Glenn E. Bartley

Regulation of carotenoid biosynthesis during tomato development.

Phytoene synthase (Psy) and phytoene desaturase (Pds) are the first dedicated enzymes of the plant carotenoid biosynthesis pathway. We report here the organ-specific and temporal expression of PDS and PSY in tomato plants. Light increases the carotenoid content of seedlings but has little effect on PDS and PSY expression. Expression of both genes is induced in seedlings of the phytoene-accumulating mutant ghost and in wild-type seedlings treated with the Pds inhibitor norflurazon. Roots, which contain the lowest levels of carotenoids in the plant, have also the lowest levels of PDS and PSY expression. In flowers, expression of both genes and carotenoid content are higher in petals and anthers than in sepals and carpels. During flower development, expression of both PDS and PSY increases more than 10-fold immediately before anthesis. During fruit development, PSY expression increases more than 20-fold, but PDS expression increases less than threefold. We concluded that PSY and PDS are differentially regulated by stress and developmental mechanisms that control carotenoid biosynthesis in leaves, flowers, and fruits. We also report that PDS maps to chromosome 3, and thus it does not correspond to the GHOST locus, which maps to chromosome 11.

Control of leaf and chloroplast development by the Arabidopsis gene pale cress.

Leaf plastids of the Arabidopsis pale cress (pac) mutant do not develop beyond the initial stages of differentiation from proplastids or etioplasts and contain only low levels of chlorophylls and carotenoids. Early in development, the epidermis and mesophyll of pac leaves resemble those of wild-type plants. In later stages, mutant leaves have enlarged intercellular spaces, and the palisade layer of the mesophyll can no longer be distinguished. To study the molecular basis of this phenotype, we cloned PAC and determined that this gene is regulated by light and has the capacity to encode an acidic, predominantly alpha-helical protein. The PAC gene appears to be a novel component of a light-induced regulatory network that controls the development of leaves and chloroplasts.

Functional analysis and purification of enzymes for carotenoid biosynthesis expressed in photosynthetic bacteria.

Publisher Summary This chapter discusses methods for the inducible expression of carotenoid biosynthesis genes in Rhodobacter capsulatus and Escherichia coli along with novel procedures for the purification of phytoene desaturases. Carotenoid desaturases are membrane-bound enzymes in both bacteria and plants. The functional expression of Neurospora crassa and Glycine max (soybean) cDNAs coding for carotenoid desaturases in mutants of the photosynthetic bacterium Rhodobacter capsulatus have been achieved. Thus, it is logical to expect functional expression in R. capsulatus of carotenoid desaturase cDNAs from other species. In addition, it is possible that cDNAs coding for carotenoid enzymes other than desaturases could be expressed in R. capsulatus carotenoid mutants. Phytoene desaturases from various organisms catalyze a different number of desaturations. Rhodobacter capsulatus phytoene desaturase converts phytoene to neurosporene, a three-step desaturation reaction. Neurospora crassa Al-1 catalyzes six desaturations. Soybean Pds1 converts phytoene to ζ-carotene, a two-step desaturation reaction. Expression of al-1 in a phytoene-accumulating strain of R. capsulatus leads to the accumulations of lycopene and 3,4 dehydrolycopene. Expression of pds in this bacterial strain results in the accumulation of ζ-carotene. Lycopene, 3,4-dehydrolycopene, and ζ-carotene are not normally accumulated in R. capsulatus .

New Genetic Tools for Rhodobacter capsulatus and Structure and Expression of Genes for Carotenoid Biosynthesis

Photosynthetic bacteria are currently used as model systems for the study of genetic, biochemical and physical aspects of photosynthesis. In this article we briefly review genetic methods that we developed for use in Rhodobacter capsulatus and we present recent results on the molecular biology of carotenoid biosynthesis.