Among the accumulators are amino acids, predominantly proline, sugars and sugar alcohols, such as sucrose or mannitol, trehalose, mannitol/sorbitol, or inositol derivatives, and more complex carbohydrates, such as fructans and raffinose-related compounds. This list is likely to expand in the future as other models are investigated. In a drought-adapted watermelon, for example, citrulline, an intermediate in the urea cycle, has been detected as a drastically accumulating metabolite during drought stress. Citrulline function may be in radical oxygen scavenging (Akashi et al., 2001, 2004). A number of experiments have been reported that attempted to engineer osmolyte accumulation into glycophytic plants that show only marginal accumulation of metabolites with the intention to improve tolerance. Table 12.2 provides a selection of such studies, also including experiments to transgenically engineer ionic stress tolerance and to engineer regulatory circuits. The range included exemplifies the major categories of genes or cDNAs that have been used in engineering: osmolytes and other protectants [chaperones, late-embryogenesis-abundant (LEA) and heat-shock proteins (HSPs)], transporters and pumps, scavengers of radicals, adjustments in hormone biosynthesis, and regulatory genes, outlined in detail at: www.plantstress.com/Files% 5CAbiotic-stress_gene.htm. Under strictly controlled growth conditions, it has been shown in many of these experiments that the plants exhibited showed increased osmotic, ionic, or temperature tolerance (Table 12.2). Often the actual increase or accumulation did not amount to concentrations found in the natural models gave rise to the ‘‘compatible osmolyte’’ concept.
These experiments demonstrated several other aspects as well. First, osmotic/ ionic abiotic stress tolerance seems to be controlled by different mechanisms depending on age or developmental stage, that is, seedling, vegetative, and reproductive stages, each seem to require stage-specific regulation of tolerance and protective determinants (Rontein et al., 2002).
Third, synthesis leading to accumulation is not necessarily the major purpose of a purported osmolyte, while the accumulation may be a pathological side effect. One example is yeast that increases the pathway leading to increased synthesis of trehalose (Hohmann, 2002). However, yeast also increases levels of enzymes that degrade trehalose, indicating that flux through the pathway (consuming reducing power) may be more important than simply making more (Fig. 12.2B). This behavior has been shown in several studies (Yale and Bohnert, 2001). Only at very high salt concentrations, which arrests or slows growth, will trehalose accumulate drastically in yeast. Such a view might then lead to a different interpretation for the compatible solutes. They might be seen as metabolic valves that adjust or lower the redox potential of cells in order to prevent or minimize production of radical oxygen species (ROS) in mitochondria and chloroplasts. Finally, attempts at engineering salinity stress tolerance have only begun to pay attention to compartment-specific strategies and the appreciation of stress tolerance as a multigenic network has not been tackled, for example, by the accumulation of different stress alleviating determinants.
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