Expression of xanthophyll biosynthetic genes during light-dependent chloroplast differentiation

Genome-based examination of chlorophyll and carotenoid biosynthesis in Chlamydomonas reinhardtii

The unicellular green alga Chlamydomonas reinhardtii is a particularly important model organism for the study of photosynthesis since this alga can grow heterotrophically, and mutants in photosynthesis are therefore conditional rather than lethal. The recently developed tools for genomic analyses of this organism have allowed us to identify most of the genes required for chlorophyll and carotenoid biosynthesis and to examine their phylogenetic relationships with homologous genes from vascular plants, other algae, and cyanobacteria. Comparative genome analyses revealed some intriguing features associated with pigment biosynthesis in C. reinhardtii; in some cases, there are additional conserved domains in the algal and plant but not the cyanobacterial proteins that may directly influence their activity, assembly, or regulation. For some steps in the chlorophyll biosynthetic pathway, we found multiple gene copies encoding putative isozymes. Phylogenetic studies, theoretical evaluation of gene expression through analysis of expressed sequence tag data and codon bias of each gene, enabled us to generate hypotheses concerning the function and regulation of the individual genes, and to propose targets for future research. We have also used quantitative polymerase chain reaction to examine the effect of low fluence light on the level of mRNA accumulation encoding key proteins of the biosynthetic pathways and examined differential expression of those genes encoding isozymes that function in the pathways. This work is directing us toward the exploration of the role of specific photoreceptors in the biosynthesis of pigments and the coordination of pigment biosynthesis with the synthesis of proteins of the photosynthetic apparatus.

Chlamydomonas reinhardtii in the landscape of pigments

This review focuses on the biosynthesis of pigments in the unicellular alga Chlamydomonas reinhardtii and their physiological and regulatory functions in the context of information gathered from studies of other photosynthetic organisms. C. reinhardtii is serving as an important model organism for studies of photosynthesis and the pigments associated with the photosynthetic apparatus. Despite extensive information pertaining to the biosynthetic pathways critical for making chlorophylls and carotenoids, we are just beginning to understand the control of these pathways, the coordination between pigment and apoprotein synthesis, and the interactions between the activities of these pathways and those for other important cellular metabolites branching from these pathways. Other exciting areas relating to pigment function are also emerging: the role of intermediates of pigment biosynthesis as messengers that coordinate metabolism in the chloroplast with nuclear gene activity, and the identification of photoreceptors and their participation in critical cellular processes including phototaxis, gametogenesis, and the biogenesis of the photosynthetic machinery. These areas of research have become especially attractive for intensive development with the application of potent molecular and genomic tools currently being applied to studies of C. reinhardtii.

Regulation of carotenoid biosynthesis in plants: evidence for a key role of hydroxymethylbutenyl diphosphate reductase in controlling the supply of plastidial isoprenoid precursors

Carotenoids are isoprenoid pigments that function as photoprotectors, precursors of the hormone abscisic acid (ABA), colorants and nutraceuticals. A major problem for the metabolic engineering of high carotenoid levels in plants is the limited supply of their isoprenoid precursor geranylgeranyl diphosphate (GGPP), formed by condensation of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) units usually synthesized by the methylerythritol phosphate (MEP) pathway in plastids. Our earlier work with three of the seven MEP pathway enzymes suggested that the first reaction of the pathway catalyzed by deoxyxylulose 5-phosphate synthase (DXS) is limiting for carotenoid biosynthesis during tomato (Lycopersicon esculentum) fruit ripening. Here we investigate the contribution of the enzyme hydroxymethylbutenyl diphosphate reductase (HDR), which simultaneously synthesizes IPP and DMAPP in the last step of the pathway. A strong upregulation of HDR gene expression was observed in correlation with carotenoid production during both tomato fruit ripening and Arabidopsis thaliana seedling deetiolation. Constitutive overexpression of the tomato cDNA encoding HDR in Arabidopsis did not increase carotenoid levels in etioplasts. By contrast, light-grown transgenic plants showed higher carotenoid levels and an enhanced seed dormancy phenotype suggestive of increased ABA levels. The analysis of double transgenic Arabidopsis plants overproducing both the enzyme taxadiene synthase (which catalyzes the production of the non-native isoprenoid taxadiene from GGPP) and either HDR or DXS showed a twofold stronger effect of HDR in increasing taxadiene levels. Together, the data support a major role for HDR in controlling the production of MEP-derived precursors for plastid isoprenoid biosynthesis.