Compounds from plants that regulate or participate in disease resistance

Disease resistance is multifactorial. The response phase includes: synthesis of phytoalexins, i.e. low molecular weight antimicrobial compounds which accumulate at sites of infection; systemically produced enzymes which degrade pathogens, e.g. chitinases, beta-1,3-glucanases and proteases; systemically produced enzymes which generate antimicrobial compounds and protective biopolymers, e.g. peroxidases and phenoloxidases; biopolymers which restrict the spread of pathogens, e.g. hydroxyproline-rich glycoproteins, lignin, callose; and compounds which regulate the induction and/or activity of the defence compounds, e.g. elicitors of plant and microbial origin, immunity signals from immunized plants and compounds which release immunity signals. Disease resistance in plants is not determined by the presence or absence of genes for resistance mechanisms, it is determined by the speed and degree of gene expression and the activity of the gene products. It is likely, therefore, that all plants have the genetic potential for resistance. This potential can be expressed systemically (immunization) after restricted inoculation with pathogens, attenuated pathogens or selected non-pathogens, or treatment with chemical substances that are produced by immunized plants or chemicals which release such signals. Immunization is effective against diseases caused by fungi, bacteria and viruses, and it has been successfully tested in the laboratory and field. Advances in science have provided information and technology to enhance resistance to plant pests. Pesticides are part of this technology, but they also contribute to a complex world problem which threatens our environment and hence our survival. The future will see the restriction of pesticide use and a greater reliance on resistant plants generated using immunization and other biological control technologies, genetic engineering and classical plant breeding. However, as with past and current technology, we will have created unique problems. The survival of our planet depends upon anticipating these problems and meeting the challenge of their solution.