Dwarfism and cytochrome P450-mediated C-6 oxidation of plant steroid hormones

PagDET2 promotes cambium cell division and xylem differentiation in poplar stem

Secondary growth of the woody tree stem is governed by meristematic cell division and differentiation in the vascular cambium. Multiple hormonal signals and endogenous developmental programs regulate vascular cambium activity. Brassinosteroids (BRs) significantly promote secondary stem growth and wood formation in poplar trees. However, the underlying regulatory mechanisms of BRs within the vascular tissue remain unclear. Genetic and anatomical approaches were used here to elucidate the role of PagDET2, the rate-limiting enzyme for BRs biosynthesis, in regulating secondary vascular cambium activity in Populus. This study showed that the elevated endogenous castasterone (CS) levels in tree stems through overexpressing PagDET2 could enhance cambium meristem cell activity and xylem (XY) differentiation to promote secondary stem growth. RNA-seq analysis revealed that genes involved in BRs response, vascular cambium cell division, XY differentiation, and secondary cell wall synthesis were up-regulated.

Seed-Specific Expression of Arabidopsis AtCYP85A2 Produces Biologically Active Brassinosteroids Such as Castasterone and Brassinolide to Improve Grain Yield and Quality in Seeds of Brachypodium Distachyon

Brachypodium distachyon is a monocotyledonous model plant that contains castasterone (CS) but no brassinolide (BL) as the end product of brassinosteroids (BR) biosynthesis, indicating dysfunction of BL synthase, which catalyzes the conversion of CS to BL. To increase BR activity, Arabidopsis cytochrome P450 85A2 (AtCYP85A2) encoding BR 6-oxidase/BL synthase was introduced into B. distachyon with the seed-specific promoters pBSU1, pAt5g10120, and pAt5g54000. RT-PCR analysis and GUS activity revealed that AtCYP85A2 was only expressed in the seeds of the transgenic plants pBSU1-AtCYP85A2::Bd21-3, pAt5g10120-AtCYP85A2::Bd21-3, and pAt5g54000-AtCYP85A2::Bd21-3. The crude enzyme prepared from the seeds of these three transgenic plants catalyzed the conversion of 6-deoxoCS to CS. The transgenic plants showed greater specific enzyme activity than the wild-type plant for the conversion of 6-deoxoCS to CS, indicating enhanced BR 6-oxidase activity in the transgenic plants. The enzyme solution also catalyzed the conversion of CS into BL. Additionally, BL was identified from the seeds of transgenic plants, verifying that seed-specific AtCYP85A2 encodes a functional BL synthase to increase BR activity in the seeds of transgenic Brachypodium. In comparison with wild-type Brachypodium, the transgenic plants showed better growth and development during the vegetative growing stage. The flowers of the transgenic plants were remarkably larger, resulting in increments in the number, size, and height of seeds. The total starch, protein, and lipid contents in transgenic plants were higher than those in wild-type plants, indicating that seed-specific expression of AtCYP85A2 improves both grain yield and quality in B. distachyon.

Drought-tolerant and drought-sensitive genotypes of maize (Zea mays L.) differ in contents of endogenous brassinosteroids and their drought-induced changes

The contents of endogenous brassinosteroids (BRs) together with various aspects of plant morphology, water management, photosynthesis and protection against cell damage were assessed in two maize genotypes that differed in their drought sensitivity. The presence of 28-norbrassinolide in rather high quantities (1-2 pg mg-1 fresh mass) in the leaves of monocot plants is reported for the first time. The intraspecific variability in the presence/content of the individual BRs in drought-stressed plants is also described for the first time. The drought-resistant genotype was characterised by a significantly higher content of total endogenous BRs (particularly typhasterol and 28-norbrassinolide) compared with the drought-sensitive genotype. On the other hand, the drought-sensitive genotype showed higher levels of 28-norcastasterone. Both genotypes also differed in the drought-induced reduction/elevation of the levels of 28-norbrassinolide, 28-norcastasterone, 28-homocastasterone and 28-homodolichosterone. The differences observed between both genotypes in the endogenous BR content are probably correlated with their different degrees of drought sensitivity, which was demonstrated at various levels of plant morphology, physiology and biochemistry.

