Production of cyanide through biological and environmental processes requires the detoxification of this metabolic poison. In the 1960s, discovery of the β-cyanoalanine synthase (β-CAS) pathway in cyanogenic plants provided the first insight on cyanide detoxification in nature. Fifty years of investigations firmly established the protective role of the β-CAS pathway in cyanogenic plants and its role in the removal of cyanide produced from ethylene synthesis in plants, but also revealed the importance of this pathway for plant growth and development and the integration of nitrogen and sulfur metabolism. This review describes the β-CAS pathway, its distribution across and within higher plants, and the diverse biological functions of the pathway in cyanide assimilation, plant growth and development, stress tolerance, regulation of cyanide and sulfide signalling, and nitrogen and sulfur metabolism. The collective roles of the β-CAS pathway highlight its potential evolutionary and ecological importance in plants.
NH4+ and wheat demonstrated an interaction between cyanide and ammonium in roots in which increasing solution ammonium concentrations decreased the enrichment from 100 mM cyanide. In contrast, with increasing solution cyanide concentrations there was an increase in the enrichment from ammonium. The results suggest increased transport and assimilation of cyanide in response to decreased nitrogen supply and perhaps to ammonium supply.
The β-cyanoalanine pathway is primarily responsible for detoxification of excess cyanide produced by plants. Recent evidence suggests that cyanide detoxification via this pathway may be involved in the response and tolerance to water deficit in plants. The aim of this study was to explore this role in Arabidopsis thaliana in greater detail. The first objective was to establish responsiveness of the pathway to the magnitude and duration of water deficit. The second objective was to examine how interruption of single genes (AtCysA1, AtCysC1 and AtNIT4) encoding enzymes of the pathway influenced the ability to metabolize cyanide and withstand water deficit. Arabidopsis plants were exposed to conditions which emulated acute and chronic water deficit, followed by measurement of tissue cyanide concentration, activity of enzymes, and physiological parameters. The results for wild-type Arabidopsis demonstrated a transient increase in cyanide concentration and β-cyanoalanine synthase activity, followed by a decrease in both. The increase in enzyme activity was localized to the tissue in direct proximity to the stress. The knockdown AtCysA1 mutant did not differ from wild-type while AtCysC1 mutants were slightly more sensitive to water deficit. The AtNIT4 mutant was the most sensitive showing decreased growth along with altered chlorophyll content under water deficit as compared to wild-type. Collectively, the results indicated that the pathway is responsive to water deficit although the severity of stress did not alter the nature of the response, implying that the capacity to remove cyanide generated during water deficit may contribute to tolerance to this stress in Arabidopsis.
The b-cyanoalanine pathway in plants detoxifies cyanide by assimilating this metabolic poison. Given the possibility that cyanide in soil could serve as an alternate source of nitrogen for plant nutrition, this study investigated whether nitrogen deprivation of wheat seedlings altered the activity of the first enzyme of the pathway (b-cyanoalanine synthase) or asparaginase. The results suggest that ambient, nontoxic concentrations of soil cyanide may serve as an alternate source of nitrogen for plants under nitrogen-limiting conditions.
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