Decomposer biomass in the rhizosphere to assess rhizodeposition

Quantification of the organic carbon released from plant roots is a challenge. These compounds of rhizodeposition are quickly transformed into CO₂ and eventually bacterial biomass to be consumed by bacterivores (protozoa and nematodes). Microbes stimulate rhizodeposition several-fold so assays under sterile conditions give an unrealistic value. Quantifying bacterial production from ³H-thymidine incorporation falls short in the rhizosphere and the use of isotopes does not allow clear distinction between labeled CO₂ released from roots or microbes. We reduced rhizodeposition in 3–5 week old barley with a 2 week leaf aphid attack and found that biomass of bacterivores but not bacteria in the rhizosphere correlated with plant–induced respiration activity belowground. This indicated top-down control of the bacteria. Moreover, at increasing density of aphids, bacterivore biomass in the rhizosphere decreased to the level in soil unaffected by roots. This suggests that difference in bacterivore biomass directly reflects variations in rhizodeposition. Rhizodeposition is estimated from plant-induced increases in bacterial and bacterivore biomass, and yield factors, maintenance requirements, and turnover rates from the literature. We use literature values that maximize requirements for organic carbon and still estimate the total organic rhizodeposition to be as little as 4–6% of the plant-induced respiration belowground.     

Simulated herbivory effects on rhizosphere organisms in pea (Pisum sativum) depended on phosphate

The effects of simulated aboveground herbivory and phosphate addition to soil on rhizosphere organisms (arbuscular mychorrhiza (AM), Rhizobium spp., bacteria, protozoa and nematodes) were studied in a 2 by 2 factorial designed pot experiment with Pea plants (Pisum sativum). Measurements were performed on 24 day old plants that were still in the nutrient acquisition phase before flowering. AM colonization and bacterial feeding nematodes were stimulated by high simulated her- bivory especially when plants were phosphate deficient. Total number of nematodes was higher with phosphate deficiency. Furthermore, non-significant peak values in soil respiration, total number of nematodes, and bacterial number were observed in phosphate deficient plants with high simulated herbivory. In phosphate amended plants, fast-growing protozoa and bacterial feeding nematodes decreased at high simulated herbivory. These results support the hypothesis that the plant regulates abundances of both AM and free-living rhizosphere organisms and thereby the amount of plant-available nutrients, according to demand via root exudation. Rhizobium spp. was significantly stimulated by phosphate addition but not affected by simulated herbivory. Metabolites produced by rhizosphere bacteria from plants exposed to high simulated herbivory in phosphate amended soil stimulated seed performance. Possible interactions between protozoa and nematodes in relation to production and composition of bacteria in the rhizosphere are discussed.     

Root herbivore identity matters in plant-mediated interactions between root and shoot herbivores

Plants are simultaneously attacked by a multitude of herbivores that affect plant responses and plant-mediated interactions in a variety of ways. So far, studies on indirect interactions between below- and aboveground herbivores have almost exclusively focused on interactions between only one root and one shoot herbivore species at the same time. Since these studies show a variety of outcomes, we test the hypothesis that root herbivore identity matters in below-/aboveground interactions. We studied the combined effects root-feeding nematodes (Pratylenchus penetrans) and wireworms (Agriotes lineatus larvae) on Plantago lanceolata and on the performance of aboveground phloem-feeding aphids (Myzus persicae) and chewing caterpillars (Chrysodeixis chalcites larvae). Since root herbivores may also affect resource availability and the microbial community in the rhizosphere, we examined resource utilization by soil microorganisms using BIOLOG EcoPlates™.

Wireworms decreased root biomass by 13%, but led to compensatory shoot growth. Nematodes and the aboveground herbivores did not affect the biomass of Plantago lanceolata. Feeding by C. chalcites larvae enhanced the concentration of aucubin in leaves, which might explain the high mortality of the caterpillars. Aphids and the belowground herbivores did not change iridoid glycoside levels in the leaves. However, the number of aphid offspring was reduced by 44% when nematodes had been added to the soil, whereas wireworms had no effect. We observed higher utilization of BIOLOG carbon sources by the soil microorganisms only in the presence of Pratylenchus penetrans. Our results suggest that the outcome of below–aboveground interactions highly depends on herbivore identity.     

