Current concepts on the pathogenicity of phytopathogenic bacteria

What are the molecular determinants that make a bacterium a plant pathogen? In the last 10-20 years, important progress has been made in answering this question. In the early 20th century soon after the discovery of infectious diseases, the first studies of pathogenicity were undertaken. These early studies relied mostly on biochemistry and led to the discovery of several major pathogenicity determinants, such as toxins and hydrolytic enzymes which govern the production of major disease symptoms. From these pioneering studies, a simplistic view of pathogenicity arose. It was thought that only a few functions were sufficient to transform a bacterium into a pathogen. This view rapidly changed when modern techniques of molecular genetics were applied to analyse pathogenicity. Modern analyses of pathogenicity determinants took advantage of the relatively simple organization of the haploid genome of pathogenic bacteria. By creating non-pathogenic mutants, a large number of genes governing bacterium-host interactions were identified. These genes are required either for host colonization or for the production of symptoms. Even though the role of motility and chemotaxis in these processes is still unclear, it is clear that a strong attachment of Agrobacterium to plant cells is a prerequisite for efficient plant transformation and disease. Other important pathogenicity factors identified with a molecular genetic approach include hydrolytic enzymes such as pectinases and cellulases which not only provide nutrients to the bacteria but also facilitate pathogen invasion into host tissues. The precise role of exopolysaccharide in pathogenicity is still under discussion, however it is has been established that it is crucial for the induction of wilt symptoms caused by Ralstonia solanacearum. Trafficking of effector proteins from the invading bacterium into the host cell emerged recently as a new central concept. In plant pathogenic bacteria, protein translocation takes place through the so-called 'type II secretion machinery' encoded by hrp genes in the bacterium. These genes are present in representatives of all the major groups of Gram negative plant pathogenic bacteria except Agrobacterium. Most of these genes have counterparts in pathogens of mammals (including those of human) and they also play a central role in pathogenicity. Additionally, recent evidence suggests that a 'type IV secretion machinery' injects bacterial proteins into host cells. This machinery, originally found to be involved in the transfer of t-DNA from Agrobacterium into plant cells, was recently shown to translocate pathogenicity proteins in pathogens of mammals such as Helicobacter pylori and Brucella. Discovery of the trafficking of proteins from the pathogen into host cells revolutionized our conception of pathogenicity. First, it rather unexpectedly established the conservation of basic pathogenicity strategies in plant and animal pathogens. Second, this discovery changes our ideas about the overall strategy (or mechanism) of pathogenicity, although we still think the end result is exploitation of host cell nutritive components. Rather than killing the host cell from outside, we envision a more subtle approach in which pathogens inject effector proteins into the host cell to effect a change in host cell biology advantageous to the pathogen. Identification of the effector proteins, of their function and of the corresponding molecular targets in the host is a new challenge which will contribute to the conception of new strategies to control diseases.     

The evolution of virulence and pathogenicity in plant pathogen populations

The term virulence has a conflicting history among plant pathologists. Here we define virulence as the degree of damage caused to a host by parasite infection, assumed to be negatively correlated with host fitness, and pathogenicity the qualitative capacity of a parasite to infect and cause disease on a host. Selection may act on both virulence and pathogenicity, and their change in parasite populations can drive parasite evolution and host-parasite co-evolution. Extensive theoretical analyses of the factors that shape the evolution of pathogenicity and virulence have been reported in last three decades. Experimental work has not followed the path of theoretical analyses. Plant pathologists have shown greater interest in pathogenicity than in virulence, and our understanding of the molecular basis of pathogenicity has increased enormously. However, little is known regarding the molecular basis of virulence. It has been proposed that the mechanisms of recognition of parasites by hosts will have consequences for the evolution of pathogenicity, but much experimental work is still needed to test these hypotheses. Much theoretical work has been based on evidence from cellular plant pathogens. We review here the current experimental and observational evidence on which to test theoretical hypotheses or conjectures. We compare evidence from viruses and cellular pathogens, mostly fungi and oomycetes, which differ widely in genomic complexity and in parasitism. Data on the evolution of pathogenicity and virulence from viruses and fungi show important differences, and their comparison is necessary to establish the generality of hypotheses on pathogenicity and virulence evolution.     

