Dynamin GTPase,a force-generating molecular switch

Dynamin is a GTPase that regulates late events in clathrin-coated vesicle formation. Our current working model suggests that dynamin is targeted to coated pits in its unoccupied or GDP-bound form, where it is initially distributed uniformly throughout the clathrin lattice. GTP/GDP exchange triggers its release from these sites and its assembly into short helices that encircle the necks of invaginated coated pits like a collar. GTP hydrolysis, which is required for vesicle detachment, presumably induces a concerted conformation change, tightening the collar. Unlike most of its GTPase cousins that serve as molecular switches, dynamin has a low affinity for GTP, a very high intrinsic rate of GTP hydrolysis and functions as a homo-oligomer. A concerted conformational change resulting from coordinated GTP hydrolysis by the dynamin oligomer might be sufficient to generate force. In this case, dynamin would be the first GTPase identified that acts as a structural protein with mechano-chemical function.     

The molecular bases of plant resistance and defense responses to aphid feeding: current status

Plant genes participating in the recognition of aphid herbivory in concert with plant genes involved in defense against herbivores mediate plant resistance to aphids. Several such genes involved in plant disease and nematode resistance have been characterized in detail, but their existence has only recently begun to be determined for arthropod resistance. Hundreds of different genes are typically involved and the disruption of plant cell wall tissues during aphid feeding has been shown to induce defense responses in Arabidopsis, Triticum, Sorghum, and Nicotiana species. Mi‐1.2, a tomato gene for resistance to the potato aphid, Macrosiphum euphorbiae (Thomas), is a member of the nucleotide‐binding site and leucine‐rich region Class II family of disease, nematode, and arthropod resistance genes. Recent studies into the differential expression of Pto‐ and Pti1‐like kinase genes in wheat plants resistant to the Russian wheat aphid, Diuraphis noxia (Mordvilko), provide evidence of the involvement of the Pto class of resistance genes in arthropod resistance. An analysis of available data suggests that aphid feeding may trigger multiple signaling pathways in plants. Early signaling includes gene‐for‐gene recognition and defense signaling in aphid‐resistant plants, and recognition of aphid‐inflicted cell damage in both resistant and susceptible plants. Furthermore, signaling is mediated by several compounds, including jasmonic acid, salicylic acid, ethylene, abscisic acid, giberellic acid, nitric oxide, and auxin. These signals lead to the development of direct chemical defenses against aphids and general stress‐related responses that are well characterized for a number of abiotic and biotic stresses. In spite of major plant taxonomic differences, similarities exist in the types of plant genes expressed in response to feeding by different species of aphids. However, numerous differences in plant signaling and defense responses unique to specific aphid–plant interactions have been identified and warrant further investigation.     

Protein phosphorylation: A molecular switch in plant signaling

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The strawberry plant defense mechanism: a molecular review

Strawberry, a small fruit crop of great importance throughout the world, has been considered a model plant system for Rosaceae, and is susceptible to a large variety of phytopathogenic organisms. Most components and mechanisms of the strawberry defense network remain poorly understood. However, from current knowledge, it seems clear that the ability of a strawberry plant to respond efficiently to pathogens relies first on the physiological status of injured tissue (pre-formed mechanisms of defense) and secondly on the general ability to recognize and identify the invaders by surface plant receptors, followed by a broad range of induced mechanisms, which include cell wall reinforcement, production of reactive oxygen species, phytoalexin generation and pathogenesis-related protein accumulation. Dissection of these physiological responses at a molecular level will provide valuable information to improve future breeding strategies for new strawberry varieties and to engineer strawberry plants for durable and broad-spectrum disease resistance. In turn, this will lead to a reduction in use of chemicals and in environmental risks. Advances in the understanding of the molecular interplay between plant (mainly those considered model systems) and various classes of microbial pathogens have been made in the last two decades. However, major progress in the genetics and molecular biology of strawberry is still needed to uncover fully the way in which this elaborate plant innate immune system works. These fundamental insights will provide a conceptual framework for rational human intervention through new strawberry research approaches. In this review, we will provide a comprehensive overview and discuss recent advances in molecular research on strawberry defense mechanisms against pathogens.     

