What makes a cherry red?: an investigation into flavonoid pathway regulation in sweet cherry (Prunus avium L.) fruit.
Date
2010
Authors
Parker, Jessie-Lee
Editors
Advisors
Tester, Mark Alfred
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Thesis
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Abstract
Colour is an important fruit quality indicator because many consumers make their selections based primarily on this trait. Inheritance of colour has been studied within sweet cherry (Prunus avium L.) populations and as a result fruit colour is thought to be determined by three genetic factors. A flesh colour factor (F) and the major skin colour factor (A) are the main determinants of fruit colour, where red pigmentation is incompletely dominant over yellow. A third factor, the minor skin colour factor (B), can produce blush skin but is epistatically masked by a dominant A allele. The pigments that colour fruit are known as anthocyanins, synthesised via the transcriptionally regulated flavonoid pathway, which also synthesizes the related secondary metabolites, condensed tannins and flavonols. In other fruit and flower species, mutations in flavonoid pathway or regulatory genes can lead to non-functional alleles that explain the inheritance of colour. However the genes encoding the genetic colour factors are not known in sweet cherry. Therefore, this research has endeavoured to study the cherry flavonoid pathway and its transcriptional regulation, with a view to determining the genetic differences responsible for yellow, blush, red and black cultivars. To achieve this aim, genes encoding flavonoid pathway enzymes and putative regulators of flavonoid synthesis were isolated from the red sweet cherry cultivar ‘Lapins’. PaMYBA1, an R2R3-MYB factor, possessing a high degree of sequence similarity with characterised anthocyanin regulators and conserved C-terminal motifs common within this type of protein, was identified. Functional characterisation of PaMYBA1 demonstrated its ability to activate transcription from the promoters of chalcone synthase (MdCHS), which encodes an enzyme that performs the first committed step in the synthesis of flavonoids, and the anthocyanin biosynthetic gene UDP-glycosyl:flavonoid-3-O-glycosyltransferase (MdUFGT). Furthermore, correlation between anthocyanin accumulation and the expression profile of PaMYBA1 in developing ‘Lapins’ fruit and light-treated blush-skinned ‘Ranier’ fruit suggest that PaMYBA1 might be an important colour factor. Transcript analysis revealed that PaMYBA1 is necessary for the production of colour in cherries; PaMYBA1 is not expressed in the solid yellow fruit of ‘Yellow Glass’ that lacks anthocyanins. However, similar levels of expression of PaMYBA1 in blush, red and black sweet cherry fruit indicate that there are additional factors that contribute to differences in colour intensity. The intense colour and increased flavonoid levels of the black sweet cherry ‘Sam’, compared with the blush and red fruits tested, correlated with a large increase in the expression of the putative tannin regulator PaMYBPA1 in this cultivar. In a functional assay, PaMYBPA1 could trans-activate not only the promoters of the tannin genes anthocyanidin reductase (VvANR) and leucaonthocyanidin reductase (VvLAR), but also of MdCHS and MdUFGT. Therefore, it is possible that PaMYBPA1 could regulate both tannin and anthocyanin synthesis, particularly when expressed at high levels. Taking into consideration the expression of flavonoid pathway genes in different sweet cherry cultivars and tissues, and under different environmental conditions, together with published scientific observations of the genetic factors contributing to fruit colour, we have developed a working model for flavonoid pathway regulation in sweet cherry fruit. Aspects of the model remain to be determined, such as the involvement of two additional anthocyanin-type MYB factors PaMYBA2 and PaMYBA3 in fruit pigmentation. However, it provides a general understanding of differences in the activity of the flavonoid pathway between sweet cherry cultivars, and moves us closer to knowing the identity of the inherited factors that determine skin and flesh colour in sweet cherry fruit.
School/Discipline
School of Agriculture, Food and Wine
Dissertation Note
Thesis (Ph.D) -- University of Adelaide, School of Agriculture, Food and Wine, 2010
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.