Balancing the battle: how oak trees defend against new pathogens

Oak trees are potent symbols of the countryside, not in the least because they are capable of growing to great size and living for many centuries. Most representative of the oak’s place in history and culture in Europe is Quercus robur  (the common, pedunculate or European oak), which is the focus of a paper by Bartholomé and colleagues in New Phytologist’s recent oak-focussed special issue. Protection of oak trees from pathogens is a topic of both strong scientific, popular and commercial interest, which is underpinned by fundamental questions of how pathogens evolve to use plants as a host and how plants evolve to counter-act this. Bartholomé and colleagues examine how infection susceptibility or resistance of the ‘naïve’ common oak to two exotic pathogens varies according to genetic makeup. They then identify genetic loci likely to be of future interest to studying the basis of resistance of the common oak to its pathogens.

Unlike mammals, plants do not have mobile immune cells and so somatic plant cells must be capable of sensing and responding to pathogens. Whether oak trees are resistant to a particular pathogen is believed to be linked to population genetics and whether plant-pathogen co-evolution has been able to occur. As such, serious outbreaks of disease often occur when a ‘naïve’ population encounters a pathogen to which it has little or no previous exposure. As Bartholomé and colleagues explain, cases where partial disease resistance to a new pathogen does occur in ‘naïve’ populations is termed exapted resistance. Exapted resistance may occur through existing resistance mechanisms to previously-encountered pathogens being at least partially effective against new pathogens.

Powdery mildew is a common disease of many plant species, caused by a variety of different pathogens. In the case of the common oak, the powdery mildew pathogens Erysiphe alphitoides and Phytophthora cinnamomi are both of non-European origin and so have presumably not been involved in significant co-evolution with European oaks. In this long-term study, common oaks in both field and controlled conditions of known heritage are examined for susceptibility to these two exotic powdery mildew pathogens. The variable of degrees of susceptibility to the two exotic powdery mildew pathogens follows a hereditary pattern, linking this to a genetic basis despite the fact that these oak populations are ‘naïve’ to the two pathogens.

In order to identify the genetic basis of the hereditary variation in susceptibility, Bartholomé and colleagues compare pre-existing genetic map data to their population susceptibility data to identify genetic loci associated with this variation. Of particular importance appear to be two regions, one on chromosome 3 and another one chromosome 10. One of these regions is particularly rich in genes known as Receptor-Like Kinases, which are known to function in recognition of broad-scale molecular markers of attacking pathogens. Also significantly enriched were genes encoding galactinol-synthases, which are thought to be involved in signalling-based responses to pathogens. By contrast, no genes that are known to be involved in recognition of traits specific to more restricted pathogens were found in significant density in the two genetic regions identified.

Identifying the genetic loci important in determining resistance or susceptibility to pathogens of the common oak will likely be important for protecting this species in the future from scenarios that can quickly become devastating, for example as we have seen with Dutch Elm Disease and Ash Dieback. This and other studies also give interesting insights into the pattern of evolution of plant responses to pathogens. Plants seem to have, on the whole, struck a balance between maintaining broad perception and response mechanisms capable of providing at least partial resistance to new pathogens, and specifically acquiring tailored immune recognition and responses to encountered pathogens.