MYB 转录因子在植物抗逆胁迫中的作用及其分子机理

摘要: 在植物抗逆反应体系中, 通过转录因子调控功能基因的表达, 是植物逆境应答反应的关键环节。作为植物体内最大的转录因子家族之一, MYB(v-myb avian myeloblastosis viral oncogene homolog)类转录因子在植物抗逆胁迫过程中起着重要的作用。文章论述了MYB转录因子的结构特征、功能特性及其分子机理, 并结合研究进展着重讨论了它们在植物抗逆胁迫中的作用。     

一氧化氮与激发子诱导的植物抗病防卫反应

来源于真菌或植物细胞壁的激发子可以诱导植物的抗性反应。一系列的信号分子,如一氧化氮、活性氧、茉莉酸、水杨酸、乙烯等都参与了激发子诱导的植物抗性反应。它们在介导激发子刺激诱发胞内抗性反应的过程中起着重要的作用。本文介绍了激发子的种类,并简述了激发了受体以及植物细胞对激发子刺激的感受与传递;重点介绍了一氧化氮在激发子诱导植物抗性反应过程中的作用,以及它与其他信号分子之间相互关系的研究进展。     

Herbivore-specific plant volatiles prime neighboring plants for nonspecific defense responses

Plants produce species-specific herbivore-induced plant volatiles (HIPVs) after damage. We tested the hypothesis that herbivore-specific HIPVs prime neighboring plants to induce defenses specific to the priming herbivore. Since Manduca sexta (specialist) and Heliothis virescens (generalist) herbivory induced unique HIPV profiles in Nicotiana benthamiana, we used these HIPVs to prime receiver plants for defense responses to simulated herbivory (mechanical wounding and herbivore regurgitant application). Jasmonic acid (JA) accumulations and emitted volatile profiles were monitored as representative defense responses since JA is the major plant hormone involved in wound and defense signaling and HIPVs have been implicated as signals in tritrophic interactions. Herbivore species-specific HIPVs primed neighboring plants, which produced 2 to 4 times more volatiles and JA after simulated herbivory when compared to similarly treated constitutive volatile-exposed plants. However, HIPV-exposed plants accumulated similar amounts of volatiles and JA independent of the combination of priming or challenging herbivore. Furthermore, volatile profiles emitted by primed plants depended only on the challenging herbivore species but not on the species-specific HIPV profile of damaged emitter plants. This suggests that feeding by either herbivore species primed neighboring plants for increased HIPV emissions specific to the subsequently attacking herbivore and is probably controlled by JA.     

Elicitors and priming agents initiate plant defense responses

Biotic elicitors produced by plant pathogens or herbivore pests rapidly activate a range of plant chemical defenses when translocated to plant tissue. The fatty acid conjugate volicitin has proven to be a robust elicitor model for studying herbivore-induced plant defense responses. Here we review the role of insect-derived volicitin (N-[17-hydroxylinolenoyl]-L-glutamine) as an authentic elicitor of defense responses, specifically as an activator of signal volatiles that attract natural enemies of herbivore pests. Comparisons are drawn between volicitin as an elicitor of plant defenses and two other classes of signaling molecules, C₆ green-leaf volatiles and C₄ bacterial volatiles that appear to prime plant defenses thereby enhancing the capacity to mobilize cellular defense responses when a plant is faced with herbivore or pathogen attack.     

B1-phytoprostanes trigger plant defense and detoxification responses

Phytoprostanes are prostaglandin/jasmonate-like products of nonenzymatic lipid peroxidation that not only occur ubiquitously in healthy plants but also increase in response to oxidative stress. In this work, we show that the two naturally occurring B(1)-phytoprostanes (PPB(1)) regioisomers I and II (each comprising two enantiomers) are short-lived stress metabolites that display a broad spectrum of biological activities. Gene expression analysis of Arabidopsis (Arabidopsis thaliana) cell cultures treated with PPB(1)-I or -II revealed that both regioisomers triggered a massive detoxification and defense response. Interestingly, expression of several glutathione S-transferases, glycosyl transferases, and putative ATP-binding cassette transporters was found to be increased by one or both PPB(1) regioisomers, and hence, may enhance the plant's capacity to inactivate and sequester reactive products of lipid peroxidation. Moreover, pretreatment of tobacco (Nicotiana tabacum) suspension cells with PPB(1) considerably prevented cell death caused by severe CuSO(4) poisoning. Several Arabidopsis genes induced by PPB(1), such as those coding for adenylylsulfate reductase, tryptophan synthase beta-chain, and PAD3 pointed to an activation of the camalexin biosynthesis pathway that indeed led to the accumulation of camalexin in PPB(1) treated leaves of Arabidopsis. Stimulation of secondary metabolism appears to be a common plant reaction in response to PPB(1). In three different plant species, PPB(1)-II induced a concentration dependent accumulation of phytoalexins that was comparable to that induced by methyl jasmonate. PPB(1)-I was much weaker active or almost inactive. No differences were found between the enantiomers of each regioisomer. Thus, results suggest that PPB(1) represent stress signals that improve plants capacity to cope better with a variety of stresses.     