Rapid genetic diversification and high fitness penalties associated with pathogenicity evolution in a plant virus

Under the gene-for-gene model of host-pathogen coevolution, recognition of pathogen avirulence factors by host resistance factors triggers host defenses and limits infection. Theory predicts that the evolution of higher levels of pathogenicity will be associated with fitness penalties and that the cost of higher pathogenicity must be much smaller than that of not infecting the host. The analysis of pathogenicity costs is of academic and applied relevance, as these are determinants for the success of resistance genes bred into crops for disease control. However, most previous attempts of addressing this issue in plant pathogens yielded conflicting and inconclusive results. We have analyzed the costs of pathogenicity in pepper-infecting tobamoviruses defined by their ability to infect pepper plants with different alleles at the resistance locus L. We provide conclusive evidence of pathogenicity-associated costs by comparison of pathotype frequency with the fraction of the crop carrying the various resistance alleles, by timescaled phylogenies, and by temporal analyses of population dynamics and selection pressures using nucleotide sequences. In addition, experimental estimates of relative fitness under controlled conditions also provided evidence of high pathogenicity costs. These high pathogenicity costs may reflect intrinsic properties of plant virus genomes and should be considered in future models of host-parasite coevolution.     

From bacterial avirulence genes to effector functions via the hrp delivery system: an overview of 25 years of progress in our understanding of plant innate immunity

Cloning the first avirulence ( avr ) gene has led not only to a deeper understanding of gene-for-gene interactions in plant disease, but also to fundamental insights into the suppression of basal defences against microbial attack. This article (focusing on Pseudomonas syringae ) charts the development of ideas and research progress over the 25 years following the breakthrough achieved by Staskawicz and coworkers. Advances in gene cloning technology underpinned the identification of both avr and hrp genes, the latter being required for the activation of the defensive hypersensitive reaction (HR) and pathogenicity. The delivery of Avr proteins through the type III secretion machinery encoded by hrp gene clusters was demonstrated, and the activity of the proteins inside plant cells as elicitors of the HR was confirmed. Key roles for avr genes in pathogenic fitness have now been established. The rebranding of Avr proteins as effectors, proteins that suppress the HR and cell wall-based defences, has led to the ongoing search for their targets, and is generating new insights into the co-ordination of plant resistance against diverse microbes. Bioinformatics-led analysis of effector gene distribution in genomes has provided a remarkable view of the interchange of effectors and also their functional domains, as the arms race of attack and defence drives the evolution of microbial pathogenicity. The application of our accrued knowledge for the development of disease control strategies is considered.     

A single gene encodes a selective toxin causal to the development of tan spot of wheat.

The identification and characterization of pathogenicity factors are essential to an understanding of the molecular events that regulate the interaction of plant-pathogenic microbes with their hosts. We have isolated the gene that encodes a host-selective toxic protein produced by the fungus Pyrenophora tritici-repentis and confirmed that this gene functions in the plant as the primary determinant of pathogenicity in the Pyrenophora-wheat interaction. These results demonstrate that a single gene encodes the production of a host-selective toxin and that transformation of this gene into a non-toxin-producing isolate of P. tritici-repentis leads to both toxin production and pathogenicity.     

Agrobacterium rhizogenes GALLS protein substitutes for Agrobacterium tumefaciens single-stranded DNA-binding protein VirE2

Agrobacterium tumefaciens and Agrobacterium rhizogenes transfer plasmid-encoded genes and virulence (Vir) proteins into plant cells. The transferred DNA (T-DNA) is stably inherited and expressed in plant cells, causing crown gall or hairy root disease. DNA transfer from A. tumefaciens into plant cells resembles plasmid conjugation; single-stranded DNA (ssDNA) is exported from the bacteria via a type IV secretion system comprised of VirB1 through VirB11 and VirD4. Bacteria also secrete certain Vir proteins into plant cells via this pore. One of these, VirE2, is an ssDNA-binding protein crucial for efficient T-DNA transfer and integration. VirE2 binds incoming ssT-DNA and helps target it into the nucleus. Some strains of A. rhizogenes lack VirE2, but they still transfer T-DNA efficiently. We isolated a novel gene from A. rhizogenes that restored pathogenicity to virE2 mutant A. tumefaciens. The GALLS gene was essential for pathogenicity of A. rhizogenes. Unlike VirE2, GALLS contains a nucleoside triphosphate binding motif similar to one in TraA, a strand transferase conjugation protein. Despite their lack of similarity, GALLS substituted for VirE2.     

