Emission of methylbutyric acid from Gypsophila paniculata L. during bud opening: Changes in amino acid catabolism

To elucidate the mechanism of methylbutyric acid emission which is responsible for the unpleasant odor of gypsophila inflorescences (Gypsophila paniculata L. 'Bristol Fairy' and 'Golan'), we investigated the activities of enzymes in the catabolic pathway of branched-chain amino acids. The continuous application of either 10 mM l-leucine, 10 mM l-isoleucine or 4.5 mM isovaleraldehyde increased the production of methylbutyric acids. When gypsophila inflorescences were treated with isovaleraldehyde, a large peak of isoamyl alcohol also appeared on the gas chromatogram. High activities of pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH) were detected in nonscented, tight and swollen buds, but the activities of these enzymes decreased in open florets. On the other hand, the activities of pyruvate dehydrogenase (PDH) and acyl-CoA hydrolase (ACH) increased during bud opening. The activities of aldehyde oxidase (AO) and aldehyde dehydrogenase (ALDH) were low throughout bud opening. These results indicate that the catabolism of branched-chain amino acids such as l-leucine in gypsophila inflorescences have two potential catabolic pathways. The first route, which finally converts l-leucine to isoamyl alcohol via isovaleraldehyde by PDC and ADH, is active before anthesis. The second route, which converts leucine to 3-methylbutyric acid via isovaleryl-CoA by PDH and ACH, becomes active in open florets. The alternation from the first to second route may correlate with the increase in emission of methylbutyric acids from open florets.