International congress on yeasts edition:11th location:Rio De Janeiro, Brazil date:15-20 August 2004
Although much correlative evidence is available, the precise determinants of freeze resistance in Saccharomyces cerevisiae are largely unknown. Genome-wide gene expression analyses of freeze-resistant and -sensitive strains have previously revealed a correlation between freeze resistance and expression of aquaporin encoding genes1. This relationship was confirmed by deletion and overexpression of AQY1 and AQY2, reducing and enhancing yeast freeze tolerance, respectively. Rapid osmotically driven water efflux upon freezing was suggested to reduce intracellular ice crystal formation and resulting cell damage. Aquaporin overexpression also improved maintenance of viability of industrial strains, without affecting commercially important characteristics. Unfortunately, further testing of the aquaporin overexpression strains in industrial conditions revealed no improvement of freeze tolerance in comparison with the control strain.
We now show that the difference in freezing rate is apparently responsible for the discrepancy between these results. Application of different cooling rates indicate that at high cooling rates aquaporin overexpression significantly improves survival while at low cooling rates the effect is much less pronounced. Differences in cultivation conditions as well as thawing rate do not seem to influence freeze tolerance in the conditions tested. Mazur and co-workers have postulated that survival after freezing is mainly affected by two factors -cellular dehydration and intracellular ice crystal formation- oppositely depending on the cooling velocity2. In accordance with this so-called two-factor hypothesis of freezing injury, we suggest that water permeability is limiting, and therefore aquaporin function advantageous, mainly in rapid freezing conditions. This finding probably hampers the use of aquaporin-mediated improvement of freeze tolerance for the development of freeze-resistant baker's yeast strains for frozen dough applications.
1. Tanghe, A., Van Dijck, P., Dumortier, F., Teunissen, A., Hohmann, S. and Thevelein J.M. Aquaporin expression correlates with freeze tolerance in baker’s yeast, and overexpression improves freeze tolerance in industrial strains. Appl. Environm. Microbiol. 68, 5981-5989, (2002).
2. Mazur, P. Cryobiology: the freezing of biological systems. Science 168, 939-949, (1970)