Harris 2004). GB occurs most abundantly in response to a variety of abiotic stress conditions by numerous organisms including bacteria, cyanobacteria, algae, fungi, animals, and many plant families such as Chenopodiaceae and Gramineae (Turkan and Demiral 2009). This metabolite is mainly located in chloroplasts and plays a vital role in the stroma adjustment and protection of thylakoid membranes, thereby maintaining the photosynthetic activity (Jagendorf and Takabe 2001) . GB protects the photosystem II (PS-II) complex at high salinity (Murata et al.

1992) and at extreme temperatures or pH (Mohanty et al. 1993) . GB also protects membranes against heat-induced destabilization and enzymes, such as RUBISCO, against osmotic stress (Mâkelâ et al. 2000). In higher plants, GB is synthesized from serine via ethanolamine, cho-line by two-step oxidation reactions that were catalyzed by choline monooxygenase and betaine aldehyde dehydrogenase, respectively (Russell et al. 1998; Ahmad and Sharma 2008; see Fig. 1.2). The insertion of serine and glycine can be taken as an indicator for the close relationship of the photorespiration (peroxisomes) to the synthesis of GB. Besides this, recently a biosynthetic pathway of GB from glycine with the involvement of two N-methyl transferase enzymes has been reported (Waditee et al. 2005). Highly tolerant genera such as Spartina and Distichlis accumulated the highest levels of GB, moderately tolerant species intermediate levels, and sensitive species hardly any GB (Rhodes and Hanson

1993). Genetic evidence that GB improves salinity tolerance has been obtained for many important agronomical crops such as tobacco, tomato, potato, barley, maize, and rice. These plants have long been a potential target for engineering GB biosynthesis pathway and thus for resistance against different environmental stress conditions (Sairam and Tyagi 2004; Turkan and Demiral 2009). The importance of N-methyltransferase for stress tolerance could also be shown for Arabidopsis . Genetically modified plants of this genus accumulated betaine to significant levels at different environmental stress conditions and hence improved seed yield (Waditee et al. 2005). A moderate stress tolerance was noted in some transgenic lines based on relative shoot growth in response to salinity, drought, and freezing. Huang et al. (2000) reported metabolic limitation in betaine production in transgenic plants. In fact, Arabidopsis thaliana, Brassica napus, and Nicotiana tabacum were transformed with bacterial choline oxidase cDNA, and their levels of GB were only between 5 and 10% of the levels found in natural betaine producers.

Beyond this, choline-fed transgenic plants synthesized substantially more GB. This result was taken as a hint that these plants require a distinct endogenous amount of choline to synthesize an adequate amount of GB (Sairam and Tyagi 2004) .

The protective effect of GB at salinity or drought could also be demonstrated by exogenous application at rice seedlings, soybean, and common beans (Ashraf and Foolad 2007; Demiral and Turkan 2006). GB pretreatment also alleviated salinity-induced peroxidation (oxidative damage) of lipid membranes of rice cultivars. Besides rice,

Fig. 1.3 Biosynthetic pathway of proline (adopted from Ahmad and Sharma 2008)

Fig. 1.3 Biosynthetic pathway of proline (adopted from Ahmad and Sharma 2008)

the correlation between the protective effect of GB and the antioxidative defense system has been observed in chilling-stressed tomato (Park et al.

2006), drought- or salt-stressed wheat (Raza et al.

2007), and salt-stressed suspension cultured tobacco BY2 cells (Hoque et al. 2007).

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