Featured Image Caption: Endophytes can be isolated from plant tissue and grown in the lab. Dual-culture assays are one way to determine if an endophyte has any antagonistic ability towards another endophyte. Photo credit: Maria Marlin
Original Article: Blumenstein, K., Bußkamp, J., Langer, G. J., Schlößer, R., Parra Rojas, N. M., & Terhonen, E., 2021. Sphaeropsis sapinea and associated endophytes in Scots Pine: Interactions and effect on the host under variable water content. Front. For. Glob. Chang, 4, 55. Link to article
The Good, The Bad, and the Fungi
Fungi have a variety of forms, habitats, and growth patterns. Some fungi live inside plant tissue; these are known as endophytes. While the term ‘endophyte’ generally refers to microbes existing in a mutually beneficial relationship with their host, sometimes, endophytes can turn pathogenic and confer very little benefit. In the case of mutualistic endophytes, the plant provides the fungus with sugars and other nutrients from photosynthesis. In return, the fungal endophyte produces chemicals that benefit the plant in some way, such as promoting plant growth or deterring potential pathogens. The antagonistic abilities of endophytes have made these resident microbes an interesting potential source of biological control for pathogens. However, some endophytes themselves can turn pathogenic if the host plant becomes stressed. The extent of environmental stress required for this lifestyle switch varies between endophyte species. Host plants may experience different levels of stress from nutrient deficiency or extreme weather events. Unfortunately, due to climate change, many plant species are undergoing increased environmental stress, such as drought. Opportunistic pathogens take advantage of these weakened plants, and plant disease occurs, devastating entire communities.
Scots pine is a conifer that is native to Europe, but known to have the widest distribution of any pine. This tree, along with its close relatives, is susceptible to attack from a fungal pathogen that causes the disease Diplodia Tip Blight. Trees suffering from this disease can lose entire shoots and branches; in rare cases, the whole tree can perish. The fungus that causes this disease, Sphaeropsis sapinea, exists inside the plant tissue as an endophyte. However, this is an example of an endophyte that quickly converts to pathogenic when the tree is stressed.
Scientists led by Blumenstein (University of Gottingen, Germany) investigated this relationship between Scots pine and its pathogenic and mutualistic fungal endophytes. To test if beneficial endophytes could inhibit the growth of S. sapinea, they co-cultured Scots pine endophyte species with the fungal pathogen in the laboratory and observed if one inhibited or overgrew the other. They also conducted a greenhouse study to address two questions: 1) Are there common endophytes aside from S. sapinea that will cause plant disease if Scots pine experiences stress? and 2) If trees are co-infected with both S. sapinea and a combination of other endophytes, will the endophytic fungal interactions jolt the plant’s immune system enough to result in lower disease severity? The researchers mimicked drought conditions by varying the levels of soil water content in the containers that held the young experimental trees. They had four categories for water content: 25%, 50%, 75%, and 100%. To address the first question, the scientists conducted single-infection experiments with 5 endophytes of interest, S. sapinea, and a control group (simply a small piece of fungal growth media with no fungus). Then, to tackle the second question, they co-infected individual trees with two endophytes, the pathogen, and the control all at once. Again, this was replicated across all water content categories. Disease was then measured in terms of length of dead tissue or lesion length.
Can Resident Endophytic Fungi Defeat a Pathogen?
The short answer is yes. However, it is worth nothing that the first experiment was conducted only in the lab. The scientists received mixed results with their lab-cultured experiment growing endophytes together with the pathogen. 26% of the tested endophytes inhibited the growth of S. sapinea, while 22% completely overgrew and covered the S. sapinea colony. About 30% of the tested endophytes had no effect on S. sapinea, and conversely, S. sapinea had no effect on them. Finally, S. sapinea inhibited and overgrew 19% of the endophytes tested.
Figure Caption: S. sapinea is a fungal pathogen that causes twig death in conifers such as Scots pine. Credit: Jacinta lluch Valero. Source: Flickr
The scientists wanted to ensure that endophytes commonly found in Scots pine would not turn pathogenic under certain drought conditions. The five endophytes tested in individual infections indeed did not produce serious death of the plant tissue. In addition, there was little variation in disease between the water content treatments, indicating that these particular levels of drought will not induce a pathogenic life stage for the tested endophytes. On the other hand, single infection with S. sapinea predictably produced significant tissue death of the shoots; 18% of the inoculated twigs perished. In addition, the mortality of twigs inoculated with S. sapinea was highest (31.9%) in the treatment with the lowest water content (25% soil water content).
Priming the Immune System?
Some studies of certain plants and endophyte communities suggest that endophyte infection activates the plant’s immune system, arming the plant against an attacking pathogen. Conversely, there is always the risk that endophyte infection could activate the plant’s stress response and indirectly facilitate pathogen growth. In the case of S. sapinea and the combination infection experiments, this was not supported; the disease and lesions caused by S. sapinea did not change due to the presence of the other endophytes. In other words, there was no evidence of an “induced systemic” tree response as a result of multiple endophyte infections. The authors mentioned that an alternate method, inoculating the endophytes first and then the pathogen (instead of at the same time), might have brought about different results. In addition, it’s worth considering that these results also indicate that endophyte infection doesn’t facilitate or increase the growth of the pathogen either.
Emphasis on the Endophytes
Endophytes produce many secondary metabolites that can confer benefits to the host, such as chemicals to deter herbivorous insects and potential pathogens. However, fungal communities are complex, and what is observed in the laboratory does not necessarily translate to living organisms. Endophytes can also cause damage themselves, especially if the plant is weakened. Stress due to environmental conditions is a leading factor in determining lifestyle switch; some endophytic species can switch to a pathogenic stage when the host is just beginning to show signs of stress. Regardless, biological control of forest pathogens using endophytes continues to be a promising area of research. Once an endophytic candidate is determined to be harmless, future studies can then focus on developing an alternate infection method that might just yield superior control of this devastating fungal disease. This model can easily translate to other pathogens and plant systems, even agricultural crops. Plants themselves could hold the key to finding a source of efficient pathogen control.