Brassinolide and [26, 28-2H6]brassinolide are differently demethylated by loss of C-26 and C-28, respectively, in Marchantia polymorpha

Characterization of Endogenous Levels of Brassinosteroids and Related Genes in Grapevines

Agronomic breeding practices for grapevines (Vitis vinifera L.) include the application of growth regulators in the field. Brassinosteroids (BRs) are a family of sterol-derived plant hormones that regulate several physiological processes and responses to biotic and abiotic stress. In grapevine berries, the production of biologically active BRs, castasterone and 6-deoxocastasterone, has been reported. In this work, key BR genes were identified, and their expression profiles were determined in grapevine. Bioinformatic homology analyses of the Arabidopsis genome found 14 genes associated with biosynthetic, perception and signaling pathways, suggesting a partial conservation of these pathways between the two species. The tissue- and development-specific expression profiles of these genes were determined by qRT-PCR in nine different grapevine tissues. Using UHPLC-MS/MS, 10 different BR compounds were pinpointed and quantified in 20 different tissues, each presenting specific accumulation patterns. Although, in general, the expression profile of the biosynthesis pathway genes of BRs did not directly correlate with the accumulation of metabolites, this could reflect the complexity of the BR biosynthesis pathway and its regulation. The development of this work thus generates a contribution to our knowledge about the presence, and diversity of BRs in grapevines.

Establishment of Biosynthetic Pathways To Generate Castasterone as the Biologically Active Brassinosteroid in Brachypodium distachyon

Gas chromatography-mass spectrometry (GC-MS) analysis revealed that castasterone and its biosynthetic precursors are found in Brachypodium distachyon. In vitro conversion experiments with crude enzyme solutions prepared from B. distachyon demonstrated the presence of the following biosynthetic sequences: campesterol → campesta-4-en-3-one → campesta-3-one → campestanol → 6-deoxocathasterone → 6-deoxoteasterone → teasterone ↔ 3-dehydroteasterone ↔ typhasterol → castasterone. campesterol → 22-hydroxycampesterol → 22-hydroxy-campesta-4-en-3-one → 22-hydroxy-campesta-3-one → 6-deoxo-3-dehydroteasterone → 3-dehydroteasterone. 6-deoxoteasterone ↔ 6-deoxo-3-dehydroteasterone ↔ 6-deoxotyphasterol → 6-deoxocastasterone → castasterone. This shows that there are campestanol-dependent and campestanol-independent pathway in B. distachyon that synthesize 24-methylated brassinosteroids (BRs). Biochemical analysis of BRs biosynthetic enzymes confirmed that BdDET2, BdCYP90B1, BdCYP90A1, BdCYP90D2, and BdCYP85A1 are orthologous to BR 5α-reductase, BR C-22 hydroxylase, BR C-3 oxidase, BR C-23 hydroxylase, and BR C-6 oxidase, respectively. Brassinolide was not identified in B. distachyon. Additionally, B. distachyon crude enzyme solutions could not catalyze the conversion of castasterone to brassinolide, and the gene encoding an ortholog of CYP85A2 (a brassinolide synthase) was not found in B. distachyon. These results strongly suggest that the end product for brassinosteroid biosynthesis which controls the growth and development of B. distachyon is not brassinolide but rather castasterone.

Occurrence of brassinosteroids in non-flowering land plants, liverwort, moss, lycophyte and fern

Endogenous brassinosteroids (BRs) in non-flowering land plants were analyzed. BRs were found in a liverwort (Marchantia polymorpha), a moss (Physcomitrella patens), lycophytes (Selaginella moellendorffii and S. uncinata) and 13 fern species. A biologically active BR, castasterone (CS), was identified in most of these non-flowering plants but another biologically active BR, brassinolide, was not. It may be distinctive that levels of CS in non-flowering plants were orders of magnitude lower than those in flowering plants. 22-Hydroxycampesterol and its metabolites were identified in most of the non-flowering plants suggesting that the biosynthesis of BRs via 22-hydroxylation of campesterol occurs as in flowering plants. Phylogenetic analyses indicated that M. polymorpha, P. patens and S. moellendorffii have cytochrome P450s in the CYP85 clans which harbors BR biosynthesis enzymes, although the P450 profiles are simpler as compared with Arabidopsis and rice. Furthermore, these basal land plants were found to have multiple P450s in the CYP72 clan which harbors enzymes to catabolize BRs. These findings indicate that green plants were able to synthesize and inactivate BRs from the land-transition stage.

