JXB Advance Access originally published online on March 26, 2009
Journal of Experimental Botany 2009 60(6):1783-1797; doi:10.1093/jxb/erp048
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© 2009 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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RESEARCH PAPER |
Molecular and functional characterization of a novel chromoplast-specific lycopene β-cyclase from Citrus and its relation to lycopene accumulation
Instituto de Agroquímica y Tecnología de Alimentos (IATA), Consejo Superior de Investigaciones Científicas (CSIC), Apartado de Correos 73, E-46100 Burjassot, Valencia, Spain
* To whom correspondence should be addressed: E-mail: mjrodrigo{at}iata.csic.es
Carotenoids are the main pigments responsible of the colouration of Citrus fruits. The β-cyclization of lycopene, catalysed by the lycopene β-cyclases (β-LCY), seems to be a key regulatory step of the carotenoid pathway. In the present study, two β-LCYs from orange fruits (Citrus sinensis), named Csβ-LCY1 and Csβ-LCY2 have been isolated and the activity of the encoded proteins was demonstrated by functional analysis. Csβ-LCY1 was expressed at low levels and remained relatively constant during fruit ripening while Csβ-LCY2 showed a chromoplast-specific expression and a marked induction in both peel and pulp of orange fruits in parallel with the accumulation of β,β-xanthophylls. The potential involvement of Csβ-LCY2 in the accumulation of lycopene, characteristic of some Citrus species such as red grapefruits, was investigated. Expression of Csβ-LCY2 and another seven carotenoid biosynthetic genes were studied in the peel and pulp of the high lycopene-accumulating grapefruit, Star Ruby, and compared with those of ordinary Navel orange. In Star Ruby, the accumulation of lycopene during fruit maturation was associated with a substantial reduction in the expression of both β-LCY2 and β-CHX genes with respect to Navel orange. Moreover, two different alleles of β-LCY2: β-LCY2a and β-LCY2b were isolated from both genotypes, and functional assays demonstrated that the lycopene β-cyclase activity of the allele b was almost null. Interestingly, Star Ruby grapefruit predominantly expressed the unfunctional β-LCY2b allele during fruit ripening whereas Navel oranges preferably expressed the functional allele. It is suggested that the presence of diverse alleles of the β-LCY2 gene, encoding enzymes with altered activity, with different transcript accumulation may be an additional regulatory mechanism of carotenoid synthesis involved in the accumulation of lycopene in red grapefruits.
Key words: Carotenoids, Citrus fruit, gene expression, grapefruit, lycopene, lycopene β-cyclase, orange fruit
Received 10 December 2008; Revised 4 February 2009 Accepted 5 February 2009
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