Protozoa,Nematoda and Lumbricidae in the rhizosphere of Hordelymus europeaus (Poaceae): faunal interactions,response of microorganisms and effects on plant growth

Interactions among protozoa (mixed cultures of ciliates, flagellates and naked amoebae), bacteria-feeding nematodes (Pellioditis pellio Schneider) and the endogeic earthworm species Aporrectodea caliginosa (Savigny) were investigated in experimental chambers with soil from a beechwood (Fagus sylvatica L.) on limestone. Experimental chambers were planted with the grass Hordelymus europeaus L. (Poaceae) and three compartments separated by 45-m mesh were established: rhizosphere, intermediate and non-rhizosphere. The experiment lasted for 16 weeks and the following parameters were measured at the end of the experiment: shoot and root mass of H. europaeus, carbon and nitrogen content in shoots and roots, density of ciliates, amoebae, flagellates and nematodes, microbial biomass (SIR), basal respiration, streptomycin sensitive respiration, ammonium and nitrate contents, phosphate content of soil compartments. In addition, leaching of nutrients (nitrogen and phosphorus) and leachate pH were measured at regular intervals in leachate obtained from suction cups in the experimental chambers. Protozoa stimulated the recovery of nitrifying bacteria following defaunation (by chloroform fumigation) and increased nitrogen losses as nitrate in leachate. In contrast, protozoa and nematodes reduced leaching of phosphate, an effect ascribed to stimulation of microbial growth early in the experiment. Earthworms strongly increased the amount of extractable mineral nitrogen whereas it was strongly reduced by protozoa and nematodes. Both protozoa and nematodes reduced the stimulatory effect of earthworms on nitrogen mineralization. Microbial biomass, basal respiration, and numbers of protozoa and nematodes increased in the vicinity of the root. Protozoa generally caused a decrease in microbial biomass whereas nematodes and earthworms reduced microbial biomass only in the absence of protozoa. None of the animals studied significantly affected basal respiration, but specific respiration of microorganisms (O₂ consumption per unit biomass) was generally higher in animal treatments. The stimulatory effect of nematodes and earthworms, however, occurred only in the absence of protozoa. The sensitivity of respiration to streptomycin suggested that protozoa selectively grazed on bacterial biomass but the bacterial/fungal ratio appeared to be unaffected by grazing of P. pellio. Earthworms reduced root biomass of H. europaeus, although shoot biomass remained unaffected, and concentrations of nitrogen in shoots and particularly in roots were strongly increased by earthworms. Both nematodes and protozoa increased plant biomass, particularly that of roots. This increase in plant biomass was accompanied by a marked decrease in nitrogen concentrations in roots and to a lesser extent in shoots. Generally, the effects of protozoa on plant growth considerably exceeded those of nematodes. It is concluded that nematodes and protozoa stimulated plant growth by non-nutritional effects, whereas the effects of earthworms were caused by an increase in nutrient supply to H. europaeus.     

Interactions between root and shoot herbivores of Ammophila arenaria in the laboratory do not translate into correlated abundances in the field

