Deficiency of valencene in mandarin hybrids is associated with a deletion in the promoter region of the valencene synthase gene

Authors: YU, QIBIN - University Of Florida; HUANG, MING - University Of Florida; JIA, HONGGE - University Of Florida; YU, YUAN - University Of Florida; Plotto, Anne; Baldwin, Elizabeth - Liz; Bai, Jinhe; WANG, NEAN - University Of Florida; GMITTER, FREDERICK - University Of Florida

Submitted to: Plant Biotechnology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2019
Publication Date: 3/13/2019
Citation: Yu, Q., Huang, M., Jia, H., Yu, Y., Plotto, A., Baldwin, E.A., Bai, J., Wang, N., Gmitter, F.G. 2019. Valencene deficiency in mandarin hybrids is associated with a deletion in the promoter region of the valencene synthase gene. Plant Biotechnology Journal. 19:101. https://doi.org/10.1186/s12870-019-1701-6.
DOI: https://doi.org/10.1186/s12870-019-1701-6

Interpretive Summary: For most fruits, including citrus, color and aroma are complex traits genetically as many chemical pathways contribute to color and aroma volatile production. It is difficult for citrus breeders to select for fruit color and aroma since the tree does not fruit for 3-5 years. Marker-assisted breeding, however, allows breeders to look for genetic markers in seedlings that relate to certain fruit traits like aroma. In a previous study citrus hybrids either produced more color or more valencene (an aroma volatile), suggesting that if valencene was not produced, then the precursor molecules were used to form color pigments. The reason for this was determined in this study to be a deletion in the promotor region of gene associated with valencene production.

Technical Abstract: Valencene is a major sesquiterpene in citrus oil, and the second most abundant volatile compound typically found in mandarin juice. We have discovered Murcott mandarin (Citrus reticulata hybrid) gene expression encoding valencene synthase (CsTps1; EC:4.2.3.22 4.2.3.75) is severely reduced. The product of CsTps1, valencene, was undetectable in Murcott. CsTps1 catalyzes the biosynthesis of linear and cyclic compounds from the 15-carbon substrate farnesyl diphosphate. A segregating population between Fortune and Murcott mandarins was investigated to identify the genomic region associated with production of valencene. Fortune produces abundant valencene. The pattern of gene expression of CsTps1 and valencene quantification is inherited as a simple Mendelian locus, which was mapped on Fortune linkage group 3. There was a clear correlation between level of CsTps1 transcript, measured by QRT-PCR, and the level of valencene in fruit among parents and their progenies. A completed cDNA sequence of CsTps1 was cloned from Fortune and Murcott mandarins, respectively, which showed extremely high similarity with known CsTps1 in Valencia sweet orange. Cloning and analyzing the promoter of CsTps1 showed a deletion of 12-nucleotides only found in Murcott, but not in Fortune and Valencia. Transient expression analysis of the Fortune CsTps1 promoter in sweet orange showed significant GUS expression, whereas the Murcott CsTps1 promoter did not show any activity. In addition, functional activity of the CsTps1 promoter in Murcott was mostly recovered by inserting the 12-nucleotide fragment. This result suggests the deletion of the 12-nucleotides in the upstream regulatory promoter regions of the CsTps1 gene is associated with the production of valencene deficiency in hybrid mandarins.