A seed coat cyanohydrin glucosyltransferase is associated with bitterness in almond (Prunus dulcis) kernels

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.

Iridoid-specific Glucosyltransferase from Gardenia jasminoides

“…The GjUGT2 cDNA contains an ORF corresponding to a protein (UGT85A24) of 481 amino acids with a predicted molecular mass of 53.6 kDa. It shares 66% amino acid identity with UGT85A9 from Prunus dulcis (29). The yield of the purified UGT85A24 protein from the bacterial culture was 2.5 mg/liter.…”

“…PSPGs of this group exhibit glucosyltransferase activity toward various acceptor substrates. For example, UGT85A19 from P. dulcis is a mandelonitrile glucosyltransferase (29), and UGT85A1 from A. thaliana exhibits glucosyltransferase activity toward zeatin (26). UGT85A24 exhibits glucosyltransferase activity toward neither mandelonitrile nor zeatin.…”

Molecular markers for kernel bitterness in almond

“…Metabolic and genetic studies of sweet and bitter almonds show that a seed coat UDP-glucosyltransferase (GT1; see Fig. 1) is associated with bitterness (Franks et al 2008). Distinct…”

Comparative Analysis of Benzoxazinoid Biosynthesis in Monocots and Dicots: Independent Recruitment of Stabilization and Activation Functions

“…Like in benzoxazinoid biosynthesis, the branchpoint enzyme homospermidine synthase evolved by repeated independent recruitment of a primary function, deoxyhypusine synthase. The analysis of widespread plant secondary pathways, such as flavonoid and cyanogenic glucoside biosynthesis, demonstrated recruitment of genes from the same gene family for the respective enzymatic steps in the pathways, such as the CYP75 flavonoid hydroxylases in angiosperm flavonoid biosynthesis (Nelson and Werck-Reichhart, 2011), CYP79 and CYP71E monooxygenases , and UGT85 glucosyltransferases in cyanogenic glucoside biosynthesis (Jones et al, 1999;Franks et al, 2008;Kannangara et al, 2011). However, the recruited genes are not necessarily orthologous …”