17.3.3  Organic solutes as metabolic protectants

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The common occurrence of these nitrogen-rich, energy-intensive compounds indicates that they must have a strong selective advantage to outweigh the cost of their synthesis. This advantage is the protective role they play with regard to enzyme function during salt stress. However, their functions have been surprisingly difficult to identify. Are they just non-toxic osmolytes or do they have a special metabolic function?

Osmolyte function

When present at high concentrations organic solutes are often termed osmolytes as they generate an osmotic pressure high enough to have an important osmotic function, especially if concentrated in a small cell compartment. This term is syn-ony-mous with the original term compatible solutes, because these organic osmolytes are understood to be compatible with enzyme activity. Mannitol at high concentrations does not inhibit enzyme activity nor does proline, glycinebetaine and many other organic solutes (Pollard and Wyn Jones 1979).

Osmoprotectant function

In addition to this osmotic function, organic solutes may have a protective function for enzymes in cells containing high concentrations of Na+ and Cl. In vitro studies with enzyme extracts showed that high concentrations of glycinebetaine prevented inhibition of enzyme activity by NaCl (Pollard and Wyn Jones 1979). Sometimes these organic solutes are present at low concentrations, but still seem to be associated with stress tolerance of plants. Various metabolic roles have been suggested such as scavenging radical oxygen and hydroxyl species, stabilising membrane processes and modifying the redox potential of cells. For example, glycinebetaine may stabilise the O2-evolving complex of photosystem II against the inhibitory effects of NaCl (Murata et al. 1992).

Identifying the functional roles of these compounds is difficult but an implied contribution to stress tolerance in maize has been established by Saneoka et al. (1995). These workers compared salt tolerance by pairs of homozygous near-isogenic lines that were either sufficient (Bet1/Bet1) or deficient (bet1/bet1) in accumulation of glycinebetaine. Stressed by a mixture of NaCl and CaCl2, growth in Bet1/Bet1 lines was less inhibited than in their bet1/bet1 sister lines, and was correlated with a much greater accumulation of glycinebetaine in leaf sap (10.9 mmol–1 L–1 in Bet1/Bet1 compared with 0.2 mmol L–1 in bet1/bet1). Carbon assimilation, leaf expansion rate, relative water content and turgor were all higher in Bet1/Bet1.

Further evidence for osmoprotection comes from transgenic plants engineered to synthesise selected organic solutes such as proline, mannitol and other polyols. Improved salt tolerance can then be linked to over expression of specific genes for synthesis of selected solutes, and some distinctions drawn between purely osmotic effects and osmoprotection of meta-bolic function. Genetically modified plants show improved tolerance to stress without an alteration in water relations, that is, transgenic and wild type have similar turgor. Such outcomes confirm osmoprotection and offer prospects of engineering crop plants to improve salt tolerance (Bohnert and Jensen 1996).