Caracterização bioquimica e molecular da doença vassoura-de-bruxa do cacaueiro (Theobroma cacao L.), causada pelo fungo Crinipellis perniciosa / Biochemical and molecular characterization of witches broom disease of cocoa (Theobroma cacao L.), caused by the fungus Crinipellis perniciosa

AUTOR(ES)
DATA DE PUBLICAÇÃO

2006

RESUMO

Witches? broom disease, caused by the hemibiotrophic fungus Crinipellis perniciosa, is one of the most important diseases of cocoa, causing dramatic production and economic losses. A better understanding of the cacao - C. perniciosa interaction and the processes that occur in the plant during disease progression becomes necessary in order to establish novel control strategies for this disease. In view of this, the present work had two main goals: 1) establishment of the biochemical modifications that occur in the host plant during disease development; and 2) identification of differentially expressed genes in in vitro grown biotrophic mycelia of C. perniciosa, which constitutes the fungal life stage that induces disease symptoms. The results of the biochemical characterization of the disease in artificially infected cocoa seedlings indicated an initial increase of secondary metabolites related to plant defenses, such as alkaloids, phenolics, and tannins. Since these mechanisms fail to control plant colonization by the fungus, a cascade of events, probably triggered by the increased levels of ethylene, boosts the contents of asparagine, sugars, malondialdehyde (MDA) and glycerol, alters the profile of fatty acids, lowers the levels of individual amino acids and photosynthetic pigments, and ultimately leads to tissue death. The important technological development of an in vitro system that allows the maintenance of the biotrophic phase of C. perniciosa, which was recently established by our group, allowed the analysis of the differential gene expression between the two mycelial phases of the life cycle of this fungus: the biothophic and the necrotrophic phases. For this study we employed the technique of DNA microarrays, analyzing 2,304 fragments obtained from the genomic DNA libraries of the Witches? Broom Genome Project, and which were selected based on their sequence similarity to pathogenicity genes of other pathogens. The results showed suppressed expression of genes related to the metabolism and transport of hexoses and to the metabolism of lipids in the biotrophic mycelia of the fungus. On the other hand, the expression of genes of the cytochrome P450 family, hydrophobins, agglutinin, laccase, several proteases, lipases, alcohol oxidase (AOX), and pol proteins of the viral retrotransposition system were induced in the biotrophic mycelia, thus suggesting their possible participation in fungal mechanisms of pathogenicity. These results represent significant advances to our understanding of the metabolism of C. perniciosa and also regarding the molecular basis underlying the progression of witches? broom disease of cocoa, and will be valuable as a basis for future investigations aimed at disease control

ASSUNTO(S)

gene expression phytopathogenic fungi cocoa vassoura-de-bruxa (fitopatologia) expressão genica cacau fungos fitopatogenicos basidiomycetes witches broom disease basidiomicetos

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