Flor Strains Research Articles

Oxygen is required to restore flor strain viability and lipid biosynthesis under fermentative conditions

To further elucidate the biosynthesis of lipids in flor strains under fermentative conditions, the transcription levels of the lipid biosynthetic genes ACS1, ACS2, ACC1, OLE1, ERG1, ERG11, ARE1 and ARE2, as well as the lipid composition and cell viability of a flor strain were compared with that of a non-flor strain during hypoxic and aerobic fermentations in the absence of lipid nutrients. While no significant differences in transcription levels or lipid compositions were observed between the two strains when oxygen was not limiting, significant differences were seen during hypoxic fermentation. In this last condition, the flor strain, in spite of higher levels of transcription of hypoxic genes, lost the abilities to desaturate fatty acids and complete ergosterol biosynthesis, and showed a dramatic loss of viability. In contrast, the non-flor strain, which showed lower transcription levels, was able to reach a balanced lipid composition and maintained a higher cell viability. One possible explanation is that the flor strain requires a higher amount of oxygen than the non-flor strain in order to carry out the oxygen-dependent steps of lipid biosynthesis under fermentative conditions.

Genetic characterization of strains of Saccharomycescerevisiae responsible for 'refermentation' in Botrytis-affected wines

Saccharomyces cerevisiae is responsible for alcoholic fermentation of wines. However, some strains can also spoil sweet Botrytis-affected wines. Three 'refermentation' strains were isolated during maturation. Characterization of those strains in regards to their fingerprint, rDNA sequence and resistance to SO2, which constituted the main source of stress in Botrytis-affected wines, was carried out. Refermentation strains could be clearly discriminated by interdelta fingerprinting. However, they exhibited close relationships by karyotyping. A part of RDN1 locus sequence was examined by using PCR-RFLP and PCR-DGGE. The resistance of refermentation strains to SO2 was performed by using real time quantitative PCR focusing on SSU1 gene. Results suggested that refermentation strains were heterozygote in 26S rDNA and their ITS1-5.8S rDNA-ITS2 region sequence revealed relationships with 'flor' strains. As described in the literature for flor strain, two out of three refermentation strains constitutively developed a higher level of SSU1 expression than the reference strains, improving their putative tolerance to SO2. Therefore, refermentation strains of S. cerevisiae had developed many strategies to survive during maturing sweet wines. Singularities in rDNA sequence and SSU1 overexpression revealed a natural adaptation. Moreover, genomic relationship between flor and refermentation strains suggested that stress sources could induced selection of survivor strains.

FLO11Is the Primary Factor in Flor Formation Caused by Cell Surface Hydrophobicity in Wild-Type Flor Yeast

Some strains of Saccharomyces cerevisiae form a biofilm called a "flor" on the surface of wine after ethanolic fermentation, but the molecular mechanism of flor formation by the wild-type flor strain involved in wine making is not clear. Previously, we found that expression of the C-terminally truncated form of NRG1 (NRG1(1-470)) on a multicopy plasmid increases the hydrophobicity of the cell surface, conferring flor formation on the non-flor laboratory strain. Here we show that in Ar5-H12, a wild-type flor haploid strain, flor formation is regulated by NRG1(1-470). Moreover, the disruptant of the wild-type flor diploid strain (Deltaflo11/Deltaflo11) show a weak ability to form the flor. The expression of FLO11 is always high in the wild-type flor strain, regardless of carbon source. Thus FLO11 is primary factor for wild-type flor strains. Furthermore, the disruptant (Deltaflo11) shows lower hydrophobicity of cell surface than the wild type. However, the hydrophobicity of the wild-type flor strains grown in ethanol medium was much higher than those grown in glucose medium. These results indicate that cell surface hydrophobicity is closely related to flor formation in wild-type flor yeasts.

