Dissecting plant secondary metabolism - constitutive chemical defences in cereals

Collectively plants synthesise a diverse array of secondary metabolites. Secondary metabolites are well known as agents that mediate pollination and seed dispersal. They may also act as chemical defenses that ward off pests and pathogens or suppress the growth of neighbouring plants. The ability to synthesise particular classes of secondary metabolite is commonly restricted to selected plant groups, and the evolution of different pathways in distinct plant lineages is likely to have been key for survival and for the generation of diversity at the organism level. An understanding of the evolution of secondary metabolism requires the characterisation of enzymes and genes for complete pathways in a broad range of plants in addition to the two model species, Arabidopsis thaliana and rice. Tracing the ancestry of the pathway components can then unravel the chain of events that led to the creation of individual pathways. This review summarises progress that has been made in the dissection of the pathways for constitutive chemical defences in cereals, namely saponins and benzoxazinoids.