Whether the inhibition of RuBISCO or PEPC activity is due to reduction of amount or activity of enzymes and what is the role of other factors in protection of this.
The importance of altered metabolism under abiotic stress for example diversion of carbon to polyol biosynthesis is exemplified by the metabolic reactions originating from the Glucose-6-P pool. The levels of glycine betaine are diverse in different varieties of maize (Brunk et al 1989).
A positive correlation has been found between level of endogenous glycine betaine and the degree of salt resistance in maize ( Saneoka et al. 1995): Therefore enhancing glycine betaine synthesis in maize may be potential method of improving stress resistance in maize. Recently bet A from E. coli has been expressed in maize ( Quan et al. 2004) and transgenic maize aquired improving chilling tolerance due to the enhanced accumulation of glycine betaine. In Aphanothece halophytica accumulated GB promoted RuBISCO activity under salinity. In barley GB protected malate dehydrogenase and puruvate kinase against salt stress. However GB protection may be only partial or does not apply to all enzymes, phosphoenol-pyruvate carboxylase for instance was not protected ( Pollard and Wyn Jones, 1979, Izui et al 2004).
C4 PEPc is phosphorylated in day time. Phosphorylation is mainly controlled by light in C4 plants. Recently method employing antibody raised against phosphorylated N terminal peptide of maize C4 PEPC (Ueno et al 2000) for quantitaive determination has been developed. Further studies suggest that phosphorylation is regulated not only by light but also over-ridden by some compensatory feedback mechanism and oxidative stress. (Ueno et al. 2000 and Agetsuma et al 2003). Enhanced phosphorylation of C4 PEPC under salt stress (Garica -Maurino et al 2003) also suggest multiple cues for regulatory phosphorylation. The protein kinase involved in this phosphorylation is calcium-independent and highly specific to PEPC, with estimated molecular weights of 30 and 37 kDa. However its correlation with cytoplasmic pH, and other factors remains undetermined.
Despite some reservations (Biehler and Foch, 1996) it is generally accepted that a reduction in internal CO2 levels resulting from stress induced stomatal closure results in elevated photorespiratory activity (Bohnert and Jensen, 1996). Sivakumar et al. [2000] found a salt stress-induced enhancement of ribulose-1,5-bisphosphate oxygenase activity while curtailing carboxylase activity in the salt resistant plant Sesbania sesban. Salt stress-induced enhancement in oxygenase activity was fully alleviated by proline even when present at a concentration as low as 50 mM.
