Abstract
BackgroundValencene is a major sesquiterpene in citrus oil and biosynthesized by valencene synthase (Cstps1; EC: 4.2.3.73) from the 15-carbon substrate farnesyl diphosphate. It is abundant in juice of some mandarins (e.g. Citrus reticulata Blanco cv. Fortune), however, it is undetectable in others (e.g. C. reticulata Blanco cv. Murcott), We have discovered that the Murcott mandarin Cstps1 gene expression is severely reduced. A previous genetic mapping study using an F1 population of Fortune × Murcott found that the segregation of valencene production in fruit exhibited a Mendelian inheritance ratio of 1:1. There was only one dominant locus associated with valencene content detected on the mandarin genetic map. The goal of this study was to understand the molecular mechanism underlying the valencene deficiency observed in some citrus hybrids.ResultsThere was a clear relationship between presence or absence of the valencene synthase gene (Cstps1) expression, and presence or absence of valencene among randomly selected mandarin hybrids. Cloning the coding regions of Cstps1 from Fortune and Murcott mandarin, and aligning with previous reported Valencia orange Cstps1 sequence, showed that they both exhibited extremely high similarity with the known Cstps1. By further cloning and analyzing the promoter region of Cstps1 from Valencia, Fortune and Murcott, a 12-nucleotide deletion at approximately − 270 bp from the Cstps1 coding region was only found in Murcott. Three binary vectors, designated as p1380-FortP-GUSin, p1380-MurcP-GUSin and p1380-MurcP(+ 12)-GUSin, were developed for promoter activity analysis. Transient over-expression of Fortune Cstps1 promoter in sweet orange showed notable GUS activity, but the Murcott Cstps1 promoter did not. In addition, by re-inserting the 12-nucleotide fragment, the activity of the Murcott Cstps1 promoter was mostly recovered.ConclusionThe deficiency of valencene production in some mandarins is probably due to a 12-nucleotide deletion in the promoter region of the Cstps1, which could be a crucial switch of Cstps1 transcription. Our results further enhanced the understanding of valencene biosynthesis in citrus.
Highlights
Valencene is a major sesquiterpene in citrus oil and biosynthesized by valencene synthase (Cstps1; EC: 4.2.3.73) from the 15-carbon substrate farnesyl diphosphate
Elston et al [8] suggested that valencene may be marker with a statistical correlation to higher quality in orange oil because it correlates with maturity, which give citrus juice flavor and fragrance. Both valencene and nootkatone are used for flavoring citrus beverages and are considered among the Relationship between valencene production and Valencene synthase (Cstps1) gene expression Valencene and other sesquiterpenes were measured in mature fruit of Fortune and Murcott mandarins, as well as nine of their F1 progenies randomly selected from the previous mapping population
We found that the gene expression of Cstps1 in mature fruit was over 2720 times higher in mandarin hybrid Temple than in Murcott [9]
Summary
Valencene is a major sesquiterpene in citrus oil and biosynthesized by valencene synthase (Cstps; EC: 4.2.3.73) from the 15-carbon substrate farnesyl diphosphate. It is abundant in juice of some mandarins It is important to understand the complex production and regulatory mechanisms of fruit flavor compounds to lead to efficient breeding strategies. The diversity of terpenes seems to stem mainly from the specific composition and expression of terpene synthases, the key enzymes in the biosynthetic pathway, and additional modification controlled by downstream enzymes [5] Despite their diversity, all terpenes originate from isopentenyl diphosphate (IPP) and its allylic isomer dimethylallyl diphosphate (DMAPP). The mevalonate (MVA) pathway synthesizes sesquiterpenes, phytosterols and ubiquinone, whereas the methylerythritol 4-phosphate (MEP) pathway produces monoterpenes, gibberellins, abscisic acid, carotenoids and the prenyl moiety of chlorophylls, plastoquinone and tocopherol in plastids [6, 7]
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