Brassinosteroids regulate pectin methylesterase activity and AtPME41 expression in Arabidopsis under chilling stress

Pectin methylesterases (PMEs) are important cell wall enzymes that may play important roles in plant chilling/freezing tolerance. We investigated the possible roles of brassinosteroids (BRs) in regulation of PMEs under chilling stress. Chilling stress or 24-epibrassinolide (eBL) treatments induced significant increases in PME activity in wild type (Col-0) seedlings of Arabidopsis. The chilling-stress-induced increases in PME activity were also found in bzr1-D mutant, a BZR1 stabilized mutant with a constitutively active BR signaling pathway, but not in bri1-116, a BR insensitive null allele of the BR receptor BRI1. The results suggest that the regulation of PME activity in Arabidopsis under chilling stress depends on the BR signaling pathway. Furthermore, we showed that the effect of chilling stress on PME activity was impaired in pme41, a knockout mutant of AtPME41. Semi-quantitative RT-PCR results showed that expression of AtPME41 was induced by chilling stress in wild type plants but not in the bri1-116 mutant. The expression of AtPME41 increased in bzr1-D and eBL treated wild type seedlings, but decreased in bri1-116 seedlings. Furthermore, ion leakage induced by low temperature were dramatically increased in both bri1-116 and pme41, while lipid peroxidation was increased in bri1-116 only. The results suggest that BRs may modulate total PME activity in Arabidopsis under chilling stress by regulating AtPME41 expression. Regulation of PME activity may serve as one of the mechanisms that BR participates in chilling tolerance of plants.     

Nuclear-localized BZR1 mediates brassinosteroid-induced growth and feedback suppression of brassinosteroid biosynthesis

Plant steroid hormones, brassinosteroids (BRs), are perceived by a cell surface receptor kinase, BRI1, but how BR binding leads to regulation of gene expression in the nucleus is unknown. Here we describe the identification of BZR1 as a nuclear component of the BR signal transduction pathway. A dominant mutation bzr1-1D suppresses BR-deficient and BR-insensitive (bri1) phenotypes and enhances feedback inhibition of BR biosynthesis. BZR1 protein accumulates in the nucleus of elongating cells of dark-grown hypocotyls and is stabilized by BR signaling and the bzr1-1D mutation. Our results demonstrate that BZR1 is a positive regulator of the BR signaling pathway that mediates both downstream BR responses and feedback regulation of BR biosynthesis.     

Brassinosteroid‐mediated reactive oxygen species are essential for tapetum degradation and pollen fertility in tomato

Phytohormone brassinosteroids (BRs) are essential for plant growth and development, but the mechanisms of BR‐mediated pollen development remain largely unknown. In this study, we show that pollen viability, pollen germination and seed number decreased in the BR‐deficient mutant d^{^im}, which has a lesion in the BR biosynthetic gene DWARF (DWF), and in the bzr1 mutant, which is deficient in BR signaling regulator BRASSINAZOLE RESISTANT 1 (BZR1), compared with those in wild‐type plants, whereas plants overexpressing DWF or BZR1 exhibited the opposite effects. Loss or gain of function in the DWF or BZR1 genes altered the timing of reactive oxygen species (ROS) production and programmed cell death (PCD) in tapetal cells, resulting in delayed or premature tapetal degeneration, respectively. Further analysis revealed that BZR1 could directly bind to the promoter of RESPIRATORY BURST OXIDASE HOMOLOG 1 (RBOH1), and that RBOH1‐mediated ROS promote pollen and seed development by triggering PCD and tapetal cell degradation. In contrast, the suppression of RBOH1 compromised BR signaling‐mediated ROS production and pollen development. These findings provide strong evidence that BZR1‐dependent ROS production plays a critical role in the BR‐mediated regulation of tapetal cell degeneration and pollen development in Solanum lycopersicum (tomato) plants.     

Somatic Embryogenesis Receptor Kinases Control Root Development Mainly via Brassinosteroid-Independent Actions in Arabidopsis thaliana

Brassinosteroids(BRs),a group of plant steroidal hormones,play critical roles in many aspects of plant growth and development.Previous studies showed that BRI1-mediated BR signaling regulates cell division and differentiation during Arabidopsis root development via interplaying with auxin and other phytohormones.Arabidopsis somatic embryogenesis receptor-like kinases(SERKs),as co-receptors of BRI1,were found to play a fundamental role in an early activation step of BR signaling pathway.Here we report a novel function of SERKs in regulating Arabidopsis root development.Genetic analyses indicated that SERKs control root growth mainly via a BR-independent pathway.Although BR signaling pathway is completely disrupted in the serk1 bak1 bkk1 triple mutant,the root growth of the triple mutant is much severely damaged than the BR deficiency or signaling null mutants.More detailed analyses indicated that the triple mutant exhibited drastically reduced expression of a number of genes critical to polar auxin transport,cell cycle,endodermis development and root meristem differentiation,which were not observed in null BR biosynthesis mutant cpd and null BR signaling mutant bri1-701.     

BAK1, an Arabidopsis LRR receptor-like protein kinase,interacts with BRI1 and modulates brassinosteroid signaling

Brassinosteroids regulate plant growth and development through a protein complex that includes the leucine-rich repeat receptor-like protein kinase (LRR-RLK) brassinosteroid-insensitive 1 (BRI1). Activation tagging was used to identify a dominant genetic suppressor of bri1, bak1-1D (bri1-associated receptor kinase 1-1Dominant), which encodes an LRR-RLK, distinct from BRI1. Overexpression of BAK1 results in elongated organ phenotypes, while a null allele of BAK1 displays a semidwarfed phenotype and has reduced sensitivity to brassinosteroids (BRs). BAK1 is a serine/threonine protein kinase, and BRI1 and BAK1 interact in vitro and in vivo. Expression of a dominant-negative mutant allele of BAK1 causes a severe dwarf phenotype, resembling the phenotype of null bri1 alleles. These results indicate BAK1 is a component of BR signaling.     

