Brassinosteroid biosynthesis anddwarf mutants

Plants enjoy their entire life exactly where they were initially rooted. Because of this fixed life pattern, plants have to devise a different type of strategy than animals to survive the numerous biotic and abiotic challenges. Many different plant hormones that act alone or in concert underpin these mechanisms. Brassinosteroids (BRs) collectively refer to plant-originated 5μ-cholestane steroids that elicit growth stimulation in nano-or micromolar concentrations. BRs that are biosynthesized using sterols as precursors are structurally similar to the cholesterol derived, mammalian steroid hormones, insect molting hormones and ecdysteroids. BRs have been known for decades to be effective in plant growth promotion. However, definitive evidence for their roles in growth and development remained unclear until the recent characterization of BRdwarf mutants isolated fromArabidopsis and other plants. This review aims to provide a cohesive summary of information obtained from the molecular genetic characterization of mutants that are defective in sterol and BR biosynthetic pathways.     

Brassinosteroid biosynthesis and inactivation

The term brassinosteroids (BRs) refers to the growth-promoting plant steroidal hormones. Various developmental programs including but not limited to cell elongation, stress tolerance, and skoto-/photo-morphogenesis are controlled by subnanomolar concentrations of BRs. Accordingly, BR mutants that are defective in BR biosynthetic or signaling pathways usually display dwarfism. Characterization of numerous BR dwarf mutants isolated from Arabidopsis , pea, tomato, and rice greatly contributed to our understanding of BR biology. Recently, an enzyme that mediates the final step in the BR biosynthetic pathways has been characterized by two different groups. The brassinolide synthases (Cytochrome P450s 85A2 and 85A3) are multifunctional enzymes that catalyze the last three consecutive steps in BR biosynthetic pathways, namely, C-6 hydroxylation, dehydrogenation, and Baeyer-Villiger type oxidation. In addition, many of the previously unknown steps have been genetically characterized. This review aims to summarize the knowledge that has been developed during the last 2–3 years in this field of BR biosynthesis and inactivation research.     

Brassinosteroid signaling positively regulates abscisic acid biosynthesis in response to chilling stress in tomato

Brassinosteroids (BRs) and abscisic acid (ABA) are essential regulators of plant growth and stress tolerance. Although the antagonistic interaction of BRs and ABA is proposed to ensure the balance between growth and defense in model plants, the crosstalk between BRs and ABA in response to chilling in tomato (Solanum lycopersicum), a warm-climate horticultural crop, is unclear. Here, we determined that overexpression of the BR biosynthesis gene DWARF (DWF) or the key BR signaling gene BRASSINAZOLE-RESISTANT1 (BZR1) increases ABA levels in response to chilling stress via positively regulating the expression of the ABA biosynthesis gene 9-CIS-EPOXYCAROTENOID DIOXYGENASE1 (NCED1). BR-induced chilling tolerance was mostly dependent on ABA biosynthesis. Chilling stress or high BR levels decreased the abundance of BRASSINOSTEROID-INSENSITIVE2 (BIN2), a negative regulator of BR signaling. Moreover, we observed that chilling stress increases BR levels and results in the accumulation of BZR1. BIN2 negatively regulated both the accumulation of BZR1 protein and chilling tolerance by suppressing ABA biosynthesis. Our results demonstrate that BR signaling positively regulates chilling tolerance via ABA biosynthesis in tomato. The study has implications in production of warm-climate crops in horticulture.     

Brassinosteroid transport

Brassinosteroids (BRs) are steroidal plant hormones that are important regulators of plant growth. These compounds are widely distributed throughout reproductive and vegetative plant tissues. This raises the question of whether or not BRs are transported over long distances between these tissues. Several lines of evidence indicate that this is not the case. Exogenous BRs move only slowly, if at all, after application to leaves; grafting BR-deficient mutants to wild-type plants has no phenotypic effect; removal of the apical bud or mature leaves does not reduce BR levels in the remaining internodes; and, in tomato, wild-type sectors do not substantially alter the growth of BR-deficient sectors when the two types are together in a variegated leaf. Although BRs do not undergo long-distance transport they may influence long-distance signalling by altering auxin transport. At the cellular level, BRs do appear to be transported. The enzymes for BR biosynthesis appear to be located within the cell, and to be associated with the endoplasmic reticulum, in particular. BR reception, on the other hand, is thought to occur on the exterior cell surface. Therefore, BRs must move from the interior of the cell to the exterior, where they are perceived by the same cell or by neighbouring cells. The existence of a feedback system, whereby bioactive BRs negatively regulate their own biosynthesis, provides further evidence that individual cells are able to both perceive and synthesize BRs.     

