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Brassinosteroids and plant steroid hormone signaling - PubMed

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Brassinosteroids and plant steroid hormone signaling

Gerard J Bishop et al. Plant Cell. 2002.

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Figure 1.
Figure 1.

Structures of Steroid Hormones. Chemical structure of brassinolide and castasterone plant steroid hormones, in comparison with the mammalian sex steroid hormones testosterone and oestradiol, and the insect steroid hormone ecdysone. Highlighted are carbon numbers of BL having oxygen moieties that are important for BR activity.

Figure 2.
Figure 2.

BR Biosynthesis Pathway. A simplified scheme of the BR biosynthesis pathway including steps of early and late C-6 oxidation that lead to the synthesis of BL (Noguchi et al., 2000; Shimada et al., 2001). Mutations in genes coding for key biosynthetic enzymes are in bold. det-2, DEETIOLATED-2, steroid 5α reductase (Li et al., 1996); dwf4, DWARF4 cytochrome P450 CYP90B, C-22 hydroxylase (Choe et al., 1998); cpd, CONSTITUTIVE PHOTOMORPHOGENIC and DWARFISM cytochrome P450 CYP90A, C-23 hydroxylase (Szekeres et al., 1996); dx, DWARF cytochrome P450 CYP85, C-6 oxidase (Bishop et al., 1996, 1999; Shimada et al., 2001); DDWF, DARK-INDUCED dwf-LIKE-1, cytochrome P450 CYP92A6, C-2 hydroxylase (Kang et al., 2001).

Figure 3.
Figure 3.

Dwarf Phenotype of bri1 Mutants in Different Plant Species. (A) Six-week-old wild-type and bri1-1 mutant Arabidopsis plants. (B) Twelve-day-old wild-type and lka mutant pea plants. (C) Seven-week-old wild-type and cu-3 mutant tomato plants. WT, wild type. Bars in (A), (B), and enlarged inset photograph of cu-3 in (C), 5 cm; bar in (C), 15 cm.

Figure 4.
Figure 4.

Schematic Structure of BRI1 and Similar LRR-RLKs and LRR receptor–like Proteins. (A) Similarities in LRR-domain topologies highlighting a similar location of loop-out island region (for details see main text). Source of primary sequence information: BRI1, BL signaling (Li and Chory, 1997); CLV2, LRR-R involved in control of plant meristem size and differentiation (Jeong et al., 1999); Cf-9, involved in plant pathogen signaling (Jones et al., 1994); and Toll, involved in Drosophila embryo development and immunity pathways (Hashimoto et al., 1988). Note: domains are not to scale and the N-terminal signal peptides are not shown. (B) Domain topology of selected LRR-RLKs highlighting the location of sequenced mutations. BRI1, location of brassinolide insensitive 1 (Li and Chory, 1997) and additional alleles (Noguchi et al., 1999; Friedrichsen et al., 2000; Bouquin et al., 2001); Xa21, (Song et al., 1995); ER, erecta-1 (Torii et al., 1996) and additional alleles (Lease et al., 2001); CLV1, clavata-1 alleles (Clark et al., 1997). Missense mutations indicated by arrows are shown on the left, whereas nonsense, deletion (Δ) and insertion (+) mutations are indicated on the right. (†), independent mutations at the same position. All alleles are strong (null alleles) unless indicated. (*), weak alleles; (**), intermediate alleles. α and β indicate the approximate regions used in domain swaps between BRI1 and Xa21 (He et al., 2000). Note: domains are not to scale, and the N-terminal signal peptide is not shown. Also note: allele bri-201 is a deletion mutant in the N-terminal region that is not shown.

Figure 5.
Figure 5.

Schematic Models Indicate a Central Role for BRI1 in BL Signaling. Three scenarios for formation of a possible BRI1 receptor complex are shown: (A) BRI1 interacts with another LRR-RLK. (B) BRI1 forms a homodimer. (C) BRI1 interacts with an LRR-receptor-like protein. In each case, the amino-acid island of BRI1 is implicated in BL sensing, either by directly binding BL or by binding an SBP. KAPP, a kinase-associated protein phosphatase, is likely to be associated with the receptor complex to inactivate it. BL activation of BRI1 leads to autophosphorylation and subsequent phosphorylation of other substrates, e.g., TRIP1 (transforming growth factor β receptor interacting protein homolog). BL-mediated activation of BRI1 leads to suppression of genes controlling BR biosynthesis, such as CPD and DWF4 (BRD, BR-downregulated genes), but promotes the expression of BR-upregulated genes (BRU). A putative role for a sterol binding protein and potential processing of this protein by BRS facilitating BL binding and BRI1 activation is indicated. Potentially, BRS may also modify BRI. (See main text for further details).

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