The presence of two types of beta-cyanoalanine synthase in germinating seeds and their responses to ethylene (original) (raw)

Germinafine seeds of many species contain fwo fypes of /i-cyanoalanine synthase (CAS. EC 4.4.1.9) thaf conveii HCN fo /?-cyanoa'lanine. One is cyfoplasmic CAS (cyt-C.AS), which is precipitated by 50 lo 609r (NHai^SO^ and has a pH optimum of 10.5. Cvtoplasmic C.^S is present at high levels in dry seed and its activity does nof increase during imbibition. The activity ot cyt-CAS is uot affected by exogenously applied efhylene (CjHji. except in riee (Or\~a sativa cv. Sasanishiki). The second fype of CAS ftiuud in seed is mitochondrial CAS (mit-CAS I. which is precipifated by 60 fo 1()''« (NHjl:SO4 and has a pH optimum of 9.5. Mitochondrial CAS is present at low levels iu dry seed, and its activity increases greatly during imbibition in the seeds of ali species fested. Exposure fo C:H^ stimulated mit-C.i^S activity in seeds of rice, barley (Hordeum yidgare cy, Hadakamugii, cucumber (Cucumis satiyus cv. Kagafushinari) and cocklebur {Xanihhmi pennsyhamcum). The increase in fhe mit-CAS activity in cocklebur in response to C^Hj ccmimeticed after a fag period of 2 to 3 h when fhe duration of soaking was short (16 hi. but commenced without a lag period when fhe seeds were soaked lor Ihree mouths. Application ot bofh chioramphenicol aud cycloheximide to the axial and eotyledonary tissues of cocklebur seeds strongly inhibifed growth as well as fhe increase in mit-CAS acfivity. It is postulated fhal the mit-CAS is syuthesized de noi-o dtiring imbibition and thai ifs acfivity is regulated by C;H4. CO;, vvhich also promotes seed genninatiou in some species. v»as ineffective in stimulating tnif-C.AS activity iu cocklebur seeds.