Biosynthesis of a cholesterol-derived brassinosteroid, 28-norcastasterone, in Arabidopsis thaliana

A metabolic study revealed that 28-norcastasterone in Arabidopsis is synthesized from cholesterol via the late C-6 oxidation pathway. On the other hand, the early C-6 oxidation pathway was found to be interrupted because cholestanol is converted to 6-oxocholestanol, but further metabolism to 28-norcathasterone was not observed. The 6-oxoBRs were found to have been produced from the respective 6-deoxoBRs administered to the enzyme solution, thus indicating that these 6-oxoBRs are supplied from the late C-6 oxidation pathway. Heterologously expressed CYP85A1 and CYP85A2 in yeast catalysed this C-6 oxidation, with CYP85A2 being much more efficient than CYP85A1. Abnormal growth of det2 and dwf4 was restored via the application of 28-norcastasterone and closer precursors. Furthermore, det2 and dwf4 could not convert cholesterol to cholestanol and cholestanol to 6-deoxo-28-norcathasterone, respectively. It is, therefore, most likely that the same enzyme system is operant in the synthesis of both 28-norcastasterone and castasterone. In the presence of S-adenosyl-L-methionine, the cell-free enzyme extract catalysed the C-24 methylation of 28-norcastasterone to castasterone, although the conversion rates of 28-norteasterone to teasterone and 28-nortyphasterol to typhasterol were much lower; this suggests that 28-norcastasterone is the primary precursor for the generation of C(28)-BRs from C(27)-BRs.

Rice CYP734As function as multisubstrate and multifunctional enzymes in brassinosteroid catabolism

Catabolism of brassinosteroids regulates the endogenous level of bioactive brassinosteroids. In Arabidopsis thaliana, bioactive brassinosteroids such as castasterone (CS) and brassinolide (BL) are inactivated mainly by two cytochrome P450 monooxygenases, CYP734A1/BAS1 and CYP72C1/SOB7/CHI2/SHK1; CYP734A1/BAS1 inactivates CS and BL by means of C-26 hydroxylation. Here, we characterized CYP734A orthologs from Oryza sativa (rice). Overexpression of rice CYP734As in transgenic rice gave typical brassinosteroid-deficient phenotypes. These transformants were deficient in both the bioactive CS and its precursors downstream of the C-22 hydroxylation step. Consistent with this result, recombinant rice CYP734As utilized a range of C-22 hydroxylated brassinosteroid intermediates as substrates. In addition, rice CYP734As can catalyze hydroxylation and the second and third oxidations to produce aldehyde and carboxylate groups at C-26 in vitro. These results indicate that rice CYP734As are multifunctional, multisubstrate enzymes that control the endogenous bioactive brassinosteroid content both by direct inactivation of CS and by the suppression of CS biosynthesis by decreasing the levels of brassinosteroid precursors.

BEN1, a gene encoding a dihydroflavonol 4-reductase (DFR)-like protein, regulates the levels of brassinosteroids in Arabidopsis thaliana

The ben1-1D (bri1-5 enhanced 1-1dominant) mutant was identified via an activation-tagging screen for bri1-5 extragenic modifiers. bri1-5 is a weak mutant allele of the brassinosteroid receptor gene, BRI1. Overexpression of BEN1 greatly enhances the defective phenotypes of bri1-5 plants. Removal of BEN1 by gene disruption in a Col-0 wild-type background, on the other hand, promotes the elongation of organs. Because BEN1 encodes a novel protein homologous to dihydroflavonol 4-reductase (DFR) and anthocyanidin reductase (BAN), BEN1 is probably involved in a brassinosteroid metabolic pathway. Analyses of brassinosteroid profiles demonstrated that BEN1 is indeed responsible for regulating the levels of several brassinosteroids, including typhasterol, castasterone and brassinolide. In vivo feeding and in vitro biochemical assays suggest that BEN1 is probably involved in a new mechanism to regulate brassinosteroid levels. These results provide additional insight into the regulatory mechanisms of bioactive brassinosteroids.

Metabolism of brassinosteroids in plants

Brassinosteroids represent a class of plant hormones. More than 70 compounds have been isolated from plants. Currently 42 brassinosteroid metabolites and their conjugates are known. This review describes the miscellaneous metabolic pathways of brassinosteroids in plants. There are some types of metabolic processes involving brassinosteroids in plants: dehydrogenation, demethylation, epimerization, esterification, glycosylation, hydroxylation, side-chain cleavage and sulfonation. Metabolism of brassinosteroids can be divided into two categories: i) structural changes to the steroidal skeleton; and ii) structural changes to the side-chain.

