Plants are grown in hostile environments where they are continuously attacked by pathogens. Many species of bacteria, nematodes, fungi, oomycetes and insects feed on plant tissue, meanwhile harming the plant and causing severe yield losses with a huge economic impact. Many efforts are being made to reduce those yield losses, ranging from; good agricultural practices, the use of effective and environmentally friendly pesticides, the search for naturally resistant cultivars or the development of genetically modified plants. To develop new plant protection strategies, fundamental knowledge is needed on how pathogens can overcome defence mechanisms of the plant.
Pathogens have evolved to “spit” proteins with specialized functions, called effectors, into plant tissue in order to evade or weaken the immune system of their host. A whole array of effector proteins can be secreted, enabling pathogens to interfere with a multitude of processes in the plant. In our research we have focused on effector proteins interfering with the production of the plant hormone salicylic acid (SA) and a group of secondary metabolites called the phenylpropanoids. SA is an important signalling molecule linked to defence, while phenylpropanoids play various roles in taste, color, defence or they act as precursors for structural compounds like lignin. Since SA and phenylpropanoids share some steps in their production process (see Figure 1), it is possible that a single effector affects both biosynthesis pathways.
Looking at all available data of effectors acting on those pathways, it becomes clear that different pathogens try to interfere in different ways. Some pathogens (most of them feeding on living cells) secrete effectors to reduce the SA content. On the other hand, insects or pathogens feeding on dead tissue will try to achieve the opposite, since an increase in SA might trigger localized cell death. For instance, the fungal effector isochorismatase decreases SA content, while the effector Bt56 of whitefly is responsible for an increase in SA content. Even when looking at one type of effector, the outcome can be different depending on the host and the invading pathogen. The effector chorismate mutase (CM) from the fungus Ustilago maydis, for example, elevates phenylpropanoid production in maize, while CM from the nematode Hirschmanniella oryzae appears to lower phenylpropanoid production in rice.
Although different effectors might have divergent effects on both SA and phenylpropanoid biosynthesis, the final result for the invading pathogen remains the same: the immune system of the plant is weakened, increasing the pathogen’s chances for a successful infection. We made a (non-exhaustive) list of effectors manipulating the SA and/or phenylpropanoid pathway in order to better understand the infection strategies of invading pathogens. However, due to the multitude of effectors and the different responses by various plants, it is difficult to draw general conclusions and each pathosystem should be judged separately. The more information that becomes available on the tactics used by pathogens, the higher the chance we can come up with effective strategies to protect the plant against pathogen invasion.
Lander Bauters, Boris Stojilković and Godelieve Gheysen published this review in Molecular Plant Pathology:
TITLE IMAGE: Plant pathogens like bacteria (red), nematodes (brown), fungi (purple) or insects (orange) secrete specialized effector proteins into their host in order to influence defence. A subgroup of these effectors can manipulate the biosynthesis of salicylic acid or phenylpropanoids. They can do so by either affecting enzymes involved in the biosynthesis or degradation of these compounds, or by regulating expression of genes involved in this process. Created with Biorender.com. All images used with permission of the authors.