Brassinosteroid-insensitive dwarf mutants of Arabidopsis accumulate brassinosteroids

Seven dwarf mutants resembling brassinosteroid (BR)-biosynthetic dwarfs were isolated that did not respond significantly to the application of exogenous BRs. Genetic and molecular analyses revealed that these were novel alleles of BRI1 (Brassinosteroid-Insensitive 1), which encodes a receptor kinase that may act as a receptor for BRs or be involved in downstream signaling. The results of morphological and molecular analyses indicated that these represent a range of alleles from weak to null. The endogenous BRs were examined from 5-week-old plants of a null allele (bri1-4) and two weak alleles (bri1-5 and bri1-6). Previous analysis of endogenous BRs in several BR-biosynthetic dwarf mutants revealed that active BRs are deficient in these mutants. However, bri1-4 plants accumulated very high levels of brassinolide, castasterone, and typhasterol (57-, 128-, and 33-fold higher, respectively, than those of wild-type plants). Weaker alleles (bri1-5 and bri1-6) also accumulated considerable levels of brassinolide, castasterone, and typhasterol, but less than the null allele (bri1-4). The levels of 6-deoxoBRs in bri1 mutants were comparable to that of wild type. The accumulation of biologically active BRs may result from the inability to utilize these active BRs, the inability to regulate BR biosynthesis in bri1 mutants, or both. Therefore, BRI1 is required for the homeostasis of endogenous BR levels.     

油菜素内酯调控作物农艺性状和非生物胁迫响应的研究进展

植物对不利环境的适应依赖于将外部胁迫信号传递到内部信号通路中,在进化过程中形成一系列的胁迫响应机制。其中,油菜素内酯(brassinosteroids, BRs)是一种类固醇激素,广泛参与植物生长发育和逆境响应过程。BRs被包括受体BRI1和共受体BAK1在内的细胞表面受体感知,继而触发信号级联,导致蛋白激酶BIN2的抑制和转录因子BES1/BZR1的激活,BES1/BZR1可直接调控数千个下游响应基因的表达。在模式植物拟南芥中的研究表明,BR的生物合成和信号转导通路成员,特别是BIN2和其下游的转录因子BES1/BZR1,可以被各种环境因子广泛地调节。本文系统总结了BR相关的最新研究进展,对BR的生物合成和信号转导是如何被复杂的环境因子所调节,以及BR与环境因子如何协同调控作物重要农艺性状、冷胁迫和盐胁迫的响应进行了综述。     

BRL1 and BRL3 are novel brassinosteroid receptors that function in vascular differentiation in Arabidopsis

Plant steroid hormones, brassinosteroids (BRs), are perceived by the plasma membrane-localized leucine-rich-repeat-receptor kinase BRI1. Based on sequence similarity, we have identified three members of the BRI1 family, named BRL1, BRL2 and BRL3. BRL1 and BRL3, but not BRL2, encode functional BR receptors that bind brassinolide, the most active BR, with high affinity. In agreement, only BRL1 and BRL3 can rescue bri1 mutants when expressed under the control of the BRI1 promoter. While BRI1 is ubiquitously expressed in growing cells, the expression of BRL1 and BRL3 is restricted to non-overlapping subsets of vascular cells. Loss-of-function of brl1 causes abnormal phloem:xylem differentiation ratios and enhances the vascular defects of a weak bri1 mutant. bri1 brl1 brl3 triple mutants enhance bri1 dwarfism and also exhibit abnormal vascular differentiation. Thus, Arabidopsis contains a small number of BR receptors that have specific functions in cell growth and vascular differentiation.     

Brassinosteroids regulate glucosinolate biosynthesis in Arabidopsis thaliana

Plants must constantly adjust their growth and defense responses to deal with the wide variety of stresses they encounter in their environment. Among phytohormones, brassinosteroids (BRs) are an important group of plant steroid hormones involved in numerous aspects of the plant lifecycle including growth, development and responses to various stresses including insect attacks. Here, we show that BRs regulate glucosinolate (GS) biosynthesis and function in insect herbivory. Preference tests and larval feeding experiments using the generalist herbivore, diamondback moth (Plutella xylostella), revealed that the larvae prefer to feed on Arabidopsis thaliana brassinosteroid insensitive 1 (bri1‐5) plants over wild‐type Ws‐2 or BRI1‐Flag (bri1‐5 background) transgenic plants, which results in an increase in larval weight. Analysis of GS contents showed that 3‐(methylsulfinyl) propyl GS (C3) levels were higher in bri1‐5 than in Ws2 and BRI1‐Flag transgenic plants, whereas sinigrin (2‐propenylglucosinolate), glucoerucin (4‐methylthiobutylglucosinolate) and glucobrassicin (indol‐3‐ylmethylglucosinolate) levels were lower in this mutant. We investigated the effect of brassinolide (BL) on GS biosynthesis in Arabidopsis and radish (Raphanus sativus L.) by monitoring the expression levels of GS biosynthetic genes, including MAM1, MAM3, BCAT4 and AOP2, which increased in a BL‐dependent manner. These results suggest that BRs regulate GS profiles in higher plants, which function in defense responses against insects.