title: Comprehending crystalline β-carotene accumulation by comparing engineered cell models and the natural carotenoid-rich system of citrus.
abstract: Genetic manipulation of carotenoid biosynthesis has become a recent focus for the alleviation of vitamin A deficiency. However, the genetically modified phenotypes often challenge the expectation, suggesting the incomplete comprehension of carotenogenesis. Here, embryogenic calli were engineered from four citrus genotypes as engineered cell models (ECMs) by over-expressing a bacterial phytoene synthase gene (CrtB). Ripe flavedos (the coloured outer layer of citrus fruits), which exhibit diverse natural carotenoid patterns, were offered as a comparative system to the ECMs. In the ECMs, carotenoid patterns showed diversity depending on the genotypes and produced additional carotenoids, such as lycopene, that were absent from the wild-type lines. Especially in the ECMs from dark-grown culture, there emerged a favoured β,β-pathway characterized by a striking accumulation of β-carotene, which was dramatically different from those in the wild-type calli and ripe flavedos. Unlike flavedos that contained a typical chromoplast development, the ECMs sequestered most carotenoids in the amyloplasts in crystal form, which led the amyloplast morphology to show a chromoplast-like profile. Transcriptional analysis revealed a markedly flavedo-specific expression of the β-carotene hydroxylase gene (HYD), which was suppressed in the calli. Co-expression of CrtB and HYD in the ECMs confirmed that HYD predominantly mediated the preferred carotenoid patterns between the ECMs and flavedos, and also revealed that the carotenoid crystals in the ECMs were mainly composed of β-carotene. In addition, a model is proposed to interpret the common appearance of a favoured β,β-pathway and the likelihood of carotenoid degradation potentially mediated by photo-oxidation and vacuolar phagocytosis in the ECMs is discussed.
authors.affiliation: Key Laboratory of Horticultural Plant Biology (Ministry of Education), National Key Laboratory of Crop Genetic Improvement, Huazhong Agricultural University, Wuhan, Hubei 430070, China.
authors.forename: Hongbo
authors.initials: H
authors.lastname: Cao
authors.forename: Jiancheng
authors.initials: J
authors.lastname: Zhang
authors.forename: Jidi
authors.initials: J
authors.lastname: Xu
authors.forename: Junli
authors.initials: J
authors.lastname: Ye
authors.forename: Ze
authors.initials: Z
authors.lastname: Yun
authors.forename: Qiang
authors.initials: Q
authors.lastname: Xu
authors.forename: Juan
authors.initials: J
authors.lastname: Xu
authors.forename: Xiuxin
authors.initials: X
authors.lastname: Deng
mesh_terms: D019883:Alkyl and Aryl Transferases / Q000235:genetics / Q000378:metabolism; D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002338:Carotenoids / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D002957:Citrus / Q000737:chemistry* / Q000201:enzymology / Q000235:genetics / Q000648:ultrastructure; D004885:Erwinia / Q000201:enzymology / Q000235:genetics / Q000276:immunology; D005638:Fruit / Q000737:chemistry / Q000201:enzymology / Q000235:genetics / Q000648:ultrastructure; D018506:Gene Expression Regulation, Plant; D005838:Genotype; D051232:Geranylgeranyl-Diphosphate Geranylgeranyltransferase; D046529:Microscopy, Electron, Transmission; D006899:Mixed Function Oxygenases / Q000235:genetics / Q000378:metabolism; D008954:Models, Biological; D012275:Oryza / Q000201:enzymology / Q000235:genetics; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified; D018087:Plastids / Q000737:chemistry* / Q000201:enzymology / Q000235:genetics / Q000648:ultrastructure; D011817:Rabbits; D046509:Tissue Culture Techniques; D014815:Vitamins / Q000235:genetics / Q000378:metabolism*; D019207:beta Carotene / Q000032:analysis / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D010940:Plant Proteins; D014815:Vitamins; D019207:beta Carotene; D002338:Carotenoids; D006899:Mixed Function Oxygenases; C079232:beta-carotene hydroxylase; D019883:Alkyl and Aryl Transferases; D051232:Geranylgeranyl-Diphosphate Geranylgeranyltransferase