Host-Pathogen Interactions XXV. Endopolygalacturonic Acid Lyase from Erwinia carotovora Elicits Phytoalexin Accumulation by Releasing Plant Cell Wall Fragments
TL;DR: It is demonstrated that endopolygalacturonic acid lyase elicits phytoalexin accumulation by releasing fragments from pectic polysaccharides in plant cell walls through heat-stable elicitor-active material solubilized from soybean cell walls.
Abstract: Heat-labile elicitors of phytoalexin accumulation in soybeans (Glycine max L. Merr. cv Wayne) were detected in culture filtrates of Erwinia carotovora grown on a defined medium containing citrus pectin as the sole carbon source. The heat-labile elicitors were highly purified by cation-exchange chromatography on a CM-Sephadex (C-50) column, followed by agarose-affinity chromatography on a Bio-Gel A-0.5m gel filtration column. The heat-labile elicitor activity co-purified with two α-1,4-endopolygalacturonic acid lyases (EC 4·2·2·2). Endopolygalacturonic acid lyase activity appeared to be necessary for elicitor activity because heat-inactivated enzyme preparations did not elicit phytoalexins. The purified endopolygalacturonic acid lyases elicited pterocarpan phytoalexins at microbial-inhibitory concentrations in the soybean-cotyledon bioassay when applied at a concentration of 55 nanograms per milliliter (1 × 10−9 molar). One of these lyases released heat-stable elicitors from soybean cell walls, citrus pectin, and sodium polypectate. The heat-stable elicitor-active material solubilized from soybean cell walls by the lyase was composed of at least 90% (w/v) uronosyl residues. These results demonstrate that endopolygalacturonic acid lyase elicits phytoalexin accumulation by releasing fragments from pectic polysaccharides in plant cell walls.
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TL;DR: Significant research progress is revealing mechanisms of MAMP perception, the host defense processes and specific host proteins that pathogen effectors target, the mechanisms of R protein activation, and the ways in which pathogenic effector suites and R genes evolve.
Abstract: The plant basal immune system can detect broadly present microbe-associated molecular patterns (MAMPs, also called PAMPs) and induce defenses, but adapted microbes express a suite of effector proteins that often act to suppress these defenses. Plants have evolved other receptors (R proteins) that detect these pathogen effectors and activate strong defenses. Pathogens can subsequently alter or delete their recognized effectors to avoid defense elicitation, at risk of a fitness cost associated with loss of those effectors. Significant research progress is revealing, among other things, mechanisms of MAMP perception, the host defense processes and specific host proteins that pathogen effectors target, the mechanisms of R protein activation, and the ways in which pathogen effector suites and R genes evolve. These findings carry practical ramifications for resistance durability and for future resistance engineering. The present review uses numerous questions to help clarify what we know and to identify areas that are ripe for further investigation.
749 citations
TL;DR: The differential regulation of expression of peroxidase isoenzymes following elicitor treatment suggests that individual isoenZymes of per oxidase may have specific functional roles in the biosynthesis of disease-lignin.
Abstract: Suspension cultures of castor bean (Ricinus communis L.) which have been treated with pectic fragment elicitor rapidly accumulate lignin as measured by derivatization with thioglycolic acid. The responsiveness of cultured cells to elicitor is dependent on the stage of culture growth. In 6-day (maximally responsive) cultures, increases in lignin are first evident 3 hours after addition of pectic fragment elicitor with maximal rates of lignin synthesis between 4 and 10 hours. The abundance of lignin in cultures after 12 hours of elicitor treatment is 10- to 20-fold higher than in untreated control cultures and can thereby account for as much as 2% of the dry cell weight. Only intermediate sizes of pectic oligomer are active as elicitors of lignin. Half-maximal accumulation of lignin occurs at 250 to 300 micrograms per milliliter of an optimal elicitor preparation with an average degree of polymerization of seven. We consider the synthesis of lignin in elicited cultures to be a mechanism of plant disease resistance which is induced by the elicitor. Plant peroxidases have been proposed to catalyze the last enzymatic steps in the biosynthesis of both lignin and hydrogen peroxide. Six extracellular isoenzymes of peroxidase (two anionic, designated A1 and A2, and four cationic, designated C2, C3, C4, and C7) are detectable in healthy suspension cultures of castor bean by native gel electrophoresis. Treatment of cultures with elicitor causes substantial changes in the activity of four of these species (A1, C2, C3, and C7). Elicitor treatment also results in the appearance of three new peroxidase isoenzymes that are not readily detectable in healthy cultures (C1, C5, and C6). Increases in the activities of these isoenzymes are concurrent with or slightly precede the accumulation of lignin in elicited 6-day cultures. By 12 hours after addition of elicitor, C1 becomes the most abundant extracellular isoperoxidase. The differential regulation of expression of peroxidase isoenzymes following elicitor treatment suggests that individual isoenzymes of peroxidase may have specific functional roles in the biosynthesis of disease-lignin.