Novel biosynthetic pathway of castasterone from cholesterol in tomato

Endogenous brassinosteroids (BRs) in tomato (Lycopersicon esculentum) seedlings are known to be composed of C27- and C28-BRs. The biosynthetic pathways of C27-BRs were examined using a cell-free enzyme solution prepared from tomato seedlings that yielded the biosynthetic sequences cholesterol --> cholestanol and 6-deoxo-28-norteasterone <--> 6-deoxo-28-nor-3-dehydroteasterone <--> 6-deoxo-28-nortyphasterol --> 6-deoxo-28-norcastasterone --> 28-norcastasterone (28-norCS). Arabidopsis CYP85A1 that was heterologously expressed in yeast mediated the conversion of 6-deoxo-28-norCS to 28-norCS. The same reaction was catalyzed by an enzyme solution from wild-type tomato but not by an extract derived from a tomato dwarf mutant with a defect in CYP85. Furthermore, exogenously applied 28-norCS restored the abnormal growth of the dwarf mutant. These findings indicate that the C-6 oxidation of 6-deoxo-28-norCS to 28-norCS in tomato seedlings is catalyzed by CYP85, just as in the conversion of 6-deoxoCS to CS. Additionally, the cell-free solution also catalyzed the C-24 methylation of 28-norCS to CS in the presence of NADPH and S-adenosylmethionine (SAM), a reaction that was clearly retarded in the absence of NADPH and SAM. Thus it seems that C27-BRs, in addition to C28-BRs, are important in the production of more active C28-BRs and CS, where a SAM-dependent sterol methyltransferase appears to biosynthetically connect C27-BRs to C28-BRs. Moreover, the tomato cell-free solution converted CS to 26-norCS and [2H6]CS to [2H3]28-norCS, suggesting that C-28 demethylation is an artifact due to an isotope effect. Although previous feeding experiments employing [2H6]CS suggested that 28-norCS was synthesized from CS in certain plant species, this is not supported in planta. Altogether, this study demonstrated for the first time, to our knowledge, that 28-norCS is not synthesized from CS but from cholesterol. In addition, CS and [2H6]CS were not converted into BL and [2H6]BL, respectively, confirming an earlier finding that the active BR in tomato seedlings is not BL but CS. In conclusion, the biosynthesis of 28-norBRs appears to play a physiologically important role in maintaining homeostatic levels of CS in tomato seedlings.

Biosynthesis and metabolism of brassinosteroids

Brassinosteroids (BRs) are steroid hormones that regulate the growth and development of plants. Detailed study of the biosynthesis of brassinolide, a C28 BR, revealed that two parallel routes, the early and late C-6 oxidation pathways, are connected at multiple steps and also are linked to the early C-22 oxidation pathway. Thus, BR biosynthetic pathways are highly networked. Furthermore, the biosynthesis of C27 BRs was shown to proceed in a similar way to that of C28 BRs. Information on enzymes and genes involved in the BR biosynthesis, as well as their regulation, has been obtained using BR-deficient and BR-insensitive mutants. In addition, the biosynthesis of sterols, which were recently recognized not only as precursors of BRs and membrane constituents, but also as modulators of plant development, is discussed. Various metabolic reactions of BRs including epimerization, oxidation, and conjugation are also summarized.

Synthesis and biological activity of 26-norbrassinolide, 26-norcastasterone and 26-nor-6-deoxocastasterone

26-Norbrassinolide, identified as a metabolite of brassinolide in cultured cells of the liverwort, Marchantia polymorpha, as well as 26-norcastasterone and 26-nor-6-deoxocastasterone were synthesized. Synthesis of these new brassinosteroids was conducted by employing the orthoester Claisen rearrangement and asymmetric dihydroxylation as key reactions. The modified rice lamina inclination test indicated that these three 26-norbrassinosteroids were less active than their corresponding C28 brassinosteroids. Growth-promoting activities were also examined by using the brassinosteroid-deficient, dwarf mutant lkb of garden pea (Pisum sativum L.). In this assay, 26-norbrassinolide was as effective as brassinolide and 26-norcastasterone was more effective than castasterone although 26-nor-6-deoxocastasterone was much less effective than 6-deoxocastasterone. Therefore, removal of C-26 of brassinosteroids does not necessarily reduce the biological activity. The role of C-26 removal in Marchantia cells remains unclear.

Brassinazole, an inhibitor of brassinosteroid biosynthesis, inhibits development of secondary xylem in cress plants (Lepidium sativum)

Brassinazole (Brz) is a specific brassinosteroid biosynthesis inhibitor. Cress plants (Lepidium sativum) grown in medium containing Brz exhibited a slight predominance of phloem differentiation at the expense of xylem differentiation and remarkable inhibition of the development of secondary xylem. This result indicates that brassinosteroids function in xylem development in vivo.