[Abstract unavailable

In the fungal pathogen Fusarium oxysporum, vegetative hyphal fusion triggers nuclear mitotic division in the invading hypha followed by migration of a nucleus into the receptor hypha and degradation of the resident nucleus. Here we examined the role of autophagy in fusion-induced nuclear degradation. A search of the F. oxysporum genome database for autophagy pathway components identified putative orthologs of 16 core autophagy-related (ATG) genes in yeast, including the ubiquitin-like protein Atg8, which is required for the formation of autophagosomal membranes. F. oxysporum Foatg8D mutants were generated in a strain harboring H1-cherry fluorescent protein (ChFP)-labeled nuclei to facilitate analysis of nuclear dynamics. The Foatg8D mutants did not show MDC-positive staining in contrast to the wild type and the FoATG8-complemented (cFoATG8) strain, suggesting that FoAtg8 is required for autophagy in F. oxysporum. The Foatg8D strains displayed reduced rates of hyphal growth, conidiation, and fusion, and were significantly attenuated in virulence on tomato plants and in the nonvertebrate animal host Galleria mellonella. In contrast to wild-type hyphae, which are almost exclusively composed of uninucleated hyphal compartments, the hyphae of the Foatg8D mutants contained a significant fraction of hyphal compartments with 2 or more nuclei. The increase in the number of nuclei per hyphal compartment was particularly evident after hyphal fusion events. Timelapse microscopy analyses revealed abnormal mitotic patterns during vegetative growth in the Foatg8D mutants. Our results suggest that autophagy mediates nuclear degradation after hyphal fusion and has a general function in the control of nuclear distribution in F. oxysporum.

General autophagy is an evolutionarily conserved process in eukaryotes, by which intracellular materials are transported into and degraded inside lysosomes or vacuoles, with the main goal of recycling those materials during periods of starvation. The molecular bases of autophagy have been widely described in Saccharomyces cerevisiae, and the specific roles of Atg proteins in the process were first characterized in this model system. Important contributions have been made in Schizosaccharomyces pombe highlighting the evolutionary similarity and, at the same time, diversity of Atg components in autophagy. However, little is known regarding signals, pathways and role of autophagy in this distant yeast. Here, we undertake a global approach to investigate the signals, the pathways and the consequences of autophagy activation. We demonstrate that not only nitrogen but several nutritional deprivations including lack of carbon, sulfur, phosphorus or leucine sources, trigger autophagy, and that the TORC1, TORC2 and MAP kinase Sty1 pathways control the onset of autophagy. Furthermore, we identify an unexpected phenotype of autophagy-defective mutants, namely their inability to survive in the absence of leucine when biosynthesis of this amino acid is impaired.

No abstract

No abstract