Flavonoids are a diverse group of phenolic secondary metabolites that occur naturally in plants and therefore form an integral component of the human diet. Many of the compounds belonging to this group are potent antioxidants in vitro and epidemiological studies suggest a direct correlation between high flavonoid intake and decreased risk of cardiovascular disease, cancer and other age-related diseases. Enhancing flavonoid biosynthesis in chosen crops may provide new raw materials that have the potential to be used in foods designed for specific benefits to human health. Using genetic modification, it was possible to generate several tomato lines with significantly altered flavonoid content and to probe the role and importance of several key enzymatic steps in the tomato flavonoid biosynthetic pathway. Most notably an up to 78-fold increase in total fruit flavonols was achieved through ectopic expression of a single biosynthetic enzyme, chalcone isomerase. In addition, chalcone synthase and flavonol synthase transgenes were found to act synergistically to up-regulate flavonol biosynthesis significantly in tomato flesh tissues.

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N. tabacum lines that lacked functional Rubisco were transformed with plasmids encoding a chloroplast transit peptide in frame with C. vinosum rbcL and stable transformants generated. However, the transgene was transcribed at a low level and no Rubisco activity or C. vinosum large subunits were detectable in any line.

Three clonal genotypes of Lotus corniculatus L. (bird's foot trefoil) were transformed with an antisense chalcone synthase (CHS) gene construct made using a stress induced CHS17 cDNA from Phaseolus vulgaris under the control of the constitutive CaMV 35S promoter and Nos terminator via Agrobacterium rhizogenes. After initial screening, ten antisense and five control co-transformation events from each recipient clonal genotype were analysed. After elicitation with glutathione, the level of tannin accumulation was found to be increased in a number of antisense root cultures derived from the low (S33) and moderate (S50) tannin recipient genotypes. Six antisense and four control transformed lines from genotype S50 were selected for more detailed study. The antisense CHS construct was found to be integrated into the genome, with a copy number ranging from 1 to 5 and antisense orientation was confirmed by PCR. In transformed root cultures, increased CHS transcript levels were noted in a number of antisense lines. Biochemical analyses of glutathione-elicited-root cultures indicated a significant increase in tannin accumulation in antisense CHS lines and mean vestitol levels were reduced. These results show that the introduction of a heterologous antisense chalcone synthase construct into L. corniculatus resulted in an unpredicted molecular and biochemical phenotype. Such findings are discussed in relation to manipulation of this complex multigene family.

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