Plants and their pathogens have an intricate web of communications that affects the outcome of their interactions. Sometimes the pathogen is successful and can cause disease, while often the host plant displays successful resistance. Flor discovered a pattern of resistance known as the gene-for-gene concept in which a single gene of the plant pathogen corresponds to a single resistance gene of the host. This knowledge has proven invaluable in plant breeding where researchers use combinations of resistance genes to build cultivars resistant to the most dangerous strains of particular plant pathogens.
Flor’s Advance in Plant Pathology in 1942
Back in the 1940s, before DNA was shown to be responsible for inheritance, a plant pathologist was studying flax rust. In 1942, Harold Flor deduced that a single gene of this fungal pathogen was responsible for the ability of the fungus to cause disease on the flax plant. He also determined that this gene in the fungus corresponded to a particular resistance gene in the flax plant. This interaction was called the gene-for-gene theory. It applies primarily to biotrophic plant pathogens such as rust fungi—organisms that require a living host to cause disease.
Both Plant Pathogen and Host Genes Affect Disease Development
The gene-for-gene theory has held up over time and applies to both fungal and bacterial plant pathogens, along with some viruses, parasitic plants, and even insects. It is quite complex, since many pathogens have multiple genes that can affect virulence—the ability to cause disease. Also, plants tend to have many different types of resistance genes. There are many different races of a number of plant pathogens. Within a species or subspecies are different types of strains known as races. Similarly, there are different types of plant varieties known as cultivars. Each race has the ability to attack different combinations of plant cultivars, depending on the mixture of resistance genes in the plant.
An interaction that results in an infection is known as a compatible reaction, while a resistant reaction is referred to as incompatible. Plant pathogens that induce a resistance reaction have avirulence genes (Avr), dominant genes that produce a product that initiates a response from the plant. For there to be resistance, the plant must have a corresponding resistance gene (R) to recognize the product.
R genes are generally dominant. Infectious pathogens have a recessive avirulence gene (avr) and do not produce any detectable product. They are able to sneak by the plant’s resistance mechanisms undetected and cause disease. With such a plant pathogen, it does not matter if the plant has any resistance genes. There is constant co-evolution of the plant and its pathogens. For instance, the current wheat rust epidemic is thought to be due to evolution of the avirulence genes of the wheat rust pathogen.
Molecular Plant Pathology Techniques are Applied to the Study of Avr Genes
The development of the gene-for-gene concept was a theoretical breakthrough in plant pathology and led to practical advances in plant breeding. Great advances have been made using the techniques of molecular plant pathology to understand the dynamics of the interaction of avirulence genes with their hosts. A large number of bacterial and fungal avirulence genes have been cloned for more detailed study. Many are being found to produce effector proteins.
Some of these molecules interact with the plant’s immune system that detects basic microbial signals, such as chitin from fungi or flagellin from bacteria. This is similar to having a human antibody detect a piece of a virus. As more is understood about the basic science of the interaction of these molecules with their host, this knowledge should facilitate additional strategies for plant breeding to obtain disease resistance. A current practical application of work on fungal effector genes involves cloning them into bacteria to screen strains of wheat in the laboratory for their resistance to wheat rust.
Avirulence Genes Induce Plant Resistance
The fields of plant pathology and plant breeding have benefited from Harold Flor’s insight that led to the development of the gene-for-gene concept. Plant pathogens with avirulence genes are able to cause disease on their host plants that carry any type of resistance genes, while those with an avirulence gene that matches a particular resistance gene will induce the plant to be resistant. These avirulence genes produce a product that is detected by the plant, primarily proteins that interact with a plant’s basic immune system. Increasing knowledge of the molecular interactions between the pathogen and its host should result in greater disease control of plant pathogens.
References:
Agrios, G.N. 2005. “”Plant Pathology.”” Fifth edition. Elsevier, Academic Press.
De Wit, P.J.G.M., Mehrab, R., Van Den Burg, H., and I. Stergiopoulos. 2009. “”Fungal effector proteins: past, present and future.”” Molecular Plant Pathology. 10(6):735-747. abstract accessed June, 2010
