-NaCI +200 mM NaCI

-NaCI +200 mM NaCI

H20 C02 57 40 52 39

H20 C02 57 40 52 39

transport protein and a regulatory protein led to glycine betaine accumulation in the transformants, in particular under high salt stress. These cells grew significantly better under salt stress than did control cells, although their growth was distinctly slower than at low salt stress (Fig. 1.6.14).

The reason for the improved growth is that the photosynthetic electron transport is less impaired by salt in the glycine betaine-containing cells (Table 1.6.6). Glycine betaine is synthesised in the chloroplast and performs its protective function principally in this organelle, where it appears to stabilise mainly the protein complexes of the photosynthetic membrane (Papa-georgiou and Murata 1995). In this it differs markedly from the osmolyte glycerol, which has no stabilising effect on the protein complexes. Betaines are so-called chaotropic compounds, i.e. zwitterions, which counteract effects caused by ionic forces. They develop their maximum protective effect specifically at those sites where complexes or subunits of proteins threaten to dissociate as a result of salt stress.

DMSP (3-Dimethyl Sulfoniopropionate)

Less is known about TSCs than about QACs. The best-known osmolyte of this group is dimethyl sulfoniopropionate (DMPS), which has been found mainly in algae, but it has also been recently found in certain grasses and Composi-tae4. The starting material for the synthesis of DMSP is the amino acid methionine, which is metabolised to DMSP in marine algae and terrestrial plants according to different pathways. Since the starting compound and the end product are the same in each case, the pathways differ in principle only in the sequence of the individual steps involved (removal of the amino group, addition of a methyl group, decarboxyla-

4 DMSP is the biogenetic precursor of the atmospheric trace gas dimethyl sulfide, which is released above all by algae.

tion). The various groups of land plants do synthesise DMSP in different ways, however, whereby they all require betaine aldehyde dehydrogenase for the last enzymatic step.

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