Shahbazi Maryam, Gilbert Matthias, Labouré Anne-Marie, Kuntz Marcel
Centre National de la Recherche Scientifique and Université Joseph Fourier, Laboratory Plastes et Différenciation Cellulaire, 38041 Grenoble, France.
Plant Physiol. 2007 Nov;145(3):691-702. doi: 10.1104/pp.107.106336. Epub 2007 Sep 14.
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.
质体末端氧化酶（PTOX）是一种质体醌氧化酶，番茄（Solanum lycopersicum）中缺乏该酶会导致“幽灵”（gh）表型，其特征为叶片杂色（有绿色和漂白区域）以及成熟果实类胡萝卜素缺乏。我们发现，PTOX缺乏主要由于gh突变体中类胡萝卜素合成能力降低，导致暴露于强光下的子叶发生光漂白，这与PTOX作为八氢番茄红素去饱和酶辅因子的已知作用一致。相比之下，当gh植株长出完全绿色的成叶并置于光漂白强光条件下时，未获得类胡萝卜素生物合成存在缺陷的证据。相反，一致的证据表明，PTOX的缺失通过扰乱质体醌氧化还原状态，使番茄叶片光合装置对光更敏感。尽管gh果实通常会漂白（很可能是由于早期发育阶段类胡萝卜素生物合成缺陷所致），但可以获得绿色成熟果实并将其置于光漂白强光条件下。同样，我们的数据表明PTOX在成年绿色果实光合电子传递调节中发挥作用，而非主要致力于类胡萝卜素生物合成。相比之下，成熟果实的类胡萝卜素去饱和主要依赖PTOX和质体完整性。总之，我们的数据显示了PTOX的双重作用。其活性在某些发育阶段对某些器官高效的类胡萝卜素去饱和是必需的，但并非对所有器官和阶段都如此，这表明存在一条与八氢番茄红素去饱和酶相关的不依赖PTOX的质体醌再氧化途径。作为第二个作用，PTOX参与绿色组织中的叶绿素呼吸机制。
