Herbicide residues in soil decrease microbe-mediated plant protection

The residues of glyphosate are found to remain in soils longer than previously reported, affecting rhizosphere microbes. This may adversely affect crop and other non-target plants because the plant's resilience and resistance largely rely on plant-associated microbes. Ubiquitous glyphosate residues in soil and how they impact mutualistic microbes inhabiting the aboveground plant parts are largely unexplored. We studied the effects of herbicide residues in soil on Epichloë sp., which are common endophytic symbionts inhabiting aerial parts of cool-season grasses. In this symbiosis, the obligate symbiont subsists entirely on its host plant, and in exchange, it provides alkaloids conferring resistance to herbivores for the host grass that invests little in its own chemical defence. We first show decreased growth of Epichloë endophytes in vitro when directly exposed to two concentrations of glyphosate or glyphosate-based herbicides. Second, we provide evidence for a reduction of Epichloë-derived, insect-toxic loline alkaloids in endophyte-symbiotic meadow fescue (F. pratensis) plants growing in soil with a glyphosate history. Plants were grown for 2 years in an open field site, and natural herbivore infestation was correlated with the glyphosate-mediated reduction of loline alkaloid concentrations. Our findings indicate that herbicides residing in soil not only affect rhizosphere microbiota but also aerial plant endophyte functionality, which emphasizes the destructive effects of glyphosate on plant symbiotic microbes, here with cascading effects on plant-pest insect interactions.

Glyphosate residues in soil can modify plant resistance to herbivores through changes in leaf quality

Glyphosate is the most widely used non-selective herbicide in the world. Glyphosate residues in soil can affect plant quality by modifying plant physiology, hormonal pathways and traits, with potential consequences for plants' interactions with herbivores. We explored these indirect effects in the context of plant-herbivore interactions in a perennial, nitrogen-fixing herb. We quantified leaf herbivory for glyphosate-exposed and control plants grown in phosphorus-fertilized and non-fertilized soils, and assessed the impacts of glyphosate treatment on traits related to plant resistance against herbivores (leaf trichome density, leaf mass per area) and performance (aboveground biomass, root:shoot ratio, nodule number, nodule activity). Moreover, we conducted a laboratory feeding experiment to compare the palatability of leaves from glyphosate-exposed and control plants to a generalist mollusc herbivore. Herbivore damage and intensity in situ increased during the growing season regardless of glyphosate or phosphorus treatment. Glyphosate treatment reduced leaf trichome density but had no effect on the other plant traits considered. Herbivore damage was negatively associated with leaf trichome density. The feeding experiment revealed no difference in the feeding probability of mollusc herbivores between glyphosate-exposed and control plants. However, there was an interaction between glyphosate treatment and initial leaf area for leaf consumption by herbivores: leaf consumption increased with increasing leaf area in both groups, but at a lower rate for glyphosate-exposed plants than for control plants. Our results show that glyphosate residues in soil have the potential to indirectly affect aboveground herbivores through changes in leaf quality, which may have mixed consequences for folivore damage.

Data on litter quality of host grass plants with and without fungal endophytes

Certain Pooideae species form persistent symbiosis with fungal endophytes of Epichloë genus. Although endophytes are known to impact the ecology and evolution of host species, their effects on parameters related with quality of plant biomass has been elusive. This article provides information about parameters related with the quality of plant litter biomass of two important grass species (Schedonorus phoenix and Schedonorus pratensis) affected by the symbiosis with fungal endophytes (Epichloë coenophiala and Epichloë uncinata, respectively). Four population origins of S. phoenix and one of S. pratensis were included. Mineral, biochemical and structural parameters were obtained from three samples per factors combination [species (and population origin)×endophyte]. This data can be potentially used in other studies which, by means of ‘data reanalyzing’ or meta-analysis, attempt to find generalizations about endophyte effects on host plant litter biomass. The present data is associated with the research article “Role of foliar fungal endophytes on litter decomposition among species and population origins” (Gundel et al., In preparation) [1].

Alkaloid Quantities in Endophyte-Infected Tall Fescue are Affected by the Plant-Fungus Combination and Environment

Many grass species are symbiotic with systemic, vertically-transmitted, asymptomatic Epichloë endophytic fungi. These fungi often produce alkaloids that defend the host against herbivores. We studied how environmental variables affect alkaloids in endophyte-infected tall fescue (Schedonorus phoenix) from three Northern European wild origins and the widely planted US cultivar ‘Kentucky-31’ (KY31). The plants were grown in identical common garden experiments in Finland and Kentucky for two growing seasons. Plants were left as controls (C) or given water (W), nutrient (N) or water and nutrient (WN) treatments. For 8–10 replications of each plant origin and treatment combination in both experiments, we analyzed ergot alkaloids, lysergic acid, and lolines. In Finland, tall fescue plants produced 50 % more ergot alkaloids compared to plants of the same origin and treatments in Kentucky. Origin of the plants affected the ergot alkaloid concentration at both study sites: the wild origin plants produced 2–4 times more ergot alkaloids than KY31, but the ergot alkaloid concentration of KY31 plants was the same at both locations. Overall lysergic acid content was 60 % higher in plants grown in Kentucky than in those grown in Finland. Nutrient treatments (N, WN) significantly increased ergot alkaloid concentrations in plants from Finland but not in plants from Kentucky. These results suggest that the success of KY31 in US is not due to selection for high ergot alkaloid production but rather other traits associated with the endophyte. In addition, the environmental effects causing variation in alkaloid production of grass-endophyte combinations should be taken into account when using endophyte-infected grasses agriculturally.

Simulated acid rain reduces the susceptibility of the European pine sawfly (Neodiprion sertifer) to its nuclear polyhedrosis virus

The study dealt with the effect of simulated acid rain (both H(2)SO(4) and HNO(3); acidities of pH 4 and pH 3) on the susceptibility of the larvae of Neodiprion sertifer to its nuclear polyhedrosis virus. Scots pines growing in a subarctic area with low ambient pollution levels were irrigated with simulated acid rain during two summers. Neodiprion larvae fed with foliage from the experimental trees were infected with a dilute virus suspension. The acid treatment of host trees had a significant effect on the proportion of virus-treated larvae alive 16 days after the virus application: there were almost no differences between the controls and the pH 4 irrigation group, but on the needles of pH 3-treated trees larval survival was twice as high as with other treatments. The direct spraying of acid water on the needles before they were fed to the larvae did not significantly affect the survival of virus infected larvae. Our results suggest that acid rain may reduce the susceptibility of Neodiprion larvae to virus disease via changes in the quality of pine foliage.

Fragmented environment affects birch leaf endophytes

The effect of environmental fragmentation on the species distribution and frequency of horizontally transmitted endophytic fungi in Betula pubescens and Betula pendula leaves was studied in an archipelago in southwestern Finland. The study system consisted of 14 islands, ranging in size and distance to the mainland, and five mainland sites. Endophytic fungi were grown out from surface-sterilized leaves. The frequency of endophytic fungi mainly depended on the size of the island, explaining 32-35% of the variation, and the distance to the mainland explaining 29-35% of the variation. The birch trees on the largest islands near the mainland had the highest endophyte frequencies. Fusicladium betulae, Gnomonia setacea and Melanconium betulinum were the most commonly isolated fungi. Foliar endophytes of birch trees are able to disperse to fairly fragmented areas, but their frequencies seem to depend on environmental isolation and size of the island.