Autophagy contributes to regulation of nuclear dynamics during vegetative growth and hyphal fusion in <i>Fusarium oxysporum</i>

<div><p>In the fungal pathogen <i>Fusarium oxysporum</i>, 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 <i>F. oxysporum</i> genome database for autophagy pathway components identified putative orthologs of 16 core autophagy-related (<i>ATG</i>) genes in yeast, including the ubiquitin-like protein Atg8, which is required for the formation of autophagosomal membranes. <i>F. oxysporum Foatg8</i>Δ mutants were generated in a strain harboring H1-cherry fluorescent protein (ChFP)-labeled nuclei to facilitate analysis of nuclear dynamics. The <i>Foatg8</i>Δ mutants did not show MDC-positive staining in contrast to the wild type and the <i>FoATG8</i>-complemented (c<i>FoATG8</i>) strain, suggesting that FoAtg8 is required for autophagy in <i>F. oxysporum</i>. The <i>Foatg8</i>Δ 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 <i>Galleria mellonella</i>. In contrast to wild-type hyphae, which are almost exclusively composed of uninucleated hyphal compartments, the hyphae of the <i>Foatg8</i>Δ 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. Time-lapse microscopy analyses revealed abnormal mitotic patterns during vegetative growth in the <i>Foatg8</i>Δ mutants. Our results suggest that autophagy mediates nuclear degradation after hyphal fusion and has a general function in the control of nuclear distribution in <i>F. oxysporum</i>.</p></div>