Possible involvement of ethylene-activatedβ-cyanoalanine synthase in the regulation of cocklebur seed germination

Abstract

A possible involvement of β-cyanoalanine synthase (CAS : EC 4.4.1.9) in germination processes of seeds was demonstrated using pre-soaked upper seeds of cocklebur (Xanthium pennsylvanicum Wallr.). Pretreatment in anoxia not only with KCN but also cysteine, as the substrates for CAS, stimulated the subsequent germination of cocklebur seeds in air. However, the effect of cysteine was manifested even in air when applied together with C 2 H 4 , and its effect was further enhanced in combination with KCN. The germination-stimulating effect of KCN was intensified by C 2 H 4 only when O 2 was present. In contrast, serine, another substrate of CAS, was effective in air only when combined with C 2 H 4 and/or KCN. The addition of cysteine greatly reduced the cyanogenic glycoside content of seeds, but increased HCN evolution. On the other hand, glutathione did not have any effect on cocklebur seed germination, HCN evolution or bound cyanogen content, suggesting that cysteine is not acting as a reducing reagent. It is suggested that CAS regulates the process of cocklebur seed germination by the dual action of enlarging the pool of amino acids and supplying sulphydryl bases, the latter being more determinately important. Serine is effective only via the former action, while cysteine would act via both.