Phenazines are secondary metabolites with broad spectrum antibiotic activity and thus show high potential in biological control of pathogens. In this study, we identified phenazine biosynthesis (phz) genes in two genome-completed plant pathogenic bacteria Pseudomonas syringae pv. tomato (Pst) DC3000 and Xanthomonas oryzae pv. oryzae (Xoo) PXO99^A. Unlike the phz genes in typical phenazine-producing pseudomonads, phz homologs in Pst DC3000 and Xoo PXO99^A consisted of phzC/D/E/F/G and phzC/E1/E2/F/G, respectively, and the both were not organized into an operon. Detection experiments demonstrated that phenazine-1-carboxylic acid (PCA) of Pst DC3000 accumulated to 13.4 μg L^–1, while that of Xoo PXO99^A was almost undetectable. Moreover, Pst DC3000 was resistant to 1 mg mL^–1 PCA, while Xoo PXO99^A was sensitive to 50 μg mL^–1 PCA. Furthermore, mutation of phzF blocked the PCA production and significantly reduced the pathogenicity of Pst DC3000 in tomato, while the complementary strains restored these phenotypes. These results revealed that Pst DC3000 produces low level of and is resistant to phenazines and thus is unable to be biologically controlled by phenazines. Additionally, phz-mediated PCA production is required for full pathogenicity of Pst DC3000. To our knowledge, this is the first report of PCA production and its function in pathogenicity of a plant pathogenic P. syringae strain.

Sclerotinia sclerotiorum is one of the most devastating necrotrophic phytopathogens.  Virulence of the hyphae of this fungus at different ages varies significantly.  Molecular mechanisms underlying this functional distinction are largely unknown.  In this study, we confirmed the effect of mycelial culture time/age on virulence in two host plants and elucidated its molecular and morphological basis.  The virulence of the S. sclerotiorum mycelia in plants dramatically decreases along with the increase of the mycelial age.  Three-day-old mycelia lost the virulence in plants.  Comparative proteomics analyses revealed that metabolism pathways were comprehensively reprogrammed to suppress the oxalic acid (OA) accumulation in old mycelia.  The oxaloacetate acetylhydrolase (OAH), which catalyzes OA biosynthesis, was identified in the S. sclerotiorum genome.  Both gene expression and protein accumulation of OAH in old mycelia were strongly repressed.  Moreover, in planta OA accumulation was strikingly reduced in old mycelia-inoculated plants compared with young vegetative mycelia-inoculated plants.  Furthermore, supply with 10 mmol L^–1 OA enabled the old mycelia infect the host plants, demonstrating that loss of virulence of old mycelia is mainly caused by being unable to accumulate OA.  Additionally, aerial mycelia started to develop from 0.5-day-old vegetative mycelia and dominated over 1-day-old mycelia grown on potato dextrose agar plates.  They were much smaller in hypha diameter and grew significantly slower than young vegetative mycelia when subcultured, which did not maintain to progenies.  Collectively, our results reveal that S. sclerotiorum aerial hyphae-dominant old mycelia fail to accumulate OA and thereby lose the virulence in host plants.

Nonhost resistance is a phenomenon that enables plants to protect themselves against the majority of potential pathogens, and thus has a great potential for application in plant protection. We recently found that CfHNNI1 (for Cladosporium fulvum host and nonhost plant necrosis inducer 1) is an inducer of plant hypersensitive response (HR) and nonhost resistance. In this study, its functional mechanism was analyzed. CfHNNI1 was a single copy gene in C. fulvum genome. The functional ORF of the CfHNNI1 cDNA was ATG3-TAG780, which showed homology with genes encoding bZIP transcription factors. The functional ORF included in frame an inner one ATG273-TAG780, which was sufficient to induce HR in plants. CfHNNI1 induced plant HR in a dose-dependent manner. CfHNNI1-induced necrosis in NahG transgenic tomato plants was significantly stronger than that in their wild type controls. However, the necrosis in Nr and def1 tomato mutants was similar to that in their corresponding wild type plants. These data demonstrate that induction of HR and nonhost resistance by CfHNNI1 is negatively regulated by salicylic acid signalling pathway but independent of ethylene and jasmonic acid signalling pathways.