pst (photoautotrophic salt tolerance):
Plants, subjected to high levels of salt concentrations, suffer severe injury or even death unless the affected plant species has a special mechanism to repel excessive salt accumulation from the cytosol.
In a review article published in the February, 2001 issue of Trends in Plant Science (6(2):66-71.), Dr. J-K Zhu at the Department of Plant Sciences, University of Arizona, discusses the various aspects of salt tolerance, its mechanism and how the future research in this area can be accelerated by the study of a newly discovered halophyte related to Arabidopsis.
The author begins by describing the mechanism adopted by plants to overcome the molecular damage due to salt. He points out that salt-tolerant transgenics are often tolerant to other stresses such as chilling, freezing, heat and drought. Early salt stress research, he points out, were confined to yeast. Studies on yeast provided insights on two important pathways for salt stress, the HOG1 (high osmolarity glycerol) pathways for adaptation to hyperosmotic stress and the calcineurin pathway for ionic stress. In yeast, Na +, K+ and Ca2+ and the pheromone response are regulated by calcineurine; yeast mutant cells showing a mutation at this locus are sensitive to Na+ and Li+.
Dr. Zhu, then turns his focus on salt tolerant research in plants. One important mechanism for stress tolerant plants, according to Dr. Zhu, is their ability to produce stress proteins and compatible osmolytes. The latter scavenge reactive oxygen species (ROS) such as O 2- , OH and H2O2 generated by salt stress. Removal of the ROS prevents damage to cellular structures. In this connection, the author discusses the pst (photoautotrophic salt tolerance1) mutant of Arabidopsis, which is much more salt tolerant than its normal counterpart.
Taking a cue from the mechanism which operates in naturally salt tolerant species, transgenics have been created with genes involved in the production of osmolytes such as mannitol, fructans, trehalose, ononitol, proline, ectoine and glycinebetaine. One interesting outcome of such transformation experiments was that salt-tolerant transgenics were also found to be tolerant to other stresses such as tolerance to chilling, freezing, heat and drought. For example, plants transformed with the barley protein HVA1 (Hordeum vulgare A1) gene were found to be stress tolerant due to detoxyfying effect of the protein produced by the alien gene. Another example includes transgenics over-expressing CBF/DREB (C-repeat)/dehydration-responsive element) proteins, which were found to be tolerant to salt, drought and freezing stresses.
Tuning his attention to the re-establishment of homeostasis in stressful environments, the Dr. Zhu stresses that Na+ accumulation must be prevented in order for the stressful plant to survive. One way to exploit this is to identify transporters which block Na+ influx contributing to enhanced salt tolerance. In addition to Na+ influx control and vacuolar compartmentation, Na+ efflux is essential to maintain a low Na+ concentration in the cytoplasm.
The author notes that recently Arabidopsis has proved to be as efficient a system for salt tolerance studies as yeast. This is one consequence of the availability of its entire genome sequences and the recent availability of microarrays and gene chips. For instance, the cloning of the salt overly sensitive (SOS) genes using these new resources, indicated that the mutation at this locus makes Arabidopsis plants more sensitive to Na+.
Dr. Zhu then discusses three recently discovered SOS genes in Arabidopsis: SOS1, SOS2 and SOS3. The SOS pathway begins with SOS3 gene product. It encodes a calcium binding protein with attached myristic acid, an unusual fatty acid. It has three EF hands for calcium binding. SOS3 forms a complex with SOS2 which is a serine/threonine kinase. Downstream, the complex interacts with SOS1, a plasma membrane antiporter* which ejects Na+ from the cell.
When a wild Arabidopsis plant is subjected to salt stress, SOS1 is activated and its expression is enhanced (upregulated) i.e. extra sodium molecules are exported out of the cell. On the other hand, the capacity in the mutants sos2 and sos3 to bind calcium is considerably reduced concomitant with excessive Na+ accumulation in the cell. Recent studies have shown that a high level of Ca2+ is necessary for activation of the SOS pathway indicating that it probably acts as a second messenger*.
It is presumed that higher plants and yeast share similar pathways such as HOG1 for osmotic regulation but none of the components have so far been detected in Arabidopsis. It needs to be emphasized here that Arabidopsis falls short of the criteria required to unravel novel processes that characterize naturally occurring halophytes and xerophytes. Fortunately, a recently discovered halophytic species, Thellungiella halophila from the eastern coast of China, has brightened the prospect of detecting determinants (genes) contributing to salt and other abiotic stresses. T. halophila, apart from being salt tolerant, is a close relative of Arabidopsis thaliana. As such it shares similar phenotypes: short stature, self-pollination, short life cycle, small genome, and is easily mutated. Also, the two species share 90% identity in cDNA sequences.
The authors conclude on a confident note that findings related to new tolerance determinants and operating pathways in T. halophila may be applicable to bioengineer salt- and stress tolerant food and other useful crops.
Calcineurin is a Ca2+/calmodulin-dependent S/T protein phosphatase 2B, which has been reported to be important in the calcium signaling pathway.