Voie de la lipoxygénase et système antioxydant dans des plantules de tomate (Lycopersicon esculentum Mill.) soumises au stress salin

Soil or growing media salinity is an increasing problem in agriculture throughout the world. Therefore, there is a real interest in a better understanding of the mechanisms set by the plants to respond to this stress. The present study concerns tomato, which is one of the most important cultivated vegetables. Tomato seeds (Lycopersicon esculentum var. 'Moneymaker') were disinfected and a salt stress (NaCl 100 mM) was applied 8 days after the onset of germination. Samples were collected after 4 days of stress. In this study, attention was focused on the antioxidant system and the lipoxygenase pathway because salt stress is often associated with an increased production of reactive oxygen species and polyunsaturated fatty acids are one of the most susceptible targets to oxidative attacks. The perception of the stress by the plantlets was confirmed by the over expression of an aquaporin-coding gene (aqua2). Concerning the antioxidant system, the activities of ascorbate peroxydase and catalase enzymes were studied but no significant variation was observed between stressed seedlings and controls. Similar results were obtained for ascorbate measurements. Complementarily, the lipoxygenase pathway was studied at three levels in the plantlets. The expression of two genes (lipoxygenase-A and oxo-phytodienoic acid reductase) was first followed at the mRNA level by RT-PCR. The oxylipin content was determined by HPLC and the lip oxygenase activity as well as the hydro peroxide degradation activity were measured spectro photometrically. The obtained results indicate that no clear accumulation of oxylipin occurs in our stress conditions. However, an increase in lip oxygenase and hydro peroxide degradation activities was observed and seems to indicate that several enzymes of the lip oxygenase pathway are involved in the plant response to salt stress.