Cytological and molecular characterization of carotenoid accumulation in normal and high-lycopene mutant oranges

Peng-Jun Lu,Chang-Jie Xu,Kun-Song Chen,Don Grierson,Si-Lin Zhong,Ting-Ting Yin,Chun-Yan Wang

doi:10.1038/s41598-017-00898-y

Abstract

Ripe Cara Cara sweet orange contains 25 times as much carotenoids in flesh as Newhall sweet orange, due to high accumulation of carotenes, mainly phytoene, lycopene and phytofluene. Only yellow globular chromoplasts were observed in Newhall flesh. Distinct yellow globular and red elongated crystalline chromoplasts were found in Cara Cara but only one type of chromoplast was present in each cell. The red crystalline chromoplasts contained lycopene as a dominant carotenoid and were associated with characteristic carotenoid sequestering structures. The increased accumulation of linear carotenes in Cara Cara is not explained by differences in expression of all 18 carotenogenic genes or gene family members examined, or sequence or abundance of mRNAs from phytoene synthase (PSY) and chromoplast-specific lycopene β-cyclase (CYCB) alleles. 2-(4-Chlorophenylthio)-triethylamine hydrochloride (CPTA) enhanced lycopene accumulation and induced occurrence of red crystalline chromoplasts in cultured Newhall juice vesicles, indicating that carotenoid synthesis and accumulation can directly affect chromoplast differentiation and structure. Norflurazon (NFZ) treatment resulted in high accumulation of phytoene and phytofluene in both oranges, and the biosynthetic activity upstream of phytoene desaturase was similar in Newhall and Cara Cara. Possible mechanisms for high carotene accumulation and unique development of red crystalline chromoplasts in Cara Cara are discussed.

Highlights

Carotenoid pigments are a large family of isoprenoid compounds essential for plant growth and development
The tremendous differences often resulted from no or trace accumulation of carotenoids in a mutant or cultivar, such as the r tomato and white-fleshed loquats caused by the loss of function mutation of SlPSY1 and EjPSY2A14, 19, and Early Moonbeam watermelon[39]
The difference in carotenoid accumulation between two sweet orange cultivars is a different case, since the ordinary orange Newhall contains normal level of carotenoids while the mutant Cara Cara contains over 20 times more in flesh during fruit development

Summary

Introduction

Carotenoid pigments are a large family of isoprenoid compounds essential for plant growth and development. The difference in accumulation of carotenoids in red and yellow cultivars can be partially explained by the variations in expression of biosynthetic genes[20]. In some cases, both nonsense mutation and loss of expression of a gene can be involved, as reported for the capsanthin/capsorubin synthase gene (CCS) in two yellow pepper cultivars[11]. Apart from sequence mutation or altered expression of carotenoid biosynthetic genes, a number of studies have shown that carotenoid accumulation was affected indirectly by chromoplast development. For Orange (Or) cauliflower mutants, high accumulation of β-carotene was explained by enhanced conversion from proplastids and other colourless plastids to chromoplasts resembling flattened sheets[23, 24]

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Conclusion