The model organism, Arabidopsis, has been useful in dissecting the role of BR in plant development and photomorphogenesis. Mutant plants that are defective in BR biosynthesis develop in the dark as if they were grown in the light. One interesting mutant, bri1, resembles BR biosynthetic mutants but is insensitive to exogenous BR. The BRI gene encodes a leucine-rich repeat receptor-like kinase (LRR-RLK), the hypothesized BR receptor, the LRR being extracellular. There are >100 LRR-RLK-class genes in Arabidopsis, and so understanding how these potential receptors function is of fundamental importance.Now, Zuhua He and colleagues1xPerception of Brassinosteroids by the extracellular domain of the receptor kinase BRI1. He, Z. et al. Science. 2000; 288: 2360–2363Crossref | PubMed | Scopus (272)See all References1 report important insight into the mechanism of receptor activation in plants. The group constructed a chimeric receptor and uncoupled two domains that are necessary for specific signaling. The XA21 LRR-RLK from rice confers bacterial resistance, and resembles BRI1, the LRR and RLK domains flanking a transmembranal domain. He et al. demonstrated that a chimeric BRI1 LRR-Xa21 RLK gene activated defense responses in rice upon treatment with BR. The defense-response markers included cell death, oxidative burst and genes upregulated in disease response. One defense-response gene was induced in a BR-dose-dependent manner. Both functional BRI1 LRR and Xa21 kinase domains were necessary for the activation, as missense mutants in either domain abolished activation. The results suggest that BR binds specifically to the BRI1 LRR and activates the kinase domain, which then signals downstream responses.This study gives a clearer understanding of the primary events of BR signaling, but also paves the way for a more general mechanism of LRR-RLK activation in plants. Research suggests a model where BR interacts with the LRR domain, activating the RLK, which then transduces the signal inside the plant cell. This opens a new path of receptor research in plants, with many questions remaining. Does BR binding activate kinase activity? What is the topography of the Bri1 protein? What are downstream components of the signaling pathway? He et al. have devised a powerful approach to dissect the molecular mechanism of receptors in plants and their research points to exciting directions for the future.