Focus on research: Plant pathologist Frank Takken
Plants and their various natural enemies are embroiled in a constant arms race. Each time a plant finds a new way to resist a specific type of fungus, the fungus develops a new strategy to circumvent the plant's resistance mechanism. Plant pathologist dr. Frank Takken studies the mechanisms involved in these attack and defence reactions between fungi and plants. His research mainly focuses on the way in which the proteins involved in these processes operate. One of his surprising discoveries: humans and plants have comparable immune proteins that cause comparable ailments when they break down.
Takken (1969) studied molecular and cellular biology in Amsterdam and obtained his doctorate from the VU, where he successfully isolated resistance genes from tomato plants. After having studied pathogens at Wageningen University for several years, he joined the Swammerdam Institute for Life Sciences (SILS) eight years ago, where he has since focused on the interaction between the tomato plant and the soil-born fungus Fusarium oxysporum.
Defence mechanisms and resistance
The majority of plants are resistant to most diseases, Takken explains. ‘Plants have a basal immune system that can be activated when they are under attack.' For example, they produce anti-microbial proteins, free oxygen radicals or thicken their cell walls. ‘The plant responds with a generic, but relatively weak response, which nevertheless is sufficient to fend off most attackers.' However, some pathogens use specialised tools to infect a healthy plant, requiring a targeted response. The relationship between the tomato plant and Fusarium is such a case in point.
Fusarium is a common fungus that causes wilt disease; one strain specifically infects tomato plants. The fungus grows upwards through the roots, into the xylem vessels, which become clogged; the plant eventually wilts and dies as a result. Fusarium is difficult to control. In some cases, the soil can be decontaminated using steam or chemicals. However, these procedures are not necessary in the case of modern tomato varieties, Takken explains: the great majority are resistant to the fungus.
Tomato plants' current resistance can be attributed to the 1-2 gene, which codes for the protein 1-2 and is a key area of focus in Takkens' research. As he explains, the protein is not easy to study. ‘In order to study a protein, you may need to attach tags or make other alterations. Unfortunately, most changes to 1-2 will immediately destroy the protein.' This is why Takken and his colleagues often use the more robust Mi-2 resistance gene, which is also found in tomato plants and makes them resistant against nematodes, aphids and whiteflies.
Strategy of the scorched earth
Resistance reactions will only occur where a root has been infected with Fusarium. In the case of I-2, this will take the form of a hypersensitivity reaction, or - in Takken's words: the strategy of the scorched earth. As soon as a fungal spore enters a cell, the plant reacts by killing off the surrounding cells, stopping the fungus from developing any further.
Takken uses an Agrobacterium-assay to study resistance reactions in living plants. This involves injecting a solution containing the Agrobacterium tumefaciens bacteria into a leaf. This bacterium is unique in its ability to introduce and express specific genes in the plant's genetic material. Using Agrobacterium, leaves can be locally transformed to express the 1-2 resistance gene.
Takken shows photographs of tobacco plants with neat round circles where the leaf has died: clear signs of an immune reaction. As he explains, this assay can be used to test different varieties of 1-2. ‘We know the I-2 protein consists of five domains: what happens when we entirely eradicate one of those domains? What happens when we separate one domain from the others, or create minor mutations in a domain? Most of these changes will nullify the resistance reaction. As it turns out, however, certain changes - even in absence of the fungus - result in a hypersensitivity reaction.
Takken used this knowledge to find out more about the various domains' functions. One domain, for example, is involved in the hypersensitivity reaction. Another domain is responsible for recognising the fungus's avirulence gene - a gene that encodes for a key protein from the fungus. Takken found that the central and largest portion of the protein played an essential part in what he refers to as its ‘switch functionality': initiating the anti-fungal resistance reaction. This domain (see visualisation) is normally responsible for binding an ADP molecule: the "off" position. As soon as the fungus is detected, the protein will change shape. As a result, the ADP will drop out, allowing for the binding of a similar substance - ATP: the switch is then set to the ‘on' position.
Auto-immune disease: a broken switch
In addition to our familiar white blood cells, humans also have an immune system similar to that of plants, Takken explains, ‘Apparently, it's an extremely old system that developed during the early stages of evolution. It can still be found in a large number of species.' The human immune system even has switch proteins identical to that in plants. Takken: ‘If anything goes wrong with the ‘switch' in these proteins, the consequences can be serious, such as the occurrence of Crohn's diseases in humans. In patients suffering from this auto immune disease, the ‘switch' is constantly in the ‘on' position, triggering a continues immune response at cellular level. We found the exact same mutation during our study of plants.'