Functional genomics of salt tolerance: The yeast overexpression approach

The classical approach to salt tolerance consisted on the physiological and biochemical description of the responses of plants to this stress. This defined several crucial factors such as salt inclusion and exclusion from the aerial part, vacuolar compartmentation, osmolyte synthesis, stomatal regulation, abscisic acid, LEA proteins and antioxidant defences. In order to convert this knowledge into biotechnology, molecular biology is needed for the genetic modification of crop plants with halotolerance genes. These genes correspond to limiting steps susceptible of improvement and are identified by genetic screens based on gain-of-function. Random insertion of transcriptional enhancers in plant genomes ("activation tagging") is being pursued but still with low efficiency. Alternatively, we have developed a strategy based on random overexpression of genes in the yeast model system. By testing yeast and plant (Arabidopsis and sugar beet) gene libraries we have identified several halotolerance genes. They correspond to processes operating at the cellular level, related to antioxidant defences and Na⁺ homeostasis and conserved between yeast and plants. They include ascorbate peroxidase, serine acetyltransferase, Na⁺ extrusion and K⁺ uptake at the plasma membrane and several targets of Na⁺ toxicity such as sulfate and uracil metabolism, RNA processing and initiation of protein synthesis. As complex biological phenomena have several limiting factors, a significant increase in salt tolerance will require the simultaneous gain of function of many halotolerance genes, a plausible but not trivial task. It is ironic, however, that now that the tools for the generation of halotolerance crops start to be available, the radical ecologism recently imposed in Europe is preventing any further progress. This will have devastating consequences for the future of Mediterranean agriculture and environment.