Nectar bacteria,but not yeast,weaken a plant–pollinator mutualism

Mutualistic interactions are often subject to exploitation by species that are not directly involved in the mutualism. Understanding which organisms act as such ‘third-party’ species and how they do so is a major challenge in the current study of mutualistic interactions. Here, we show that even species that appear ecologically similar can have contrasting effects as third-party species. We experimentally compared the effects of nectar-inhabiting bacteria and yeasts on the strength of a mutualism between a hummingbird-pollinated shrub, Mimulus aurantiacus, and its pollinators. We found that the common bacterium Gluconobacter sp., but not the common yeast Metschnikowia reukaufii, reduced pollination success, seed set and nectar consumption by pollinators, thereby weakening the plant–pollinator mutualism. We also found that the bacteria reduced nectar pH and total sugar concentration more greatly than the yeasts did and that the bacteria decreased glucose concentration and increased fructose concentration whereas the yeasts affected neither. These distinct changes to nectar chemistry may underlie the microbes'' contrasting effects on the mutualism. Our results suggest that it is necessary to understand the determinants of microbial species composition in nectar and their differential modification of floral rewards to explain the mutual benefits that plants and pollinators gain from each other.     

Limiting the cost of mutualism: the defensive role of elongated gynophore in the leafflower–moth mutualism

Mutualisms are interactions from which both partners benefit but may collapse if mutualists’ costs and benefits are not aligned. Host sanctions are one mechanism whereby hosts selectively allocate resources to the more cooperative partners and thereby reduce the fitness of overexploiters; however, many mutualisms lack apparent means of host sanctions. In mutualisms between plants and pollinating seed parasites, such as those between leafflowers and leafflower moths, pollinators consume subsets of the seeds as larval food in return for their pollination service. Plants may select against overexploiters by selectively aborting flowers with a heavy egg load, but in many leafflower species, seeds are fully eaten in some fruits, suggesting that such a mechanism is not present in all species. Instead, the fruits of Breynia vitis-idaea have stalk-like structures (gynophore) through which early-instar moth larvae must bore to reach seeds. Examination of moth mortality in fruits with different gynophore lengths suggested that fruits with longer gynophore had higher moth mortality and, therefore, less seed damage. Most moth mortality occurred at the egg stage or as early larval instar before moths reached the seeds, consistent with the view that gynophore functions to prevent moth access to seeds. Gynophore length was unaffected by plant size, extent of moth oviposition, or geography; thus, it is most likely genetically controlled. Because gynophores do not elongate in related species whose pollinators oviposit directly into the ovary, the gynophore in B. vitis-idaea may have evolved as a defense to limit the cost of the mutualism.     

The effect of disturbance on an ant–plant mutualism

Protective ant–plant mutualisms—where plants provide food or shelter to ants and ants protect the plants from herbivores—are a common feature in many ecological communities, but few studies have examined the effect of disturbance on these interactions. Disturbance may affect the relationship between plants and their associated ant mutualists by increasing the plants’ susceptibility to herbivores, changing the amount of reward provided for the ants, and altering the abundance of ants and other predators. Pruning was used to simulate the damage to buttonwood mangrove (Conocarpus erectus) caused by hurricanes. Pruned plants grew faster than unpruned plants, produced lower levels of physical anti-herbivore defenses (trichomes, toughness), and higher levels of chemical defenses (tannins) and extrafloral nectaries. Thus, simulated hurricane damage increased plant growth and the amount of reward provided to ant mutualists, but did not have consistent effects on other anti-herbivore defenses. Both herbivores and ants increased in abundance on pruned plants, indicating that the effects of simulated hurricane damage on plant traits were propagated to higher trophic levels. Ant-exclusion led to higher leaf damage on both pruned and upruned plants. The effect of ant-exclusion did not differ between pruned and unpruned plants, despite the fact that pruned plants had higher ant and herbivore densities, produced more extrafloral nectaries, and had fewer physical defenses. Another common predator, clubionid spiders, increased in abundance on pruned plants from which ants had been excluded. I suggest that compensatory predation by these spiders diminished the effect of ant-exclusion on pruned plants.     

