Molecular and functional characterization of a novel chromoplast-specific lycopene β-cyclase from Citrus and its relation to lycopene accumulation

Abstract

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.

DOES <i>Beta vulgaris</i> L. HAVE LCYB GENE?

“…The enzyme catalyze the biosynthesis of carotenoid in Arabidopsis thaliana, maize, and rice encoded by a single gene LCYB,in tomatoes and other plants encoded by family gene: the two enzymes which similar to LCYB are LYCB1/CRTL-B1 (expressed inchloroplasts) and LCYB2/CRTL-B2/CYCB (expressed inchromoplas) (Ruiz-Sola et al, 2012), which in citrus grape fruits LCYB is encoded by two genes,CitLCYB1 and CitLCYB2 which have a similarity of 93%. On the other than, Capsanthin Capsorubin Synthase (CCS) in chilli and Neoxanthin Synthase (NSY) on tomato and potato has a mechanism similar toLCYB (Alquézar et al, 2009). Remain a question does β-carotene syntesis in beet catalyzed by one of those enzymes sgared by other plant or there is a different enzymes doing so or regulated through different pathway?…”

Pigments in Citrus

“…A newly isolated gene of LCYB, CsLCYB2, showed an allele-differential, chromoplast-specific expression, and a marked induction in both peel and pulp of oranges and grapefruits in parallel with the accumulation of β,β-xanthophylls. It was different from CsLCYB1 that was expressed at low levels and remained relatively constant during fruit ripening [67]. One allele of CsLCYB2, expressed only in normally pigmented fruits, has normal LCYB activity, while the other, preferentially expressed in red-fleshed grapefruit, loses that activity and likely leads to the lycopene accumulation [63,67].…”

Mechanism of Carotenoid Accumulation in Citrus Fruit

“…Recently, genes encoding enzymes for the main steps of carotenoid metabolism have been isolated and their expression has been characterized in plants (Alquézar et al, 2009;Kato et al, 2004Kato et al, , 2006Kita et al, 2007). During fruit ripening, transcriptional regulation of carotenoid genes appears to be the major mechanism by which the biosynthesis and accumulation of specific carotenoids are regulated.…”