Saccharomyces cerevisiae

Folia Microbiol. 52 (1), 35–38 (2007)

http://www.biomed.cas.cz/mbu/folia/

Colony Density Influences Invasive and Filamentous Growth in Saccharomyces cerevisiae A. LUCACCIONIa, G. MORPURGOa, A. ACHILLIb, C. BARBERIOc , E. CASALONEc, N. BABUDRIa * aDipartimento di Biologia Cellulare e Ambientale, Università di Perugia, 06100 Perugia, Italy

e-mail [email protected] bDipartimento di Genetica e Microbiologia, Università di Pavia, 27100 Pavia, Italy cDipartimento di Biologia Animale e Genetica, Università di Firenze, 50125 Florence, Italy

Received 3 May 2006 Revised version 18 September 2006

ABSTRACT. The effect of colony density on the dimorphic switch was determined in natural strains of Saccharomyces cerevisiae. In some strains invasiveness and pseudohyphal (PH) growth were highly sensitive to colony density; moreover, strains constitutively able to invade the substrate with PH formation positively influenced the invasiveness but not the PH growth of a different strain less prone to the dimorphic switch.

S. cerevisiae is a dimorphic yeast (Mösch 2002); in certain growth conditions, it may switch from spherical cells to filaments of adherent, elongated cells that form pseudohyphae (PH) and may invade the substrate. Solid medium invasion is facilitated by PH growth but it may occur even without PH. Critical depletion of nutrients, such as nitrogen starvation, was shown by Gimeno et al. (1992) to induce PH growth in diploids which is considered as an important adaptive response that allows a starving fungal colony to forage for nutrients (Lee and Elion 1999; Mösch 2002; Palecek et al. 2002; Braus et al. 2003). Since its discovery, the phenomenon has been extensively characterized; other environmental signals such as poor carbon sources and short chain alcohols were shown to induce PH and invasive growth (Dickinson 1996; Cullen and Sprague 2000; Lorenz et al. 2000). Much of the signal transduction mechanism has been uncovered, although many challenges to complete understanding of yeast dimorphism still remain. Indeed, signal-transduction pathways are complex biological systems which allow cells to physiologically adapt to changing environments. The two best characterized pathways regulating filament formation are the cAMP-dependent protein kinase and the STE MAPK cascade. These pathways are activated by external signals and are implicated in the regulation of the FLO11 gene which codes for a cell-surface flocculin that promotes agar invasion and cell adhesion (Gagiano et al. 2002; Palecek et al. 2002; Trachtulcová et al. 2003). The response to nitrogen limitation has been studied in yeast natural populations and also in laboratory strains by lowering the diammonium sulfate concentration or by adding an analogue of histidine to the medium; a wide distribution of dimorphism in yeast populations and a great variability with respect to the dimorphic switch ability has been shown (Casalone et al. 2005). In most studies, the authors did not plate single cells from dilute suspensions; instead they observed the response with heavy streaks. However, Wright et al. (1993) showed in the haploid strain D273-10B MATα that the formation of PH on colonies derived from single-plated cells could be observed only when a maximum of 500 cells were plated in a Petri dish of 150 mm diameter. At higher cell concentrations PH formation in this strain was completely inhibited. Therefore, we decided to study the relevance of the colony density–cell concentration to PH growth and to invasive phenotype in natural strains of S. cerevisiae.

MATERIALS AND METHODS The strains used are listed in Table I. All strains are diploid non-fresh isolates of yeast collections. SGU162, SGU339, SGU398 and SGU413 are from the yeast collection of the Department of Animal Biology and Genetics of the University of Florence (Italy) and were isolated from grape samples (Casalone et al. 2005). DBVPG1892 and DBVPG1893 are from the Industrial Yeasts Collection of the University of Perugia (DBVPG), Italy. Ps93 and Ps168 are mutants obtained from UV-irradiated spores of the strain 1014 (DBVPG *Corresponding author.