Peculiarities of strains adapted to Sardinian sherry-like wine ageing conditions

Saccharomyces cerevisiae flor yeasts, which are subjected to stressful conditions during wine ageing, exhibit a number of characteristics which distinguish them from non-flor S. cerevisiae wine strains. In the present work, 22 flor and 14 non-flor S. cerevisiae wine strains are compared, in order to elucidate other possible peculiarities of these yeasts. The results obtained demonstrate that in contrast to the homothallic nature of the non-flor strains, 77% of the flor strains exhibit two variants of a semi-homothallic life cycle. Moreover, the flor-forming ability is shown to be inversely correlated to spore viability and the utilisation of maltose and galactose.

FLO11 is essential for flor formation caused by the C-terminal deletion of NRG1 in Saccharomyces cerevisiae

The flor strains of Saccharomyces cerevisiae form a flor on the surface of wine after alcoholic fermentation. High hydrophobicity of the cell surface is suggested to be important for flor formation by the flor wine yeasts. However, the molecular mechanism of flor formation is not clear. We found that expression of C-terminal deleted NRG1 lacking its two C2H2 zinc finger motifs (NRG1(1-470)) on the multicopy plasmid conferred the ability to form a flor to a non-flor laboratory strain. The cell surface hydrophobicity of NRG1(1-470) was higher than of the non-flor strain. Disruption of the Nrg1p-repressed gene FLO11, which encodes a cell surface glycoprotein that functions as a flocculin or an adhesin, abolished flor formation. Moreover, expression of FLO11 on a multicopy plasmid could also cause flor formation. These results indicate that FLO11 is essential for flor formation by NRG1(1-470). In addition, the results suggest that the C-terminal truncated form of Nrg1p exerts a dominant negative effect on FLO11 repression, resulting in FLO11 expression and, thus, flor formation.

FLO11 is essential for flor formation caused by the C-terminal deletion of NRG1 in Saccharomyces cerevisiae

The flor strains of Saccharomyces cerevisiae form a flor on the surface of wine after alcoholic fermentation. High hydrophobicity of the cell surface is suggested to be important for flor formation by the flor wine yeasts. However, the molecular mechanism of flor formation is not clear. We found that expression of C-terminal deleted NRG1 lacking its two C 2H 2 zinc finger motifs ( NRG1 1–470) on the multicopy plasmid conferred the ability to form a flor to a non-flor laboratory strain. The cell surface hydrophobicity of NRG1 1–470 was higher than of the non-flor strain. Disruption of the Nrg1p-repressed gene FLO11, which encodes a cell surface glycoprotein that functions as a flocculin or an adhesin, abolished flor formation. Moreover, expression of FLO11 on a multicopy plasmid could also cause flor formation. These results indicate that FLO11 is essential for flor formation by NRG1 1–470. In addition, the results suggest that the C-terminal truncated form of Nrg1p exerts a dominant negative effect on FLO11 repression, resulting in FLO11 expression and, thus, flor formation.

A genetic study of natural flor strains of Saccharomyces cerevisiae isolated during biological ageing from Sardinian wines.

In this study, three flor strains of Saccharomyces cerevisiae were genetically characterized. They were isolated from biofilms on Sardinian sherry-like wines produced at family-run wineries where pure cultures of yeasts were not used. The study aimed to investigate the life cycle of these naturally-occurring flor strains, using a genetic procedure supplemented by analysis of subsequent meiotic generations. A semi-homothallic life cycle was found in three strains that could be helpful in a genetic improvement programme.

Pulse field gel electrophoresis study of the karyotype of some Saccharomyces cerevisiae wine strains

Wine yeasts have been shown to be characterised by marked chromosome‐length polymorphism. Study of this polymorphism could explain some of the genetic characteristics of these strains and, furthermore, lead to their precise identification, which is still an unresolved problem in the farm‐produce industry. With this aim in mind, we characterised the electrophoretic karyotype of 52 flor strains, isolated from the films of the following Sardinian wines: Cannonau, Malvasia di Bosa and Vernaccia di Oristano.