BRI-ghtening the Pathway to Steroid Hormone Signaling Events in Plants

Plant biologist are pumped up about steroids! While it component of the plant cell wall and because XETs can has long beenknown that plants can synthesizea variety catalyze the breaking and rejoining of these molecules, of steroidal compounds, a biological role for these aniBRU1 may be necessary for cell expansion. Additionally, mal hormones in plant growth was not evident. Some expression of the TCH4 gene, a BRand touch-induced of these substances are related to the powerful insect XET in Arabidopsis, was found to be highest in BRmolting hormone ecdysone, providing the plant with a induced elongating tissues. Although it may not be a potent defense mechanism against hungry larval invaddriving force in the process, an increase in cell wall ers. A clue to their importance in plant growth and develloosening provided by XET activity isa likely prerequisite opment came when a group of sterol-derived comfor cell elongation to occur. pounds were partially purifed from pollen of Brassica However, the wake-up call for most of us came with napis. These so called brassins were shown to possess last year’s discovery of mutants in Arabidopsis that are remarkable hormone-like effects when applied exogedefective in the synthesis of brassinolide. The excitenously to plant tissues (Mitchell et al.,1970). The growthment revolves around the reclassification of a group of promoting effects of chemical extracts prepared from dwarf mutants that include the previoulsy known depollen had been known since the 1930s. However, it etiolated2 (det2), dwarf1 (dwf1)/diminuto(dim1) and the was not until 1979 that a major breakthrough in this identification of several new mutants including constitufield was made when chemists solved the structure of tive photomorphogenesis and dwarf (cpd) (reviewed in brassinolide (Grove et al., 1979), the most biologically Yokota, 1997). Alleles of dwf1 and cpd are also known as potent of all the brassinosteroids (BRs) (Figure 1). This cbb1 and cbb3, respectively, because as rosette plants 40-year-long drought in plant steroid hormone research their phenotype is reminiscent of a head of cabbage. may be understandable given that biochemists required These plants are extremely stunted with dark-green, over 500 lb of bee-collected pollen as starting material compact rosette leaves, and if they produce an infloresfor the purification of only 10 mg of product! While brascence at all, the flowers are sterile. In darkness, these sinolide is most abundant in pollen, using sensitive anamutants show a morphology similar to light-grown seedlytic techniques, it can be found in most tissues of the lings including a short hypocotyl, open apical hook, explant and ubiquitously in the plant kingdom (Mandava panded cotyledons, early leaf initiation, and activation 1988). of a variety of lightand stress-regulated genes. Hence, With a sufficient quantity of pure compound in hand, with good reason, several of these dwarf mutants were physiologists were now able to more rigorously examine thought previously to be defective in genes encoding the effects of BRs on plant growth and development. signaling pathway components for light-regulated deThey demonstrated that, when applied to tissues in exvelopment. Cloning of DET2 (Li et al., 1996) and CPD tremely small amounts, brassinolide promoted cell elon(Szekeres et al., 1996) revealed that these genes engation, organ bending, cell differentiation and a host of coded proteins with similarity to mammalian steroid 5aother effects in a variety of plants (reviewed in Sakurai reductases and a cytochrome P450/steroid hydroxyand Fujioka, 1993). Thus, BRs fulfill several of the key lase, respectively, which suggested a role in steroid requirements to be classified alongside the five classical biosynthesis. Treatment of plants with brassinolide fully plant hormones (Kende and Zeevaart, 1997) auxin, ethylrestored the normal etiolation phenotypes of darkene, gibberellic acid, abscisic acid, and cytokinin: they grown det2, (Li et al., 1996), cpd and dwf1/dim1 (Szeare produced ina broad range of species and are biologkeres et al., 1996), and cbb1 and cbb3 (Kauschmann et ically active at very low concentrations (nM to pM). al., 1996). Brassinolide treatment also promoted a norStill, plant scientists have been slow to accept that mal wild-type growth habit in these mutants when grown BRs are true hormones. Critics bemoaned that before in the light. det2 and cpd mutants are defective in differBRs can take their place among the “big-five” classical ent steps in brassinolide biosynthesis (Figure 1). Applihormones, they must be shown to possess a unique cation of various biosynthetic precursors of brassinolide biological activity. Plagued by reports that many of the to cpd seedlings allowed the precise positioning of their functions ascribed to BRs could be simply explained block in the pathway, and this block is at a point different by synergy with auxin or the other growth-promoting from that predicted for det2 mutants, on the basis of hormones, bio-assays specific for brassinolide were dethe functional conservation of mammalian steroid 5aveloped (reviewed in Clouse, 1996). Molecular genetic reductases and DET2. In an amazing demonstration of approaches to study the action of BRs have now re-