The deleterious effects of non-protein amino acids from desert plants on human and animal health

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Since plants lack the ability to remove themselves from sites of predation, they have evolved alternative defences to keep predators at bay. Mechanical defences include camouflage, the addition of thorns and spikes as well as growing in locations not easily accessed by herbivores. Chemical defences include the synthesis of compounds that are toxic to predatory species and can also inhibit or retard the growth of other plant species in a process termed “allelopathy”. Amongst these chemicals is a reservoir of non-protein amino acids that mediate toxicity. These non-protein amino acids can be charged by tRNA synthetases and subsequently mis-incorporated into nascent polypeptides, resulting in aberrant, dysfunctional proteins that can be toxic to predators. Some species such as the bruchid beetle Bruchus rufimanus (Chrysomelidae), which feeds on the jack bean Canavalia ensiformis (Fabaceae) plant, have evolved advanced tRNA synthetases that are able to discriminate between protein and non-protein amino acids, thus remain unaffected. Other species including livestock and humans that do not possess such selectivity are susceptible to plant toxins. In this brief review we discuss the mechanisms of action and consequences of exposure to plant-derived non-protein amino acids with a focus on those derived from plants from arid environments.