Phytosterols are important structural components of plant membranes and stabilize phospholipid bilay-ers in plant cell membranes. Moreau et al. (2002) and Piironen et al. (2000) have recently reviewed phytos-terols in foods and their health promoting properties. Phytosterols have received increased attention in the last 10 years because of their cholesterol-lowering properties. The primary interest in tomato phytos-terols has focused on ripening related events involving phospholipids and their catabolism mediated by phospholipase D (Whitaker et al. 2001; Pinhero et al. 2003). Transformation of tomato with an antisense phospholipase D cDNA construct reduced phospholipase D activity 30 to 40% and resulted in firmer fruit with enhanced lycopene content, vitamin C, and flavor (Oke et al. 2003). Juice and sauce prepared from the transgenic fruit exhibited increased viscosity and levels of major flavor volatiles. Increased membrane stability due to decreased phospholipase D activity may account for the observed enhancement in fruit constituents.
Related studies highlight the potential for genetic modification of the, albeit low, composition of fatty acids in tomato fruit and effect changes in fatty acid composition and flavor volatile compounds. Using metabolic profiling, Schauer et al. (2005) determined that fruit fatty acids 16:0, 18:0, and 18:2 were higher in all wild tomato species with the exception of S. pimpinellifolium. Wang et al. (2001) demonstrated that expression of the yeast A9-desaturase gene in tomato resulted in changes in leaf fatty acid profiles that were accompanied by changes in volatiles derived from fatty acids. Using a similar strategy, Cook et al. (2002) expressed a A6-desaturase transgene in tomato resulting in fruit that produced y-linolenic acid, octadecatetraenoic acid, and a reduced linoleic: a-linolenic acid ratio. Additional studies suggest that a chloroplast-targeted lipoxygenase, TomloxC, can utilize both linoleic and linolenic acids as substrates to generate volatile flavor compounds (Chen et al. 2004).
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