Functional analyses of Populus euphratica brassinosteroid biosynthesis enzyme genes DWF4 (PeDWF4) and CPD (PeCPD) in the regulation of growth and development of Arabidopsis thaliana

DWF4 and CPD are key brassinosteroids (BRs) biosynthesis enzyme genes. To explore the function of Populus euphratica DWF4 (PeDWF4) and CPD (PeCPD), Arabidopsis thaliana transgenic lines (TLs) expressing PeDWF4, PeCPD or PeDWF4 plus PeCPD, namely PeDWF4-TL, PeCPD-TL and PeCP/DW-TL, were characterized. Compared with wild type (WT), the changes of both PeDWF4-TL and PeCPD-TL in plant heights, silique and hypocotyls lengths and seed yields were similar, but in bolting time and stem diameters, they were opposite. PeCP/DW-TL was more in plant heights and the lengths of primary root, silique, and fruit stalk, but less in silique numbers and seed yields than either PeDWF4-TL or PeCPD-TL. PeDWF4 and PeCPD specially expressed in PeDWF4-TL or PeCPDTL, and the transcription level of PeDWF4 was higher than that of PeCPD. In PeCP/DW-TL, their expressions were all relatively reduced. Additionally, the expression of PeDWF4 and PeCPD differentially made the expression levels of AtDWF4, AtCPD, AtBR6OX2, AtFLC, AtTCP1 and AtGA5 change in the TLs. The total BRs contents were PeDWF4-TL greater than PeCP/DW-TL greater than WT greater than PeCPD-TL. These results imply that PeDWF4 is functionally not exactly the same as PeCPD and there may be a synergistic and antagonistic effects in physiology between both of them in the regulation of plant growth and development.

The Cytochrome P450 Gene CsCYP85A1 Is a Putative Candidate for Super Compact-1 (Scp-1) Plant Architecture Mutation in Cucumber (Cucumis sativus L.)

The dwarf or compact plant architecture is an important trait in plant breeding. A number of genes controlling plant height have been cloned and functionally characterized which often involve in biosynthesis or signaling of plant hormones such as brassinosteroids (BRs). No genes for plant height or vine length have been cloned in cucurbit crops (family Cucurbitaceae). From an EMS-induced mutagenesis population, we identified a super compact (SCP) mutant C257 which was extremely dwarf due to practically no internode elongation. Under dark growing condition, C257 did not undergo skotomorphogenesis and its mutant phenotype could be rescued with exogenous application of brassinolide (BL), suggesting SCP might be a BR-deficient mutant. Segregation analysis revealed a single recessive gene scp-1 that was responsible for the SCP mutation. Map-based cloning combined with a modified MutMap identified CsCYP85A1, a member of the plant cytochrome P450 monooxygenase gene family, as the most possible candidate gene for scp-1, which encodes a BR-C6-oxidase in the BR biosynthesis pathway. We show that a SNP within the second exon of scp-1 candidate gene caused the SCP phenotype. Three copies of the CsCYP85A gene are present in the cucumber genome, but only the scp-1/CsCYP85A1 gene seemed active. The expression of CsCYP85A1 was higher in flowers than in the leaves and stem; its expression in the wild type (WT) was feedback regulated by BL application. Its expression was reduced in C257 as compared with the WT. This was the first report of map-based cloning of a plant height gene in cucurbit crops. The research highlighted the combined use of linkage mapping, an improved MutMap method and allelic diversity analysis in natural populations in quick cloning of simply inherited genes in cucumber. The roles of CsCYP85A1 in regulation of internode elongation in cucumber was discussed.

