Antimicrobial Compounds and Resistance

Abstract

Many antimicrobial compounds produced by plants have important roles in their resistance to infection by bacteria, fungi and nematodes. The defensive compounds may be broadly classified into phytoanticipins which are constitutive, and phytoalexins which are synthesized in response to challenge by microorganisms. The two groups of secondary metabolites include a wide range of chemical families produced by different biosynthetic pathways. Phytoanticipins are primarily involved in non-host rather than varietal resistance. Many pathogens have evolved mechanisms of tolerance to their host's phytoanticipins and phytoalexins. Tolerance often involves enzymatic degradation, and transfer of genes for such detoxification can extend host range. Activation of phytoalexin biosynthesis is associated with the hypersensitive reaction or the formation of necrotic lesions. Accumulation of phytoalexins is highly localized. Two strategies appear to have evolved in pathogens to overcome the phytoalexin response, either the defence is not activated or the pathogen tolerates the presence of the inhibitor. Antimicrobial compounds from plants have potential as fungicides themselves or in providing classes of compounds suitable for development as biocides. The biosynthetic pathways to phytoanticipins and phytoalexins offer targets for genetic manipulation to engineer resistance. Progress in the development of our understanding of the role of antimicrobial compounds in resistance is critically discussed, with particular emphasis on the application of molecular genetics to unravel plant-microbe interactions.