On the role of β-cyanoalanine synthase (CAS) in metabolism of free cyanide and ferri-cyanide by rice seedlings

A study was conducted to investigate the contribution of β-cyanoalanine synthase (CAS) to the botanical metabolism of free cyanide and iron cyanides. Seedlings of rice (Oryza sativa L. cv. XZX 45) were grown hydroponically and then amended with free cyanide (KCN) or ferri-cyanide [K3Fe(CN)6] into the growth media. Total cyanide, free cyanide, and Fe3+/Fe2+ in aqueous solution were analyzed to identify the speciation of K3Fe(CN)6. Activity of CAS in different parts of the rice seedlings was also assayed in vivo and results indicated that dissociation of K3Fe(CN)6 to free cyanide in solution was negligible. Almost all of the applied KCN was removed by rice seedlings and the metabolic rates were concentration dependent. Phyto-transport of K3Fe(CN)6 was apparent, but appreciable amounts of cyanide were recovered in plant tissues. The metabolic rates of K3Fe(CN)6 were also positively correlated to the concentrations supplied. Rice seedlings exposed to KCN showed a considerable increase in the CAS activity and roots had higher CAS activity than shoots, indicating that CAS plays an important role in the botanical assimilation of KCN. However, no measurable change of CAS activity in different parts of rice seedlings exposed to K3Fe(CN)6 was detected, suggesting that K3Fe(CN)6 is likely metabolized by rice directly through an unknown pathway rather than the β-cyanoalanine pathway.