Dual role of the plastid terminal oxidase in tomato

The plastid terminal oxidase (PTOX) is a plastoquinol oxidase whose absence in tomato (Solanum lycopersicum) results in the ghost (gh) phenotype characterized by variegated leaves (with green and bleached sectors) and by carotenoid-deficient ripe fruit. We show that PTOX deficiency leads to photobleaching in cotyledons exposed to high light primarily as a consequence of reduced ability to synthesize carotenoids in the gh mutant, which is consistent with the known role of PTOX as a phytoene desaturase cofactor. In contrast, when entirely green adult leaves from gh were produced and submitted to photobleaching high light conditions, no evidence for a deficiency in carotenoid biosynthesis was obtained. Rather, consistent evidence indicates that the absence of PTOX renders the tomato leaf photosynthetic apparatus more sensitive to light via a disturbance of the plastoquinone redox status. Although gh fruit are normally bleached (most likely as a consequence of a deficiency in carotenoid biosynthesis at an early developmental stage), green adult fruit could be obtained and submitted to photobleaching high light conditions. Again, our data suggest a role of PTOX in the regulation of photosynthetic electron transport in adult green fruit, rather than a role principally devoted to carotenoid biosynthesis. In contrast, ripening fruit are primarily dependent on PTOX and on plastid integrity for carotenoid desaturation. In summary, our data show a dual role for PTOX. Its activity is necessary for efficient carotenoid desaturation in some organs at some developmental stages, but not all, suggesting the existence of a PTOX-independent pathway for plastoquinol reoxidation in association with phytoene desaturase. As a second role, PTOX is implicated in a chlororespiratory mechanism in green tissues.