Efficient gene replacement and direct hyphal transformation in Sclerotinia sclerotiorum.

Homologous recombination is required for gene-targeted procedures such as gene disruption and gene replacement. Ku80 is part of the non-homologous end-joining DNA repair mechanism in many organisms. We identified and disrupted the Ku80 homologue in Sclerotinia sclerotiorum and generated heterokaryon mutants enriched with Ku80-deficient nuclei (ssku80). Sclerotial formation and pathogenicity of ssku80 mutants were normal on tomato fruits. The frequencies of homologous recombination in these strains were much higher than those of the wild type when transformed with a cna1 (encoding calcineurin) replacement construct. We coupled the increase in homologous recombination with a direct BIM-LAB-mediated transformation procedure, which utilizes compressed air to assist the transforming DNA in penetrating fungal hyphae of S. sclerotiorum. We found this method to be efficient and reproducible, and it did not alter the fitness of the mutants. We also demonstrated the first case of direct transformation of sclerotia. Nourseothricin was introduced as a selectable marker in S. sclerotiorum. The tools and procedures described will improve our ability to study gene function in S. sclerotiorum and are most likely to be adaptable for use in other plant pathogens.

Type 2A phosphoprotein phosphatase is required for asexual development and pathogenesis of Sclerotinia sclerotiorum.

Sclerotinia sclerotiorum is a necrotrophic, omnivorous plant pathogen with worldwide distribution. Sclerotia of S. sclerotiorum are pigmented, multihyphal structures that play a central role in the life and infection cycles of this pathogen. Plant infection depends on the formation of melanin-rich infection cushions, and secretion of hydrolytic enzymes and oxalic acid. Type 2A Ser/Thr phosphatases (PP2As) are involved in the regulation of a variety of cellular process. In the presence of cantharidin, a PP2A-specific inhibitor, hyphal elongation and sclerotia numbers were impaired whereas sclerotial size increased. We partially inactivated PP2A by antisense expression of the gene (pph1) encoding the PP2A catalytic subunit. When antisense expression was induced, almost complete cessation of fungal growth was observed, indicative of a crucial role for PP2A in fungal growth. RNAi-based gene silencing was employed to alter the expression of the 55-kDa R2 (B regulatory subunit). Isolates in which rgb1 RNA levels were decreased were slow growing, but viable. Melanin biosynthesis, infection-cushion production, and pathogenesis were significantly impaired in the rgb1 mutants, yet theses mutants were pathogenic on wounded leaves. Reduced ERK (extracellular signal-regulated kinases)-like mitogen-activated protein kinase (MAPK) function conferred a reduction in NADPH oxidase and PP2A activity levels, suggesting a functional link between MAPK, reactive oxygen species, and PP2A activity in S. sclerotiorum.