Abstract
Filamentous fungi consist of continuum of multinucleate cells called hyphae, and proliferate by means of hyphal tip growth. Accordingly, research interest has been focusing on hyphal tip cells, but little is known about basal cells in colony interior that do not directly contribute to proliferation. Here, we show that autophagy mediates degradation of basal cell components in the filamentous fungus Aspergillus oryzae. In basal cells, enhanced green fluorescent protein (EGFP)-labeled peroxisomes, mitochondria, and even nuclei were taken up into vacuoles in an autophagy-dependent manner. During this process, crescents of autophagosome precursors matured into ring-like autophagosomes to encircle apparently whole nuclei. The ring-like autophagosomes then disappeared, followed by dispersal of the nuclear material throughout the vacuoles, suggesting the autophagy-mediated degradation of whole nuclei. We also demonstrated that colony growth in a nutrient-depleted medium was significantly inhibited in the absence of functional autophagy. This is a first report describing autophagy-mediated degradation of whole nuclei, as well as suggesting a novel strategy of filamentous fungi to degrade components of existing hyphae for use as nutrients to support mycelial growth in order to counteract starvation.
Highlights
Autophagy is the process whereby cytoplasmic compounds and organelles are sequestered into vacuoles for degradation and recycling [1]
To test the hypothesis that cytoplasmic organelles in basal cells of a filamentous fungus are degraded through autophagy, we used several A. oryzae strains in which established markers of organelles are labeled by enhanced green fluorescent protein (EGFP) fusion proteins
These results suggested that autophagy mediates the uptake of peroxisomes, mitochondria, and nuclei in basal cells of A. oryzae to recycle their components as nutrients
Summary
Autophagy is the process whereby cytoplasmic compounds and organelles are sequestered into vacuoles for degradation and recycling [1]. In yeast autophagy is known to play a key role in degrading cytoplasmic organelles such as mitochondria [2] and peroxisomes [3], and recycling them as nutrients in order for cells to survive starvation. The outer membrane of the autophagosomes fuses with the vacuolar membrane to release the inner membrane and its contents into the vacuoles [1]. Degradation products such as amino acids are translocated to the cytoplasm via vacuolar membrane transporters to be reused [4]
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