What we are reading in February, 2018
By: Camille Truong and C. Guillermo Bueno
Invited contributors: Lena Neuenkamp and Maret Gerz
Plant growth is affected by a wide range of antagonist interactions with mycorrhizal fungi. For example ECM fungi have positive effects on ECM plant growth, while arbuscular mycorrhizal (AM) plant competitors have negative effects on ECM plant growth. In soils containing natural ECM inoculum (collected near pines), the growth of pine seedlings (Pinus muricata) and ECM colonization of fine roots were not affected by artificial inoculations with the ECM fungi Suillus pungens, nor by competition from the AM plant Baccharis pilularis On the other hand, in soil collected far from pine vegetation (lacking natural ECM inoculum), pine seedling growth was enhanced by Suillus inoculations and reduced by competition from Baccharis. The positive growth effect of Suillus inoculations was also much greater (5 times higher) than the negative effect of Baccharis on pine seedling growth (1.5 times lower). In addition, early Suillus inoculations affected more negatively the growth of Baccharis than late inoculations. This very ingenious experimental design demonstrates the importance of timing in shaping antagonist interactions between ECM and AM associations. In the Pinus muricata system, it seems that both the availability of ECM symbionts and competition with AM plants affect pine seedling growth, but an early ECM colonization can partially buffer the effect of competition from AM plants. The use of two natural soils presenting different ECM inoculum illustrate how ECM symbiont availability may vary between sites in correlation with the mosaic of vegetation and the spatial proximity to host plants.
Peay, KG. 2018. Timing of mutualist arrival has a greater effect on Pinus muricata seedling growth than interspecific competition. Journal of Ecology, 106: 514–523.
Species complex are common in fungi. By apprehending their genetic structure, detecting gene flow between populations, and identifying potential drivers of speciation, population genetic methods can help to delimit species. The edible truffle Tuber indicum is an ectomycorrhizal ascomycete showing high morphological and molecular variability, for example in spore ornamentation. Previous studies have identified two groups that were either considered as two distinct species (T. indicum and T. himalayense) or as geographical ecotypes of a single species. The genetic structure of 31 populations from Southwest China was studied based on ITS and simple sequence repeats. Two clades were clearly delimited as two different species entities. In addition, widespread haplotypes in certain areas were distributed along major river basins, suggesting that long-distance dispersal of these two species was enhanced by water. Indeed, even though large rivers can act as barriers to the movements of animal dispersers (mostly wild rodents), ascospores are frequently washed into the rivers by rains and can spread on both sides of the river. The Himalaya-Hengduan Mountain (HHM) region of southwest China is considered a biodiversity hotspot that served as a refuge for a variety of organisms during the Quaternary glacial period. The present distribution pattern of these species may therefore have been influenced by river expansion during the formation of the HHM mountain range in the Mid-Eocene.
Qiao, P et al. 2018. Phylogeography and population genetic analyses reveal the speciation of the Tuber indicum complex. Fungal Genetics and Biology: https://doi.org/10.1016/j.fgb.2018.02.001
The priming effect is the variation in microbial decomposition rate of soil organic matter (SOM) after the input of fresh organic matter. Competition for energy and nutrient acquisition induce shifts between microbial communities specialized for various sources of SOM: fast-growing r-strategists benefit by extracting energy from easily available substrates (stoichiometric decomposition) while slow-growing K-strategists decompose more recalcitrant sources of SOM to release nutrients (nutrient mining). Interactions between C and N availability therefore drive the strength and extent of the priming effect. In Madagascar, the authors studied how climatic parameters (mean annual temperature) affect the priming effect by modifying the edaphic variables and selecting for different soil microbial communities. It seems that under cold climates the priming effect was generated mostly by fast-growing SOM decomposers. Conversely, warmer climates enhanced the priming effect generated by nutrient mining. By affecting the mechanisms of priming effect, the consequences of climate change on soil carbon sequestration may become larger than previously assumed.
Razanamalala, K et al. 2018. Soil microbial diversity drives the priming effect along climate gradients: a case study in Madagascar. The ISME Journal, 12: 451–462.
Litter production and decomposition are the main doors to nutrient re-cycling, providing the source of nutrients to soil microbes and plants, and thus to all ecosystem organisms. In the last decade, an increasing attention is given to the fraction of belowground litter (root litter), which turned out to exceed sometimes the leaf litter share in the soil and even being the main source for stabilized soil organic matter. However, we are still lacking a general understanding of the interaction of this belowground fraction with the aboveground litter and with key plant traits, such as plant mycorrhizal types in the context of decay and decomposition processes. This is what Jacobs et al investigated in a factorial experiment crossing soil origins (from AM, ECM or mix of tree dominated forests) and litter type (root, leaf, both or none for either AM, ECM or both dominating tree species) see the figure below. Using an experimental mesocosm approach (see below), they found that root litter of AM plants decompose faster than ECM plants. As well that litter decompose better “at home” in the same local soil conditions, especially for AM litter in soils coming from AM dominated forests. Finally, they found that AM leaf litter decompose better in the presence of root litter of AM plants. All in all, this paper points out to intrinsic structural differences between the decomposing pattern of AM vs ECM litter, and the overlooked relevance of the belowground litter share. More research would be needed to unveil the main actors in this film. Which microbes are leading to these differences? What really happens in the “home effect” for AM plants? How root litter facilitate the decomposition of leaf litter for AM plants? Now the questions are opened up for the new exciting research to come!
