Developing plants that are immune to fungal diseases can perhaps best be compared to the development of flu vaccines. After all, making a plant immune to one form of fungus is not too hard but, just like flu viruses, fungal plant pathogens continue to develop new variants as they adapt to and elude the plant's immune system. It is this that often makes it difficult to grow plants that remain immune to a fungal pathogen over many years. Although it is possible, scientists need to work hard to achieve it, and growers need to grow plants with enough immune genes to prevent new outbreaks of disease. This is the subject of Pierre de Wit’s farewell address as Professor of Phytopathology at Wageningen University, part of Wageningen UR, on Thursday 5 June 2014.
In the 1980s and 1990s, Pierre de Wit and his research group were responsible for a better understanding of the gene-for-gene model in phytopathology. Fungi have genes that produce proteins, called effectors, which cause disease in plants. Luckily, some plants also have immune genes that produce receptors to recognise the effectors and so defend themselves against the fungus. When they do, the fungus is unable to develop and reproduce. To transfer this immunity, de Wit’s research group cross-fertilised wild species containing these immune genes with crops such as tomato plants.
Fungi are spread through the air using billions of spores – the seeds of the fungus. Because there are so many, there is always the chance of a spontaneous mutation, resulting in the fungus suddenly producing a slightly different effector protein. This protein is no longer recognised by the immune receptor, so that the pathogen can enter the plant and develop. When this altered pathogen produces spores, these spores will all make the new protein and therefore also no longer be recognised by the plant as an invader. The result is that all the plants of the same species in a particular area become diseased.
Development new immune genes
In nature, the result is a race. As they evolve, wild plants also spontaneously develop new immune genes through mutation and recombination, in a manner somewhat comparable to that of our own immune genes, which produce antibodies. Plants with new immune genes can therefore continue to defend themselves against the pathogen, so that the fungus has no chance. The progeny of these plants will therefore also be able to defend themselves, resulting in an increase in the number of immune plants and making it more difficult for the pathogen to survive. When spores develop with yet another effector protein, the cycle repeats itself.
Plant breeders try to accelerate this race to the advantage of the plants. They search for immune genes in wild plants which they transfer to the cultivated plants, either through cross-fertilisation or genetic modification. The aim is to develop varieties of tomatoes and potatoes, for example, which are immune to new genetic variants of pathogens. However, until recently plant breeders were only able to look at the outside of a plant (is it healthy or not), and the pathogens produced such huge quantities of spores that any success was short-lived.
Identification immune genes
Pierre de Wit showed how this gene-for-gene system works at the molecular level, and was therefore one of the pioneers of molecular phytopathology. Together with other research groups, such as that run by Jonathan Jones at the Sainsbury Laboratory in Norwich, he laid the foundation for the use of molecular technology in the study of the interaction between plants and their pathogens. For example, he has identified more than ten effector proteins in the Cladosporium fulvum fungus, and ten immune genes in the wild tomato plant that recognise these effectors.
Tomato breeding without fungicides
Thanks to plant breeding, tomatoes have been grown in the Netherlands without the need for fungicides for many years. However, tomato growers do need to make sure that they use varieties with enough immune genes. ‘To give an example, the resistance of a plant with just one immune gene is broken through 10 000 000 000 000 000 000 000 000 times faster than that of a plant with five immune genes’, explains Professor de Wit. ‘Yet it is relatively easy to develop varieties with five immune genes using molecular technology, by cross-breeding and tracing genes using molecular markers, or by transferring genes in genetic modification’.