Involvement of the small GTPase Rac in the defense responses of tobacco to pathogens

During the hypersensitive response (HR), plants accumulate reactive oxygen species (ROS) that are likely generated at least in part by an NADPH oxidase similar to that found in mammalian neutrophils. An essential regulator of mammalian NADPH oxidase is the small GTP-binding protein Rac. To investigate whether Rac also regulates the pathogen-induced oxidative burst in plants, a dominant negative form of the rice OsRac1 gene was overexpressed in tobacco carrying the N resistance gene. Following infection with Tobacco mosaic virus (TMV), DN-OsRacl plants developed smaller lesions than wild-type plants, accumulated lower levels of lipid peroxidation products, and failed to activate expression of antioxidant genes. These results, combined with the demonstration that superoxide and hydrogen peroxide levels were reduced in DN-OsRacl tobacco developing a synchronous HR triggered by transient expression of the TMV p50 helicase domain or the Pto and AvrPto proteins, suggest that ROS production is impaired. The dominant negative effect of DN-OsRacl could be rescued by transiently overexpressing the wild-type OsRac1 protein. TMV-induced salicylic acid accumulation also was compromised in DN-OsRacl tobacco. Interestingly, while systemic acquired resistance to TMV was not impaired, nonhost resistance to Pseudomonas syringae pv. maculicola ES4326 was suppressed. Thus, the effect DN-OsRac1 expression exerts on the resistance signaling pathway appears to vary depending on the identity of the inoculated pathogen.     

A small GTPase molecular switch regulates epigenetic centromere maintenance by stabilizing newly incorporated CENP-A

Epigenetic mechanisms regulate genome activation in diverse events, including normal development and cancerous transformation. Centromeres are epigenetically designated chromosomal regions that maintain genomic stability by directing chromosome segregation during cell division. The histone H3 variant CENP-A resides specifically at centromeres, is fundamental to centromere function and is thought to act as the epigenetic mark defining centromere loci. Mechanisms directing assembly of CENP-A nucleosomes have recently emerged, but how CENP-A is maintained after assembly is unknown. Here, we show that a small GTPase switch functions to maintain newly assembled CENP-A nucleosomes. Using functional proteomics, we found that MgcRacGAP (a Rho family GTPase activating protein) interacts with the CENP-A licensing factor HsKNL2. High-resolution live-cell imaging assays, designed in this study, demonstrated that MgcRacGAP, the Rho family guanine nucleotide exchange factor (GEF) Ect2, and the small GTPases Cdc42 and Rac, are required for stability of newly incorporated CENP-A at centromeres. Thus, a small GTPase switch ensures epigenetic centromere maintenance after loading of new CENP-A.     

植物与植食性昆虫相互作用的分子机制

在长期的协同进化中,植物建立起应对昆虫取食为害的精密而又复杂的防御机制,植物转录组调控中防御应答基因的表达及防御物质的合成因不同的昆虫取食方式而异。一般来说,咀嚼式口器昆虫取食时造成大面积组织伤害,可诱导植物产生伤害反应;而刺吸式口器昆虫因其特殊的口针取食,诱导植物激活病原体相关的防御途径。不同的防御途径激活不同的识别机制和信号途径。本文从信号识别和转导上综述了不同食性的昆虫取食植物时所引发的防御反应,分析了昆虫-植物相互作用关系的分子机制。     

Preliminary studies on differential defense responses induced during plant communication

We compared the expression patterns of three representative genes in undamaged tomato and tobacco plants in response to exposure to either tomato or tobacco fed on by Helicoverpa armigera (cotton bollworm). When tomato and tobacco, two species of one family, were incubated in the chambers with the tomato plants damaged by the cotton bollworm, the expression of the PR1, BGL2, and PAL genes was up-regulated in leaves of both plants. However, the levels of gene expression were significantly higher in the tomato than that in the tobacco. In addition, the activities of enzymes, peroxidase, polyphenol oxidase, and lipoxygenase were found to be higher in the tomato than those in the tobacco. Similar results were obtained when the damaged plants were replaced by the tobacco.     

