Accumulation of carotenoids and expression of carotenoid biosynthetic genes during maturation in citrus fruit.
The relationship between carotenoid accumulation and the expression of carotenoid biosynthetic genes during fruit maturation was investigated in three citrus varieties, Satsuma mandarin (Citrus unshiu Marc.), Valencia orange (Citrus sinensis Osbeck), and Lisbon lemon (Citrus limon Burm.f.). We cloned the cDNAs for phytoene synthase (CitPSY), phytoene desaturase (CitPDS), zeta-carotene (car) desaturase (CitZDS), carotenoid isomerase (CitCRTISO), lycopene beta-cyclase (CitLCYb), beta-ring hydroxylase (CitHYb), zeaxanthin (zea) epoxidase (CitZEP), and lycopene epsilon-cyclase (CitLCYe) from Satsuma mandarin, which shared high identities in nucleotide sequences with Valencia orange, Lisbon lemon, and other plant species. With the transition of peel color from green to orange, the change from beta,epsilon-carotenoid (alpha-car and lutein) accumulation to beta,beta-carotenoid (beta-car, beta-cryptoxanthin, zea, and violaxanthin) accumulation was observed in the flavedos of Satsuma mandarin and Valencia orange, accompanying the disappearance of CitLCYe transcripts and the increase in CitLCYb transcripts. Even in green fruit, high levels of beta,epsilon-carotenoids and CitLCYe transcripts were not observed in the juice sacs. As fruit maturation progressed in Satsuma mandarin and Valencia orange, a simultaneous increase in the expression of genes (CitPSY, CitPDS, CitZDS, CitLCYb, CitHYb, and CitZEP) led to massive beta,beta-xanthophyll (beta-cryptoxanthin, zea, and violaxanthin) accumulation in both the flavedo and juice sacs. The gene expression of CitCRTISO was kept low or decreased in the flavedo during massive beta,beta-xanthophyll accumulation. In the flavedo of Lisbon lemon and Satsuma mandarin, massive accumulation of phytoene was observed with a decrease in the transcript level for CitPDS. Thus, the carotenoid accumulation during citrus fruit maturation was highly regulated by the coordination of the expression among carotenoid biosynthetic genes. In this paper, the mechanism leading to diversity in beta,beta-xanthophyll compositions between Satsuma mandarin and Valencia orange was also discussed on the basis of the substrate specificity of beta-ring hydroxylase and the balance of expression between upstream synthesis genes (CitPSY, CitPDS, CitZDS, and CitLCYb) and downstream synthesis genes (CitHYb and CitZEP).
Kato M
，Ikoma Y
，Matsumoto H
，Sugiura M
，Hyodo H
，Yano M
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Functional characterization and comparison of lycopene epsilon-cyclase genes in Nicotiana tabacum.
Lycopene epsilon-cyclase (ε-LCY) is a key enzyme in the carotenoid biosynthetic pathway (CBP) of higher plants. In previous work, we cloned two Ntε-LCY genes from allotetraploid tobacco (Nicotiana tabacum), Ntε-LCY2 and Ntε-LCY1, and demonstrated the overall effect of Ntε-LCY genes on carotenoid biosynthesis and stress resistance. However, their genetic and functional characteristics require further research in polyploid plants.
Here, we used CRISPR/Cas9 to obtain Ntε-LCY2 and Ntε-LCY1 mutants in allotetraploid N.tabacum K326. Ntε-LCY2 and Ntε-LCY1 had similar promoter cis-acting elements, including light-responsive elements. The Ntε-LCY genes were expressed in roots, stems, leaves, flowers, and young fruit, and their highest expression levels were found in leaves. Ntε-LCY2 and Ntε-LCY1 genes responded differently to normal light and high light stress. Both the Ntε-LCY2 and the Ntε-LCY1 mutants had a more rapid leaf growth rate, especially ntε-lcy2-1. The expression levels of CBP genes were increased in the ntε-lcy mutants, and their total carotenoid content was higher. Under both normal light and high light stress, the ntε-lcy mutants had higher photosynthetic capacities and heat dissipation levels than the wild type, and this was especially true of ntε-lcy2-1. The reactive oxygen species content was lower in leaves of the ntε-lcy mutants.
In summary, the expression patterns and biological functions of the Ntε-LCY genes Ntε-LCY1 and Ntε-LCY2 differed in several respects. The mutation of Ntε-LCY2 was associated with a greater increase in the content of chlorophyll and various carotenoid components, and it enhanced the stress resistance of tobacco plants under high light.
Song W
，Wei F
，Gao S
，Dong C
，Hao J
，Jin L
，Li F
，Wei P
，Guo J
，Wang R
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《BMC PLANT BIOLOGY》
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