Transfer of T-DNA and Vir proteins to plant cells by Agrobacterium tumefaciens induces expression of host genes involved in mediating transformation and suppresses host defense gene expression

Agrobacterium tumefaciens is a plant pathogen that incites crown gall tumors by transferring to and expressing a portion of a resident plasmid in plant cells. Currently, little is known about the host response to Agrobacterium infection. Using suppressive subtractive hybridization and DNA macroarrays, we identified numerous plant genes that are differentially expressed during early stages of Agrobacterium-mediated transformation. Expression profiling indicates that Agrobacterium infection induces plant genes necessary for the transformation process while simultaneously repressing host defense response genes, thus indicating successful utilization of existing host cellular machinery for genetic transformation purposes. A comparison of plant responses to different strains of Agrobacterium indicates that transfer of both T-DNA and Vir proteins modulates the expression of host genes during the transformation process.     

Plant innate immunity: An updated insight into defense mechanism

Plants are invaded by an array of pathogens of which only a few succeed in causing disease. The attack by others is countered by a sophisticated immune system possessed by the plants. The plant immune system is broadly divided into two, viz. microbial-associated molecular-patterns-triggered immunity (MTI) and effector-triggered immunity (ETI). MTI confers basal resistance, while ETI confers durable resistance, often resulting in hypersensitive response. Plants also possess systemic acquired resistance (SAR), which provides long-term defense against a broad-spectrum of pathogens. Salicylic-acid-mediated systemic acquired immunity provokes the defense response throughout the plant system during pathogen infection at a particular site. Trans-generational immune priming allows the plant to heritably shield their progeny towards pathogens previously encountered. Plants circumvent the viral infection through RNA interference phenomena by utilizing small RNAs. This review summarizes the molecular mechanisms of plant immune system, and the latest breakthroughs reported in plant defense. We discuss the plant–pathogen interactions and integrated defense responses in the context of presenting an integral understanding in plant molecular immunity.     

Metabolic and miRNA profiling of TMV infected plants reveals biphasic temporal changes

Plant viral infections induce changes including gene expression and metabolic components. Identification of metabolites and microRNAs (miRNAs) differing in abundance along infection may provide a broad view of the pathways involved in signaling and defense that orchestrate and execute the response in plant-pathogen interactions. We used a systemic approach by applying both liquid and gas chromatography coupled to mass spectrometry to determine the relative level of metabolites across the viral infection, together with a miRs profiling using a micro-array based procedure. Systemic changes in metabolites were characterized by a biphasic response after infection. The first phase, detected at one dpi, evidenced the action of a systemic signal since no virus was detected systemically. Several of the metabolites increased at this stage were hormone-related. miRs profiling after infection also revealed a biphasic alteration, showing miRs alteration at 5 dpi where no virus was detected systemically and a late phase correlating with virus accumulation. Correlation analyses revealed a massive increase in the density of correlation networks after infection indicating a complex reprogramming of the regulatory pathways, either in response to the plant defense mechanism or to the virus infection itself. Our data propose the involvement of a systemic signaling on early miRs alteration.     

反式-2-己烯醛在植物防御反应中的作用

反式-2-己烯醛是绿色植物释放的一种小分子挥发性物质,在调节植物生长发育和抵抗各种环境胁迫中发挥重要作用。已有研究表明,反式-2-己烯醛可抑制植物根系生长,具有较高的抑菌和抗虫活性,也可以作为植物间的“信使”来传递防御信号。该文系统综述了反式-2-己烯醛的生物合成、代谢途径及其在生物胁迫防御反应中的重要作用,提出了研究...     

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.     

Non-invasive microelectrode potassium flux measurements as a potential tool for early recognition of virus–host compatibility in plants

Diseases caused by plant viruses are widespread, resulting in severe economic losses worldwide. Understanding the cellular basis of defense responses and developing efficient diagnostic tools for early recognition of host specificity to viral infection is, therefore, of great importance. In this work, non-invasive ion selective microelectrodes (the MIFE technique) were used to measure net ion fluxes in mesophyll tissue of host (potato, tomato, tobacco) and non-host (sugar beet and periwinkle) plants in response to infection with Potato virus X (PVX). These results were complemented by FLIM (Fluorescence Lifetime Imaging) measurements of PVX-induced changes in intracellular Ca²⁺ concentrations. Our results demonstrate that, unlike in other plant–pathogen interactions, Ca²⁺ signaling appears to be non-essential in recognition of the early stages of viral infection. Instead, we observed significant changes in K⁺ fluxes as early as 10 min after inoculation. Results of pharmacological experiments and membrane potential measurements pointed out that a significant part of these fluxes may be mediated by depolarization-activated outward-rectifying K⁺ channels. This may suggest that viral infections trigger a different mechanism of plant defense signaling as compared to signals derived from other microbial pathogens; hence, altered Ca²⁺ fluxes across the plasma membrane may not be a prerequisite for all elicitor-activated defense reactions. Clearly pronounced host specificity in K⁺ flux responses suggests that the MIFE technique can be effectively used as a screening tool for the early diagnostics of virus–host compatibility.     

