Drought- and Salt-Tolerant Populations of the Xero-Halophyte Mediterranean Shrub Atriplex halimus L. Exhibit Contrasting Proline and Glycinebetaine Metabolism

To determine the effects of ionic and nonionic osmotic stresses, seedlings from two populations of the xero-halophyte species Atriplex halimus L. Djelfa (salt-affected site) and Tamanrasset (non-saline arid area) were exposed for 10 days in nutrient solution to NaCl (100 or 300 mM) or to polyethylene glycol (PEG 6000; 50 or 100 g L-1). Ions, proline and glycinebetaine were analyzed in relation to osmotic potential. Enzymes activities involved in proline or glycinebetaine synthesis were compared to the corresponding genes expression. Seedlings from Djelfa were more resistant to NaCl while those from Tamanrasset were more resistant to PEG. Salt-resistance in Djelfa was associated with greater Na+ accumulation and the maintenance of K+ concentration. Both ions significantly contributed to osmotic adjustment in salt-stress conditions. Proline accumulated mainly in response to nonionic water stress while glycinebetaine accumulated preferentially in response to salinity. Proline accumulation resulted from a stress-induced stimulation of Delta 1-pyrroline-5-carboxylate reductase and ornithine delta-aminotransferase activities in Tamanrasset and from an inhibition of proline dehydrogenase activity in Djelfa. Glycinebetaine accumulation in Djelfa was associated to an increase in betaine aldehyde dehydrogenase activity while Tamarasset produced the glycinebetaine precursor choline in limiting amounts. Although genes coding for these enzymes were upregulated in most cases, there was a poor correlation between gene expression and enzyme activities suggesting important posttranscriptional regulation. The gene coding for proline/glycinebetaine transporter ProT was upregulated indicating a role of long-distance transport of osmotic compounds in plant resistance to environmental constraints.