The molecular basis of a peculiar epistasis of tangerine over yellow-flesh in tomato fruits.

DAVID E. KACHANOVSKY; TAL ISAACSON; JOSEPH HIRSCHBERG

Masidon, Wisconsin

Congreso; PAA/ Solanaceae 2006. Genomics Meets Biodiversity; 2006

Aaccumulation of the red pigment lycopene during ripening of tomato fruits is controlled primarily by transcriptional regulation of carotenoid biosynthesis enzymes. The gene Psy1 encodes a chromoplast-specific phytoene synthase, which catalyzes the first committed step in the pathway. The mutation yellow-flesh (r) is in complete linkeage with Psy1 and abolishes carotenoids in the fruit. The gene CrtISO encodes a carotene isomerase that converts cis-lycopene to all-trans lycopene. In tangerine (t), a recessive null mutation in CrtISO, the fruits accumulate poly-cis carotenoids instead of trans-lycopene. Segregation in F2 of a cross between yellow-flesh and tangerine gave 9:3:4 ratio of wt:r:t phenotypes instead of the expected Mendelian 9:3:3:1, indicating that t is epistatic on r. In linear biosynthetic pathways mutations in genes for early enzymes in a pathway are epistatic on later ones and hence r is supposed to be epistatic on t. The molecular basis for this peculiarity was resolved when transcript levels of Psy-1 and CrtISO were measured in wt and single and double mutants. It was determined that in yellow-flesh transcription of Psy1 is reduced ca. 1000 folds relative to wt. In the tangerine/yellow-flesh double mutants, transcription of Psy1 is up-regulated 100 folds as compared to yellow-flesh, thus enabling the production of enough phytoene to allow the progress of pathway. These results suggest a feedback regulation mechanism of transcription of a carotenoid gene by products of the pathway, and provide a molecular explanation for epistasis of t over r.