The Role of Carotenogenic Metabolic Flux in Carotenoid Accumulation and Chromoplast Differentiation: Lessons From the Melon Fruit

Carotenoids have various roles in plant physiology. Plant carotenoids are synthesized in plastids and are highly abundant in the chromoplasts of ripening fleshy fruits. Considerable research efforts have been devoted to elucidating mechanisms that regulate carotenoid biosynthesis, yet, little is known about the mechanism that triggers storage capacity, mainly through chromoplast differentiation. The Orange gene (OR) product stabilizes phytoene synthase protein (PSY) and triggers chromoplast differentiation. OR underlies carotenoid accumulation in orange cauliflower and melon. The OR's 'golden SNP', found in melon, alters the highly evolutionary conserved Arginine¹⁰⁸ to Histidine and controls β-carotene accumulation in melon fruit, in a mechanism yet to be elucidated. We have recently shown that similar carotenogenic metabolic flux is active in non-orange and orange melon fruit. This flux probably leads to carotenoid turnover but known carotenoid turnover products are not detected in non-orange fruit. Arrest of this metabolic flux, using chemical inhibitors or mutations, induces carotenoid accumulation and biogenesis of chromoplasts, regardless of the allelic state of OR. We suggest that the 'golden SNP' induces β-carotene accumulation probably by negatively affecting the capacity to synthesize downstream compounds. The accumulation of carotenoids induces chromoplast biogenesis through a metabolite-induced mechanism. Carotenogenic turnover flux can occur in non-photosynthetic tissues, which do not accumulate carotenoids. Arrest of this flux by the 'golden SNP' or other flux-arrest mutations is a potential tool for the biofortification of agricultural products with carotenoids.

A 'golden' SNP in CmOr governs the fruit flesh color of melon (Cucumis melo)

The flesh color of Cucumis melo (melon) is genetically determined, and can be white, light green or orange, with β-carotene being the predominant pigment. We associated carotenoid accumulation in melon fruit flesh with polymorphism within CmOr, a homolog of the cauliflower BoOr gene, and identified CmOr as the previously described gf locus in melon. CmOr was found to co-segregate with fruit flesh color, and presented two haplotypes (alleles) in a broad germplasm collection, one being associated with orange flesh and the second being associated with either white or green flesh. Allelic variation of CmOr does not affect its transcription or protein level. The variation also does not affect its plastid subcellular localization. Among the identified single nucleotide polymorphisms (SNPs) between CmOr alleles in orange versus green/white-flesh fruit, a single SNP causes a change of an evolutionarily highly conserved arginine to histidine in the CmOr protein. Functional analysis of CmOr haplotypes in an Arabidopsis callus system confirmed the ability of the CmOr orange haplotype to induce β-carotene accumulation. Site-directed mutagenesis of the CmOr green/white haplotype to change the CmOR arginine to histidine triggered β-carotene accumulation. The identification of the 'golden' SNP in CmOr, which is responsible for the non-orange and orange melon fruit phenotypes, provides new tools for studying the Or mechanism of action, and suggests genome editing of the Or gene for nutritional biofortification of crops.

Genetic and chemical characterization of an EMS induced mutation in Cucumis melo CRTISO gene

In order to broaden the available genetic variation of melon, we developed an ethyl methanesulfonate mutation library in an orange-flesh 'Charentais' type melon line that accumulates β-carotene. One mutagenized M2 family segregated for a novel recessive trait, a yellow-orange fruit flesh ('yofI'). HPLC analysis revealed that 'yofI' accumulates pro-lycopene (tetra-cis-lycopene) as its major fruit pigment. The altered carotenoid composition of 'yofI' is associated with a significant change of the fruit aroma since cleavage of β-carotene yields different apocarotenoids than the cleavage of pro-lycopene. Normally, pro-lycopene is further isomerized by CRTISO (carotenoid isomerase) to yield all-trans-lycopene, which is further cyclized to β-carotene in melon fruit. Cloning and sequencing of 'yofI' CRTISO identified two mRNA sequences which lead to truncated forms of CRTISO. Sequencing of the genomic CRTISO identified an A-T transversion in 'yofI' which leads to a premature STOP codon. The early carotenoid pathway genes were up regulated in yofI fruit causing accumulation of other intermediates such as phytoene and ζ-carotene. Total carotenoid levels are only slightly increased in the mutant. Mutants accumulating pro-lycopene have been reported in both tomato and watermelon fruits, however, this is the first report of a non-lycopene accumulating fruit showing this phenomenon.