CAROTENOID BIOSYNTHESIS DURING TOMATO FRUIT-DEVELOPMENT

Tomato (Lycopersicon esculentum Mill. cv Ailsa Craig) fruit, at five s tages of development, have been analyzed for their carotenoid and chlo rophyll (Chi) contents, in vitro activities of phytoene synthase, phyt oene desaturase, and lycopene cyclase, as well as expression of the ph ytoene synthase (Psy) and phytoene desaturase (Pds) genes. During ripe ning, the total carotenoids increased with a concomitant decrease in C hi. Although the highest carotenoid content (consisting mainly of lyco pene and a-carotene) was found in ripe fruit, the greatest carotenogen ic enzymic activities were found in green fruit. Phytoene synthase was located in the plastid stroma, whereas the metabolism of phytoene was associated with plastid membranes during all stages of fruit developm ent. The in vitro products of phytoene desaturation altered from being predominantly phytofluene and zeta-carotene in chloroplasts to becomi ng mainly lycopene in chromoplasts. The expression of Psy was detected in breaker and ripe fruit, as well as flowers, but was not detectable by northern blot analysis in leaves or green fruits. The Pds gene tra nscript was barely detectable in green fruit and leaves but was expres sed in flowers and breaker fruit. These results suggest that transcrip tion of Psy and Pds is regulated developmentally, with expression bein g considerably elevated in chromoplast-containing tissues. Antiserum t o the Synechococcus phytoene synthase cross-reacted with phytoene synt hase of green fruit only on western blots and not with the enzyme from ripe fruit. In contrast, a monoclonal antibody to the Psy gene produc t only cross-reacted with phytoene synthase from ripe fruit. The enzym es from green and ripe fruit had different molecular masses of 42 and 38 kD, respectively. The absence of detectable Psy and Pds mRNA in gre en tissues using northern blot analyses, despite high levels of phytoe ne synthase and desaturase activity, lends support to the hypothesis o f divergent genes encoding these enzymes.