Garden Soaps - intimacy, deceit and betrayal in the plant world.
Pests and Pathogens.
Plants are food for a wide range of animals - including many we recognise as garden or agricultural pests, but they are also food (in the general sense of a supply of raw materials and energy) for pathogens such as bacteria, fungi and viruses. Consequently, plants have evolved many defence strategies to protenct themselves against grazing animals, pests and pathogens. These range from spines and thorns, through unpleasant tasting or toxic chemicals, to mechanisms for switching off the genes of invading viruses. In their turn, pests and pathogens have evolved a variety of counter-measures, including the ability to conceal their identity, break down toxic chemicals and disable plant alarm systems.
Understanding the interaction between plants and their pathogens is crucial in the breeding and management of crops to both reduce crop losses and our dependence on chemical controls. These objectives are important if we are to develop economically and environmentally sustainable agricultural systems. It is estimated that 20-40% of the world’s food production is lost to pests and pathogens, even after a range of controls have been applied, and that without these controls losses would approach 80%. The cost of losses and controls are significant; in the UK in 1996 more than £30 million of wheat was lost to disease, even though £80 million was spent on wheat fungicides.
A fascinating question for scientists is why most plants are able to resist the attacks of most plant pathogens? This general resistance to infection is called non-host resistance. This contrasts with the kind of resistance that allows plants to defend themselves against specific damaging pathogens (this is called host resistance). If researchers could understand non-host resistance they might be able to improve the general resistance of plants to pathogen attack.
Interactions between plants and pathogens are often complex and intimate. Susceptibility or resistance to a particular disease can be determined by a single pair of genes; one in the plant and the other in the pathogen. These genes can exist in different forms (alleles) in the plant, which determine susceptibility or resistance to the disease. Different forms, or alleles, of genes in the pathogen make the pathogen either virulent or avirulent (able, or unable, to infect the plant). A plant will only successfully resist attack when it carries the resistance allele and is attacked by a pathogen carrying the avirulence allele. Natural genetic changes (mutations) in either plants or pathogens generate new virulence and resistance alleles, continuously changing the relationship between the host plant and pest or pathogen. Indeed, this ‘breakdown’ of disease resistance leads to a constant ‘race’ between plant breeders and pathogens in which breeders try to predict which types of resistance genes will give the most long-lasting disease resistance in their ornamental or crop varieties. This is one of the main reasons for the regular appearance of new varieties of plants for farms and gardens.
One way to make disease resistance less likely to breakdown, and therefore longer term, or durable, is to use several different resistance genes that give resistance to the same disease. The probability that a pathogen will undergo, all at the same time, the genetic changes needed to overcome 3 or 4 resistance genes is very low. Although breeding for multi-gene resistance is more complex than working with single genes, modern approaches to biology, such as genomics, are providing the knowledge and tools to make durable disease resistance more achievable.