Akiko Maruyama

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.

Mechanism of Action of C2H4 in Promoting the Germination of Cocklebur Seeds. III. A Further Enhancement of Priming Effect With Nitrogenous Compounds and C2H4 Responsiveness of Seeds

Efficiency of organic or inorganic osmotica for seed priming of cocklebur (Xanthium pennsylvanicum Wallr.) revealed that KNO3 was the most promising, and was more effective than mannitol or other salts at the same concentration (200 mM) and was independent of the C2H4 action. However, KNO3 applied as a priming reagent enhanced the effect of C2H4 or that of the water stress imposed with mannitol. Unlike the action of mannitol, both KNO3M and C2H4 augmented the pool size of amino acids in seed cells. However, below 50 mM KNO3 imposing no stress only slightly, though insignificantly, affected the germinability as well as the levels of total cyanogen. On the other hand, at a high concentration which imposed water stress on the seeds, 200 mM KNO3 remarkably elevated the contents of both cyanogenic glycosides and lipids in the excised cotyledons. When C2H4 was added with KNO3, the level of cyanogenic compounds significantly increased but when added without KNO3, the contrary effect was shown. Hence the enhancement of the mannitol-induced priming effect by nitrogenous reagents in cocklebur seeds could be implicated in the accumulation of cyanogenic compounds. Unlike cocklebur, both common chickweed and barnyard grass seeds are very responsive to 30 mM KNO3 on germination, and such species abundantly contain cyanogen. The amount of cyanogen was further augmented by contact with KNO3 at only 30 mM. The role of NO-3 -dependent cyanogenesis is highlighted in relation to germination response of seeds.