The resource-mediated modular structure of a non-symbiotic ant–plant mutualism

1. Plant–animal mutualisms are key processes that influence community structure, dynamics, and function. They reflect several neutral and niche-based mechanisms related to plant–animal interactions. 2. However, the strength with which these processes influence community structure depends on functional traits that influence the interactions between mutualistic partners. In mutualisms involving plants and ants, nectar is the most common reward, and traits such as quantity and quality can affect ant species' responses by influencing their recruitment rates and aggressiveness. 3. In this study, nectar traits that mediate ant–plant defensive mutualisms were manipulated to test whether resource quantity and quality affect the structure of ant–plant interaction networks. A downscaling approach was used to investigate the interaction network between ant species and individual plants of the extrafloral nectary-bearing terrestrial orchid Epidendrum secundum. 4. We found a short-term reorganization of the ant assemblage that caused the interaction networks to become more specialised and modular in response to a more rewarding nectar gradient. Furthermore, the ant species tended to narrow their foraging range by limiting their associations to one or a few individual plants. 5. This study shows that ant species' responses to variable resource traits play an important role in the structure of the ant–plant interaction network. We suggest that more rewarding nectar enhanced aggressiveness and a massive recruitment of some ant species, leading to lower niche overlap and thus a less connected and more specialised network.     

Lack of fidelity revealed in an insect–fungal mutualism after invasion

Symbiont fidelity is an important mechanism in the evolution and stability of mutualisms. Strict fidelity has been assumed for the obligate mutualism between Sirex woodwasps and their mutualistic Amylostereum fungi. This assumption has been challenged in North America where the European woodwasp, Sirex noctilio, and its fungal mutualist, Amylostereum areolatum, have recently been introduced. We investigate the specificity of the mutualism between Sirex and Amylostereum species in Canada, where S. noctilio co-infests Pinus with native Sirex nigricornis and its mutualist, Amylostereum chailletii. Using phylogenetic and culture methods, we show that extensive, reciprocal exchange of fungal species and strains is occurring, with 75.3 per cent of S. nigricornis carrying A. areolatum and 3.5 per cent of S. noctilio carrying A. chailletii. These findings show that the apparent specificity of the mutualism between Sirex spp. and their associated Amylostereum spp. is not the result of specific biological mechanisms that maintain symbiont fidelity. Rather, partner switching may be common when shifting geographical distributions driven by ecological or anthropogenic forces bring host and mutualist pairs into sympatry. Such novel associations have potentially profound consequences for fitness and virulence. Symbiont sharing, if it occurs commonly, may represent an important but overlooked mechanism of community change linked to biological invasions.     

Plant–bacteria interactions in the removal of pollutants

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Highlights► Rhizobacteria degrade a wide range of pollutants and efficiently colonize plant roots. ► Plants have an effect on the selection of their own rhizospheric microorganisms. ► Catabolic pathways can be induced by natural secondary plant products. ► Horizontal gene transfer has an important role in bioremediation. ► Manipulation of plant/microbe interactions could improve rhizoremediation outcomes.     

Consumer–resource dynamics of indirect interactions in a mutualism–parasitism food web module

Food web dynamics are well known to vary with indirect interactions, classic examples including apparent competition, intraguild predation, exploitative competition, and trophic cascades of food chains. Such food web modules entailing predation and competition have been the focus of much theory, whereas modules involving mutualism have received far less attention. We examined an empirically common food web module involving mutualistic (N ₂) and parasitic (N ₃) consumers exploiting a resource of a basal mutualist (N ₁), as illustrated by plants, pollinators, and nectar robbers. This mutualism–parasitism food web module is structurally similar to exploitative competition, suggesting that the module of two consumers exploiting a resource is unstable. Rather than parasitic consumers destabilizing the module through (?,?) indirect interactions, two mechanisms associated with the mutualism can actually enhance the persistence of the module. First, the positive feedback of mutualism favors coexistence in stable limit cycles, whereby (+,?) indirect interactions emerge in which increases in N ₂ have positive effects on N ₃ and increases in N ₃ have negative effects on N ₂. This (+,?) indirect interaction arising from the saturating positive feedback of mutualism has broad feasibility across many types of food web modules entailing mutualism. Second, optimization of resource exploitation by the mutualistic consumer can lead to persistence of the food web module in a stable equilibrium. The mutualism–parasitism food web module is a basic unit of food webs in which mutualism favors its persistence simply through density-dependent population dynamics, rather than parasitism destabilizing the module.     

