Thesis defense of Ahmed EL MOUKHTARI

Interplaying roles of silicon and proline on salt stress tolerance in Medicago – Ensifer meliloti symbiosis

Salinity is one of the main abiotic stresses limiting plant growth and productivity. The exogenous supply of silicon (Si) or proline has been described for their beneficial effect on the tolerance of plants to abiotic stresses, particularly salinity stress. The aim of this thesis was to study the effect of the exogenous supply of Si and / or proline on the tolerance of alfalfa to salt stress, by focusing on the mechanisms of tolerance induced in salt-stressed plants in response Si and / or proline application. Results showed that Si supply reduces the effects of salinity and induces a significant accumulation of proline and improves seed germination, embryo viability and reserve mobilization. Si also triggers high catalase (CAT) and superoxide dismutase (SOD) activities and reduces malondialdehyde and hydrogen peroxide contents in salt-stressed seedlings. During plant growth, exogenous supply of Si and / or proline results in a considerable accumulation of some compatible solutes, such as proline, glycine betaine and soluble sugars together with an enhanced antioxidant enzyme activity, such as SOD, CAT, ascorbate peroxidase and glutathione reductase. This improved leaf relative water content, reduced oxidative stress and therefore restores growth and photosynthetic activity of salt-stressed plants. Moreover, the application of proline and / or Si modulates the expression of genes encoding enzymes of proline metabolism, such as Pyroline-5-carboxylate synthetase 1 (P5CS1), P5CS2, Ornithine aminotransferase, Proline dehydrogenase 1, and P5C dehydrogenase as well as Low silicon 2 gene encoding a silicon transporter. In conclusion, our results demonstrate that the exogenous supply of proline and/or Si could be a promising way to mitigate the toxic effect of salinity stress.

Key-word: Medicago; Ensifer meliloti; Salinity; Silicon; Proline; Germination; Growth; Photosynthesis; Proline metabolism; Si transporter.