Cyanogenic Eucalyptus nobilis is polymorphic for both prunasin and specific β-glucosidases

Cyanogenesis (i.e. the evolution of HCN from damaged plant tissue) requires the presence of two biochemical pathways, one controlling synthesis of the cyanogenic glycoside and the other controlling the production of a specific degradative β-glucosidase. The sole cyanogenic glycoside in Eucalyptus nobilis was identified as prunasin (D-mandelonitrile β-D-glucoside) using HPLC and GC-MS. Seedlings from three populations of E. nobilis were grown under controlled conditions and 38% were found to be acyanogenic, a proportion far greater than reported for any other cyanogenic eucalypt. A detailed study of the acyanogenic progeny from a single open-pollinated parent found that 23% lacked a cyanogenic β-glucosidase, 32% lacked prunasin and 9% lacked both. Of the remaining seedlings initially identified as acyanogenies, 27% contained either trace amounts of β-glucosidase or prunasin, while 9% contained trace amounts of both. Results support the hypothesis that the two components necessary for cyanogenesis are inherited independently. Trace amounts are likely to result from the presence of non-specific β-glucosidases or the glycosylation of the cyanohydrin intermediate by non-specific UDP glycosyl transferases.