Cell‐wall invertases,key enzymes in the modulation of plant metabolism during defence responses

Most plant–pathogen interactions do not result in pathogenesis because of pre‐formed defensive plant barriers or pathogen‐triggered activation of effective plant immune responses. The mounting of defence reactions is accompanied by a profound modulation of plant metabolism. Common metabolic changes are the repression of photosynthesis, the increase in heterotrophic metabolism and the synthesis of secondary metabolites. This enhanced metabolic activity is accompanied by the reduced export of sucrose or enhanced import of hexoses at the site of infection, which is mediated by an induced activity of cell‐wall invertase (Cw‐Inv). Cw‐Inv cleaves sucrose, the major transport sugar in plants, irreversibly yielding glucose and fructose, which can be taken up by plant cells via hexose transporters. These hexose sugars not only function in metabolism, but also act as signalling molecules. The picture of Cw‐Inv regulation in plant–pathogen interactions has recently been broadened and is discussed in this review. An interesting emerging feature is the link between Cw‐Inv and the circadian clock and new modes of Cw‐Inv regulation at the post‐translational level.     

Complex regulation of gene expression, photosynthesis and sugar levels by pathogen infection in tomato

The infection of plants with pathogens results in the induction of defence reactions as well as changes in carbohydrate metabolism. On the one hand, the pathogen attempts to manipulate the carbohydrate metabolism of the plant for its own advantage. On the other, the plant has to reorganize carbon fluxes to ensure fight against the pathogen. In order to further investigate the connection between pathogen infection and carbohydrate metabolism, the effects of two types of pathogen, biotrophic and necrotrophic, on gene expression, endogenous sugar levels and photosynthesis of tomato plants were analysed. Photosynthetic gene expression was downregulated on infection with Pseudomonas syringae and Botrytis cinerea . In contrast, expression of a sink-specific gene encoding a cell wall invertase and of defence genes was induced by both pathogens. These results provide evidence for a co-regulation of defence, sink and photosynthetic gene expression in planta in response to both types of pathogen. The brassinosteroid-containing plant restorative ComCat enhanced resistance against B. cinerea and counter-regulated the repression of photosynthetic gene expression. Endogenous sugar levels decreased and the hexose to sucrose ratio increased on treatment with B. cinerea . The application of chlorophyll fluorescence imaging revealed the spatio-temporal heterogeneity of the pathogen response. At 24 h after infection, inhibition of photosynthetic electron transport was restricted to the direct vicinity of the infection site, which was surrounded by a circle of increased photosynthetic activity. The photosynthesis of the remaining leaf was not affected at this stage. These results show the usefulness of chlorophyll fluorescence imaging for the assessment of the complex spatio-temporal changes and for the definition of the areas relevant for other types of determination, e.g. gene expression.     

植物过氧化物酶研究进展

过氧化物酶 [peroxidase,POD,EC1 .1 1 .1 .7(X) ]是广泛存在于各种动物、植物和微生物体内的一类氧化酶。催化由过氧化氢参与的各种还原剂的氧化反应 :RH2 H2 O2 →2 H2 O R。植物过氧化物酶的研究可追溯到 1 80 9年用愈创树脂为底物进行的颜色反应。但直到一个世纪之后才开展此酶的分离和命名。已知的催化反应底物超过 2 0 0种 ,以及多种过氧化物和辅助因子。迄今被研究最深入的应首推辣根过氧化物酶 (horseradish pero-xidase,HRP)。早在 1 94 0年 ,Thorell即用电泳方法从部分纯化的辣根组织中区分出 2种不同的 HRP,之后此酶…     

Nutrition acquisition strategies during fungal infection of plants

In host-pathogen interactions, efficient pathogen nutrition is a prerequisite for successful colonization and fungal fitness. Filamentous fungi have a remarkable capability to adapt and exploit the external nutrient environment. For phytopathogenic fungi, this asset has developed within the context of host physiology and metabolism. The understanding of nutrient acquisition and pathogen primary metabolism is of great importance in the development of novel disease control strategies. In this review, we discuss the current knowledge on how plant nutrient supplies are utilized by phytopathogenic fungi, and how these activities are controlled. The generation and use of auxotrophic mutants have been elemental to the determination of essential and nonessential nutrient compounds from the plant. Considerable evidence indicates that pathogen entrainment of host metabolism is a widespread phenomenon and can be accomplished by rerouting of the plant's responses. Crucial fungal signalling components for nutrient-sensing pathways as well as their developmental dependency have now been identified, and were shown to operate in a coordinate cross-talk fashion that ensures proper nutrition-related behaviour during the infection process.     

