Diaminopimelate decarboxylase catalyses the last step in lysine biosynthesis: the decarboxylation of meso-diaminopimelate to form lysine. Lysine biosynthesis only occurs in plants and microorganisms and is essential for their growth. Therefore, this pathway represents a target for antimicrobial and herbicide treatments. This is the first structural characterisation of diaminopimelate decarboxylase from a plant, Arabidopsis thaliana. The enzyme’s crystal structure has been solved by X-ray crystallography, its quaternary architecture examined using analytical ultra-centrifugation and its shape in solution confirmed using small angle X-ray scattering. While many structural elements appear to be conserved between plants and bacteria there are differences present between them, particularly within the active site, which may be vital in the design of herbicides that selectively inhibit plant variants of this essential biosynthetic enzyme.