Some effects of host-plant physiology on resistance to viruses

Abstract

Although the subject for discussion is the physiology of disease resistance, I propose to take a more restricted topic and to deal only with variations in the degree and type of resistance shown towards viruses by susceptible hosts. I do this because we know a little of some of the conditions that affect the reactions of susceptible plants, but we know nothing of the causes that render most plants immune from most viruses. With viruses, as with other kinds of pathogens, immunity is the common state of plants and susceptibility is the exceptional. Indeed, all uninjured plants appear to be immune from all viruses, and though there are a few viruses that can infect a wide range of different plants through suitable wounds, most viruses have only a narrow host range even as wound parasites. Presumably immunity is con­ferred on most species because their cells either contain substances that are antagonistic towards individual viruses or they lack substances that are essential for multiplication of the viruses. Extracts of many plants do contain substances that act as inhibitors of infectivity when mixed with viruses in vitro , but whether such substances play any part in conferring resistance in vivo is uncertain. We know nothing of the growth requirements of any viruses that attack flowering plants, but it is perhaps worth noting that there are bacterial viruses that lyse certain bacteria only when specific amino-acids are supplied to the culture media. When we consider the types of resistance shown by susceptible hosts towards viruses, we find some that are common to other forms of infectious diseases but also additional ones peculiar to virus diseases. This could be expected, for viruses possess an unusual combination of properties. They are the only pathogens that are both obligate and wound parasites. For their spread from plant to plant most of them depend on the activities of certain vectors, of which the most common are leaf­-sucking insects. A further peculiarity is their ability to cause systemic infections, for in most hosts viruses spread from a single entry point to invade all the vegetative parts of the plant that are still actively growing. It is this ability to permeate the whole system of infected plants that gives us with virus diseases the only authentic examples in plant pathology of acquired resistance to disease and provides perhaps the biggest contrast between virus and other types of disease. There are two rather different types of behaviour that come under this heading, both of which simulate conditions usually considered more typical of diseases of animals than of plants.