Plant–Pathogen Interactions: What Microarray Tells About It?

Plant defense responses are mediated by elementary regulatory proteins that affect expression of thousands of genes. Over the last decade, microarray technology has played a key role in deciphering the underlying networks of gene regulation in plants that lead to a wide variety of defence responses. Microarray is an important tool to quantify and profile the expression of thousands of genes simultaneously, with two main aims: (1) gene discovery and (2) global expression profiling. Several microarray technologies are currently in use; most include a glass slide platform with spotted cDNA or oligonucleotides. Till date, microarray technology has been used in the identification of regulatory genes, end-point defence genes, to understand the signal transduction processes underlying disease resistance and its intimate links to other physiological pathways. Microarray technology can be used for in-depth, simultaneous profiling of host/pathogen genes as the disease progresses from infection to resistance/susceptibility at different developmental stages of the host, which can be done in different environments, for clearer understanding of the processes involved. A thorough knowledge of plant disease resistance using successful combination of microarray and other high throughput techniques, as well as biochemical, genetic, and cell biological experiments is needed for practical application to secure and stabilize yield of many crop plants. This review starts with a brief introduction to microarray technology, followed by the basics of plant–pathogen interaction, the use of DNA microarrays over the last decade to unravel the mysteries of plant–pathogen interaction, and ends with the future prospects of this technology.     

Advances in understanding the mechanism of resistance to anthracnose and induced defence response in tea plants

The tea plant (Camellia sinensis) is susceptible to anthracnose disease that causes considerable crop loss and affects the yield and quality of tea. Multiple Colletotrichum spp. are the causative agents of this disease, which spreads quickly in warm and humid climates. During plant–pathogen interactions, resistant cultivars defend themselves against the hemibiotrophic pathogen by activating defence signalling pathways, whereas the pathogen suppresses plant defences in susceptible varieties. Various fungicides have been used to control this disease on susceptible plants, but these fungicide residues are dangerous to human health and cause fungicide resistance in pathogens. The problem-solving approaches to date are the development of resistant cultivars and ecofriendly biocontrol strategies to achieve sustainable tea cultivation and production. Understanding the infection stages of Colletotrichum, tea plant resistance mechanisms, and induced plant defence against Colletotrichum is essential to support sustainable disease management practices in the field. This review therefore summarizes the current knowledge of the identified causative agent of tea plant anthracnose, the infection strategies and pathogenicity of C. gloeosporioides, anthracnose disease resistance mechanisms, and the caffeine-induced defence response against Colletotrichum infection. The information reported in this review will advance our understanding of host–pathogen interactions and eventually help us to develop new disease control strategies.     

Plant amine oxidases “on the move”: An update

Amine oxidases (AOs) catalyse the oxidative de-amination of polyamines, ubiquitous polycationic compounds involved in important events of cell life. They include the copper-containing amine oxidases (CuAOs; EC 1.4.3.6) and the flavin-containing polyamine oxidases (PAOs; EC 1.5.3.11). The main physiological role of these moonlighting proteins has been linked to compartment-specific H₂O₂ synthesis in different phases of development and differentiation as well as in the course of defence mechanisms against pathogens and abiotic stress. Moreover, several studies evidenced a correlation of AO expression levels with physiological stages characterized by intense metabolism, such as cell division or organ formation, thus leaving open the hypothesis that AOs may have also a role in the regulation of cell cycle through the modulation of polyamine cellular content. This update will deal with recent reports on the involvement of CuAOs and PAOs in abiotic (salt) stress, wound-healing and host–pathogen interactions.     

