Inheritance, Gene Expression, and Valencene Quantification in a Mandarin Hybrid Deficient in Valencene Synthase CsTps1

We have discovered that in Murcott mandarin (Citrus reticulata hybrid) expression of the gene encoding valencene synthase (CsTps1; EC:4.2.3.22 4.2.3.75) is severely reduced. Valencene synthase (CsTps1) catalyzes the biosynthesis of linear and cyclic compounds from the 15-carbon substrate farnesyl diphosphate. The product of CsTps1, valencene, is a major sesquiterpene in citrus oils, and the second most abundant volatile compound typically found in mandarin juice. We have investigated a segregating population between Fortune and Murcott mandarin. Fortune produces abundant valencene, whereas valencene is undetectable in Murcott. Phenotypic segregation was determined from 80 individuals and was 34:46 (non-detected: valencene), not significantly different from a 1:1 ratio (χ2=0.134). There was a clear correlation between level of CsTps1 transcript, measured by QRT-PCR, and the level of valencene in fruit from parents and their progeny. The pattern of gene expression of CsTps1 and valencene quantification is inherited as a simple Mendelian locus. The results indicated that valencene synthase is a single gene which controls the production of valencene. Sequence comparisons revealed that the CsTps1 gene is composed of seven exons and six introns, and encodes a 548-amino acid protein for Fortune, Murcott and Temple. Temple is also a mandarin hybrid cultivar with abundant valencene. Fortune and Temple have identical 548-amino acid protein sequences, whereas Murcott has only 98% similarity with Fortune and Temple based on the partially obtained sequences. Quantitative Trait Locus (QTL) analysis identified two loci associated with valencene abundance: 1) between scaffold_3:8,031,886 and 10,000,894; 2) between scaffold_3:16,819 and 699 21,924,004. The CsTps1 gene is located in neither of the two regions. This might suggest trans-regulatory elements required for regulation of CsTps1. Cloning and comparisons of the upstream regulatory promoter regions of CsTps1 gene may reveal the functional differences of CsTps1 between Murcott and Fortune.