Plants under attack II.

In many diseases of plants there is a well defined genetic relationship between host and pathogen, the host possessing resistance genes and the pathogen possessing avirulence genes. Where these are cognate their products interact and trigger a variety of resistance responses, resulting in an incompatible interaction. Where they are not cognate the pathogen is not recognised and invades the host causing a compatible interaction. A few resistance genes and a larger number of avirulence genes have been cloned and this information has afforded us new perspectives of the recognition phenomenon itself and the means by which virulent pathogens avoid being recognised. Mechanisms of resistance range from preformed antimicrobial compounds to a series of components that are induced as a response to microbial challenge. Most of these induced reactions are preceded by and may be triggered by an oxidative burst which results in the cross-linking of cell wall proteins and also causes the death of one to several cells. This localized death of host cells, termed the hypersensitive response, provides an explanation for resistance to obligate parasites which require living host tissue but not for resistance to facultative parasites which colonise dying or dead tissue. However, the hypersensitive response appears to be intimately involved with several other defence responses such as the synthesis and accumulation of phytoalexins, lignification and the production of hydrolase enzymes that attack the invader. Salicylic acid has proved fundamental to the effecting of these defence responses and also to the phenomenon of systemic acquired resistance whereby a previous challenge from a pathogen induces the expression of enhanced resistance to a second challenge by the same or another organism. Despite these sophisticated defences plants still succumb to attack. Two of the weapons to which they are vulnerable and which are produced by many successful pathogens are toxins and enzymes. In some instances an understanding of these has pointed to novel methods of control supplementary to that obtained by conventional breeding for resistance.