Apart from satisfying scientific curiosity, the value that can be associated with knowledge about plant reactions to high salinity, leading to tolerance or explaining sensitivity, can be significant. One incentive is the agronomical value that crops might acquire if they could be made salt tolerant. Such modification, if accomplished without major yield penalties, would provide food security through yield stability in areas where crop production predominantly satisfies daily sustenance. Also, if more fresh water could be made available for growing urban populations, instead of its present primary use in agriculture, human lives could improve in many countries. On the other hand, the engineering of truly halophytic crops might generate incentives to utilize land that has not previously been under the plow, which could then endanger saline wetlands, estuaries, or semideserts that provide refuge for endemic organisms. Also, using saline groundwater for irrigation possibly represents a short-lived achievement, necessitating leaching of land by fresh water. In their majority, present crops are glycophytes depending on fresh water to approach their yield potential; their metabolism and growth are, however, affected at low concentrations of sodium, 50–100 mM NaCl, equivalent to ~20% of seawater strength. While it seems possible to engineer salinity tolerance at this level or at even somewhat higher concentrations of NaCl (Apse and Blumwald, 2002; Ward et al., 2003), productivity might still be compromised. Results are already available from a number of experiments that used a controlled environment to gauge differences between a progenitor species and its engineered lines (Apse et al., 1999; Ellul et al., 2003; Garg et al., 2002; Jang et al., 2003; Kasuga et al., 1999; Lee et al., 2004; Mckersie et al., 1996; Mittova et al., 2003; Quintero et al., 1996; Romero et al., 1997; Roxas et al., 1997; Shi et al., 2003; Singla-Pareek et al., 2003; Sulpice et al., 2003; Urano et al., 2004; Van Camp et al., 1996; Wang et al., 2003; Wu et al., 2004; Zhifang and Loescher, 2003). The results with single gene engineering attempts, exemplified by these references that mostly targeted biochemical and physiological characters during the last decade, have identified a number of genes whose altered expression affects tolerance or sensitivity but agronomical benefits have yet to be documented. The characters that emerged as providing increased tolerance tend to support membrane and protein integrity, the synthesis of carbohydrates and N-containing compounds, energy provision, detoxification reactions, and a variety of transport proteins that establish ion and metabolite homeostasis.
New approaches have become possible as information emerged about the nature of the genetic elements that control expression of stress alleviating biochemical determinants, and about how hormonal changes elicit stress response pathways (Mukhopadhyay et al., 2004; Nagaoka and Takano, 2003; Novillo et al., 2004; Perruc et al., 2004; Sakamoto et al., 2004; Teige et al., 2004; Villalobos et al., 2004; Winicov and Bastola, 1999; Zhang et al., 2004). This new frontier and orientation of abiotic stress research has profited from, and in many cases has been initiated by, work on the presently most tractable genetic model, Arabidopsis thaliana (Hasegawa et al., 2000a, b; Ishitani et al., 1997; Serrano and Rodriguez- Navarro, 2002; Shinozaki and Yamaguchi-Shinozaki, 1999; Shinozaki et al., 2003; Tester and Davenport, 2003; Ward et al., 2003; Xiong et al., 2002a; Zhu, 2002). In this chapter, we will review the classical approaches and then outline recent developments with special emphasis on results that have become possible with the advent of a new age: genome sequences, transcript profiles, protein dynamics, and, especially, a large number of phenotyped and functionally identified mutant lines for Arabidopsis, as well as a growing number of other plants, including crop species.
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