Academic journal article Genetics

Spore Germination Requires Ferrichrome Biosynthesis and the Siderophore Transporter Str1 in Schizosaccharomyces Pombe

Academic journal article Genetics

Spore Germination Requires Ferrichrome Biosynthesis and the Siderophore Transporter Str1 in Schizosaccharomyces Pombe

Article excerpt

IN Schizosaccharomyces pombe, cells of opposite mating types respond to nitrogen starvation by forming precursor diploid cells through conjugation (Sabatinos and Forsburg 2010). These precursor diploid cells undergo a specialized mode of division known as meiosis (Marston and Amon 2004; Shigehisa et al. 2010; Blyth et al. 2018). During mei- osis, homologous chromosomes and their sister chromatids are successively segregated to generate four haploid sets of chromosomes that are embedded into four spores. After a maturation process, the spore tetrad is enclosed in an ascus sack. In S. pombe, spore tetrads are spontaneously released from the ascus after a prolonged period of time (Hatanaka and Shimoda 2001). At this stage, spores are dispersed into the environment. One hallmark of spore properties is that they exhibit high levels of resistance to harsh treatments and environmental stresses, including dehydration, chemical exposure, and extreme temperature variations. The strong resistance of S. pombe spores is primarily due to the presence of a specific protective shell called the outer spore wall (OSW), or spore coat (Garcia et al. 2006; Bonazzi et al. 2014; Fukunishi et al. 2014). The OSW acts as a strong bar- rier that protects the fungal spore content against detrimen- tal environmental conditions.

When favorable growth conditions return, spores exit from dormancy and undertake a developmental program called germination (Hatanaka and Shimoda 2001; Dworkin and Shah 2010; Bonazzi et al. 2014). This program allows quies- cent spores to go through metabolic and morphological changes that result in re-entry into the mitotic cell cycle to resume vegetative growth. Germination of S. pombe spores is a process that includes the following developmental stages. First, the end of the dormancy period is marked by the loss of spore refractility under light microscopy examination (Hatanaka and Shimoda 2001). It is also accompanied by a transient decrease in optical absorbance of the spore suspen- sion (Hatanaka and Shimoda 2001). Second, spores approx- imately double their size in an isotropic manner. Third, there is a local rupture of the OSW to form the polar cap. This hatching step is followed by emergence of a projection (germ tube) at one side of the swollen spore in a process called outgrowth (Hatanaka and Shimoda 2001; Bonazzi et al. 2014). At this stage, outgrowing spores adopt a pear-shaped form. Fourth, there is a progressive extension of the polarized tube that grows away in the opposite direction of the spore side. Fifth, segregation of the replicated chromosomal mate- rial occurs, allowing the new daughter cell to acquire its own DNA content and break up by septation from the mother spore body (Bonazzi et al. 2014).

Although the development of germinating spores requires nutrients to drive re-entry in vegetative growth, each devel- opmental stage has a number of specific nutrient require- ments. For instance, activation of dormant S. pombe spores is induced in vitro by simply adding glucose to the culture so- lution, as observed by the loss of spore refractility by phase- contrast microscopy (Shimoda 1980). However, the addition of glucose alone is insufficient to drive the subsequent de- velopmental stages such as isotropic swelling, hatching of the OSW, and outgrowth. In this context, a recent study of S. pombe spores has revealed that the essential transition metal copper is dispensable for entry into germination and isotropic swelling but it is strictly required for outgrowth and genera- tion of nascent daughter cells (Plante et al. 2017). On the basis of these observations, it is expected that iron, one of the transition metals most utilized as cofactor for the function of cellular enzymes, may also be required for the developmental process of spore germination.

Acquisition of iron in S. pombe has been primarily studied in the case of mitotically growing cells. Three strategies for iron assimilation in S. …

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