Microbes could help oak trees cope with environmental change. Publishing February 11 in the Cell Press journal Cell Host & Microbe, a study observing oaks growing in a natural woodland found that the trees' above- and below-ground microbiomes were resilient to drought, nutrient scarcity, and exposure to pathogenic beetles and bacteria. The trees showed subtle changes to their root-associated microbiota after prolonged drought, suggesting they can recruit beneficial bacteria under stressful conditions.
"As environmental stressors are increasing, one of the key adaptations that trees have is their microbiome," says senior author and microbial ecologist James McDonald of the University of Birmingham. "If we can get a more mechanistic understanding of how host-microbe interactions help trees navigate and tolerate drought, it might open up the opportunity to improve tolerance, for example by inoculating trees with beneficial microbes."
Microbes are known to support plant health and productivity by improving soil nutrient availability, protecting against pathogens, and modulating plant immunity. However, most studies of plant microbiomes have been in fast-growing plants with short lifecycles, and the few studies in trees have focused on trees that are only a few years old. Some tree species, such as oaks, can live for hundreds of years, and almost nothing is known about how the microbiomes of such long-lived plants function.
"Climate change is happening really quite rapidly, but trees are long-lived, sessile organisms that take a long time to adapt to changes, and many of our trees are not well equipped," says senior author Sandra Denman, a plant pathologist at Forest Research (Forestry Commission UK).
To test whether physiologically stressful conditions impact the microbiomes of mature trees, the researchers experimentally manipulated the environmental conditions of 144 35-year-old sessile oak trees growing in a natural woodland in Norfolk, UK. They built rain-excluding enclosures around some of the trees to simulate drought and ringbarked other trees—which prevents plants from transporting water and nutrients between the roots and leaves—to simulate both water and nutrient scarcity. They also inoculated a subset of trees in each group with beetles and bacteria that are associated with acute oak decline (AOD), a fatal disease that causes oak trees to develop oozing, necrotic wounds.
To track microbiome changes, the team collected samples at four time points over the course of two years and used DNA sequencing to characterize bacteria and fungi associated with the trees' leaves, stems, and roots.
"The scale of our study is pretty unique because we were able to study a large number of trees in the same area, where you have the same soil and the same amount of sunshine and wind and so on, and manipulate their conditions to overlay these different types of environmental stress," says Denman.
The researchers showed that semi-mature oak trees have distinct microbial communities associated with their leaves, stems, and roots. Rain exclusion, ringbarking, and the development of AOD symptoms had only minimal effects on the composition of these microbial communities.
After prolonged rain exclusion, the trees' root microbiomes showed subtle changes including an increased abundance of Actinobacteriota bacteria, which are linked to drought tolerance, and an increased abundance of bacterial and fungal genera that have potential growth-promoting properties.
"Even as the trees were showing physiological changes and the soil was becoming a lot drier, their microbiome remained quite stable," says first author Usman Hussain, a microbiologist and molecular biologist at Bangor University and the University of the West of England. "This highlights a potential role for oak-associated microbial communities in maintaining forest ecosystem stability."
The lack of a significant microbiome alteration in trees that developed symptoms of AOD could be because the trees were only around 35 years old, whereas AOD typically affects trees that are 50 years or older, the researchers say.
In the future, the team plans to investigate the molecular mechanisms that allow microbes to endow their hosts with increased resilience. They emphasize that more research is needed to understand how tree microbiomes change over the course of their hosts' long lives and to compare trees in different locations.
"We should start to think about how changes in climate and environmental perturbation might influence not just disease severity, but also biogeochemical cycles, and the role that trees play in carbon sequestration," says McDonald.
