To date, vitamin A deficiency remains a significant global health problem. Improving the carotenoid content in food crops to provide provitamin A could alleviate this nutritional problem. To achieve this goal, a complete carotenoid biosynthetic pathway and well-characterized regulatory mechanism (s) are required. The carotenoid biosynthetic pathway remains uncertain at its isomerization steps although most parts are well established. Despite ongoing regulation studies of carotenoid biosynthesis in several model organisms, few of these studies involve members of the Grass family (Poaceae), which contains most of the agronomically important crops. Thus, I chose maize as a model for the Grass family to study the regulation of the carotenoid biosynthetic pathway.
The first committed step of the carotenoid biosynthetic pathway is catalyzed by phytoene synthase or PSY, which is thought to be a rate-controlling enzyme. Therefore, genes encoding PSY were selected to study the regulation of carotenoid biosynthesis in maize. My data showed that the maize genome possesses three gene copies encoding functional PSY enzymes. Transcript assays revealed that PSY1 but not PSY2 or PSY3 mRNA levels were correlated with carotenoid accumulation in endosperm. On the other hand, the up-regulation of PSY2 transcripts was associated with leaf carotenogenesis during greening. Maize PSY3 expresses predominately in root and embryo tissues, where carotenoids accumulate at negligible levels. PSY3 transcript levels in roots are induced in response to various abiotic stresses and are associated with accumulation of abscisic acid (ABA).
The carotenoid composition analysis of maize kernel mutants also led to the genetic definition of a new isomerization step required for carotenoid biosynthesis. HPLC analysis revealed that dark-grown tissues, such as roots and etiolated leaves of the maize y9 mutant condition accumulation of the product of phytoene desaturase (PDS), 9,15,9'-tri-cis-ζ-carotene. This finding suggests that maize Y9 encodes a factor required for an isomerase activity which is necessary for converting the product of PDS (9,15,9'-tri-cis-ζ-carotene) into the suitable ZDS substrate (9,9'-di-cis-ζ-carotene); this enzyme activity I named Z-ISO (15-cis zeta-carotene isomerase). In the future, cloning of the maize y9 locus will be critical for elucidation of the isomerization reactions in the carotenoid biosynthetic pathway of plants.
|Advisor:||Wurtzel, Eleanore T.|
|Commitee:||Calhoun, David H., Cheng, Hai-Ping, Kennelly, Edward, Short, Timothy W.|
|School:||City University of New York|
|School Location:||United States -- New York|
|Source:||DAI-B 69/03, Dissertation Abstracts International|
|Keywords:||Carotenoid, Carotenoid biosynthesis, Phytoene synthase|
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