Over the past decades a growing body of literature has presented proof of the possible interactions between foliar and root herbivores. These effects can be positive, negative or neutral in either direction, depending on the species and the involved mechanism. Most of these studies however concern experiments under controlled conditions. Whether these interactions affect the distribution of herbivores under natural conditions still largely remains an open question. This study examined interactions between root feeding nematodes and shoot feeding aphids on Ammophila arenaria in the laboratory. We subsequently addressed the question whether expectations from this experiment are reflected in correlations between plant related variables and the abundance of both herbivores in the field. We demonstrated that nematodes and aphids can negatively affect each other in a controlled microcosm. In the field however no significant correlations between nematode and aphid abundances could be detected. There, shorter plants with a more vital leaf set and a higher root density supported the highest numbers of aphids. Plants with a lower root density and higher root vitality held more migratory endoparasitic nematodes, while more nematode cysts were found among roots with a low vitality. A certain plant property can furthermore affect above‐ and belowground herbivores in the opposite direction, such as root density in this case. This study suggests that effects of root herbivores on foliar herbivores or vice versa seem to be blurred in a field situation where other variables related to plant vitality and water content structure the herbivore populations. Therefore, caution should be used in generalising the prevalence of these interactions between the above‐ and belowground fauna, based solely on laboratory experiments.     

Interactions among plants,bacteria, and fungi reduce extracellular enzyme activities under long‐term N fertilization

Atmospheric nitrogen (N) deposition has enhanced soil carbon (C) stocks in temperate forests. Most research has posited that these soil C gains are driven primarily by shifts in fungal community composition with elevated N leading to declines in lignin degrading Basidiomycetes. Recent research, however, suggests that plants and soil microbes are dynamically intertwined, whereby plants send C subsidies to rhizosphere microbes to enhance enzyme production and the mobilization of N. Thus, under elevated N, trees may reduce belowground C allocation leading to cascading impacts on the ability of microbes to degrade soil organic matter through a shift in microbial species and/or a change in plant–microbe interactions. The objective of this study was to determine the extent to which couplings among plant, fungal, and bacterial responses to N fertilization alter the activity of enzymes that are the primary agents of soil decomposition. We measured fungal and bacterial community composition, root–microbial interactions, and extracellular enzyme activity in the rhizosphere, bulk, and organic horizon of soils sampled from a long‐term (>25 years), whole‐watershed, N fertilization experiment at the Fernow Experimental Forest in West Virginia, USA. We observed significant declines in plant C investment to fine root biomass (24.7%), root morphology, and arbuscular mycorrhizal (AM) colonization (55.9%). Moreover, we found that declines in extracellular enzyme activity were significantly correlated with a shift in bacterial community composition, but not fungal community composition. This bacterial community shift was also correlated with reduced AM fungal colonization indicating that declines in plant investment belowground drive the response of bacterial community structure and function to N fertilization. Collectively, we find that enzyme activity responses to N fertilization are not solely driven by fungi, but instead reflect a whole ecosystem response, whereby declines in the strength of belowground C investment to gain N cascade through the soil environment.     

Above‐ and belowground insect herbivores mediate the impact of nitrogen eutrophication on the soil food web in a grassland ecosystem

Insect herbivores are important drivers of ecosystem processes in grasslands, and can mediate the grassland's response to environmental change. For example, recent evidence shows that above‐ and belowground herbivory, individually and in combination, can modify how a plant community responds to nitrogen (N) eutrophication, an important driver of global change. However, knowledge about how such effects extend to the associated soil food web is lacking. In a mesocosm experiment, we investigated how communities of soil nematodes – an abundant and functionally important group of soil organisms – responded to above‐ and belowground insect herbivory at contrasting N levels. We found that the strongest influence of above‐ and belowground herbivory on the nematode community appeared at elevated N. The abundance of root‐feeding nematodes increased when either above‐ or belowground insect herbivores were present at elevated N, but when applied together the two herbivore types cancelled out one another's effect. Additionally, at elevated N aboveground herbivory increased the abundance of fungal‐feeders relative to bacterial‐feeders, which indicates changes in decomposition pathways induced by N and herbivory. Belowground herbivory increased the abundance of omnivorous nematodes. The shifts in both the herbivorous and detrital parts of the soil food web demonstrate that above‐ and belowground herbivory does not only mediate the response of the plant community to N eutrophication, but in extension also the soil food web sustained by the plant community. We conclude that feedbacks between effects of above‐ and belowground herbivory mediate the response of the grassland ecosystem to N eutrophication.     