482 citations
TL;DR: How modern genomic approaches, including complete genome sequencing of Eca and Ech may open the door to a new understanding of the potential subtlety and complexity of soft rot erwiniae and their interactions with plants is discussed.
Abstract: SUMMARY
The soft rot erwiniae, Erwinia carotovora ssp. atroseptica (Eca), E. carotovora ssp. carotovora (Ecc) and E. chrysanthemi (Ech) are major bacterial pathogens of potato and other crops world-wide. We currently understand much about how these bacteria attack plants and protect themselves against plant defences. However, the processes underlying the establishment of infection, differences in host range and their ability to survive when not causing disease, largely remain a mystery. This review will focus on our current knowledge of pathogenesis in these organisms and discuss how modern genomic approaches, including complete genome sequencing of Eca and Ech, may open the door to a new understanding of the potential subtlety and complexity of soft rot erwiniae and their interactions with plants.
Taxonomy: The soft rot erwiniae are members of the Enterobacteriaceae, along with other plant pathogens such as Erwinia amylovora and human pathogens such as Escherichia coli, Salmonella spp. and Yersinia spp. Although the genus name Erwinia is most often used to describe the group, an alternative genus name Pectobacterium was recently proposed for the soft rot species.
Host range: Ech mainly affects crops and other plants in tropical and subtropical regions and has a wide host range that includes potato and the important model host African violet ( Saintpaulia ionantha ). Ecc affects crops and other plants in subtropical and temperate regions and has probably the widest host range, which also includes potato. Eca , on the other hand, has a host range limited almost exclusively to potato in temperate regions only.
Disease symptoms: Soft rot erwiniae cause general tissue maceration, termed soft rot disease, through the production of plant cell wall degrading enzymes. Environmental factors such as temperature, low oxygen concentration and free water play an essential role in disease development. On potato, and possibly other plants, disease symptoms may differ, e.g. blackleg disease is associated more with Eca and Ech than with Ecc.
Useful websites: http://www.scri.sari.ac.uk/TiPP/Erwinia.htm, http://www.ahabs.wisc.edu:16080/pernalab/erwinia/index.htm, http://www.tigr.org/tdb/mdb/mdbinprogress.html, http://www.sanger.ac.uk/Projects/E_carotovora/.
379 citations
TL;DR: The present review considers the process of phytoalexin induction, at the molecular level, from the fungal elicitor to the early changes in host gene expression associated with its action.
Abstract: Summary
1. Induced resistance of plants to incompatible races of their microbial pathogens is often characterized by rapid cell death (the hypersensitive response) and the accumulation of low-Mr antimicrobial compounds termed phytoalexins. There is much indirect evidence to support a major role for phytoalexin induction as an event determining host resistance, and genetical analysis of race-specific pathogen-host interactions predicts that induced resistance occurs via interaction between a pathogen avirulence gene product and a host resistance gene product. Elicitors (inducing agents) of phytoalexin accumulation are possible candidates for the active products directly or indirectly resulting from the expression of microbial avirulence genes. The present review considers the process of phytoalexin induction, at the molecular level, from the fungal elicitor to the early changes in host gene expression associated with its action.
2. A number of microbial polysaccharides, glycoproteins, pectic enzymes, peptides and fatty acids have potent elicitor activity.
3. The results of studies on the purification and characterization of microbial elicitors depend on the nature of the bioassay used. Methods for elicitor extraction may lead to artifacts or loss of race specificity.
4. Elicitors isolated from plant pathogens may be race-specific or race-non-specific. In cases where only race-non-specific elicitors can be shown, race-specific induction of phytoalexins may result from the action of enhancer or suppressor molecules. Fungal glucans have been proposed as candidates to act in both these roles.