Graphical abstract of the factorial design with soil, litter type and mycorrhizal types as the main factors.
Jacobs LM, Sulman BN, Brzostek ER, Feighery JJ, Phillips RP. 2018. Interactions among decaying leaf litter, root litter and soil organic matter vary with mycorrhizal type. J Ecol., 106:502–513. https://doi.org/10.1111/1365-2745.12921
This paper belongs to a quite interesting special issue in the Journal of Ecology: MYCORRHIZAL FUNGI AS DRIVERS AND MODULATORS OF ECOSYSTEM PROCESSES, published in March 2018.
Interactions between communities of plants and arbuscular mycorrhizal (AM) fungi shape fundamental ecosystem properties. Experimental evidence suggests that compositional changes in plant and AM fungal communities should be correlated, but empirical data from natural ecosystems is scarce.
PhD student Lena Neuenkamp and co-authors fill this gap of knowledge with their recently published study in New Phytologist, where they clearly show that compositional changes of plant and AM fungal communities were correlated across three stages of grassland succession. The authors also reveal that strength of plant-AM fungal correlation weakened during succession following cessation of grassland management, which was brought about by changes in the proportion of plants exhibiting different AM status. Plant-AM fungal correlation was strong when the abundance of obligate AM plants was high, and declined as the proportion of facultative AM plants increased.
The findings of this study indicate that the extent to which plants rely on AM symbiosis can determine how tightly communities of plants and AM fungi are interlinked, regulating community assembly of both symbiotic partners. Further, the results of this study imply that restoration of ecosystems with obligate AM plant dominated vegetation, such as calcareous grasslands, might benefit from the re-introduction of local AM fungal communities, especially if the ecosystem is already heavily degraded.
Graphical abstract: Showing how the strength of correlation between plant and AM fungal communities in grasslands depends on the abundance of obligatory AM plants in the plant community. The higher is the abundance of obligatory AM plants, the stronger are plant and AM fungal communities related.
Neuenkamp, L., Moora, M., Öpik, M., Davison, J., Gerz, M., Männistö, M., Jairus, T., Vasar, M. and Zobel, M. 2018. The role of plant mycorrhizal type and status in modulating the relationship between plant and arbuscular mycorrhizal fungal communities. New Phytol. doi:10.1111/nph.14995
Mycorrhizal symbiosis affects the realized niches of plant species
Mechanisms of coexistence has fascinated ecologists for a long time and one of the proposed ways is minimizing competition by niche differentiation. According to this, to coexist, species must differ in their realized niches (i.e. coexisting species must have distinct resource and habitat requirements). Traditionally, the realized niches are thought to be affected by competitors, but recent hypotheses state that symbiotic relationships could also be important.
Therefore, in a paper published in the Journal of Ecology, we investigated whether and how the associations with symbiotic mycorrhizal fungi alter the realized niches of vascular plant species, using plant species co-occurrence data from the Netherlands and plant mycorrhizal trait data. We found that, indeed, plants with different mycorrhizal statuses and types had distinct environmental preferences. In addition, the ranges of environmental conditions which plant species tolerate (niche widths), depend on the mycorrhizal status and mycorrhizal type. Specifically, facultatively mycorrhizal plant species had wider niches compared to obligately and non-mycorrhizal plants, indicating that the ability of the faculatively mycorrhizal plants to regulate the presence of the symbiosis plays a key role in determining the range of habitats these plants can occupy. Regarding mycorrhizal types, ecto- and ericoid mycorrhizal plants had wider niches than plant species with other mycorrhizal types. For the ectomycorrhizal plants the underlying mechanisms can possibly be the higher diversity of ectomycorrhizal fungal symbionts and positive plant-soil feedbacks, whereas for the ericoid plants the mechanisms remain unclear.
The differences in plant niches among distinct groups of plants indicate that mycorrhizal symbiosis is an important contributing factor to plant coexistence, and this information could also help predicting vegetation change due to climate change or human impact.
Realized niche volume for obligately (OM) mycorrhizal, facultatively mycorrhizal (FM) and non-mycorrhizal (NM) status, for arbuscular (AM), dual (AEM), ecto- (EcM), ericoid (ErM) and orchid (OrM) mycorrhizal type, and for flexible (FL) and inflexible (IFL) plants. (Graph 2b from the paper.)
Gerz, M., Bueno, C. G., Ozinga, W. A., Zobel, M., & Moora, M. 2018. Niche differentiation and expansion of plant species are associated with mycorrhizal symbiosis. Journal of Ecology, doi: 10.1111/1365-2745.12873