Toward a mechanistic understanding of competition in vascular-feeding herbivores: an empirical test of the sink competition hypothesis

Recent evidence suggests that competitive interactions among herbivores are mostly indirect and mediated by plant responses to herbivory. Most studies, however, emphasize chewing insects and secondary chemistry, thus ignoring the diverse group of vascular-parasites that may be more likely to compete through induced changes in phytonutrients. Using an aboveground phloem-feeding aphid (Myzus persicae) and a belowground gall-forming nematode (Meloidogyne incognita) on tobacco plants, we assessed the importance of competition via induced host–plant sinks. In a series of experimental trials, nematode root herbivory caused 55 and 72% declines in the growth and fecundity of aphids, respectively. Aphids, on the other hand, did not impact nematode performance. Therefore, we predicted that nematodes out-compete M. persicae by attenuating the magnitude of aphid-induced sinks. Through a combination of invertase enzyme measurements and stable isotope (¹³C and ¹⁵N) enrichment, we found evidence that both herbivores act as mobilizing sinks. Aphids attracted photoassimilates to feeding aggregations on leaves and nematode galls accumulated resources in the roots. Levels of invertase enzymes, for example, were more than fourfold higher in nematode galls than in surrounding root tissue. Yet we found no evidence supporting a sink competition model for aphid–nematode interactions. The strength of aphid-induced leaf sinks was entirely unaffected by nematode presence, and vice versa. Thus, induced host–plant sinks appear to be a common strategy employed by vascular parasites to manipulate the physiology of their host, but multi-sink competition may be limited to herbivores that co-occur on the same tissue type and/or plants under growth-limited abiotic conditions.     

Facultative bacterial endosymbionts benefit pea aphids Acyrthosiphon pisum under heat stress

Abstract 1. Natural populations of pea aphids in California contain at least two facultative bacterial secondary symbionts (pea aphid secondary symbiont, PASS, or pea aphid rickettsia, PAR) in a range of frequencies throughout the state.
2. Two pea aphid clones without either of these facultative associates failed to reproduce in the first 8 days after the final moult if they had been heat-stressed for a period of about 4 h at 39 °C as 1-day-old larvae in the laboratory.
3. Aphids infected artificially with PASS, however, were able to produce up to 48% of the normal complement of offspring produced by PASS-positive aphids that had not been heat-stressed. Clones infected artificially with PAR did not have the same advantage as those with PASS after heat stress.
4. In aphids without PASS or PAR, heat stress reduced the number of bacteriocytes (in which the obligate primary symbiont, Buchnera , resides) to 7% of non-heat-stressed aphids, while aphids with only PASS retained 70% of their bacteriocytes. Bacteriocytes in aphids with PAR but not PASS were reduced to 42% of controls.
5. When larvae were heat-stressed as older instars (5 days old), a similar pattern emerged, though the effect of heat stress was less extreme. Clones containing PASS produced the most offspring, three to 14 times as many as aphids without PASS or PAR. Aphids with PAR only, or PASS and PAR together, had reduced or no advantage over aphids without facultative symbionts.
6. Aphids of all clones that had been heat-stressed as later instars gave birth to a variable number of stillborn offspring. Aphids without facultative symbionts produced the most stillborn larvae.
7. Field studies showed a higher incidence of PASS in aphids collected in California in summer compared with aphids from the same sites collected 2–4 months earlier. The difference was significant in two of three widely dispersed locations.     