5. Molecular genetic approaches to the identification of phytopathogenic bacterial avirulence genes may help to prove or disprove the role of elicitors of the phytoalexin response as agents responsible for the induction of host resistance. Similar analyses, involving genetic transformation, should soon be possible for phytopathogenic fungi.
6. Studies on the nature of host receptors for microbial elicitors are still in their infancy. Such receptors are probably localized in the plant plasma membrane, and elicitation results in often striking changes in host membrane properties.
7. Cyclic 3′,5′-adenosine monophosphate and polyamines do not appear to act in plants as intracellular transducers of the phytoalexin response. Interest is now being shown in a possible role for calcium in intracellular signalling.
8. Plant cells contain endogenous elicitor molecules whose synthesis or release may play a role in the intercellular transmission of the phytoalexin response. The main candidates for endogenous elicitors are pectic fragments of the host cell wall, although no direct evidence for their involvement in plant-pathogen interactions is available. Pectic fragments may act as synergists in parallel with, rather than as couplers in series with, microbial elicitors. The role of ethylene as a response coupler for induced resistance phenomena is difficult to assess.
9. Some elicitors may themselves move to the host cell nucleus, although whether this is their effective site of action remains unclear.
10. Induction of the phytoalexin response is associated with specific changes in host gene expression related to the selective induction of new mRNA species and enzyme activities.
11. Progress is now being made in the characterization of enzymes specific for phytoalexin biosynthesis. Enzyme induction and/or infection appears to involve rapid modulation of gene transcription, although some post-translational events may also be involved in determining induction patterns.
12. Work is now commencing on the sequencing of the genes encoding elicitor-inducible enzymes. Multigene families have been identified for phenylalanine ammonia-lyase and chalcone synthase.
13. Phytoalexin accumulation may be accompanied by the rapid induction of ethylene biosynthesis and the expression of genes encoding activities involved in the synthesis of hydroxyproline-rich glycoproteins.
14. Pectic endogenous elicitors of phytoalexin accumulation are also active as inducers of systemic proteinase inhibitor synthesis in some solanaceous species.
15. Results of studies on the mechanisms of signal transmission and differential gene expression in relation to active defence reactions should be of relevance to many other aspects of the plant's response to environmental stimuli at the physiological, biochemical and molecular genetical levels.
378 citations
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TL;DR: Evidence is presented which supports the view that accumulation of phytoalexins at the site of attempted infection in plants is supported and a hypothesis is presented that suggests that all of the abiotic and some of the biotic elicitors stimulate phy toalexin accumulation by causing the release of an endogenous elicitor from the cell walls of plants.
Abstract: Some of the more important evidence is presented which supports the view that accumulation of phytoalexins at the site of attempted infection in plants. The accumulation of phytoalexins represents only one of a number of disease-resistance mechanisms in plants. Experiments are described demonstrating that the enzymes that catalyze the synthesis of phytoalexins are themselves synthesized de novo when plant cells are exposed to microbes of other effective stimuli. Evidence is also presented showing that phytoalexins, once accumulated, are catabolized or detoxified by many microorganisms as well as by the plants themselves. Elicitors are the major concern of this review. The literature is summarized in which elicitors are shown to be not only constituents of some microbes but also to be present in the cell walls of plants. A hypothesis is presented that suggests that all of the abiotic and some of the biotic elicitors stimulate phytoalexin accumulation by causing the release of an endogenous elicitor from the cell walls of plants. 108 references, 9 figures.
851 citations
TL;DR: This paper presents a meta-analysis of the literature on induced resistance and its applications to state-of-the-art materials science and discusses the role of nanofiltration in the evolution of resistance.
Abstract: INTRODUCTION . APPROACHES TO RESISTANCE STUDIES .. Genetically Different Plants . Genetically Different Pathogens. . Environmenta71y Altered Resistance . Ontogenic Changes in Resistance . Induced Resistance .. Induced Susceptibility . HISTOLOGY AND CYTOLOGY OF RESISTANCE .. Viruses . Bacteria . Fungi . Nematodes . MODIFICATION OF CELL WALLS FOR DEFENSE .. The Cuticle .
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