The CauliflowerOrGene Encodes a DnaJ Cysteine-Rich Domain-Containing Protein That Mediates High Levels of β-Carotene Accumulation

Abstract

Despite recent progress in our understanding of carotenogenesis in plants, the mechanisms that govern overall carotenoid accumulation remain largely unknown. The Orange (Or) gene mutation in cauliflower (Brassica oleracea var botrytis) confers the accumulation of high levels of β-carotene in various tissues normally devoid of carotenoids. Using positional cloning, we isolated the gene representing Or and verified it by functional complementation in wild-type cauliflower. Or encodes a plastid-associated protein containing a DnaJ Cys-rich domain. The Or gene mutation is due to the insertion of a long terminal repeat retrotransposon in the Or allele. Or appears to be plant specific and is highly conserved among divergent plant species. Analyses of the gene, the gene product, and the cytological effects of the Or transgene suggest that the functional role of Or is associated with a cellular process that triggers the differentiation of proplastids or other noncolored plastids into chromoplasts for carotenoid accumulation. Moreover, we demonstrate that Or can be used as a novel genetic tool to induce carotenoid accumulation in a major staple food crop. We show here that controlling the formation of chromoplasts is an important mechanism by which carotenoid accumulation is regulated in plants.

Arabidopsis Cotyledon-Specific Chloroplast Biogenesis Factor CYO1 Is a Protein Disulfide Isomerase

“…DnaJ proteins are molecular chaperones that specifically regulate DnaK-like proteins involved in protein folding. Hsp70s, DnaK homologs in chloroplasts, play an important role in a number of processes (Liu et al, 2005;Lu et al, 2006). However, CYO1 lacks the J domain responsible for stimulating DnaK ATPase activity.…”

“…Or is involved in the differentiation of noncolored plastids into chromoplasts for carotenoid accumulation (Lu et al, 2006). BSD2 is required for posttranslational regulation of the L subunit of ribulose-1,5-bis-phosphate carboxylase/oxygenase (Rubisco) (Brutnell et al, 1999).…”

Carotenoid Biosynthesis Regulatory Mechanisms in Plants

“…The Or gene has been introduced into the potato cultivar Desiree where expression of the Or transgene in tubers of transgenic lines led to increased levels of violaxanthin, lutein, and β-carotene. Their total carotenoid level was approximately six-fold higher than in the non-transformed or vector-only controls 43 .…”

“…The Y gene has been mapped to chromosome 3 by Bonierbale et al 42 . Recently, Lu et al 43 demonstrated that the Or gene mutation in cauliflower induces differentiation of β-carotene-containing chromoplasts in non-pigmented curd tissue. Chromoplasts are carotenoid-accumulating plastids found in many fruits and flowers, whereas the Or gene is a dominant mutation that confers orange pigmentation to the normally unpigmented curd and apical meristem of cauliflower.…”

“…Chromoplasts are carotenoid-accumulating plastids found in many fruits and flowers, whereas the Or gene is a dominant mutation that confers orange pigmentation to the normally unpigmented curd and apical meristem of cauliflower. This gene is highly expressed in tissues containing non-green plastids, such as apical shoots and young leaves in cauliflower and Arabidopsis 43 . The Or gene mutation induces accumulation of high levels of β-carotene in various tissues that are normally devoid of carotenoids.…”