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industrial strain collection); they invade SD, SLAD (see below) and YPD with PH formation, a “constitutive” phenotype which is due to recessive monogenic mutations in unlinked genes, whose products have not yet been discovered (Barberio et al. 2006). The T2-3D strain (Wenzel et al. 1992) did not invade substrate and did not form PH in any medium (unpublished results) and was not able to undergo dimorphic switch in poor nitrogen media. Strains Ps93 and Ps168 were used as positive control of invasiveness and PH growth, and T2-3D as negative control. SGU and DBVPG series strains have been chosen at random from the yeast collections. Media. The complete medium YPD and the minimal medium SD was prepared according to Sherman (1991); the minimal medium with 1/1000 of diammonium sulfate with respect to SD (SLAD) was described by Gimeno et al. (1992). Invasive growth assay and PH observation. Cells (102–106) from fresh overnight cultures in YPD were plated on YPD, SD and SLAD; heavy streaks were also done. After 5 d at 28 °C, the cells were washed off under a stream of water by rubbing with a bent glass rod. The colonies entered into the agar were counted under a stereomicroscope. The presence of PH was observed in an optical microscope at 400× magnification. In the single-cell assay a strain was considered positive for invasiveness of the substrate when most colonies entered the agar (“+” in Table I); when most colonies penetrated with PH growth it was considered positive for both phenotypes (“+PH”). A reliable count was possible only at 100 colonies per plate, while it was less accurate but still feasible at 1000 cells per plate. For higher concentrations, we considered a strain to be positive for penetration (with or without PH growth) when we observed a background of cells penetrating the substrate, negative when the number of penetrating colonies was negligible.

RESULTS AND DISCUSSION A complex picture comes out from the data on SD and SLAD reported in Table I. The strains SGU339 and SGU398, as heavy streaks, did not invade SD but they invaded SLAD with PH, which showed that they adapted to the low nitrogen concentration by penetration as well as by PH growth. Instead, they penetrated both SD and SLAD equally well when plated as single cells but they did not form PH on SD; therefore, as single cells, they were adaptive only for PH growth. Strains SGU162, SGU413, DBVPG1892, DBVPG1893 and T2-3D, as heavy streaks, did not penetrate either SD or SLAD; however, they behaved very differently when plated as single cells: (i) SGU162 invaded SLAD but not SD with PH growth at 102, 103 and 104 cells per plate

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which indicates that they adapted to a low nitrogen concentration; (ii) DBVPG1893 invaded SLAD but not SD without PH at all cell concentrations which showed that they adapted to the low nitrogen concentration only for penetration but not for PH growth; (iii) DBVPG1892 penetrated both SD and SLAD but it did not form PH – it was constitutive for penetration but not for PH formation; T2-3D penetrated both SD and SLAD only at 102 and 103 cells per plate but it formed PH only on SLAD. We also determined the invasive growth and PH formation on YPD (Table I). Strains SGU339, SGU413 and T2-3D penetrated with PH as heavy streaks. As single cells, SGU339 and SGU413 penetrated with PH at all concentrations while the strain T2-3D penetrated at all concentrations but formed PH only at 106 cells per plate. The strain SGU398 penetrated YPD only at 102 cells per plate without PH and the strain SGU162 penetrated at 102 and 103 cells per plate with PH. The behavior of the strains Ps93 and Ps168, which were previously shown to invade SD, SLAD and YPD (Barberio et al. 2006) was not affected by cell concentration. These results showed that in most cases colony density–cell concentration influences negatively invasive growth and PH formation in the nonconstitutive strain. We did not investigate the cause(s) of the phenomenon but we can speculate that the colonies communicate with each other by some yet unknown substance(s). Actually, besides mating hormones, a yeast-produced signaling molecule has been identified, viz. ammonium; this, however, cannot explain our data because ammonia is not formed in a medium without amino acids (cf. Palková et al. 1997). Interactions among yeast cells or colonies is a primitive social behavior which has been described earlier. Varon and Choder (2000) showed that yeast cells in a colony communicate with each other by producing thin filaments which penetrate adjacent cells, linking cells together. Matmati et al. (2002) showed that colonies grown on YPD are more thermotolerant and thermoresistant than cells plated from liquid suspensions; interestingly, this phenomenon was not shown by “artificial” colonies, i.e. colonies obtained by point inoculation of ≈1–2 × 106 cells on YPD, indicating that crowding alone is not sufficient to endow cells with higher thermotolerance and thermoresistance. Kuthan et al. (2003) showed that yeast strains freshly isolated from nature formed fluffy colonies whose cells are connected by an extracellular matrix, while laboratory strain colonies are smooth. The fluffy appearance is lost after some generations in a rich medium, following extensive changes in gene expression. Here we observed another phenomenon that could be explained by interactions among colonies: we plated 1000 cells of the strain SGU398 together with 1000 cells of strains Ps168 and Ps93 on YPD; as described above, Ps168 and Ps93 invade YPD with PH at whichever cellular concentration whilst SGU398 penetrated YPD only at 100 cells per plate without PH. Ps168 and Ps93 formed colonies well distinguishable from SGU398. After washing, one should expect only colonies with the phenotype of the strains Ps68 and Ps93; instead, we also observed ≈100 colonies whose appearance was that of the strain SGU398 which showed that they were able to penetrate the agar without PH (Fig. 1). This phenomenon did not need the contact between the two types of colonies. We repeated this experiment twice along with controls and we always got the same result. Although preliminary, we conside-