The ecological effects of the ant–hemipteran mutualism: A meta-analysis

Ant–hemipteran mutualism has a variety of ecological effects on the host plants, but the magnitude and moderators of those effects are poorly known. We evaluated this issue by conducting a meta-analysis based on 49 published studies. Results showed that the mutualism had significant protective effects on the host plans, although those effects did not lead to enhanced plant growth or reproductive performance. Both herbivores and predators on plants were strongly suppressed by the mutualism; a similar pattern was also detected for fruit removal. The ecological effects of the mutualism tended to be more consistent and stronger in tropical and subtropical regions, whereas in temperate regions, none of the tested variables was significantly affected by the mutualism. The protective effects of the mutualism on plants were independent of plant life form and the invasiveness of ants. The effect of the mutualism on predators varied with plant life form. The study confirmed that the ant–hemipteran mutualism has a wide range of ecological influences on plants and highlights the significance of a common and wide-spread mutualism.     

Ungulate saliva inhibits a grass–endophyte mutualism

Fungal endophytes modify plant–herbivore interactions by producing toxic alkaloids that deter herbivory. However, studies have neglected the direct effects herbivores may have on endophytes. Antifungal properties and signalling effectors in herbivore saliva suggest that evolutionary pressures may select for animals that mitigate the effects of endophyte-produced alkaloids. Here, we tested whether saliva of moose (Alces alces) and European reindeer (Rangifer tarandus) reduced hyphal elongation and production of ergot alkaloids by the foliar endophyte Epichloë festucae associated with the globally distributed red fescue Festuca rubra. Both moose and reindeer saliva reduced the growth of isolated endophyte hyphae when compared with a treatment of distilled water. Induction of the highly toxic alkaloid ergovaline was also inhibited in plants from the core of F. rubra''s distribution when treated with moose saliva following simulated grazing. In genotypes from the southern limit of the species'' distribution, ergovaline was constitutively expressed, as predicted where growth is environmentally limited. Our results now present the first evidence, to our knowledge, that ungulate saliva can combat plant defences produced by a grass–endophyte mutualism.     

A mechanistic molecular test of the plant-sanction hypothesis in legume–rhizobia mutualism

The origin and persistence of mutualism is difficult to explain because of the widespread occurrence of exploitative, ‘cheating’ partners. As a policing strategy stabilising intraspecific cooperation, host sanctions against non-N₂ fixing, cheating symbionts have been proposed to stabilise mutualism in legume-rhizobium symbiosis. Mechanism of penalisations would include decreased nodular rhizobial viability and/or early nodule senescence. We tested these potential mechanisms of penalisations in split-root experiments using two soybean varieties and two rhizobial strains, a cooperative, normal N₂-fixing strain and an isogenic non-fixing derivative. We found no differences in the number of viable rhizobia recovered from nodules and no differential expression of a nodular senescence molecular marker. Thus, our results do not support the hypothesis of plant sanctions acting against cheating rhizobia in our experimental conditions.     

Global dynamics of a mutualism–competition model with one resource and multiple consumers

Journal of Mathematical Biology - Recent simulation modeling has shown that species can coevolve toward clusters of coexisting consumers exploiting the same limiting resource or resources, with...     

Intraspecific competition favors ant–plant protective mutualism

The dependency of the anti-herbivore defense on ant–plant protective mutualism often varies depending on abiotic and biotic conditions. Although intraspecific competition is a primary interaction between neighboring plants, its effects on ant–plant mutualisms have yet to be sufficiently elucidated. In order to determine the effects of intraspecific competition and competitor genotype on ant–plant mutualisms, I conducted competition and ant-removal experiments and examined their effects on damage to the leaves of Urena lobata var. tomentosa plants. I found that larger numbers of worker ants visited the plants growing with non-siblings than plants growing alone and that plants growing with non-siblings had a higher shoot to root ratio and secreted greater volumes of extrafloral nectar than plants growing alone and/or with siblings. Under the presence of both sibling and non-sibling competitors, I observed that when ants were removed from plants, those grown with conspecific neighbors were characterized by a higher percentage of damaged leaf area than plants harboring ants. The effect of ant exclusion on leaf damage was more pronounced in plants grown with non-siblings than those grown near siblings. However, when the plants were grown alone, I detected no significant difference in percentage leaf damage between the ant-excluded and ant-harboring plants. The results indicate that neighboring plants can exert strong effects on ant–plant protective mutualisms, thereby highlighting the need to take into consideration plant–plant interactions in studies on these mutualistic associations.     