Symbiont infection affects aphid defensive behaviours

Aphids harbour both an obligate bacterial symbiont, Buchnera aphidicola, and a wide range of facultative ones. Facultative symbionts can modify morphological, developmental and physiological host traits that favour their spread within aphid populations. We experimentally investigated the idea that symbionts may also modify aphid behavioural traits to enhance their transmission. Aphids exhibit many behavioural defences against enemies. Despite their benefits, these behaviours have some associated costs leading to reduction in aphid reproduction. Some aphid individuals harbour a facultative symbiont Hamiltonella defensa that provides protection against parasitoids. By analysing aphid behaviours in the presence of parasitoids, we showed that aphids infected with H. defensa exhibited reduced aggressiveness and escape reactions compared with uninfected aphids. The aphid and the symbiont have both benefited from these behavioural changes: both partners reduced the fitness decrements associated with the behavioural defences. Such symbiont-induced changes of behavioural defences may have consequences for coevolutionary processes between host organisms and their enemies.     

Feeding behavior and performance of Nasonovia ribisnigri on grafts,detached leaves,and leaf disks of resistant and susceptible lettuce

Aphids are dependent on the phloem sap of plants as their only source of nutrients. Host‐plant resistance in lettuce, Lactuca sativa L. (Asteraceae), mediated by the Nr gene is used to control the lettuce aphid Nasonovia ribisnigri (Mosely) (Hemiptera: Aphididae). The resistance is located in the phloem; however, the exact mechanism of resistance is unknown. In this study, we investigated whether the resistance factor (or factors) is synthesized in the root or in the shoot. The feeding behavior and performance of avirulent N. ribisnigri were studied on grafts of resistant and susceptible lettuce. In addition, the persistence of resistance in excised lettuce tissue was measured, by studying the feeding behavior and performance of N. ribisnigri on detached leaves and leaf disks of resistant lettuce. It appears that the resistance factor encoded by the Nr gene is produced in the shoots: aphid feeding was reduced on resistant shoots grafted on susceptible roots, whereas aphids were able to feed on grafts of susceptible shoots on resistant roots. Partial loss of resistance was observed after detachment of leaves and excision of leaf disks from resistant plants. Aphids fed longer on excised resistant plant tissue compared with intact resistant plants; however, compared with excised plant tissue of the susceptible cultivar, the time spent on feeding was shorter, indicating resistance was not completely lost. Our findings caution against the use of excised leaf material for aphid resistance bioassays.     

Effects of defoliation intensity on soil food-web properties in an experimental grassland community

We established a greenhouse experiment based on replicated mini‐ecosystems to evaluate the effects of defoliation intensity on soil food‐web properties in grasslands. Plant communities, composed of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and plantain (Plantago lanceolata) with well‐established root and shoot systems, were subjected to five defoliation intensity treatments: no trimming (defoliation intensity 0, or DI 0), and trimming of all plant material to 35 cm (DI 1), 25 cm (DI 2), 15 cm (DI 3) and 10 cm (DI 4) above soil surface every second week for 14 weeks. Intensification of defoliation reduced shoot production and standing shoot and root mass of plant communities but increased their root to shoot ratio. Soil microbial activity and biomass decreased with intensification of defoliation. Concentrations of NO₃–N in soil steadily increased with intensifying defoliation, whereas NH₄–N concentrations did not vary between treatments. Numbers of microbi‐detritivorous enchytraeids, bacterial‐feeding rotifers and bacterial‐feeding nematodes steadily increased with intensifying defoliation, while the abundance of fungal‐feeding nematodes was significantly enhanced only in DI 3 and DI 4 relative to DI 0. The abundance of herbivorous nematodes per unit soil mass was lower in DI 3 and DI 4 than in DI 0, DI 1 and DI 2, but when calculated per unit root mass, their abundance tended to increase with defoliation intensity. The abundance of omnivorous and predatory nematodes appeared to be highest in the most intensely defoliated systems. The ratio of abundance of fungal‐feeding nematodes to that of bacterial‐feeding nematodes was not significantly affected by defoliation intensity. The results infer that defoliation intensity may significantly alter the structure of soil food webs in grasslands, and that defoliation per se is able to induce patterns observed in grazing studies in the field. The results did not support hypotheses that defoliation per se would cause a shift between the bacterial‐based and fungal‐based energy channels in the decomposer food web, or that herbivore and detritivore densities in soil would be highest under intermediate defoliation. Furthermore, our data for microbes and microbial feeders implies that the effects of defoliation intensity on soil food‐web structure may depend on the duration of defoliation and are therefore likely to be dynamic rather than constant in nature.     

