A seed coat cyanohydrin glucosyltransferase is associated with bitterness in almond (Prunus dulcis) kernels
Tricia K. Franks A D , Abbas Yadollahi B , Michelle G. Wirthensohn A , Jennifer R. Guerin A , Brent N. Kaiser A , Margaret Sedgley C and Christopher M. Ford AA School of Agriculture, Food and Wine, The University of Adelaide, PMB1 Glen Osmond, SA 5064, Australia.
B Department of Horticultural Sciences, Tarbiat Modares University, Tehran, Iran.
C Faculty of The Sciences,The University of New England, Armidale, NSW 2351, Australia.
D Corresponding author. Email: tricia.franks@adelaide.edu.au
Functional Plant Biology 35(3) 236-246 https://doi.org/10.1071/FP07275
Submitted: 22 November 2007 Accepted: 3 March 2008 Published: 23 April 2008
Abstract
The secondary metabolite amygdalin is a cyanogenic diglucoside that at high concentrations is associated with intense bitterness in seeds of the Rosaceae, including kernels of almond (Prunus dulcis (Mill.), syn. Prunus amygdalus D. A. Webb Batsch). Amygdalin is a glucoside of prunasin, itself a glucoside of R-mandelonitrile (a cyanohydrin). Here we report the isolation of an almond enzyme (UGT85A19) that stereo-selectively glucosylates R-mandelonitrile to produce prunasin. In a survey of developing kernels from seven bitter and 11 non-bitter genotypes with polyclonal antibody raised to UGT85A19, the enzyme was found to accumulate to higher levels in the bitter types in later development. This differential accumulation of UGT85A19 is associated with more than three-fold greater mandelonitrile glucosyltransferase activity in bitter kernels compared with non-bitter types, and transcriptional regulation was demonstrated using quantitative-PCR analysis. UGT85A19 and its encoding transcript were most concentrated in the testa (seed coat) of the kernel compared with the embryo, and prunasin and amygdalin were differentially compartmentalised in these tissues. Prunasin was confined to the testa and amygdalin was confined to the embryo. These results are consistent with the seed coat being an important site of synthesis of prunasin as a precursor of amygdalin accumulation in the kernel. The presence of UGT85A19 in the kernel and other tissues of both bitter and non-bitter types indicates that its expression is unlikely to be a control point for amygdalin accumulation and suggests additional roles for the enzyme in almond metabolism.
Additional keywords: cyanogenic glucoside, stereo-selectivity, testa.
Acknowledgements
The authors acknowledge with much thanks the following contributors: Andrew Lacey for making the field site available and Tony Spiers for maintaining the trees; Jana Kolesik for helping with the collection of tissues from the field; Birger Møller (Royal Veterinary and Agricultural University, Denmark) for providing the UGT85B1-encoding cDNA and Gayle Baldock (Australian Wine Research Institute) for HPLC-MS analysis of in vitro assay products. This research was supported under the Australian Research Council’s Linkage funding scheme (project number 0560480) together with the Almond Board of Australia.
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