Many plant bacteriologists, if not all, feel that their particular microbe should appear in any list of the most important bacterial plant pathogens. However, to our knowledge, no such list exists. The aim of this review was to survey all bacterial pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate the bacterial pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 458 votes from the international community, and allowed the construction of a Top 10 bacterial plant pathogen list. The list includes, in rank order: (1) Pseudomonas syringae pathovars; (2) Ralstonia solanacearum; (3) Agrobacterium tumefaciens; (4) Xanthomonas oryzae pv. oryzae; (5) Xanthomonas campestris pathovars; (6) Xanthomonas axonopodis pathovars; (7) Erwinia amylovora; (8) Xylella fastidiosa; (9) Dickeya (dadantii and solani); (10) Pectobacterium carotovorum (and Pectobacterium atrosepticum). Bacteria garnering honourable mentions for just missing out on the Top 10 include Clavibacter michiganensis (michiganensis and sepedonicus), Pseudomonas savastanoi and Candidatus Liberibacter asiaticus. This review article presents a short section on each bacterium in the Top 10 list and its importance, with the intention of initiating discussion and debate amongst the plant bacteriology community, as well as laying down a benchmark. It will be interesting to see, in future years, how perceptions change and which bacterial pathogens enter and leave the Top 10.

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Top 10 plant pathogenic bacteria in molecular plant pathology

Xanthomonads are bacterial plant pathogens that cause diseases on many plant species, including important crops. Key to pathogenicity of most Xanthomonas pathovars is a Hrp-type III secretion (T3S) system that translocates effector proteins into plant cells. Within the eukaryotic cell, the effectors are thought to perform a variety of tasks to support bacterial virulence, proliferation, and dissemination. We are only beginning to understand the host targets of different effectors. The largest effector family found in Xanthomonas spp. is the AvrBs3/PthA or TAL (transcription activator-like) family. TAL effectors act as transcriptional activators in the plant cell nucleus. Specificity of TAL effectors is determined by a novel modular DNA-binding domain. Here, we describe the discovery of TAL effectors and their structure, activity, and host targets.

Xanthomonas AvrBs3 Family-Type III Effectors: Discovery and Function

SUMMARY Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicola cause bacterial blight and bacterial leaf streak of rice (Oryza sativa), which constrain production of this staple crop in much of Asia and parts of Africa. Tremendous progress has been made in characterizing the diseases and breeding for resistance. X. oryzae pv. oryzae causes bacterial blight by invading the vascular tissue, while X. oryzae pv. oryzicola causes bacterial leaf streak by colonizing the parenchyma. In rice there are 29 major genes for resistance to bacterial blight, but so far only a few quantitative resistance loci for bacterial leaf streak. Over 30 races of X. oryzae pv. oryzae have been reported. Both pathogens exhibit genetic variation among isolates. Mechanisms of pathogenesis and resistance have begun to be elucidated. Members of the AvrBs3/PthA family of transcription activator-like effectors play a major role in the virulence of X. oryzae pv. oryzae and possibly X. oryzae pv. oryzicola. Cloning of six rice resistance genes for bacterial blight and one from maize effective against bacterial leaf streak has uncovered a diversity of structure and function, some shared by genes involved in defence in animals. This article reviews research that spans a century. It also presents a perspective on challenges for sustainable control, and opportunities that interactions of X. oryzae pathovars with rice present as models for understanding fundamental aspects of bacterial pathogenesis of plants and plant disease resistance, as well as other aspects of plant and microbial biology, with implications also for animal innate immunity.

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Xanthomonas oryzae pathovars: model pathogens of a model crop.

New evidence suggests a role for the plant growth hormone auxin in pathogenesis and disease resistance. Bacterial infection induces the accumulation of indole-3-acetic acid (IAA), the major type of auxin, in rice (Oryza sativa). IAA induces the expression of expansins, proteins that loosen the cell wall. Loosening the cell wall is key for plant growth but may also make the plant vulnerable to biotic intruders. Here, we report that rice GH3-8, an auxin-responsive gene functioning in auxin-dependent development, activates disease resistance in a salicylic acid signaling- and jasmonic acid signaling-independent pathway. GH3-8 encodes an IAA-amino synthetase that prevents free IAA accumulation. Overexpression of GH3-8 results in enhanced disease resistance to the rice pathogen Xanthomonas oryzae pv oryzae. This resistance is independent of jasmonic acid and salicylic acid signaling. Overexpression of GH3-8 also causes abnormal plant morphology and retarded growth and development. Both enhanced resistance and abnormal development may be caused by inhibition of the expression of expansins via suppressed auxin signaling.

/pdf/activation-of-the-indole-3-acetic-acid-amido-synthetase-gh3-2bdbdv4c65.pdf

Activation of the Indole-3-Acetic Acid–Amido Synthetase GH3-8 Suppresses Expansin Expression and Promotes Salicylate- and Jasmonate-Independent Basal Immunity in Rice

Plant pathogenic bacteria of the genus Xanthomonas cause a variety of diseases in economically important monocotyledonous and dicotyledonous crop plants worldwide Successful infection and bacterial multiplication in the host tissue often depend on the virulence factors secreted including adhesins, polysaccharides, LPS and degradative enzymes One of the key pathogenicity factors is the type III secretion system, which injects effector proteins into the host cell cytosol to manipulate plant cellular processes such as basal defense to the benefit of the pathogen The coordinated expression of bacterial virulence factors is orchestrated by quorum-sensing pathways, multiple two-component systems and transcriptional regulators such as Clp, Zur, FhrR, HrpX and HpaR Furthermore, virulence gene expression is post-transcriptionally controlled by the RNA-binding protein RsmA In this review, we summarize the current knowledge on the infection strategies and regulatory networks controlling secreted virulence factors from Xanthomonas species

Regulation and secretion of Xanthomonas virulence factors

The plant-pathogenic prokaryote Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight, one of the most important diseases of rice. The bacterium is a model organism for the analysis of plant-pathogen interaction, because more than 30 races differing in virulence and 25 resistance genes in rice have been reported to date. We present here the complete genome sequence of Xoo strain MAFF 311018. The size of the genome was 4,940,217 bp, in a single circular chromosome. The genome structure of Xoo MAFF 311018 was characterized by large numbers of effector (avr) genes of the avrBs3/pth family and insertion sequences (ISs). RFLP analysis of diverse strains using ISXo1 as a probe suggests that the prevalence of mobile elements in this species, which can bring about genome inversions and rearrangement, may have played a major role in generating the high degree of genetic diversity and race differentiation characteristic of this pathogen. The Xoo MAFF 311018 sequence was also highly similar to those of X. axonopodis pv. citri and X. campestris pv. campestris with the exception of the large number of effectors and IS elements, and numerous inversions and rearrangements.

https://www.jircas.go.jp/sites/default/files/publication/jarq/39-04-08_0.pdf

Genome Sequence of Xanthomonas oryzae pv. oryzae Suggests Contribution of Large Numbers of Effector Genes and Insertion Sequences to Its Race Diversity

Aeni Sasaki

Papers

Genome Sequence of Xanthomonas oryzae pv. oryzae Suggests Contribution of Large Numbers of Effector Genes and Insertion Sequences to Its Race Diversity