Uptake, assimilation and toxicity of cyanogenic compounds in plants: facts and fiction

Cyanide is a simple nitrogenous compound that arises from both anthropogenic and natural sources. Plants vary considerably in their physiological and biochemical responses to different species of exogenous cyanides from reduced growth to inhibition on enzymatic activities. Also, great differences in uptake, assimilation and toxicity between free cyanide and iron cyanide have been observed. Unlike botanical uptake of free cyanide chiefly achieved by simple diffusion, iron cyanides have long been considered membrane impermeable and a protein-mediated uptake mode has been proposed. Biological fate of cyanides in plant materials is highly dependent on speciation of cyanides present. Natural development of degradation of free cyanide in plants is very obvious, where the β-cyanoalanine pathway has been widely distributed in higher plants and the production of asparagine and aspartate associated with cyanide assimilation is suggestive. Because phytodissociation of iron cyanides into free cyanide in plant materials is not a mandatory process involved in phytoassimilation, plants probably metabolized them through an undiscovered degradation pathway rather than the β-cyanoalanine pathway. Available information shows phytoassimilation of endogenous cyanide into nitrogen metabolism; however, additional efforts to fully elucidate presence of essential enzymes involved and their proteomic or DNA expression quantitatively are needed to prove and clarify phyto-benefits of assimilation of exogenous cyanide in plant nutrition.