When and where plant‐soil feedback may promote plant coexistence: a meta‐analysis

Plant‐soil feedback (PSF) theory provides a powerful framework for understanding plant dynamics by integrating growth assays into predictions of whether soil communities stabilise plant–plant interactions. However, we lack a comprehensive view of the likelihood of feedback‐driven coexistence, partly because of a failure to analyse pairwise PSF, the metric directly linked to plant species coexistence. Here, we determine the relative importance of plant evolutionary history, traits, and environmental factors for coexistence through PSF using a meta‐analysis of 1038 pairwise PSF measures. Consistent with eco‐evolutionary predictions, feedback is more likely to mediate coexistence for pairs of plant species (1) associating with similar guilds of mycorrhizal fungi, (2) of increasing phylogenetic distance, and (3) interacting with native microbes. We also found evidence for a primary role of pathogens in feedback‐mediated coexistence. By combining results over several independent studies, our results confirm that PSF may play a key role in plant species coexistence, species invasion, and the phylogenetic diversification of plant communities.     

Mechanism of control of root-feeding nematodes by mycorrhizal fungi in the dune grass Ammophila arenaria

Root-feeding herbivores can affect plant performance and the composition of natural plant communities, but there is little information about the mechanisms that control root herbivores in natural systems. This study explores the interactions between the pioneer dune grass Ammophila arenaria, arbuscular mycorrhizal fungi (AMF) and the root-feeding nematode Pratylenchus penetrans. Our objectives were to determine whether AMF can suppress nematode infection and reproduction and to explore the mechanisms of nematode control by AMF. A sequential inoculation experiment and a split-root experiment were designed to analyse the importance of plant tolerance and resistance and of direct competition between AMF and P. penetrans for the root herbivore and the plant. Root infection and multiplication of P. penetrans were significantly reduced by the native inoculum of AMF. Plant preinoculation with AMF further decreased nematode colonization and reproduction. Nematode suppression by AMF did not occur through a systemic plant response but through local mechanisms. Our results suggest that AMF are crucial for the control of root-feeding nematodes in natural systems and illustrate that locally operating mechanisms are involved in this process.     

Interaction between a fungal endophyte and root herbivores of Ammophila arenaria

The effect of an endophytic fungus (Acremonium strictum) on plant-growth related parameters of marram grass (Ammophila arenaria), and its potential as a protective agent against root herbivores (Pratylenchus dunensis and Pratylenchus penetrans, root-lesion nematodes) was investigated in two inoculation experiments under different conditions. Acremonium strictum-inoculated plants showed increased plant development in terms of root biomass in the first experiment and increased number of tillers in the second experiment and biomass was less suppressed by nematodes than the Acremonium strictum-free plants. In neither experiment did Acremonium strictum reduce multiplication of root herbivores. On the contrary, Acremonium strictum-inoculated plants seemed to increase herbivore multiplication. Plants infected with P. penetrans benefitted more from the endophytic fungus than those with P. dunensis in terms of total biomass. The effect of Acremonium strictum on interspecific competition was also analyzed by plant inoculation with both nematode species. In Acremonium strictum-free plants with mixed nematode inoculum, the total number of nematodes, compared to numbers observed in one-species inoculation, was less than expected, suggesting that interspecific competition took place. In Acremonium strictum-inoculated plants no interspecific competition was observed. Plants inoculated with P. dunensis, P. penetrans and Acremonium strictum showed decreased total biomass compared to Acremonium strictum-free plants inoculated with the same nematodes. The implications of increased tillering and root growth of plants with Acremonium endophytes are discussed in relation to the sand stabilizing role of Ammophila arenaria in coastal dunes.     

Specificity in host-fungus associations: Do mutualists differ from antagonists?

Summary Physically intimate interactions between organisms are assumed to be highly specific, yet intimate mutualisms exhibiting little specificity are common and important in many communities. We compare host records for ectomycorrhizal fungi (mutualists) to those for biotrophic shoot fungi and necrotrophic root fungi (both antagonists) in order to test two alternative predictions: (1) intimate physical associations (biotrophy) are more specific than less intimate ones (necrotrophy); (2) antagonisms are more specific than mutualisms. Specificity of fungi for hosts supports prediction (1): ectomycorrhizal fungi and shoot biotrophs are more host specific than root necrotrophs. Fungal symbiont ranges of hosts supports prediction (2): woody hosts are associated with a greater number of mutualistic fungi than antagonistic fungi. The numbers of fungi in the three groups infecting hosts are all significantly positively correlated. This result suggests that some hosts are resistant to fungal invasion and others are quite susceptible. Thus, plants may not be able to erect selective barriers to only antagonistic fungi. The marked asymmetry of specificity from the perspectives of hosts vs fungi suggests that evolutionary and ecological processes act differently on partners in symbioses.     

The diversity and ecological significance of Protozoa

The unicellular eukaryotes are currently grouped in the kingdom Protista, together with their multicellular relatives. The inclusion of protozoa, algae and water moulds in a single taxon has resulted in nomenclatural problems, academic homelessness, and a reduction in their teaching. There are around 40 000 described protozoan protist species. Protozoa are principally grazers of bacteria, increasing mineralization and making nutrients more available to other organisms; most are aquatic, but they are also widespread animal parasites and symbionts. Their biomass, role in food chains, roles as mutualists and pathogens, and value as biomonitors are reviewed. To assess the role of protozoa in ecosystems more accurately, the current poor taxonomic standards in ecological work on protozoa must be improved. The manpower to respond to existing and new challenges in the field is declining as protozoology disappears from university courses and this problem needs to be addressed.     

Systemic fungicides reduce the abundance of yeast-like symbiotes and the performance of the planthopper Delphacodes kuscheli (Hemiptera: Delphacidae)

Delphacodes kuscheli is the main vector of Mal de Río Cuarto, the most important viral disease of maize in Argentina. This planthopper harbours obligate fungal mutualists, known as yeast-like symbiotes (YLS), which play a key role in the host life cycle. Here, we evaluated the efficacy of six systemic fungicides, prothioconazole (P), prothioconazole & trifloxystrobin (PT), pyraclostrobin & epoxiconazole (PE), pyraclostrobin, epoxiconazole & fluxapyroxad (PEF), picoxystrobin & ciproconazole (PC), and tebuconazole (T) on the reduction of the abundance of YLS in nymphs and newly emerged adults, and the impact of such reduction on the vector performance. All fungicides, except PE, reduced the number of YLS in nymphs treated from 3rd instar five days after starting treatments. When 3rd instar nymphs were allowed to reach adulthood, the fungicides P, PT and PC caused the highest nymphal mortality, significantly reduced the number of YLS in newly emerged adults, lengthened development time and negatively affected morphometric variables of females. There was also a trend towards a higher occurrence of brachypters. In females, the fungicides P, PT and PC caused a greater reduction of YLS when nymphs were treated to these fungicides from 3rd instar compared with nymphs treated from 4th instar. In males, the YLS number was significantly less when nymphs were fed on plants treated with P, PT, PC, and T in 3rd instar. This showed that fungicide treatments caused a greater reduction of YLS when they were applied in early stages of development. Our results provide an effective method to reduce the abundance of YLS in D. kuscheli, contributing to understand the ecological role that these symbionts could be playing in the success of this maize planthopper pest.