Phytoene synthase genes in tomato (Solanumlycopersicum L.) - new data on the structures, the deduced amino acid sequences and the expression patterns

The fruit of tomato (Solanum lycopersicum L.) is a berry: red, fleshy and rich in seeds. Its colour is due to the high content of lycopene whose synthesis is activated by the phytoene synthase 1 (PSY1) enzyme, encoded by Psy1 which is distinct from Psy2. In the present study, we report on the genomic structures of the Psy1 and Psy2 genes and on their transcription patterns in different tomato tissues. Our results have completely clarified the structure of the Psy1 and Psy2 genes in the coding sequence region. The two genes were shown to have an highly conserved structure, with seven exons being almost identical and six introns being much more variable. For Psy1 and Psy2, respectively, the sequenced regions were 4527 and 3542 bp long, the coding sequences were 1239 bp and 1317 bp long, whereas the predicted protein sequences were 412 and 438 amino acids. The two proteins are almost identical in the central region, whereas most differences are present in the N-terminus and C-terminus. Quantitative real time PCR analysis showed that Psy2 transcript was present in all tested plant tissues, whereas Psy1 transcript could be detected in chromoplast-containing tissues, particularly in fruit where it activates and boosts lycopene accumulation. Interestingly, the organ with the highest relative content of Psy2 transcript is the petal and not the leaf. Psy1 is a Psy2 paralog derived through a gene duplication event that have involved other genes encoding rate controlling enzymes of the carotenoid pathway. Duplicate genes have been recruited to allow carotenoid synthesis in petals and fruits. However, recruitment of carotenoid metabolism for fruit pigmentation could have occurred later in the evolution, either because phytoene synthase gene duplication occurred later or because the fruit pigmentation process required a more sophisticated mechanism involving tight control of the transcription of other genes.