Antagonistic plant defense system regulated by phytohormones assists interactions among vector insect, thrips and a tospovirus

The western flower thrips (Frankliniella occidentalis) is a polyphagous herbivore that causes serious damage to many agricultural plants. In addition to causing feeding damage, it is also a vector insect that transmits tospoviruses such as Tomato spotted wilt virus (TSWV). We previously reported that thrips feeding on plants induces a jasmonate (JA)-regulated plant defense, which negatively affects both the performance and preference (i.e. host plant attractiveness) of the thrips. The antagonistic interaction between a JA-regulated plant defense and a salicylic acid (SA)-regulated plant defense is well known. Here we report that TSWV infection allows thrips to feed heavily and multiply on Arabidopsis plants. TSWV infection elevated SA contents and induced SA-regulated gene expression in the plants. On the other hand, TSWV infection decreased the level of JA-regulated gene expression induced by thrips feeding. Importantly, we also demonstrated that thrips significantly preferred TSWV-infected plants to uninfected plants. In JA-insensitive coi1-1 mutants, however, thrips did not show a preference for TSWV-infected plants. In addition, SA application to wild-type plants increased their attractiveness to thrips. Our results suggest the following mechanism: TSWV infection suppresses the anti-herbivore response in plants and attracts its vector, thrips, to virus-infected plants by exploiting the antagonistic SA-JA plant defense systems.     

Polygalacturonases, polygalacturonase-inhibiting proteins and pectic oligomers in plant-pathogen interactions

Polygalacturonases (PGs) are produced by fungal pathogens during early plant infection and are believed to be important pathogenicity factors. Polygalacturonase-inhibiting proteins (PGIPs) are plant defense proteins which reduce the hydrolytic activity of endoPGs and favor the accumulation of long-chain oligogalacturonides (OGs) which are elicitors of a variety of defense responses. PGIPs belong to the superfamily of leucine reach repeat (LRR) proteins which also include the products of several plant resistance genes. A number of evidence demonstrates that PGIPs efficiently inhibit fungal invasion.     

Making sense of hormone crosstalk during plant immune responses

In response to biotic stress, crosstalk between plant hormonal signaling pathways prioritizes defense over other cellular functions. Some plant pathogens take advantage of this regulatory system by mimicking hormones that interfere with host immune responses to promote virulence. Here we discuss the various roles that crosstalk may play in response to pathogens with different infection strategies.     

Attacking the defenders: plant viruses fight back

Plants use RNA silencing mechanisms and produce short-interfering RNA (siRNA) molecules in a defense response against viral infection. To counter this defense response, viruses produce suppressor proteins, which can block the host silencing pathway or interfere with its function in plant cells. The targets for many viral suppressors and the mechanisms by which they function in plant cells are still largely unknown. Recent reports describe that the 2b suppressor of the Cucumber mosaic virus binds ARGONAUTE and that the P0 suppressor of Polerovirus targets ARGONAUTE to degradation. Another report has revealed that the V2 suppressor of tomato yellow mosaic virus binds the coiled-coil protein suppressor of the gene-silencing SGS3 homolog. These reports provide novel insight into the mechanisms developed by viruses to disable the defense system of the plant.     

Biochemical characterization of compatible plant-viral interaction: A case study with a begomovirus-kenaf host-pathosystem

Yellow vein mosaic disease of mesta, a compatible plant virus interaction, poses a serious threat to mesta cultivation in India. Plants respond to invasion by pathogens with multi-component defense responses particularly in incompatible interaction. With the aim of understanding, a biochemical approach was attempted to study the cellular redox status in early stages of yellow vein mosaic virus infection associated with different ages plants of Hibiscus cannabinus. Comparative analysis of GSH and GSSG content in infected and control plants of different ages indicated that infected plants are under oxidative or nitrosative stress condition. A significant change was observed in Glutathione Reductase, Catalase and Ascorbate Peroxidase level in early stage of infection. We also showed microscopic evidence of nitrosylated thiols in infected leaves, stems and roots of H. cannabinus. Furthermore, we identified few defense related proteins in infected plant using MALDI TOF mass spectrometric analysis.Key words: mesta yellow vein mosaic virus, cellular redox status, redox active enzymes, nitrosylated thiol, defense proteins