The roles of lipids in disease resistance and fruit ripening of tomato

The role of lipids in disease resistance and fruit ripening of tomato was investigated in this study, by expressing a yeast Δ-9 desaturase gene in the transgenic plants, in vitro experiments of unsaturated fatty acids on the fungal spore germination, and by direct application of unsaturated fatty acids on tomato plants. Introduction of the yeast Δ-9 desaturase gene into tomato changed its fatty acid composition. There was dramatic increases of palmitoleic acid in the transgenic plants. There were at least five new fatty acid species found in the transgenic plants, which are not detectable in the wild-type tomato plants. These results suggest that plant fatty acid biosynthesis could be altered by diverting the normal fatty acid synthesis pathway through introducing a single gene controlling the earliest step of fatty aid desaturation. It was discovered that transgenic plants possess high resistance to a wide spectrum of tomato pathogens including fungi, bacteria and viruses. The resistance to tomato powdery mildew was extensively studied. It was found that transgenic plants have constitutively higher lipid peroxidation levels and even higher increases when challenged with pathogens, confirming the involvement of lipid peroxidation in the powdery mildew resistance of tomato. The presence of palmitoleic acid in cuticle layer of transgenic tomato contributed to resistance at invasion stage by inhibiting spore germination and germ tube growth. These results also demonstrate that the modification of cutin chemical composition is one of the approaches of engineering disease-resistant plants. An investigation of a series of fatty acids revealed some important structural features of the fatty acid molecule required for their inhibitory effects on powdery mildew spore germination. Monounsaturated fatty acids with a single Δ-7 or $2$-7 double bond have the highest inhibitory activity on spore germination. The results also suggested that lipids are signal molecules in plant disease resistance. The faster fruit ripening and higher levels of volatile compounds in transgenic fruits indicated that the unsaturated fatty acids and their peroxidation are the driving force for the ethylene production during tomato fruit ripening. Engineering fatty acid biosynthesis is shown to be one of the approaches to modify flavor compound profile of tomato fruits.