Phenotypic characterization of tannin–protein complex degrading bacteria from faeces of goat

Tannin–protein complex degrading bacteria after enrichment were isolated from unadapted goat faecal samples. Based on the morphological, hemolytic and biochemical characters, the isolates were categorized in two groups comprising GF1–GF4 and GF5–GF6. All the isolates were gram-positive cocci, catalase negative belonging to different strains of Group D streptococci, Enterococcus faecalis. Among six isolates, GF1 was the most resistant that could tolerate up to 4% of tannic acid in the medium with no significant change in the morphology. Tannase activity was detected in all the isolates, indicating their tannin degrading potential while gallate decarboxylase activity was detected only in three isolates GF1, GF2 and GF6.     

Mild disintegration methods of microalgae–bacteria flocs from wastewater treatment

The use of microalgae in biological wastewater treatment has been widely studied. However, there is a dearth of information about estimating the microalgae and bacteria concentrations. In order to maintain a stable algal-bacterial system, it is necessary to quantify both the algal and bacterial biomasses. Typically, microalgae and bacteria from flocs in activated sludge contribute to better biomass settleability. However, flocs cause problems when it comes to estimating the individual biomass concentrations of microalgae and bacteria in a symbiotic algae-bacteria aggregate. This study aimed to find the best disintegration treatment with low influence on the viability of the microalgal cell determined by its photosynthetic activity. In the present work, biological (enzyme solution), chemical (formaldehyde), mechanical (glass bead-beating), and physical (sonication) treatments were performed on microalgae–bacteria flocs (ALBA flocs) to disintegrate the community as a pre-treatment step in order to develop a method for estimating the algal and bacterial concentration and to quantify the degree of disintegration. The effectiveness of the methods to disintegrate ALBA flocs in descending order are the following: sonication, bead-beating, formaldehyde and enzyme application. Sonication treatment (40 W, 6 min) showed the best disintegration performance of the microalgal-bacterial flocs, up to 90 % with 17 % loss of the algal photosynthetic activity. Bead-beating (3 mm diameter, 80 s) achieved 80 % of disintegration with only 6 % loss of its photosynthetic activity. These results demonstrate the possibility of mild disintegration of compact ALBA flocs without having any adverse impact on the microalgae cell. After these treatments, it becomes possible to estimate the individual biomass concentrations of algae and bacteria manually such as with a cell-counting chamber.     

The consequences of synthetic auxin herbicide on plant–herbivore interactions

Although herbicide drift is a common side effect of herbicide application in agroecosystems, its effects on the ecology and evolution of natural communities are rarely studied. A recent shift to dicamba, a synthetic auxin herbicide known for ‘drifting’ to nontarget areas, necessitates the examination of drift effects on the plant–insect interactions that drive eco-evo dynamics in weed communities. We review current knowledge of direct effects of synthetic auxin herbicides on plant–insect interactions, focusing on plant herbivory, and discuss potential indirect effects, which are cascading effects on organisms that interact with herbicide-exposed plants. We end by developing a framework for the study of plant–insect interactions given drift, highlighting potential changes to plant developmental timing, resource quantity, quality, and cues.     

Host–microbe interactions: bacteria

A selection of World Wide Web sites relevant to papers published in this issue of Current Opinion in Microbiology.     

Degradation of polypropylene–poly-L-lactide blend by bacteria isolated from compost

Abstract

Polypropylene (PP) degrading bacteria (P1 to P16) were isolated from compost using enrichment technique. Five isolates (P3, P6, P8, P10, and P13) were selected based on their degradation abilities. These isolates were identified as Bacillus spp. through biochemical characteristics and 16S rDNA sequence analysis. The isolates were tested for their ability to degrade blends of PP and poly-L-lactide (PLLA) (PP80 and PP80C6) in minimal media as well as in soil. In minimal media, the growth of bacteria increased with time, showing utilization of blend as carbon source. The protein content was estimated at the end of 15?days and maximum amount was secreted by isolate P8 indicating maximum potential to degrade polymers compared to other isolates. Scanning electron microscopy (SEM) results revealed the formation of biofilm on the polymer surface. Fourier-transform infrared spectroscopy (FTIR) analysis showed the formation of new bond at 2123?cm^?1 and breakage of old C=O ester bond at 1757?cm^?1 in case of polymer PP80C6. Thermogravimetric analysis (TGA) showed decrease in thermal stability of polymers after degradation. The carbon dioxide evolved from sample was measured and biodegradation degree was also calculated. The degree of biodegradation shown by the isolate P8 was 12% and the P6 was 10%. The results demonstrated that Bacillus species isolated from composted samples in this study provided promising evidence for the biodegradation of polypropylene and poly-L-lactide (PP-PLLA) blends in the environment.