Possible Participation of β-Cyanoalanine Synthase in Increasing the Amino Acid Pool of Cocklebur Seeds in Response to Ethylene during the Pre-germination Period

The amino acid content in cocklebur (Xanthium pennsylvanicum Wallr.) seeds was increased by ethylene, which stimulated their germination, regardless of whether they were non-dormant or secondarily dormant. This increase in amino acid content coincided with the increased activities of β-cyanoalanine synthase (CAS, EC 4.4.1.9) in response to ethylene. KCN and/or cysteine, the substrates of CAS, also increased the amino acid content in both non-dormant and secondarily dormant cocklebur seeds. The degrees of the increased amino acid content corresponded roughly to the germination rates of the seeds reported previously. The actual involvement of CAS in the germination process in cocklebur seeds was demonstrated by incorporation into asparagine and aspartate from 14CN which was fed to the cotyledon segments of both non-dormant and secondarily dormant cocklebur seeds. In this case, the incorporation of 14CN was augmented by ethylene, and incorporated more abundantly in the cotyledons of secondarily dormant seeds. Moreover, ethylene decreased the cysteine + cystine content in both the axial and cotyledon tissues, but increased asparagine and aspartate regardless of whether they were non-dormant or secondarily dormant. This suggests that CAS responsiveness to ethylene participates in supplying asparagine and aspartate and in increasing the amino acid pool of cocklebur seeds during the pre-germination period.