Gene-for-gene interactions: bacterial avirulence proteins specify plant disease resistance

Resistance of plants to bacterial pathogens is often controlled by corresponding genes for resistance and avirulence in host and pathogen, respectively. Fifty years after discovery of the genetic basis of gene-for-gene interactions, several avirulence and plant resistance genes have been isolated and are being studied on the molecular level. Tremendous progress has been made due to a better understanding of type III secretion systems that are required for bacterial pathogenicity. We are beginning to grasp how the plant actually recognizes bacterial avirulence determinants. The current view is that the bacterium translocates avirulence proteins into the host cell by the Hrp type III secretion system and that recognition occurs in the plant cell.     

The invasion-associated type-III protein secretion system in Salmonella – a review

The genetic determinants that confer upon Salmonella the ability to enter non-phagocytic cells are largely encoded in a pathogenicity island located at centisome 63 of the bacterial chromosome. Molecular genetic analysis has revealed that this region encodes a specialized protein secretion system that mediates the export and/or translocation of putative signaling proteins into the host cell. This protein secretion system, which has been termed type III or contact-dependent, has also been identified in other plant and animal pathogens that have, in common, the ability to interact with eukaryotic host cells in an intimate manner.     

pH dependency of sclerotial development and pathogenicity revealed by using genetically defined oxalate‐minus mutants of Sclerotinia sclerotiorum

The devastating plant pathogen Sclerotinia sclerotiorum produces copious (up to 50 mM) amounts of oxalic acid, which, for over a quarter century, has been claimed as the pathogenicity determinant based on UV‐induced mutants that concomitantly lost oxalate production and pathogenicity. Such a claim was made without fulfilling the molecular Koch's postulates because the UV mutants are genetically undefined and harbour a developmental defect in sclerotial production. Here, we generated oxalate‐minus mutants of S. sclerotiorum using two independent mutagenesis techniques, and tested the resulting mutants for growth at different pHs and for pathogenicity on four host plants. The oxalate‐minus mutants accumulated fumaric acid, produced functional sclerotia and have reduced ability to acidify the environment. The oxalate‐minus mutants retained pathogenicity on plants, but their virulence varied depending on the pH and buffering capacity of host tissue. Acidifying the host tissue enhanced virulence of the oxalate‐minus mutants, whereas supplementing with oxalate did not. These results suggest that it is low pH, not oxalic acid itself, that establishes the optimum conditions for growth, reproduction, pathogenicity and virulence expression of S. sclerotiorum. Exonerating oxalic acid as the primary pathogenicity determinant will stimulate research into identifying additional candidates as pathogenicity factors towards better understanding and managing Sclerotinia diseases.     

Phospholipase C: Diverse functions in plant biotic stress resistance and fungal pathogenicity

Phospholipase C (PLC) generates various second messenger molecules and mediates phospholipid hydrolysis. In recent years, the important roles of plant and fungal PLC in disease resistance and pathogenicity, respectively, have been determined. However, the roles of PLC in plants and fungi are unintegrated and relevant literature is disorganized. This makes it difficult for researchers to implement PLC-based strategies to improve disease resistance in plants. In this comprehensive review, we summarize the structure, classification, and phylogeny of the PLCs involved in plant biotic stress resistance and fungal pathogenicity. PLCs can be divided into two groups, nonspecific PLC (NPC) and phosphatidylinositol-specific PLC (PI-PLC), which present marked differences in phylogenetic evolution. The products of PLC genes in fungi play significant roles in physiological activity and pathogenesis, whereas those encoded by plant PLC genes mediate the immune response to fungi. This review provides a perspective for the future control of plant fungal diseases.     

Effector proteins of phytopathogenic bacteria: bifunctional signals in virulence and host recognition

Phytopathogenic bacteria deliver effectors of disease into plant hosts via a Type III secretion system. These Type III effectors have genetically determined roles in virulence. They also are among the components recognized by the putative receptors of the plant innate immune system. Recent breakthroughs include localization of some of these Type III effectors to specific host cell compartments, and the first dissection of pathogenicity islands that carry them.     

Do plant and human pathogens have a common pathogenicity strategy?

