A gain-of-function mutation in a cytokinin receptor triggers spontaneous root nodule organogenesis

Legume root nodules originate from differentiated cortical cells that reenter the cell cycle and form organ primordia. We show that perception of the phytohormone cytokinin is a key element in this switch. Mutation of a Lotus japonicus cytokinin receptor gene leads to spontaneous development of root nodules in the absence of rhizobia or rhizobial signal molecules. The mutant histidine kinase receptor has cytokinin-independent activity and activates an Escherichia coli two-component phosphorelay system in vivo. Mutant analysis shows that cytokinin signaling is required for cell divisions that initiate nodule development and defines an autoregulated process where cytokinin induction of nodule stem cells is controlled by shoots.     

Hippo pathway inhibits Wnt signaling to restrain cardiomyocyte proliferation and heart size

Genetic regulation of mammalian heart size is poorly understood. Hippo signaling represents an organ-size control pathway in Drosophila, where it also inhibits cell proliferation and promotes apoptosis in imaginal discs. To determine whether Hippo signaling controls mammalian heart size, we inactivated Hippo pathway components in the developing mouse heart. Hippo-deficient embryos had overgrown hearts with elevated cardiomyocyte proliferation. Gene expression profiling and chromatin immunoprecipitation revealed that Hippo signaling negatively regulates a subset of Wnt target genes. Genetic interaction studies indicated that β-catenin heterozygosity suppressed the Hippo cardiomyocyte overgrowth phenotype. Furthermore, the Hippo effector Yap interacts with β-catenin on Sox2 and Snai2 genes. These data uncover a nuclear interaction between Hippo and Wnt signaling that restricts cardiomyocyte proliferation and controls heart size.     

A cytokinin perception mutant colonized by Rhizobium in the absence of nodule organogenesis

In legumes, Nod-factor signaling by rhizobia initiates the development of the nitrogen-fixing nodule symbiosis, but the direct cell division stimulus that brings about nodule primordia inception in the root cortex remains obscure. We showed that Lotus japonicus plants homozygous for a mutation in the HYPERINFECTED 1 (HIT1) locus exhibit abundant infection-thread formation but fail to initiate timely cortical cell divisions in response to rhizobial signaling. We demonstrated that the corresponding gene encodes a cytokinin receptor that is required for the activation of the nodule inception regulator Nin and nodule organogenesis.     

文冠果组织培养的玻璃化控制研究

以文冠果的茎段为材料,通过外植体取材、培养条件、细胞分裂素质量浓度、培养基中钙质量浓度4个方面控制文冠果组织培养中的玻璃化问题。结果表明:沙藏后的种子萌发的小植株的玻璃化程度较轻于多年生植株;玻璃化程度随着光照的增加而降低,最适光照为6 000 lx;随着细胞分裂素6-BA质量浓度的增加玻璃化程度加剧,其最适质量浓度为1 mg/L;随着钙质量浓度的增加玻璃化程度抑制,当钙质量浓度为880 g/L时,效果最为明显。综合以上4个方面,基本可以控制玻璃化对文冠果组培苗的影响,从而为文冠果组织培养的产业化提供了可能。     

Loss of a callose synthase results in salicylic acid-dependent disease resistance

Plants attacked by pathogens rapidly deposit callose, a beta-1,3-glucan, at wound sites. Traditionally, this deposition is thought to reinforce the cell wall and is regarded as a defense response. Surprisingly, here we found that powdery mildew resistant 4 (pmr4), a mutant lacking pathogen-induced callose, became resistant to pathogens, rather than more susceptible. This resistance was due to mutation of a callose synthase, resulting in a loss of the induced callose response. Double-mutant analysis indicated that blocking the salicylic acid (SA) defense signaling pathway was sufficient to restore susceptibility to pmr4 mutants. Thus, callose or callose synthase negatively regulates the SA pathway.     

A plant miRNA contributes to antibacterial resistance by repressing auxin signaling

Plants and animals activate defenses after perceiving pathogen-associated molecular patterns (PAMPs) such as bacterial flagellin. In Arabidopsis, perception of flagellin increases resistance to the bacterium Pseudomonas syringae, although the molecular mechanisms involved remain elusive. Here, we show that a flagellin-derived peptide induces a plant microRNA (miRNA) that negatively regulates messenger RNAs for the F-box auxin receptors TIR1, AFB2, and AFB3. Repression of auxin signaling restricts P. syringae growth, implicating auxin in disease susceptibility and miRNA-mediated suppression of auxin signaling in resistance.     