Regulatory mechanisms of host plant defense responses to arbuscular mycorrhiza

Arbuscular mycorrhizal (AM) fungi colonize the roots of over 80% of terrestrial plant species, forming mutually beneficial symbioses. During the colonization process, symbiotic partners recognize each other, and undergo observable morphological and physiological changes; indicating that symbiosis formation involves multiple factors that are finely regulated. Sometimes host plants generate a transient, weak, defense response. This response and its down-regulation play a very important role in the development of AM symbioses. Although AM fungi can infect a wide range of host root tissues, which host defense may play a crucial role is hypothesized from the fact that hyphal expansion is only observed in the root cortex.
We discuss five defense mechanisms. (1) The degradation of exogenous elicitors. The host’s weak defense response may be due to the degradation of the exogenous elicitor chitin, or the prevention of release of an endogenous inductor from the plant cell wall. (2) The inactivation of defense signal molecules. Some defense signal molecules such as hydrogen peroxidase, salicylic acid (SA), and jasmonic acid (JA), are inactivated in host plants. This helps to avoid the turn-on of defense-related genes and facilitate mycorrhizal formation. (3) The regulation of plant hormones and plant photosynthates. Plant hormone levels and plant photosynthate metabolism both change during AM colonization. These mechanisms need further exploration. (4) Changes in levels of phosphorous (P), and (iso)flavonoids. High P levels can induce some defense genes to express hydrogen peroxidase, chitinase, and glucanase. These gene products can repress colonization by AM fungi. The plant defense response regulatory effect for different (iso)flavonoids varies, and their levels are regulated by P. (5) The suppressed expression of symbiotic genes. Some symbiosis-related genes inhibit plant defense responses, but it is still unclear which mechanisms underlie gene regulation. We provide here a theoretical basis for research into AM symbiosis that may promote study of host plant resistance and the mechanisms of symbiosis formation.
We provide a deeper insight into the signal transduction pathways of mycorrhization that will aid understanding and analysis of plant defense mechanisms in the AM context. The on-going development of genome sequencing technology will contribute greatly to the detailed study of symbiosis-related genes, and pathogenesis-related protein genes. These related genes may be induced to express corresponding proteins, be repressed, postpone expression or even shutdown, or both may work together to form symbioses. Elucidation of these features will help us understand the roles that plant defenses play in mycorrhizal formation; providing an unprecedented opportunity for research into mycorrhizal molecular biology and the interaction of symbiotic partners, and allowing the underlying mechanisms to be gradually uncovered.     

Regulatory mechanisms of host plant defense responses to arbuscular mycorrhiza

Arbuscular mycorrhizal (AM) fungi colonize the roots of over 80% of terrestrial plant species, forming mutually beneficial symbioses. During the colonization process, symbiotic partners recognize each other, and undergo observable morphological and physiological changes; indicating that symbiosis formation involves multiple factors that are finely regulated. Sometimes host plants generate a transient, weak, defense response. This response and its down-regulation play a very important role in the development of AM symbioses. Although AM fungi can infect a wide range of host root tissues, which host defense may play a crucial role is hypothesized from the fact that hyphal expansion is only observed in the root cortex.
We discuss five defense mechanisms. (1) The degradation of exogenous elicitors. The host’s weak defense response may be due to the degradation of the exogenous elicitor chitin, or the prevention of release of an endogenous inductor from the plant cell wall. (2) The inactivation of defense signal molecules. Some defense signal molecules such as hydrogen peroxidase, salicylic acid (SA), and jasmonic acid (JA), are inactivated in host plants. This helps to avoid the turn-on of defense-related genes and facilitate mycorrhizal formation. (3) The regulation of plant hormones and plant photosynthates. Plant hormone levels and plant photosynthate metabolism both change during AM colonization. These mechanisms need further exploration. (4) Changes in levels of phosphorous (P), and (iso)flavonoids. High P levels can induce some defense genes to express hydrogen peroxidase, chitinase, and glucanase. These gene products can repress colonization by AM fungi. The plant defense response regulatory effect for different (iso)flavonoids varies, and their levels are regulated by P. (5) The suppressed expression of symbiotic genes. Some symbiosis-related genes inhibit plant defense responses, but it is still unclear which mechanisms underlie gene regulation. We provide here a theoretical basis for research into AM symbiosis that may promote study of host plant resistance and the mechanisms of symbiosis formation.
We provide a deeper insight into the signal transduction pathways of mycorrhization that will aid understanding and analysis of plant defense mechanisms in the AM context. The on-going development of genome sequencing technology will contribute greatly to the detailed study of symbiosis-related genes, and pathogenesis-related protein genes. These related genes may be induced to express corresponding proteins, be repressed, postpone expression or even shutdown, or both may work together to form symbioses. Elucidation of these features will help us understand the roles that plant defenses play in mycorrhizal formation; providing an unprecedented opportunity for research into mycorrhizal molecular biology and the interaction of symbiotic partners, and allowing the underlying mechanisms to be gradually uncovered.     

The Rop GTPase: an emerging signaling switch in plants

G proteins are ubiquitous molecular switches in eukaryotic signal transduction, but their roles in plant signal transduction had not been clearly established until recent studies of the plant-specific Rop subfamily of RHO GTPases. Rop participates in signaling to an array of physiological processes including cell polarity establishment, cell growth, morphogenesis, actin dynamics, H₂O₂ generation, hormone responses, and probably many other cellular processes in plants. Evidence suggests that plants have developed unique molecular mechanisms to control this universal molecular switch through novel GTPase-activating proteins and potentially through a predominant class of plant receptor-like serine/threonine kinases. Furthermore, the mechanism by which Rop regulates specific processes may also be distinct from that for other GTPases. These advances have raised the exciting possibility that the elucidation of Rop GTPase signaling may lead to the establishment of a new paradigm for G protein-dependent signal transduction in plants.