Fig. 1. Influence of the constitutive strain Ps93 on the strain SGU398 on YPD medium. The appearance of colonies is shown on YPD after washing (magnification 400×); in the center is a colony of the Ps93 strain with PH, the other colonies are of the strain SGU398, without PH (when plated without the constitutive strain Ps93, the strain SGU398 at a concentration of 1000 cells per plate did not invade the substrate at all; for more details see the text).

red these data as indicative of a possible influence of strains constitutively able to invade substrate on others less prone to the dimorphic switch. This phenomenon is not in contrast with the observation that penetration and PH formation are negatively related to cell concentration; it just indicates that the presence of colonies of the constitutive strains affects the cell concentration, at which colonies of a nonconstitutive strain can penetrate the substrate.

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We also reconsider the hypothesis that dimorphic switch evolved to allow starved cells to search for nutrition. Our data confirm that natural strains may form PH and/or invade YPD. Invasive growth on rich growth media, including YPD, was observed by Casalone et al. (2005); it has been suggested that in haploids filamentation begins when glucose becomes limiting for vegetative growth (Cullen and Sprague 2000). Then the haploid invasive growth, as well as the diploid dimorphic switch (Gimeno et al. 1992) are considered to represent an adaptation to nutrient limitation. Therefore, we expected the invasiveness to occur at high cell concentrations; we observed that, if more cells were plated, more nutrients should be exhausted from the medium and more catabolites were accumulated in it. However, the SGU398 strain penetrated YPD only at 100 cells per plate but it did not do so at higher cell concentrations. Similarly, the SGU162 strain penetrated with PH only at 100 and 1000 cells per plate but not at higher concentrations (Table I). These data suggest that starvation for nutrients cannot be considered as the sole selective pressure for development of dimorphic switch in S. cerevisiae. That developmental pressures other than starvation can select for dimorphic switch in S. cerevisiae was also shown by the fact that oxygen limitation, osmotic and thermal shock have an important role in filamentous growth (Wright et al. 1993; Zaragoza and Gancedo 2000). Moreover, it has been shown that addition of compounds that alter cell membrane or cell wall, induce PH growth and invasiveness (Zaragoza and Gancedo 2000). Filamentation and invasiveness could therefore provide S. cerevisiae cells with the ability to search for nutrients but they could also allow cells to escape from stressful environments. This research was supported by the Fondo per gli Investimenti della Ricerca di Base project no. RBA U01 LM9Z of Ministero dell’Istruzione e della Ricerca Scientifica, Italy. We thank M. Paulis for skilled technical assistance. REFERENCES BARBERIO C., BIANCHI L., PINZAUTI F., LODI T., FERRERO I., POLSINELLI M., CASALONE C.: Isolation and characterization of morphological mutants in a natural Saccharomyces cerevisiae strain. Can.J.Microbiol., in press (2006). BRAUS G.H., GRUNDMANN O., BÜCKER S., MÖSCH H.U.: Amino acid starvation and Gcn4p regulate adhesive growth and FLO11 gene expression in Saccharomyces cerevisiae. Mol.Biol.Cell 14, 4272–4284 (2003). CASALONE E., BARBERIO C., CAPPELLINI L., POLSINELLI M.: Characterization of Saccharomyces cerevisiae natural populations for pseudohyphal growth and colony morphology. Res.Microbiol. 