超表达苹果细胞分裂素响应基因MdCRF6影响花青苷积累和盐胁迫抗性

【目的】克隆苹果细胞分裂素响应因子基因Md CRF6(cytokinin response factor 6),鉴定其在调节花青苷积累和盐胁迫抗性中的作用,揭示Md CRF6的功能,为研究细胞分裂素信号途径和果树生长发育调控提供理论依据。【方法】以‘嘎啦’苹果(Malus domestica‘Gala’)为研究材料,利用同源序列比对和PCR技术,分离细胞分裂素响应基因Md CRF6。使用MEGA 5.0软件构建苹果Md CRF6与拟南芥CRFs间系统进化树;通过SMART软件和DNAMAN软件分析Md CRF6蛋白的保守结构域。利用实时荧光定量PCR方法检测该基因对细胞分裂素和盐胁迫的响应。通过电泳迁移率试验(EMSA),验证Md CRF6原核表达蛋白对DRE作用元件的绑定。构建Md CRF6植物超表达载体,并通过农杆菌介导的遗传转化获得转Md CRF6苹果愈伤组织。比较野生型和转基因苹果愈伤组织在花青苷积累和盐抗性方面的差异,结合基因表达分析,初步鉴定Md CRF6在调节花青苷积累和盐胁迫方面的生物学功能。【结果】分离得到了苹果细胞分裂素响应因子基因Md CRF6(基因序列号:MDP0000783818),该基因开放阅读框(ORF)为1 047 bp,编码含有348个氨基酸的蛋白。进化树分析和氨基酸序列比对结果表明,苹果Md CRF6蛋白在N端包含保守的CRF结构域,在C端包含保守的AP2/ERF结构域,并且与拟南芥At CRF6同源性最高。基因表达分析显示该基因具有细胞分裂素和盐胁迫响应,分别在10μmol·L-1 BA和100 mmol·L-1 Na Cl处理3 h和6 h时表达量最高。EMSA试验结果验证Md CRF6原核表达蛋白能够绑定DRE序列。在苹果愈伤组织中超表达Md CRF6,发现Md CRF6转基因苹果愈伤组织花青苷积累受到抑制,同时苹果愈伤组织的抗盐性降低。基因表达分析显示,Md CRF6显著抑制花青苷合成基因和盐响应相关基因的表达。【结论】苹果Md CRF6与拟南芥At CRF6具有高度的同源性,其参与植物对细胞分裂素和盐胁迫的响应。在苹果愈伤组织中超量表达Md CRF6抑制其花青苷积累,降低植物抗盐性。推测苹果Md CRF6可能通过结合花青苷合成基因以及抗盐相关基因的启动子,抑制基因的表达,从而负调节花青苷积累和抗盐性。     

Regulation of Wnt signaling and embryo patterning by an extracellular sulfatase

The developmental signaling functions of cell surface heparan sulfate proteoglycans (HSPGs) are dependent on their sulfation states. Here, we report the identification of QSulf1, the avian ortholog of an evolutionarily conserved protein family related to heparan-specific N-acetyl glucosamine sulfatases. QSulf1 expression is induced by Sonic hedgehog in myogenic somite progenitors in quail embryos and is required for the activation of MyoD, a Wnt-induced regulator of muscle specification. QSulf1 is localized on the cell surface and regulates heparan-dependent Wnt signaling in C2C12 myogenic progenitor cells through a mechanism that requires its catalytic activity, providing evidence that QSulf1 regulates Wnt signaling through desulfation of cell surface HSPGs.     

Cytokinin activation of Arabidopsis cell division through a D-type cyclin

Cytokinins are plant hormones that regulate plant cell division. The D-type cyclin CycD3 was found to be elevated in a mutant of Arabidopsis with a high level of cytokinin and to be rapidly induced by cytokinin application in both cell cultures and whole plants. Constitutive expression of CycD3 in transgenic plants allowed induction and maintenance of cell division in the absence of exogenous cytokinin. Results suggest that cytokinin activates Arabidopsis cell division through induction of CycD3 at the G1-S cell cycle phase transition.     

细胞分裂素的信号转导机制

细胞分裂素的信号转导是一个"磷酸接力传递"(phosphorelay)过程。细胞分裂素结合受体组氨酸激酶(Arabidopsis histidine kina-ses,AHK)使其磷酸化,并将磷酸基团转移给胞质中的磷酸转运蛋白(Arabidopsis histidine-phosphotransfer proteins,AHPs),磷酸化的AHPs进入细胞核并将磷酸基团转移到A型和B型反应调节因子(Arabidopsis response regulators,ARRs)上。B型ARR是一类转录因子,可激活A型ARR基因的转录,在众多参加细胞分裂素信号传导的转录因子中起主要作用。文中综述了近年来关于细胞分裂素信号转导机制的研究进展,并进行了展望。     

Antagonistic control of disease resistance protein stability in the plant immune system

Pathogen recognition by the plant immune system is governed by structurally related, polymorphic products of disease resistance (R) genes. RAR1 and/or SGT1b mediate the function of many R proteins. RAR1 controls preactivation R protein accumulation by an unknown mechanism. We demonstrate that Arabidopsis SGT1b has two distinct, genetically separable functions in the plant immune system: SGT1b antagonizes RAR1 to negatively regulate R protein accumulation before infection, and SGT1b has a RAR1-independent function that regulates programmed cell death during infection. The balanced activities of RAR1 and SGT1, in concert with cytosolic HSP90, modulate preactivation R protein accumulation and signaling competence.     

