期刊
BMC PLANT BIOLOGY
卷 19, 期 1, 页码 -出版社
BMC
DOI: 10.1186/s12870-019-2078-2
关键词
Carotenoid; zeta-Carotene isomerase; Citrus fruit; gene expression; Mutant; Pigmentation; Ripening
资金
- Ministerio Economia y Competitividad, Spain [AGL2012-34576, AGL2015-70218]
- Ministerio Ciencia, Innovacion y Universidades, Spain [RTI2018-095131-BI00]
- Israel Science Foundation [850/13]
- COST_Action [CA15136]
- CaRed (Spanish Carotenoid Network, Ministerio de Ciencia, Innovacion y Universidades, Spain) [BIO2017-90877-REDT]
Background Fruit coloration is one of the main quality parameters of Citrus fruit primarily determined by genetic factors. The fruit of ordinary sweet orange (Citrus sinensis) displays a pleasant orange tint due to accumulation of carotenoids, representing beta,beta-xanthophylls more than 80% of the total content. 'Pinalate' is a spontaneous bud mutant, or somatic mutation, derived from sweet orange 'Navelate', characterized by yellow fruits due to elevated proportions of upstream carotenes and reduced beta,beta-xanthophylls, which suggests a biosynthetic blockage at early steps of the carotenoid pathway. Results To identify the molecular basis of 'Pinalate' yellow fruit, a complete characterization of carotenoids profile together with transcriptional changes in carotenoid biosynthetic genes were performed in mutant and parental fruits during development and ripening. 'Pinalate' fruit showed a distinctive carotenoid profile at all ripening stages, accumulating phytoene, phytofluene and unusual proportions of 9,15,9 '-tri-cis- and 9,9 '-di-cis-zeta-carotene, while content of downstream carotenoids was significantly decreased. Transcript levels for most of the carotenoid biosynthetic genes showed no alterations in 'Pinalate'; however, the steady-state level mRNA of zeta-carotene isomerase (Z-ISO), which catalyses the conversion of 9,15,9 '-tri-cis- to 9,9 '-di-cis-zeta-carotene, was significantly reduced both in 'Pinalate' fruit and leaf tissues. Isolation of the 'Pinalate' Z-ISO genomic sequence identified a new allele with a single nucleotide insertion at the second exon, which generates an alternative splicing site that alters Z-ISO transcripts encoding non-functional enzyme. Moreover, functional assays of citrus Z-ISO in E.coli showed that light is able to enhance a non-enzymatic isomerization of tri-cis to di-cis-zeta-carotene, which is in agreement with the partial rescue of mutant phenotype when 'Pinalate' fruits are highly exposed to light during ripening. Conclusion A single nucleotide insertion has been identified in 'Pinalate' Z-ISO gene that results in truncated proteins. This causes a bottleneck in the carotenoid pathway with an unbalanced content of carotenes upstream to beta,beta-xanthophylls in fruit tissues. In chloroplastic tissues, the effects of Z-ISO alteration are mainly manifested as a reduction in total carotenoid content. Taken together, our results indicate that the spontaneous single nucleotide insertion in Z-ISO is the molecular basis of the yellow pigmentation in 'Pinalate' sweet orange and points this isomerase as an essential activity for carotenogenesis in citrus fruits.
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