Pyrrolizidine alkaloids (PAs) are the class of secondary metabolites that evolved as a powerful tool in the plant defensive interactions against herbivores. The occurrence of PAs in the plant world is scattered in several unrelated botanic families with special abundance in Asteraceae, Boraginaceae and Fabaceae. Homospermidine synthase (HSS) was recognized as a key enzyme that catalyzes homospermidine formation from polyamines. The studies of HSS kinetic and gene sequence revealed that it is of polyphyletic origin and raised as a result of deoxyhypusine synthase (DHS) gene duplication. The ability of PAs production occurred independently at least four times in course of plant evolution. The PAs biosynthesis is tightly correlated with growth phase and biomass production. It is supposed that PAs biosynthesis is individually regulated in different lineages of plants. The PAs with a 1,2 unsaturated necine skeleton show toxic activity (hepatoxicity, carcinogenicity, genotoxicity, teratogenocity and cytotoxicity). It is though that pyrrolic esters formation during the detoxication process in the liver is the main mechanism of PAs toxicity. The pyrrolic esters are highly reactive and tend to bind rapidly with nucleophilic macromolecules including DNA and DNA-protein inducing hepatotoxicity or tumorigenecity. The problem of PAs toxicity cause the restrictions in the production and sale of herbal products. This review encompasses the present status of the pyrrolizidine alkaloids in the plants studies and summarize the topics of chemistry, biosynthesis, evolution including the involved genes, the factors affecting on biosynthesis, accumulation and toxicity of PAs.
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