BRs are steroidal plant hormones that are essential regulators of the expansion, division, and differentiation of plant cells. Functional BR signal transduction is required for normal expression of developmental pathways leading to organ elongation, vascular differentiation, leaf expansion, seed germination, and responses to the environment. The structural characterization in 1979 of the most active endogenously occurring BR, brassinolide (BL), demonstrated for the first time that a steroid signal could modulate the growth and development of plants at nanomolar concentrations, analogous to the profound effect of steroid hormones on animal development (Grove et al. 1979). Physiological experiments in the 1980s showed that under specific cultural conditions, exogenous application of BRs could have dramatic effects on overall plant growth, fruit set, grain filling, and adaptation to stress (Ikekawa and Zhao 1991; Kamuro and Taktsuto 1999). The subsequent identification of BR-deficient mutants in Arabidopsis, rice, tomato and pea with dramatic developmental phenotypes, coupled with the demonstration that these mutant phenotypes could be rescued by exogenous BR application, provided genetic confirmation of the absolute requirement of BRs for normal plant development (Bishop et al. 1996; Li et al. 1996; Szekeres et al. 1996; Nomura et al. 1997; Koka et al. 2000; Hong et al. 2002).

Research on BR signal transduction began in the early 1990s with studies of BR-regulated gene expression and the identification of the brassinosteroid-insensitive 1 mutant (bril), which exhibited an extremely dwarfed phenotype, dark-green curled leaves, reduced male fertility, delayed senescence and reduced apical dominance (Clouse et al. 1993, 1996; Kauschmann et al. 1996). The phenotype of bril suggested a critical role for the BRI1 gene in BR signaling. Cloning of the gene in Arabidopsis revealed that BRI1 encoded a leucine-rich repeat receptor-like kinase (LRR RLK), consistent with a possible role for this protein as the BR receptor (Li and Chory 1997). Subsequent work confirmed BRI1 as a plant steroid receptor with obvious structural differences from the widely studied nuclear steroid receptor superfamily found in animals (Wang et al. 2001; Kinoshita et al. 2005). Thus, plants and animals both require steroid signaling molecules for normal development but present strikingly different pathways for steroid signaling.

Work in several laboratories during the past decade has provided finer details of BRI1 action, identified BRI1-interacting proteins, revealed several downstream regulators required for propagation of the BR signal, and cataloged numerous BR-regulated genes. BR signaling has been extensively reviewed recently (Wang and He 2004; Li 2005; Vert et al. 2005; Belkhadir and Chory 2006; Li and Jin 2007) and this chapter will provide an overview of each component of the BR signal transduction pathway, emphasizing studies in the past few years that have expanded our understanding of the molecular mechanisms of BR signaling and its role in plant development.

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