Visualization of dynamics of plant-pathogen interaction by novel combination of chlorophyll fluorescence imaging and statistical analysis: differential effects of virulent and avirulent strains of P. syringae and of oxylipins on A. thaliana

Pathogen infection leads to defence induction as well as to changes in carbohydrate metabolism of plants. Salicylic acid and oxylipins are involved in the induction of defence, but it is not known if these signalling molecules also mediate changes in carbohydrate metabolism. In this study, the effect of application of salicylic acid and the oxylipins 12-oxo-phytodienoic acid (OPDA) and jasmonic acid on photosynthesis was investigated by kinetic chlorophyll fluorescence imaging and compared with the effects of infection by virulent and avirulent strains of Pseudomonas syringae. Both pathogen strains and OPDA caused a similar change in fluorescence parameters of leaves of Arabidopsis thaliana. The response to OPDA appeared faster compared with that to the pathogens and persisted only for a short time. Infiltration with jasmonic acid or salicylic acid did not lead to a localized and distinct fluorescence response of the plant. To capture the faint early symptoms of the plant response, a novel algorithm was applied identifying the unique fluorescence signature-the set of images that, when combined, yield the highest contrast between control and infected leaf segments. Unlike conventional fluorescence parameters, this non-biased approach indeed detected the infection as early as 6 h after inoculation with bacteria. It was posssible to identify distinct fluorescence signatures characterizing the early and late phases of the infection. Fluorescence signatures of both infection phases were found in leaves infiltrated with OPDA.     

Spatial and temporal dynamics of primary and secondary metabolism in Phaseolus vulgaris challenged by Pseudomonas syringae

Many defense mechanisms contribute to the plant immune system against pathogens, involving the regulation of different processes of the primary and secondary metabolism. At the same time, pathogens have evolved mechanisms to hijack the plant defense in order to establish the infection and proliferate. Localization and timing of the host response are essential to understand defense mechanisms and resistance to pathogens (Rico et al. 2011). Imaging techniques, such as fluorescence imaging and thermography, are a very valuable tool providing spatial and temporal information about a series of plant processes. In this study, bean plants challenged with two pathovars of Pseudomonas syringae have been investigated. Pseudomonas syringae pv. phaseolicola 1448A and P. syringae pv. tomato DC3000 elicit a compatible and incompatible interaction in bean, respectively. Both types of host–pathogen interaction triggered different changes in the activity of photosynthesis and the secondary metabolism. We conclude that the combined analysis of leaf temperature, chlorophyll fluorescence and green fluorescence emitted by phenolics allows to discriminate compatible from incompatible P. syringae–Phaseolus vulgaris interactions in very early times of the infection, prior to the development of symptoms. These can constitute disease signatures that would allow an early identification of emerging plagues in crops.     

Activation of defence reactions in Solanaceae: where is the specificity?

When a potential pathogen attempts to infect a plant, biochemical and molecular communication takes place and leads to the induction of plant defence mechanisms. In the case of efficient defence, visible symptoms are restricted and the pathogen does not multiply (incompatible interaction); when defence is inefficient, the plant becomes rapidly infected (compatible interaction). During the last 30 years, a growing body of knowledge on plant-pathogen interactions has been gathered, and a large number of studies investigate the induction of various plant defence reactions by pathogens or by pathogen-derived compounds. However, as most papers focus on incompatible interactions, there is still a lack of understanding about the similarities and differences between compatible and incompatible situations. This review targets the question of specificity in Solanaceae-pathogen interactions, by comparing defence patterns in plants challenged with virulent or avirulent pathogens (or with pathogen-associated molecular patterns from these). A special emphasis is made on analysing whether defence reactions in Solanaceae depend primarily on the type of elicitor, on the plant genotype/species, or on the type of interaction (compatible or incompatible).     

