Many plant species or ecotypes of species, most conspicuously salt-tolerant species, alter the intracellular osmotic potential when salt stressed, and this has given rise to the concept of ‘‘osmotic adjustment,’’ which, although not unchallenged, has become a widely accepted concept (Bohnert and Shen, 1999; Rontein et al., 2002). The increase of metabolically inert compounds would provide both a sink for unutilized products of primary metabolism and assure continued water influx into the plant. The topic has received much attention and has been reviewed extensively (Apse and Blumwald, 2002; Hasegawa et al., 2000b; Hill et al., 2004; Hohmann, 2002; Knight et al., 1997; Verslues and Bray, 2004). The concept is based on many observations demonstrating the accumulation of a number of metabolites that are normally present in low concentrations in response to an osmotic or ionic imbalance. As the determinants that control entire (metabolic) pathways have emerged, engineering has moved to altering expression of transcription factors (TFs), and components of calcium-dependent responses that elicit phospho-relay regulatory systems that respond to signals caused by various abiotic stresses (Table 12.2) (Himmelbach et al., 2003; Qi and Spalding, 2004; Serrano et al., 1999; Shinozaki et al., 2003; Teige et al., 2004; Zhu, 2003).
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