Phosphorelay and transcription control in cytokinin signal transduction

The past decade has seen substantial advances in knowledge of molecular mechanisms and actions of plant hormones, but only in the past few years has research on cytokinins begun to hit its stride. Cytokinins are master regulators of a large number of processes in plant development, which is known to be unusually plastic and adaptive, as well as resilient and perpetual. These characteristics allow plants to respond sensitively and quickly to their environments. Recent studies have demonstrated that cytokinin signaling involves a multistep two-component signaling pathway, resulting in the development of a canonical model of cytokinin signaling that is likely representative in plants. This Viewpoint outlines this general model, focusing on the specific example of Arabidopsis, and introduces the STKE Connections Maps for both the canonical module and the specific Arabidopsis Cytokinin Signaling Pathway.     

Role of the Arabidopsis glucose sensor HXK1 in nutrient,light, and hormonal signaling

Glucose modulates many vital processes in photosynthetic plants. Analyses of Arabidopsis glucose insensitive2 (gin2) mutants define the physiological functions of a specific hexokinase (HXK1) in the plant glucose-signaling network. HXK1 coordinates intrinsic signals with extrinsic light intensity. HXK1 mutants lacking catalytic activity still support various signaling functions in gene expression, cell proliferation, root and inflorescence growth, and leaf expansion and senescence, thus demonstrating the uncoupling of glucose signaling from glucose metabolism. The gin2 mutants are also insensitive to auxin and hypersensitive to cytokinin. Plants use HXK as a glucose sensor to interrelate nutrient, light, and hormone signaling networks for controlling growth and development in response to the changing environment.     

Regulation of cell polarity and protrusion formation by targeting RhoA for degradation

The Rho family of small guanosine triphosphatases regulates actin cytoskeleton dynamics that underlie cellular functions such as cell shape changes, migration, and polarity. We found that Smurf1, a HECT domain E3 ubiquitin ligase, regulated cell polarity and protrusive activity and was required to maintain the transformed morphology and motility of a tumor cell. Atypical protein kinase C zeta (PKCzeta), an effector of the Cdc42/Rac1-PAR6 polarity complex, recruited Smurf1 to cellular protrusions, where it controlled the local level of RhoA. Smurf1 thus links the polarity complex to degradation of RhoA in lamellipodia and filopodia to prevent RhoA signaling during dynamic membrane movements.     

Positive regulation of T cell activation and integrin adhesion by the adapter Fyb/Slap

The molecular adapter Fyb/Slap regulates signaling downstream of the T cell receptor (TCR), but whether it plays a positive or negative role is controversial. We demonstrate that Fyb/Slap-deficient T cells exhibit defective proliferation and cytokine production in response to TCR stimulation. Fyb/Slap is also required in vivo for T cell-dependent immune responses. Functionally, Fyb/Slap has no apparent role in the activation of known TCR signaling pathways, F-actin polymerization, or TCR clustering. Rather, Fyb/Slap regulates TCR-induced integrin clustering and adhesion. Thus, Fyb/Slap is the first molecular adapter to be identified that couples TCR stimulation to the avidity modulation of integrins governing T cell adhesion.     

Four-jointed is a Golgi kinase that phosphorylates a subset of cadherin domains

The atypical cadherin Fat acts as a receptor for a signaling pathway that regulates growth, gene expression, and planar cell polarity. Genetic studies in Drosophila identified the four-jointed gene as a regulator of Fat signaling. We show that four-jointed encodes a protein kinase that phosphorylates serine or threonine residues within extracellular cadherin domains of Fat and its transmembrane ligand, Dachsous. Four-jointed functions in the Golgi and is the first molecularly defined kinase that phosphorylates protein domains destined to be extracellular. An acidic sequence motif (Asp-Asn-Glu) within Four-jointed was essential for its kinase activity in vitro and for its biological activity in vivo. Our results indicate that Four-jointed regulates Fat signaling by phosphorylating cadherin domains of Fat and Dachsous as they transit through the Golgi.     

Olfactory plasticity is regulated by pheromonal signaling in Caenorhabditis elegans

Population density-dependent dispersal is a well-characterized strategy of animal behavior in which dispersal rate increases when population density is higher. Caenorhabditis elegans shows positive chemotaxis to a set of odorants, but the chemotaxis switches from attraction to dispersal after prolonged exposure to the odorants. We show here that this plasticity of olfactory behavior is dependent on population density and that this regulation is mediated by pheromonal signaling. We show that a peptide, suppressor of NEP-2 (SNET-1), negatively regulates olfactory plasticity and that its expression is down-regulated by the pheromone. NEP-2, a homolog of the extracellular peptidase neprilysin, antagonizes SNET-1, and this function is essential for olfactory plasticity. These results suggest that population density information is transmitted through the external pheromone and endogenous peptide signaling to modulate chemotactic behavior.