Glycerol and other polyols are used as osmoprotectants by many organisms. Several yeasts and other fungi can take up glycerol by proton symport. To identify genes involved in active glycerol uptake in Saccharomyces cerevisiae we screened a deletion mutant collection comprising 321 genes encoding proteins with 6 or more predicted transmembrane domains for impaired growth on glycerol medium. Deletion of STL1, which encodes a member of the sugar transporter family, eliminates active glycerol transport. Stl1p is present in the plasma membrane in S. cerevisiae during conditions where glycerol symport is functional. Both the Stl1 protein and the active glycerol transport are subject to glucose-induced inactivation, following identical patterns. Furthermore, the Stl1 protein and the glycerol symporter activity are strongly but transiently induced when cells are subjected to osmotic shock. STL1 was heterologously expressed in Schizosaccharomyces pombe, a yeast that does not contain its own active glycerol transport system. In S. pombe, STL1 conferred the ability to take up glycerol against a concentration gradient in a proton motive force-dependent manner. We conclude that the glycerol proton symporter in S. cerevisiae is encoded by STL1.

A comparison of 42 yeast species with respect to growth in the presence of high NaCl concentration and characteristics of glycerol uptake is presented. The yeast species were classified into four classes on the basis of their ability to grow in the presence of 1, 2, 3 or 4 M NaCl. Considering that two different types of active-transport systems for glycerol uptake have been described, Na+/glycerol and H+/glycerol symports, glycerol transport was investigated by testing for proton uptake upon glycerol addition in cells incubated in the absence and in the presence of NaCl. Only strains belonging to the two higher classes of salt tolerance showed constitutive active glycerol uptake, and could accumulate glycerol internally against a concentration gradient. Five of these strains exhibited a H+/glycerol symport. All the other strains showed evidence of the activity of a salt-dependent glycerol uptake similar to that described in the literature for Debraryomyces hansenii. The strains within the two lower classes of salt tolerance showed, to varying degrees, glycerol active uptake only when glycerol was used as the carbon and energy source, suggesting that this uptake system is involved in glycerol catabolism. The results within this work suggest that active glycerol uptake provides a basis for high halotolerance, helping to maintain a favourable intracellular concentration of glycerol. The relation between the constitutive expression of such carriers and a higher level of salt-stress resistance suggests that this may be an evolutionary advantage for growth under such conditions.

Many yeast species can utilize glycerol, both as a sole carbon source and as an osmolyte. In Saccharomyces cerevisiae, physiological studies have previously shown the presence of an active uptake system driven by electrogenic proton symport. We have used transposon mutagenesis to isolate mutants affected in the transport of glycerol into the cell. Here we present the identification of YGL084c, encoding a multimembrane‐spanning protein, as being essential for proton symport of glycerol into S. cerevisiae. The gene is named GUP1 (glycerol uptake) and, for growth on glycerol, is important as a carbon and energy source. In addition, in strains deficient in glycerol production it also provides osmotic protection by the addition of glycerol. Another open reading frame (ORF), YPL189w, presenting a high degree of homology to YGL084c, similarly appears to be involved in active glycerol uptake in salt‐containing glucose‐based media in strains deficient in glycerol production. Analogously, this gene is named GUP2. To our knowledge, this is the first report on a gene product involved in active transport of glycerol in yeasts. Mutations with the same phenotypes occurred in two other ORFs of previously unknown function, YDL074c and YPL180w.

Eadie-Hofstee plots of glycerol uptake in wild-type Saccharomyces cerevisiae W303-1A grown on glucose showed the presence of both saturable transport and simple diffusion, whereas an fps1delta mutant displayed only simple diffusion. Transformation of the fps1delta mutant with the glpF gene, which encodes glycerol transport in Escherichia coli, restored biphasic transport kinetics. Yeast extract-peptone-dextrose-grown wild-type cells had a higher passive diffusion constant than the fps1delta mutant, and ethanol enhanced the rate of proton diffusion to a greater extent in the wild type than in the fps1delta mutant. In addition, the lipid fraction of the fps1delta mutant contained a lower percentage of phospholipids and a higher percentage of glycolipids than that of the wild type. Fps1p, therefore, may be involved in the regulation of lipid metabolism in S. cerevisiae, affecting membrane permeability in addition to fulfilling its specific role in glycerol transport. Simultaneous uptake of glycerol and protons occurred in both glycerol- and ethanol-grown wild-type and fps1delta cells and resulted in the accumulation of glycerol at an inside-to-outside ratio of 12:1 to 15:1. Carbonyl cyanide m-chlorophenylhydrazone prevented glycerol accumulation in both strains and abolished transport in the fps1delta mutant grown on ethanol. Likewise, 2,4-dinitrophenol inhibited transport in glycerol-grown wild-type cells. These results indicate the presence of an Fps1p-dependent facilitated diffusion system in glucose-grown cells and an Fps1p-independent proton symport system in derepressed cells.

Plant diseases caused by fungal pathogens are responsible for major crop losses worldwide, with a significant socio-economic impact on the life of millions of people who depend on agriculture-exclusive economy. This is the case of the Witches' Broom Disease (WBD) affecting cacao plant and fruit in South and Central America. The severity and extent of this disease is prospected to impact the growing global chocolate market in a few decades. WBD is caused by the basidiomycete fungus Moniliophthora perniciosa. The methods used to contain the fungus mainly rely on chemical fungicides, such as copper-based compounds or azoles. Not only are these highly ineffective, but also their utilization is increasingly restricted by the cacao industry, in part because it promotes fungal resistance, in part related to consumers' health concerns and environmental awareness. Therefore, the disease is being currently tentatively controlled through phytosanitary pruning, although the full removal of infected plant material is impossible and the fungus maintains persistent inoculum in the soil, or using an endophytic fungal parasite of Moniliophthora perniciosa which production is not sustainable. The growth of Moniliophthora perniciosa was reported as being antagonized in vitro by some yeasts, which suggests that they could be used as biological control agents, suppressing the fungus multiplication and containing its spread. Concurrently, some yeast-based products are used in the protection of fruits from postharvest fungal spoilage, and the extension of diverse food products shelf-life. These successful applications suggest that yeasts can be regarded a serious alternative also in the pre-harvest management of WBD and other fungal plant diseases. Yeasts' GRAS (Generally Recognized as Safe) nature adds to their appropriateness for field application, not raising major ecological concerns as do the present more aggressive approaches. Importantly, mitigating WBD, in a sustainable manner, would predictably have a high socioeconomic impact, contributing to diminish poverty in the cacao-producing rural communities severely affected by the disease. This review discusses the importance/advantages and the challenges that such a strategy would have for WBD containment, and presents the available information on the molecular and cellular mechanisms underlying fungi antagonism by yeasts.