Plant-mediated interactions between shoot-feeding aphids and root-feeding nematodes depend on nitrate fertilization

The plant metabolite composition is modulated by various abiotic and biotic factors including nutrient availability and herbivory. In turn, induced changes in plant quality can affect herbivore performance and mediate indirect interactions between spatially separated herbivores sharing a host. Studies on plant-mediated herbivore interactions have been carried out at single fertilization regimes only, but we hypothesized that nutrient availability modifies these interactions. Therefore, we studied the interactions between two vascular tissue herbivores, the aboveground feeding aphid Brevicoryne brassicae and the belowground infesting nematode Heterodera schachtii, on Arabidopsis thaliana grown under two nitrate fertilization conditions (varying by 33 %). Furthermore, we investigated plant growth and primary metabolic responses to fertilization and herbivore treatments, which could potentially mediate these interactions, as the herbivores may act as metabolic sinks. Whereas nematodes had no effects on aphids, aphid presence influenced nematodes in opposite directions, depending on fertilization: at low nitrate supply, aphids had a promoting effect on nematodes, whereas at high nitrate fertilization they lowered the nematode infestation compared to control plants. Plants produced significantly more biomass under high nitrate supply but C and N contents were not altered. Primary metabolite profiles differed only marginally between roots of both fertilization treatments in plants with and without aphids, indicating that nematodes may respond to these or other metabolic modifications, which are caused by minute environmental changes, in a sensitive way. Our results highlight the need to consider the importance of plant nutrient availability on the outcome of interactions between co-occurring herbivores in future studies.     

The effect of soil warming on bulk soil vs. rhizosphere respiration

There has been considerable debate on whether root/rhizosphere respiration or bulk soil respiration is more sensitive to long-term temperature changes. We investigated the response of belowground respiration to soil warming by 3 °C above ambient in bare soil plots and plots planted with wheat and maize. Initially, belowground respiration responded more to the soil warming in bare soil plots than in planted plots. However, as the growing season progressed, a greater soil-warming response developed in the planted plots as the contribution of root/rhizosphere respiration to belowground respiration declined. A negative correlation was observed between the contribution of root/rhizosphere respiration to total belowground respiration and the magnitude of the soil-warming response indicating that bulk soil respiration is more temperature sensitive than root/rhizosphere respiration. The dependence of root/rhizosphere respiration on substrate provision from photosynthesis is the most probable explanation for the observed lower temperature sensitivity of root/rhizosphere respiration. At harvest in late September, final crop biomass did not differ between the two soil temperature treatments in either the maize or wheat plots. Postharvest, flux measurements during the winter months indicated that the response of belowground respiration to the soil-warming treatment increased in magnitude (response equated to a Q ₁₀ value of 5.7 compared with ∼2.3 during the growing season). However, it appeared that this response was partly caused by a strong indirect effect of soil warming. When measurements were made at a common temperature, belowground respiration remained higher in the warmed subplots suggesting soil warming had maintained a more active microbial community through the winter months. It is proposed that any changes in winter temperatures, resulting from global warming, could alter the sink strength of terrestrial ecosystems considerably.     