Sulfur-enhanced defence: effects of sulfur metabolism, nitrogen supply, and pathogen lifestyle

Evidence from field experiments indicates differential roles of sulfur and nitrogen supply for plant resistance against pathogens. Dissection of these observations in defined pathosystems and controlled nutritional conditions indicates an activation of plant sulfur metabolism in several incompatible and compatible interactions. Contents of cysteine and glutathione as markers of primary sulfate assimilation and stress response show increases in ARABIDOPSIS THALIANA upon infection, coinciding with the synthesis of sulfur-containing defence compounds. Similar increases of thiols were observed with necrotrophic, biotrophic, and hemibiotrophic pathogens. Sulfate supply was found to be neutral or beneficial for tolerance against fungal but neutral for bacterial pathogens under IN VITRO conditions. According to various reports and own observations the effects of nitrogen supply appeared to be neutral or harmful, depending on the pathogen. The activation of sulfur metabolism was a consequence of activation of gene expression as revealed by macroarray analysis of an A. THALIANA/ALTERNARIA BRASSICICOLA pathosystem. This activation appeared to be largely independent from sufficient or optimal sulfate supply and from the established sulfate deficiency response. The data suggest that plant-pathogen interactions and sulfur metabolism are linked by jasmonic acid as signal.     

Early events in host-pathogen interactions.

Research focused on early events in host-pathogen interactions has provided new insights into fundamental aspects of microbial pathogenicity and plant responses. Considerable progress has been made in understanding regulation of the delivery of pathogenicity determinants from bacteria into plant cells, signal cascades involved in fungal pathogenicity, the co-ordinating role of the plant cytoskeleton in plant defence and calcium flux as a primary signalling function during the hypersensitive reaction.     

Plant-microbe interactions to probe regulation of plant carbon metabolism

Plant growth and development is dependent on coordinated assimilate production, distribution and allocation. Application of biochemical and molecular techniques substantially contributed to a better understanding of these processes, although the underlying regulatory mechanisms are still not fully elucidated and attempts to improve crop yield by modulating carbon partitioning were only partially successful. Plant pathogens also interfere with source–sink interaction. To this end they have evolved a wide range of sophisticated strategies to allow their systemic spread, suppression of plant defence and induction of sink function to support nutrient acquisition for their growth. Studying compatible interactions of plants and pathogens like viruses, bacteria and fungi can be exploited to investigate different levels of source–sink regulation. The identification of microbial factors and their host targets involved in regulation of plant primary metabolism may allow developing novel strategies to increase crop yield. Here we will discuss recent studies on plant–microbe interactions aimed at elucidating mechanisms of compatibility.     

The targeting of plant cellular systems by injected type III effector proteins

The battle between phytopathogenic bacteria and their plant hosts has revealed a diverse suite of strategies and mechanisms employed by the pathogen or the host to gain the higher ground. Pathogens continually evolve tactics to acquire host resources and dampen host defences. Hosts must evolve surveillance and defence systems that are sensitive enough to rapidly respond to a diverse range of pathogens, while reducing costly and damaging inappropriate misexpression. The primary virulence mechanism employed by many bacteria is the type III secretion system, which secretes and translocates effector proteins directly into the cells of their plant hosts. Effectors have diverse enzymatic functions and can target specific components of plant systems. While these effectors should favour bacterial fitness, the host may be able to thwart infection by recognizing the activity or presence of these foreign molecules and initiating retaliatory immune measures. We review the diverse host cellular systems exploited by bacterial effectors, with particular focus on plant proteins directly targeted by effectors. Effector–host interactions reveal different stages of the battle between pathogen and host, as well as the diverse molecular strategies employed by bacterial pathogens to hijack eukaryotic cellular systems.     

Pathological hormone imbalances

Plant hormones play important roles in regulating developmental processes and signalling networks involved in plant responses to a wide range of biotic and abiotic stresses. Salicylic acid (SA), jasmonates (JA) and ethylene (ET) are well known to play crucial roles in plant disease and pest resistance. However, the roles of other hormones such as abscisic acid (ABA), auxin, gibberellin (GA), cytokinin (CK) and brassinosteroid (BL) in plant defence are less well known. Much progress has been made in understanding plant hormone signalling and plant disease resistance. However, these studies have mostly proceeded independently of each other, and there is limited knowledge regarding interactions between plant hormone-mediated signalling and responses to various pathogens. Here, we review the roles of hormones other than SA, JA and ET in plant defence and the interactions between hormone-mediated signalling, plant defence and pathogen virulence. We propose that these hormones may influence disease outcomes through their effect on SA or JA signalling.