156, 191–200 (2005). CULLEN P.J., SPRAGUE G.F.: Glucose depletion causes haploid invasive growth in yeast. Proc.Nat.Acad.Sci.USA 97, 13619–13624 (2000). DICKINSON J.R.: ‘Fusel’ alcohols induce hyphal-like extensions and pseudohyphal formation in yeast. Microbiology 142, 1391–1397 (1996). GAGIANO M., BAUER F.F., PRETORIUS I.S.: The sensing of nutritional status and the relationship to filamentous growth in Saccharomyces cerevisiae. FEMS Yeast Res. 2, 433–470 (2002). GIMENO C., LJUNGDAHL P.O., STYLES C.A., FINK G.R.: Unipolar cell divisions in the yeast S. cerevisiae lead to filamentous growth: regulation by starvation and RAS. Cell 68, 1077–1090 (1992). LEE B.N., ELION E.A.: The MAPKKK Ste11 regulates vegetative growth through a kinase cascade of shared signaling components. Proc.Nat.Acad.Sci.USA 96, 12679–12684 (1999). LORENZ M.C., CUTLER N.S., HEITMAN J.: Characterization of alcohol-induced filamentous growth in Saccharomyces cerevisiae. Mol. Biol.Cell 11, 183–199 (2000). KUTHAN M., DEVAUX F., JANDEROVÁ B., SLANINOVÁ I., JACQ C., PALKOVÁ Z.: Domestication of wild Saccharomyces cerevisiae is accompanied by changes in gene expression and colony morphology. Mol.Microbiol. 47, 745–754 (2003). MATMATI N., MORPURGO G., BABUDRI N., MARINI A.: The influence of colonial organization on thermotolerance and thermoresistance in Saccharomyces cerevisiae. J.Basic Microbiol. 42, 345–354 (2002). MÖSCH H.-U.: Molecular biology of fungal development, pp. 1–27 in H.D. Osiewacz (Ed.): Molecular Biology of Fungal Development. Marcel Dekker, New York–Basel 2002. PALECEK S.P., PARIKH A.S., KRON S.J.: Sensing, signaling and integrating physical processes during Saccharomyces cerevisiae invasive and filamentous growth. Microbiology 148, 893–907 (2002). PALKOVÁ Z., JANDEROVÁ B., GABRIEL J., ZIKÁNOVÁ B., POSPÍŠEK M., FORSTOVÁ J.: Ammonia mediates communication between yeast colonies. Nature 390, 532–536 (1997). SHERMAN F.: Getting started with yeast. Meth.Enzymol. 194, 3–21 (1991). TRACHTULCOVÁ P., FRÝDLOVÁ I., JANATOVÁ I., DOROSH A., HAŠEK J.: The W303 genetic background affects the isw2Δ mutant phenotype in Saccharomyces cerevisiae. Folia Microbiol. 48, 745–753 (2003). VARON M., CHODER M.: Organization and cell–cell interaction in starved Saccharomyces cerevisiae colonies. J.Bacteriol. 182, 3877– 3880 (2000). WENZEL T.J., VAN DEN BERG M.A., VISSER W., VAN DEN BERG J.A., DE STEENSMA H.Y.: Characterization of Saccharomyces cerevisiae mutants lacking the E1α subunit of the pyruvate dehydrogenase complex. Eur.J.Biochem. 209, 697–705 (1992). WRIGHT R.M., REPINE T., REPINE J.E.: Reversible pseudohyphal growth in haploid Saccharomyces cerevisiae is an aerobic process. Curr.Genet. 23, 388–391 (1993). ZARAGOZA O., GANCEDO J.M.: Pseudohyphal growth is induced in Saccharomyces cerevisiae by a combination of stress and cAMP signaling. Antonie van Leeuwenhoek 78, 187–194 (2000).