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Abstract
During fungal spore germination, a resting spore returns to a conventional mode of cell division and resumes vegetative growth, but the requirements for spore germination are incompletely understood. Here, we show that copper is essential for spore germination in Schizosaccharomyces pombe Germinating spores develop a single germ tube that emerges from the outer spore wall in a process called outgrowth. Under low-copper conditions, the copper transporters Ctr4 and Ctr5 are maximally expressed at the onset of outgrowth. In the case of Ctr6, its expression is broader, taking place before and during outgrowth. Spores lacking Ctr4, Ctr5, and the copper sensor Cuf1 exhibit complete germination arrest at outgrowth. In contrast, ctr6 deletion only partially interferes with formation of outgrowing spores. At outgrowth, Ctr4-GFP and Ctr5-Cherry first co-localize at the spore contour, followed by re-location to a middle peripheral spore region. Subsequently, they move away from the spore body to occupy the periphery of the nascent cell. After breaking of spore dormancy, Ctr6 localizes to the vacuole membranes that are enriched in the spore body relative to the germ tube. Using a copper-binding tracker, results showed that labile copper is preferentially localized to the spore body. Further analysis showed that Ctr4 and Ctr6 are required for copper-dependent activation of the superoxide dismutase 1 (SOD1) during spore germination. This activation is critical because the loss of SOD1 activity blocked spore germination at outgrowth. Taken together, these results indicate that cell-surface copper transporters and SOD1 are required for completion of the spore germination program.
Highlights
During fungal spore germination, a resting spore returns to a conventional mode of cell division and resumes vegetative growth, but the requirements for spore germination are incompletely understood
Haploid spores are more resistant to different environmental stresses than vegetative cells that proliferate in mitosis [3, 4]
The results revealed that at the end of spore germination, the plasma membrane of a new daughter cell is the preferred location of Ctr4 and Ctr5 where their presence is highly enriched as observed by fluorescence microscopy
Summary
Cells of the opposite mating types conjugate and form precursor diploid cells that can switch from mitosis to meiosis [1]. The transcript levels of ctr4ϩ and ctr5ϩ are extinguished with the concomitant disappearance of their encoded proteins At this stage, the meiosis-specific copper transporter Mfc is expressed and subsequently appears at the forespore membrane of ascospores where it serves to transport copper for accurate and timely meiotic differentiation under low-copper conditions [21]. Ctr4ϩ and ctr5ϩ genes are primarily co-expressed in response to low concentrations of copper through Cuf, with peaks of expression at the end of isotropic swelling and at the onset of outgrowth. Our findings highlight the essential requirement of copper and copper transport proteins for the developmental process of spore germination and outgrowth under lowcopper conditions
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