Creation of salt-insensitive 3′(2′),5′-bisphosphate nucleotidase by modeling and mutagenesis approach

Insertion of a specific fungal 3'-phosphoadenosine-5'-phosphatase motif into a plant homologue improves halotolerance and drought tolerance of plants

Soil salinity and drought are among the most serious agricultural and environmental problems of today. Therefore, investigations of plant resistance to abiotic stress have received a lot of attention in recent years. In this study, we identified the complete coding sequence of a 3′-phosphoadenosine-5′-phosphatase protein, ApHal2, from the halotolerant yeast Aureobasidium pullulans. Expression of the ApHAL2 gene in a Saccharomyces cerevisiae hal2 mutant complemented the mutant auxotrophy for methionine, and rescued the growth of the hal2 mutant in media with high NaCl concentrations. A 21-amino-acids-long region of the ApHal2 enzyme was inserted into the Arabidopsis thaliana homologue of Hal2, the SAL1 phosphatase. The inserted sequence included the META motif, which has previously been implicated in increased sodium tolerance of the Hal2 homologue from a related fungal species. Transgenic Arabidopsis plants overexpressing this modified SAL1 (mSAL1) showed improved halotolerance and drought tolerance. In a medium with an elevated salt concentration, mSAL1-expressing plants were twice as likely to have roots in a higher length category in comparison with the wild-type Arabidopsis and with plants overexpressing the native SAL1, and had 5% to 10% larger leaf surface area under moderate and severe salt stress, respectively. Similarly, after moderate drought exposure, the mSAL1-expressing plants showed 14% increased dry weight after revitalisation, with no increase in dry weight of the wild-type plants. With severe drought, plants overexpressing native SAL1 had the worst rehydration success, consistent with the recently proposed role of SAL1 in severe drought. This was not observed for plants expressing mSAL1. Therefore, the presence of this fungal META motif sequence is beneficial under conditions of increased salinity and moderate drought, and shows no drawbacks for plant survival under severe drought. This demonstrates that adaptations of extremotolerant fungi should be considered as a valuable resource for improving stress-tolerance in plant breeding in the future.

Salt and oxidative stress tolerance in Debaryomyces hansenii and Debaryomyces fabryi

We report the characterization of five strains belonging to the halotolerant highly related Debaryomyces hansenii/fabryi species. The analysis performed consisted in studying tolerance properties, membrane characteristics, and cation incell amounts. We have specifically investigated (1) tolerance to different chemicals, (2) tolerance to osmotic and salt stress, (3) tolerance and response to oxidative stress, (4) reactive oxygen species (ROS) content, (5) relative membrane potential, (6) cell volume, (7) K(+) and Na(+) ion content, and (8) membrane fluidity. Unexpectedly, no direct relationship was found between one particular strain, Na(+) content and its tolerance to NaCl or between its ROS content and its tolerance to H(2)O(2). Results show that, although in general, human origin D. fabryi strains were more resistant to oxidative stress and presented shorter doubling times and smaller cell volume than food isolated D. hansenii ones, strains belonging to the same species can be significantly different. Debaryomyces fabryi CBS1793 strain highlighted for its extremely tolerant behavior when exposed to the diverse stress factors studied.

Biodiversity in sulfur metabolism in hemiascomycetous yeasts

The evolution of the metabolism of sulfur compounds among yeast species was investigated. Differences between species were observed in the cysteine biosynthesis pathway. Most yeast species possess two pathways leading to cysteine production, the transsulfuration pathway and the O-acetyl-serine (OAS) pathway, with the exception of Saccharomyces cerevisiae and Candida glabrata, which only display the transsulfuration pathway, and Schizosaccharomyces pombe, which only have the OAS pathway. An examination of the components of the regulatory network in the different species shows that it is conserved in all the species analyzed, as its central component Met4p was shown to keep its functional domains and its partners were present. The analysis of the presence of genes involved in the catabolic pathway shows that it is evolutionarily conserved in the sulfur metabolism and leads us to propose a role for two gene families which appeared to be highly conserved. This survey has provided ways to understand the diversity of sulfur metabolism products among yeast species through the reconstruction of these pathways. This diversity could account for the difference in metabolic potentialities of the species with a biotechnological interest.