Phytoene synthase-2 enzyme activity in tomato does not contribute to carotenoid synthesis in ripening fruit

The characteristic yellow fruit phenotype of the r,r mutant and Psy-1 (phytoene synthase-1) antisense tomatoes is due to a mutated or down-regulated phytoene synthase protein, respectively, resulting in the virtual absence of carotenoids. Based on detailed carotenoid determinations Psy-1 appeared to barely contribute to the formation of carotenoids in chloroplast-containing tissues. Despite the virtual absence of carotenoids in ripe fruit the formation of phytoene in vitro was detected in fruit of both mutants. When [14C]isopentenyl pyrophosphate (IPP) was used as the substrate for phytoene synthase a reduction (e.g. r,r mutant, 5-fold) in the formation of phytoene was observed with an accumulation (e.g. r,r mutant, 2-fold) of the immediate precursor geranylgeranyl pyrophosphate (GGPP). Contrastingly, reduced phytoene synthase activity was not detected when [3H]GGPP was used as the substrate. The profile of phytoene formation during ripening was also different in the down-regulated mutants compared to the wild-type. Using specific primers, RT-PCR analysis detected the presence of Psy-2 transcripts in the down-regulated mutants and wild-type throughout fruit development and ripening. These data were supported by the detection of phytoene synthase protein on western blots. Both GGPP formation and phytoene desaturation were elevated in these mutants. Therefore, it appears that despite the absence of carotenoids in ripe fruit, both the mutants have the enzymic capability to synthesize carotenoids in this tissue. Implications of the data with respect to the regulation of carotenoid formation and the channelling of prenyl lipid precursors in tomato (and its potential manipulation) are discussed.