SMALL AND ROUND GRAIN is involved in the brassinosteroid signaling pathway which regulates grain size in rice
Rong MiaoQibing LinPenghui CaoChunlei ZhouMiao FengJie LanSheng LuoF. ZhangHongmin WuQixian HaoHai ZhengTengfei MaYunshuai HuangChangling MouThanhliem NguyenZhijun ChengXiuping GuoShijia LiuLing JiangJianmin Wan
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ABSTRACT Grain size is a key determinant of 1,000‐grain weight, one of three factors determining grain yield. However, the complete regulatory network controlling grain size has not been fully clarified. Here, we identified a rice mutant, named small and round grain ( srg ) that exhibits semi‐dwarf stature and small grain size. Cytological analysis showed that cell length and number of spikelet epidermal cells of the srg mutant are reduced, indicating that SRG controls grain size by promoting cell elongation and increasing cell number. SRG encodes a kinesin belonging to the kinesin‐1 subfamily and is extensively expressed in different plant tissues with relatively high expression in young panicles. SRG protein is mainly located in the nucleus and cell membrane. Expression of the SRG gene was induced by brassinolide through the brassinosteroid (BR) responsive factor OsWRKY53 and SRG protein was phosphorylated by BR‐activated kinase OsBSK3 to prevent its degradation. In addition, microtubule (MT) morphology was abnormal and disordered in the srg and cr‐1 mutants. These findings suggest that BR likely stabilizes orderly assembly and arrangement of MTs by stabilizing SRG proteins, thus promoting grain size. SRG overexpression lines produced more tillers and significantly larger and heavier grains to increase 1,000‐grain weight, suggesting that SRG has potential to increase grain yield. Our study indicated that SRG is a new BR responsive factor and BR might regulate grain size by influencing the expression of SRG .Keywords:
Brassinosteroid
Brassinolide
Brassinosteroids are steroidal hormones essential for the growth and development of plants. Brassinolide, the most biologically active brassinosteroid, has a seven-membered lactone ring that is formed by a Baeyer-Villiger oxidation of its immediate precursor castasterone. Despite its potential key role in controlling plant development, brassinolide synthase has not been identified. Previous work has shown that the formation of castasterone from 6-deoxocastasterone is catalyzed by members of the CYP85A family of cytochrome P-450 monooxygenases. A null mutation in the tomato Dwarf (CYP85A1) gene, extreme dwarf (d(x)), causes severe dwarfism due to brassinosteroid deficiency, but the d(x) mutant still produces fruits. Here, we show that d(x) fruits contain brassinolide at a higher level than wild-type fruits and that a new CYP85A gene, CYP85A3, is preferentially expressed in tomato fruits. Tomato CYP85A3 catalyzed the Baeyer-Villiger oxidation to produce brassinolide from castasterone in yeast, in addition to the conversion of 6-deoxocastasterone to castasterone. We also show that Arabidopsis CYP85A2, which was initially characterized as castasterone synthase, also has brassinolide synthase activity. Exogenous application of castasterone and brassinolide to the Arabidopsis cyp85a1/cyp85a2 double mutant suggests that castasterone can function as an active brassinosteroid but that its conversion into brassinolide is necessary for normal vegetative development in Arabidopsis. We postulate that castasterone is the major active brassinosteroid during vegetative growth in tomato, whereas brassinolide may play an organ-specific role in fruit development in this species.
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Overview of the recent progress in the research of brassinosteroids, a new class of phytohormones, is described. This article covers 1) the chemical synthesis of natural C28 brassinosteroids and their deuterio-labeled compounds and precursor sterols, 2) related brassinosteroid analogs and their biological activity, 3) the role of synthetic organic chemistry in the clarification of brassinolide biosynthesis, and 4) its role in the analysis of brassinosteroid dwarf mutants.
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Brassinosteroids are polyhydroxysteroids that are involved in different plants’ biological functions, such as growth, development and resistance to biotic and external stresses. Because of its low abundance in plants, much effort has been dedicated to the synthesis and characterization of brassinosteroids analogs. Herein, we report the synthesis of brassinosteroid 24-nor-5β-cholane type analogs with 23-benzoate function and 22,23-benzoate groups. The synthesis was accomplished with high reaction yields in a four-step synthesis route and using hyodeoxycholic acid as starting material. All synthesized analogs were tested using the rice lamina inclination test to assess their growth-promoting activity and compare it with those obtained for brassinolide, which was used as a positive control. The results indicate that the diasteroisomeric mixture of monobenzoylated derivatives exhibit the highest activity at the lowest tested concentrations (1 × 10−8 and 1 × 10−7 M), being even more active than brassinolide. Therefore, a simple synthetic procedure with high reaction yields that use a very accessible starting material provides brassinosteroid synthetic analogs with promising effects on plant growth. This exploratory study suggests that brassinosteroid analogs with similar chemical structures could be a good alternative to natural brassinosteroids.
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From immature seed of Phaseolus vulgaris, a new brassinosteroid, 6-deoxodihydrohomo- dolichosterone, as well as dolichosterone and brassinolide have been identified in addition to 6-deoxodihydrodolichosterone, 6-deoxodihydrocastasterone, castasterone and dolicholide which had already been reported therein. Extensive GC/MS analysis revealed that the number of brassinosteroids fully or partially characterized from the seed totals thirty, including unknown brassinosteroids.
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The aim of the study was to examine the effect of exogenous 24-epibrassinolide on its uptake and content of endogenous brassinosteroids in wheat seedlings. 24-Epibrassinolide was applied at two concentrations (0.1 and 2.0 μM) and in three different methods: by soaking seeds, by drenching and by spraying plants. Brassinosteroids were determined by high-performance liquid chromatography combined with electrospray mass spectrometry. Three important brassinosteroids, 24-epibrassinolide, brassinolide and castasterone, were detected in the wheat leaves, but their contents varied with leaf insertion and plant age. Increased 24-epibrassinolide content in the leaf tissue was found when this hormone was applied by soaking or drenching. Additionally the seed treatment influenced brassinosteroid balance in seedlings. The growth response of wheat seedlings treated with 24-epibrassinolide has been also investigated.
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When brassinosteroid (BR)-deficient mutant (det2) or wild-type (WT) seedlings were treated with brassinolide (BL), the most active BR, for 3 h, the abundance of PIN4 and PIN7 transcripts decreased, and there were fewer PIN4 and PIN7 transcripts in det2 than in the WT. This suggests that BL selectively regulates the PIN gene in a complex manner.
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