Carotenoids, important lipid-soluble antioxidants in photosynthetic tissues, are known to be completely absent in rice endosperm. Many studies, involving transgenic manipulations of carotenoid biosynthesis genes, have been performed to get carotenoid-enriched rice grain. Study of genes involved in their biosynthesis can provide further information regarding the abundance/absence of carotenoids in different tissues. We have identified 16 and 34 carotenoid biosynthesis genes in rice and Populus genomes, respectively. A detailed analysis of the domain structure of carotenoid biosynthesis enzymes in rice, Populus and Arabidopsis has shown that highly conserved catalytic domains, along with other domains, are present in these proteins. Phylogenetic analysis of rice genes with Arabidopsis and other characterized carotenoid biosynthesis genes has revealed that homologous genes exist in these plants, and the duplicated gene copies probably adopt new functions. Expression of rice and Populus genes has been analyzed by full-length cDNA- and EST-based expression profiling. In rice, this analysis was complemented by real-time PCR, microarray and signature-based expression profiling, which reveal that carotenoid biosynthesis genes are highly expressed in light-grown tissues, have differential expression pattern during vegetative/reproductive development and are responsive to stress.
Tocochromanols are an important group of plastidic lipophilic antioxidants that form an essential part of human diet and play important functions in photosynthetic organisms by protecting them from photo-oxidation, lipid peroxidation and membrane damage. Molecular genetics and genomics-based approaches have revealed the genes required for synthesis of these compounds in model organisms like rice, Arabidopsis and Synechocystis. To create a positive impact on human nutrition and health, the levels of total and specific tocochromanols have been altered in various agricultural crops by metabolic engineering. To understand the mechanisms involved in higher tocochromanol levels of wheat seeds and its germ, the tocochromanol biosynthesis pathway was investigated in wheat. The focus of this research was towards isolation of genes involved in wheat tocochromanol biosynthesis, and homologous and heterologous transgenic manipulation to alter their content and composition. Functional characterisation of TaHydroxyphenylpyruvate dioxygenase and Ta纬-Tocopherol methyltransferase-overexpressing transgenic Arabidopsis plants revealed alterations in tocochromanol content and composition, which suggests better growth of these plants in the presence of sorbitol. TaHydroxyphenylpyruvate dioxygenase-overexpressing transgenic wheat, Triticum aestivum L. plants also showed 2.4-fold increase in tocochromanol content, which may have nutritional as well as antioxidative roles. Further characterisation and field trials of these transgenic lines can provide us more insight about the antioxidative roles of tocochromanols.
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