Brassinosteroid Mutants of Crops

Plant steroid hormones, brassinosteroids (BRs), were originally isolated from extracts of pollen because of their growth-promoting properties and their potential use for enhancing crop production. Mutants in the biosynthesis, metabolism, and signaling of brassinolide (BL), the most bioactive BR, are important resources in helping to establish BRs essential role in plant growth and development. The dark green and distinctive dwarf phenotype of BR-related mutants identified in pea, tomato, and rice highlights the importance of BRs in crops. These mutants are helping to elucidate both the conserved and the unique features of BR biosynthesis and signaling. Such insights are providing the key knowledge and understanding that will enable the development of strategies towards the production of crops with enhanced qualities.     

油菜素内酯生物合成与功能的研究进展

植物激素油菜素内酯广泛调节植物的生长发育及对外界环境因子变化的反应,在作物上的应用也已引起人们的广泛兴趣。通过遗传学等手段对相关突变体及功能基因的研究为其生物合成与功能研究提供了基础。本文总结了油菜素内酯在植物各组织内的分布、生物合成、相关合成突变体及其编码基因的性质、生理功能以及与其它激素间的相互作用等。     

油菜素内酯生物合成与功能的研究进展

植物激素油菜素内酯广泛调节植物的生长发育及对外界环境因子变化的反应, 在作物上的应用也已引起人们的广泛兴趣。通过遗传学等手段对相关突变体及功能基因的研究为其生物合成与功能研究提供了基础。本文总结了油菜素内酯在植物各组织内的分布、生物合成、相关合成突变体及其编码基因的性质、生理功能以及与其它激素间的相互作用等。     

油菜素甾醇类与生长素的相互作用

油菜素甾醇类(brassinosteroid,BR)和生长素是两类重要的植物激素,二者在许多生理功能上存在相关性。近年来的研究表明,BR与生长素能协同调节基因表达,二者在代谢、运输和信号转导途径等不同层次上存在相互作用,并且这两种信号与其他信号转导途径,如激素信号转导途径和光信号转导途径之间也存在信号对话。现对BR与生长素之间这种复杂的相互作用进行评述。     

Brassinosteroid signaling and application in rice

Combined approaches with genetics, biochemistry, and proteomics studies have greatly advanced our understanding of brassinosteroid (BR) signaling in Arabidopsis. However, in rice, a model plant of monocot and as well an important crop plant, BR signaling is not as well characterized as in Arabidopsis. Recent studies by forward and reverse genetics have identified a number of either conserved or specific components of rice BR signaling pathway, bringing new ideas into BR signaling regulation mechanisms. Genetic manipulation of BR level or BR sensitivity to improve rice yield has established the great significance of BR research achievements.     

Brassinosteroid signaling: novel downstream components emerge

Continued genetic screening and analysis of Arabidopsis mutants has extended our view of brassinosteroid signaling beyond hormone perception to downstream events involving a negative cytoplasmic regulator and nuclear localized positive activators of the brassinosteroid response.     

Brassinosteroid signal transduction--choices of signals and receptors

Small signaling molecules that mediate cell-cell communication are essential for developmental regulation in multicellular organisms. Among them are the steroids and peptide hormones that regulate growth in both plants and animals. In plants, brassinosteroids (BRs) are perceived by the cell surface receptor kinase BRI1, which is distinct from the animal steroid receptors. Identification of components of the BR signaling pathway has revealed similarities to other animal and plant signal transduction pathways. Recent studies demonstrated that tomato BRI1 (tBRI1) perceives both BR and the peptide hormone systemin, raising new questions about the molecular mechanism and evolution of receptor-ligand specificity.     

Brassinosteroid is involved in strawberry fruit ripening

Although brassinosteroid (BR) has been suggested to play a role in strawberry fruit ripening, the defined function of this hormone remains unclear in the fruit. Here, BR content and BR receptor gene FaBRI1 expression were analysed during ??Akihime?? strawberry fruit development. We found that BR levels increased during the later developmental stages, and the mRNA expression levels of FaBRI1 increased rapidly from white to initial red stages, suggesting that BR is associated with fruit ripening. This was further confirmed by exogenous application of BR and its inhibitor brassinazole (BZ) to big-green fruit, which significantly promoted and inhibited strawberry fruit ripening, respectively. More importantly, down-regulation of FaBRI1 expression in de-greening fruit markedly retarded strawberry red-colouring. In conclusion, we have provided physiological and molecular evidence to demonstrate that BR plays a role in strawberry fruit ripening. In addition, both BR content and FaBRI1 expression reached their peak levels in small-green fruit, suggesting that BR might also be involved in early strawberry fruit development. Further experiments are required to validate the role of BR in strawberry fruit cell division.