How root microbiota helps trees adapt to climatic constraints: a case study on black poplar.

GOALS

Forests cover 31% of mainland France and play an important ecological, economic and societal role. They are very vulnerable to global changes, in particular temperature increases and lower levels of precipitation. Different silvicultural management strategies can be envisaged to cope with future climate change and mitigate the abiotic or biotic stresses that will affect trees. Today, it is possible to choose tree species that are most adapted/resistant to future climate predictions in accordance with soil fertility and water reserves.

The use of beneficial micro-organisms could constitute an additional lever. Indeed, it is now well demonstrated that trees are massively colonised by complex communities of micro-organisms, called the microbiome, which strongly contribute to the nutrition and health of their host.

Black poplar in the face of environmental change

Analyses of genetic diversity and phenotypic plasticity suggest the existence of a large gene pool that could facilitate population adaptation in the context of climatic changes. However, results indicate that black poplars growing in the Mediterranean and oceanic zones do not respond in the same way to changes in the environment, each population having developed distinct strategies for adapting to changing environmental conditions. The populations originating from the Loire essentially respond by modifying their morphology, whereas the populations originating from the Drôme respond more by adapting their physiology. Finally, the transplantation of the more northerly Loire origin into a southern environment led to stronger growth, in the absence of water stress. From a practical point of view, this could suggest that future climatic changes, especially increasing temperatures, could favour the northern populations of black poplar, if water stress remains in check.

Concerning the microbiota, it has been shown that it differs significantly from the microbiota of other poplar species already studied. Its composition is very strongly influenced by the soil (sediment) in which the seedlings grow. The results suggest that it is possible, on the one hand, to transfer a potentially more efficient non-native microbiota to seedlings using the soil as inoculum and, on the other hand, to predict the microbiota that seedlings planted in a non-native area will acquire by knowing the local microbiota. Finally, the results suggest that the resistance of black poplar to water stress could be partly conditioned by its microbiota and that the ectomycorrhizal fungus of the genus Geopora could improve this resistance. These encouraging preliminary results now require experimental validation to demonstrate that the identified members of the microbiota do indeed have a role in improving or deteriorating the studied physiological traits of black poplar.

Microbial ecology and ecophysiology of the tree

These results were obtained using an integrated experimental approach combining microbial ecology and tree ecophysiology, with two experiments being carried out jointly: in the first, the young seedlings were subjected to water stress, and in the second, the seedlings were confronted with a more significant climatic change by being transplanted to two regions of France with contrasting climates (Drôme and Loire). During these experiments, the growth of each seedling, the development of its root and aerial system, its capacity to absorb and transfer water, as well as its microbiota are characterised and compared.  The existence of links between the presence of certain micro-organisms and the development of seedlings was thus explored.

Data from the first series of experiments, sampled in November 2017, are currently being processed. A second series of experiments carried out during the summer of 2018 is now under analysis.

The first results highlight the significant impact of the tree and environmental factors on the composition and taxonomic and functional structure of the root microbiota and the need to consider the tree and its microbiota as a "meta-organism" in its own right.