Brassinosteroids and plant steroid hormone signaling

Steroids play a role as essential hormones in plants as well as in animals. Plants produce numerous steroids and sterols, some of which are recognized as hormones in animals (Geuns, 1978; Jones and Roddick, 1988). Brassinolide (BL) is the most bioactive form of the growth-promoting plant steroids termed brassinosteroids (BRs). Grove et al. (1979) purified 4 mg of BL from 40 kg of bee-collected rape pollen to determine its structure, which shows similarity to animal steroid hormones. Due to its low concentration, the identification of BL took 10 years of dedicated work on the part of U.S. Department of Agriculture researchers at a cost of over one million U.S. dollars (Mandava, 1988). Today, over 40 naturally occurring BRs are known; these carry an oxygen moiety at the C-3 position in combination with others at the C-2, C-6, C-22 or C-23 positions (Bishop and Yokota, 2001). Initial interest in brassinosteroids was based on the growth-promoting properties of pollen extracts studied by Mitchell et al. (1970). Although results from these early experiments revealed potential hormonal activities, many of the observed BR-induced effects appeared to be similar to those of other known plant hormones. The definition of BRs as hormones therefore did not gain wide acceptance, and the difficulty in obtaining and quantifying BRs restricted the related research. However, through the dedication and perseverance of a select group of researchers, numerous BRs were identified and metabolism studies performed, enabling the elucidation of the biochemical pathway leading to the production of BL. Many reviews have covered this early research period (Adam and Marquardt, 1986; Mandava, 1988; Sakurai and Fujioka, 1993; Clouse, 1997; Sasse, 1997; Yokota, 1997) and an excellent book provides intriguing details on related subjects (see Sakurai et al., 1999). The observation that Arabidopsis mutations abolishing the biosynthesis of BL result in a dwarf phenotype that can be restored to a wild-type phenotype by externally provided BL and intermediates of BR biosynthesis (Li et al., 1996; Szekeres et al., 1996) resulted in a wider acceptance that BRs are essential plant hormones. Characterization of BR-insensitive mutants showing a similar dwarf phenotype has subsequently identified key genes in BR signaling (Clouse et al., 1996; Li and Chory, 1997; Li and Nam, 2002). Recent advances in the study of plant steroid hormone biosynthesis and signaling are highlighted by several reviews (Clouse and Sasse, 1998; Szekeres and Koncz, 1998; Altmann, 1999; Li and Chory, 1999; Schumacher and Chory, 2000; Bishop and Yokota, 2001; Friedrichsen and Chory, 2001; Mussig and Altmann, 2001).