A Reverse-Genetics Mutational Analysis of the Barley HvDWARF Gene Results in Identification of a Series of Alleles and Mutants with Short Stature of Various Degree and Disturbance in BR Biosynthesis Allowing a New Insight into the Process

Brassinosteroids (BRs) are plant steroid hormones, regulating a broad range of physiological processes. The largest amount of data related with BR biosynthesis has been gathered in Arabidopsis thaliana, however understanding of this process is far less elucidated in monocot crops. Up to now, only four barley genes implicated in BR biosynthesis have been identified. Two of them, HvDWARF and HvBRD, encode BR-6-oxidases catalyzing biosynthesis of castasterone, but their relation is not yet understood. In the present study, the identification of the HvDWARF genomic sequence, its mutational and functional analysis and characterization of new mutants are reported. Various types of mutations located in different positions within functional domains were identified and characterized. Analysis of their impact on phenotype of the mutants was performed. The identified homozygous mutants show reduced height of various degree and disrupted skotomorphogenesis. Mutational analysis of the HvDWARF gene with the “reverse genetics” approach allowed for its detailed functional analysis at the level of protein functional domains. The HvDWARF gene function and mutants’ phenotypes were also validated by measurement of endogenous BR concentration. These results allowed a new insight into the BR biosynthesis in barley.

Evolutionary trails of plant steroid genes

Plant steroids - brassinosteroids (BRs) and their precursors, phytosterols - play a major role in plant growth, development, stress tolerance, and have high potential for agricultural applications. Currently, this prospect is limited by a lack of information about their evolution and expression dynamics (spatial and temporal) across plant species. The increasing number of sequenced genomes offers an opportunity for evolutionary studies that might help to prioritize functional analyses with the aim to improve crop yield and stress tolerance. In this review we provide a glimpse of the origin, evolution, and functional conservation of phytosterol and BR genes in the green plant lineage using comparative sequence and expression analyses of publicly available datasets.

From squalene to brassinolide: the steroid metabolic and signaling pathways across the plant kingdom

The plant steroid hormones, brassinosteroids (BRs), and their precursors, phytosterols, play major roles in plant growth, development, and stress tolerance. Here, we review the impressive progress made during recent years in elucidating the components of the sterol and BR metabolic and signaling pathways, and in understanding their mechanism of action in both model plants and crops, such as Arabidopsis and rice. We also discuss emerging insights into the regulations of these pathways, their interactions with other hormonal pathways and multiple environmental signals, and the putative nature of sterols as signaling molecules.

TCP1 modulates brassinosteroid biosynthesis by regulating the expression of the key biosynthetic gene DWARF4 in Arabidopsis thaliana

Brassinosteroids (BRs) are essential phytohormones regulating normal plant growth and development. TCP1, a gene thought to be involved in floral organ symmetric control, was identified as a genetic suppressor of a weak BR receptor mutant, bri1-5, in an activation-tagging genetic screen. TCP1 encodes a putative transcription factor possessing a basic helix-loop-helix domain. The dominant allele of TCP1, tcp1-1D, suppresses the defective phenotypes of bri1-5. Overexpression of a dominant-negative form of TCP1, TCP1-SRDX, with a 12-amino acid repressor sequence fused to TCP1 at its C terminus, results in dwarfed plants resembling BR-deficient or insensitive mutants. The defective phenotypes can be rescued by exogenously applied brassinolide but cannot be recovered by auxins, gibberellins, or cytokinins. BR profile assay (quantitative analysis of BR biosynthetic intermediates) strongly suggests that TCP1 expression level positively coordinates with the function of DWARF4 (DWF4), a key enzyme in BR biosynthesis. Real-time RT-PCR analysis further demonstrated that TCP1 regulates the transcription levels of DWF4, and chromatin immunoprecipitation experiments showed that TCP1 indeed interacts with the DWF4 promoter. Confocal microscopy indicated that TCP1 is mainly confined to the nucleus. The expression of TCP1 appears to be regulated by BR levels. These studies demonstrate another level of regulation through which BRs mediate plant growth and development.