Recently, a novel 'two-step' model of pathogenicity has been described that suggests host-cell-derived vasculoproliferative factors play a crucial role in the pathogenesis of bacillary angiomatosis, a disease caused by the human pathogenic bacterium Bartonella henselae. The resulting proliferation of endothelial cells could be interpreted as bacterial pathogens triggering the promotion of their own habitat: the host cell. Similar disease mechanisms are well known in the plant pathogen Agrobacterium tumefaciens, which causes crown gall disease. There are notable similarities between the pathogenicity of A. tumefaciens leading to tumourous disease in plants and to the B. henselae-triggered proliferation of endothelial cells in humans. Here, we hypothesize that this pathogenicity strategy might be common to several bacterial species in different hosts owing to shared pathogenicity factors.     

Genetic background affects pathogenicity island function and pathogen emergence in Streptomyces

With few exceptions, thaxtomin A (ThxA), a nitrated diketopiperazine, is the pathogenicity determinant for plant‐pathogenic Streptomyces species. In Streptomyces scabiei (syn. S. scabies), the ThxA biosynthetic cluster is located within a 177‐kb mobile pathogenicity island (PAI), called the toxicogenic region (TR). In S. turgidiscabies, the ThxA biosynthetic cluster is located within a 674‐kb pathogenicity island (PAIst). The emergence of new plant pathogens occurs in this genus, but not frequently. This raises the question of whether the mobilization of these pathogenicity regions, through mating, is widespread and whether TR and PAIst can confer plant pathogenicity. We showed that ThxA biosynthetic clusters on TR and PAIst were transferred into strains from five non‐pathogenic Streptomyces species through mating with S. scabiei and S. turgidiscabies. However, not all of the transconjugants produced ThxA and exhibited the virulence phenotype, indicating that the genetic background of the recipient strains affects the functionality of the ThxA biosynthetic cluster and therefore would be expected to affect the emergence of novel pathogenic Streptomyces species. Thxs have been patented as natural herbicides, but have yet to be commercialized. Our results also demonstrated the potential of the heterologous production of ThxA as a natural and biodegradable herbicide in non‐pathogenic Streptomyces species.     

Mnh6, a nonhistone protein, is required for fungal development and pathogenicity of Magnaporthe grisea

Mnh6, a nonhistone protein containing an HMG1 box, was isolated from the rice blast fungus, Magnaporthe grisea. In the current study, we utilized an MNH6-deletion mutant to investigate the role of Mnh6 in the disease cycle of M. grisea. The Deltamnh6 mutant exhibited pleiotropic effects on fungal morphogenesis, including reduction in mycelial growth, conidiation, appressorium development, plant penetration, and infectious growth in host cells. Furthermore, Deltamnh6 mutant had greatly reduced pathogenicity on barley and rice compared to the wild-type. The reintroduction of an intact copy of MNH6 into the Deltamnh6 mutant restored morphological features and pathogenicity, suggesting that Mnh6 is required for fungal development, effective pathogenicity, and completion of the disease cycle of M. grisea.     

青枯劳尔氏菌多基因家族III型效应蛋白在植物病害发展及防御系统中的作用

青枯劳尔氏菌是导致多种重要经济作物毁灭性枯萎（bacterial wilt）的一种土传病害,严重危害热带和亚热带地区食品安全。该细菌通过III型分泌系统（T3SS）向寄主细胞注射大量效应蛋白（T3Es）。效应蛋白是把双刃剑,既可诱导植物感病,又能激活植物防御系统。具有特殊重复结构的效应蛋白被归类成多基因家族,各家族成员协同致病,但其分子机制尚不清楚。本文围绕近年来有关多基因家族效应蛋白结构、功能和致病性等方面最新进展进行综述,为青枯菌致病机理和病害防治提供新思路。     

Secondary plant metabolites in phytoremediation and biotransformation

For millennia, secondary plant metabolites have antagonized microorganisms, insects and humans alike, ultimately generating a complex and dynamic mixture of facultative and obligate interactions from symbioses to pathogenicity. Secondary plant metabolites have an important role in developing the myriad of organic pollutant-degrading enzymes found in nature. The link between secondary plant metabolites and enzymatic diversity has yet to be exploited, with potential applications in fields as varied as pest management, bioremediation and fine chemical production.