Interacciones virus–hongo–insecto en la enfermedad del chancro resinoso de los pinos / interactions virus–fungus– insect in pine pitch canker disease

Laburpena

The ascomycetous fungus Fusarium circinatum Nirenberg & O’Donell is the causal agent of pine pitch canker disease (PPC), one of the most devastating forest diseases worldwide. PPC has been reported in fourteen countries throughout America, Africa, Asia and Europe. So that, it is though that more than ten million hectares of pine forests and plantations are threatened by this disease in Europe. F. circinatum is mainly considered a necrotroph when it infects conifers via stem, however its ecological role as an endophyte in several herbaceous plants as well as a possible saprophyte in litter have been recently reported. The overall aim of this Thesis was to analyze the interactions that take place among members of the PPC pathosystem. For this purpose, we carried out six scientific works focused on the study of relationships between three main elements of the PPC: viruses, fungi and insects. Firstly, a review about the use of fungal viruses (mycoviruses) in biocontrol of forest diseases was elaborated. Secondly, the complete genome of F. circinatum was used as query for in silico annotating five candidate genes putatively related with fungal virulence. The proposed annotation was based in the comparison with previously characterized genes in other Fusarium species and gene ontology. Fungus-virus interaction was addressed in a third study by high throughput sequencing of RNA. In this work, we studied the anti-viral response performed by the fungus against three previously described Mitovirus spp. (FcMV1, FcMV2-1 and FcMV2-2). As a result, new data about FcMV2-2´s genome were provided in addition to new insights about the RNA silencing process in the fungus. In a fourth study, deletion mutants of F. circinatum lacking the earlier annotated gene Fcrho1 were generated. Our results revealed not lethal effects of gene deletion even though Fcrho1 was an effective regulator of vegetative growth in vitro. Otherwise, we evaluated the effect of single and co-infection by FcMV1 and FcMV2-2 in laccase enzyme production, mycelial growth and fungal pathogenicity in Pinus radiata D. Don seedlings. The results of this article showed a main cryptic effect of virus infection in the fungus although FcMV1 significantly increased in planta virulence of its host. This study also proved a rather high rate of extracellular laccase activity in F. circinatum. On a larger scale, we studied the mycobiota phoretically associated with two bark beetles (Coleoptera; Scolytinae) considered as main forest pests (i.e. Tomicus piniperda L. and Tomicus destruens Wollaston) in both infected and asymptomatic pine stands. Fungal community showed a moderate diversity and was dominated by Sydowia polyspora (Bref. & Tavel) E. Müll, a fungus considered as an endophyte, a saprophyte as well as a pathogen of conifers. This Thesis provided novel information about ecological interactions between trees, insects and filamentous fungi under an approach based on a network analysis. The results show the high complexity of interaction virus-fungus-insect in the pathosystem of the PPC and provide information on forest pathology from the point of view of virology applied to biocontrol as well as on cell biology and ecology of communities applied to the protection of forests.