Identification and genetic analysis of normal and mutant phytoene synthase genes of tomato by sequencing, complementation and co-suppression.

A tomato phytoene synthase gene, Psy1, has recently been isolated as the clone GTOM5 and shown by sequence identity to be the gene from which the major fruit-ripening cDNA clone TOM5 was derived. Sequence analysis of transcripts from two allelic yellow-fruited tomato mutants, mapped to chromosome 3, has shown the lack of carotenoids in fruit of these mutants to be due to the production of aberrant TOM5 transcripts which are unlikely to encode a functional phytoene synthase enzyme. In one mutant (yellow flesh) the aberrant transcript contained a sequence that, by its strong hybridization to a wide size range of genomic fragments, appeared to be repeated many times within the genome. Southern and PCR analysis of the phytoene synthase genes in the mutant revealed restriction fragment length polymorphisms, suggesting that the production of altered mRNAs was associated with specific genomic rearrangements. Constitutive over-expression of a TOM5 cDNA clone in transgenic mutant plants restored synthesis of the carotenoid lycopene in ripening fruit and also led to unscheduled pigment production in other cell types. In some mutant plants transformed with the TOM5 cDNA construct, inhibition of carotenoid production in immature green fruit, leaves and flowers was observed, due to the phenomenon of co-suppression, indicating that different insertion events with the same gene construct can lead to overexpression or co-suppression in transgenic plants. Green organs of these plants were susceptible to photobleaching, due to the lack of carotenoids. These results suggest the existence of separate Psy genes for carotenoid synthesis in green organs.