Probing behaviour of the green peach aphid Myzus persicae on resistant Prunus genotypes

Electrical penetration graphs of Myzus persicae (Sulzer) (Homoptera: Aphididae) feeding behaviour on four resistant and two susceptible genotypes of peach (Prunus persica L. Batsch) and related species showed that resistance was mainly linked to (i) reduced duration of phloem sap uptake, (ii) reduced percentage of pattern E1 (salivary secretion into sieve elements) followed by pattern E2 (sap ingestion) and (iii) increased number of shifts from E1 to E2 and back. These results suggest the unsuitability of phloem sap, and thus repetitive failures to initiate sustained ingestion. Extensive comparisons of the EPGs also revealed more specific trends. Aphids on the most susceptible cultivar GF305 produced significantly longer potential drops than on other peach genotypes. On the resistant Rubira, aphids generated more penetrations before the first E occurred, indicating the possible presence of a resistance factor before the phloem was reached. The clone P1908 of the wild species Prunus davidiana displayed traits of both susceptibility (less but longer probes) and resistance. In particular, aphids produced more E1, suggesting difficulties in preparing sieve elements before feeding. The aphid probing process could be correlated with aphid settling behaviour and bionomics, as previously reported, and gave evidence for the existence of different mechanisms underlying resistance in the tested genotypes against M. persicae.     

The Influence of Previous Experience and Starvation on Aphid Feeding Behavior

It was hypothesized that (1) previous experience of aphids on a host plant leads to differences in their feeding behavior relative to aphids without previous experience on it and that (2) a change in the physiological state of the aphid modifies their experience-induced behavior. Using electronic recording, the feeding behavior of the aphid Sitobion fragariae (Walker) on wheat Triticum aestivum L. and oat Avena sativa L. was examined, comparing aphids with or without previous experience on a given host and with or without a period of starvation before assessing probing behavior. All comparisons were performed within a single aphid clone to minimize the effect of genetic variation. Feeding behavior on wheat was significantly affected by previous experience and starvation. The effect of previous experience interacted with the host plant where feeding behavior was tested. Aphids feeding on wheat following previous experience on wheat showed a longer time and a higher number of pathway activities and less time in waveform F (i.e., mechanical stylet work and penetration difficulties) than did aphids feeding on wheat after a previous experience on oat. No differences in the time from the beginning of the recording until the first salivation into the sieve elements were found. When aphids were subjected to a period of starvation, the time devoted to xylem ingestion increased compared with that of nonconstrained aphids. These results are discussed in terms of factors affecting foraging decisions.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

Foliar aphid feeding recruits rhizosphere bacteria and primes plant immunity against pathogenic and non-pathogenic bacteria in pepper

Background and Aims

Plants modulate defence signalling networks in response to different biotic stresses. The present study evaluated the effect of a phloem-sucking aphid on plant defence mechanisms in pepper (Capsicum annuum) during subsequent pathogen attacks on leaves and rhizosphere bacteria on roots.

Methods

Plants were pretreated with aphids and/or the chemical trigger benzothiadiazol (BTH) 7 d before being challenged with two pathogenic bacteria, Xanthomonas axonopodis pv. vesicatoria (Xav) as a compatible pathogen and X. axonopodis pv. glycines (Xag) as an incompatible (non-host) pathogen.

Key Results

Disease severity was noticeably lower in aphid- and BTH + aphid-treated plants than in controls. Although treatment with BTH or aphids alone did not affect the hypersensitive response (HR) against Xag strain 8ra, the combination treatment had a synergistic effect on the HR. The aphid population was reduced by BTH pretreatment and by combination treatment with BTH and bacterial pathogens in a synergistic manner. Analysis of the expression of the defence-related genes Capsicum annum pathogenesis-related gene 9 (CaPR9), chitinase 2 (CaCHI2), SAR8·2 and Lipoxygenase1 (CaLOX1) revealed that aphid infestation resulted in the priming of the systemic defence responses against compatible and incompatible pathogens. Conversely, pre-challenge with the compatible pathogen Xav on pepper leaves significantly reduced aphid numbers. Aphid infestation increased the population of the beneficial Bacillus subtilis GB03 but reduced that of the pathogenic Ralstonia solanacearum SL1931. The expression of defence-related genes in the root and leaf after aphid feeding indicated that the above-ground aphid infestation elicited salicylic acid and jasmonic acid signalling throughout the whole plant.

Conclusions

The findings of this study show that aphid feeding elicits plant resistance responses and attracts beneficial bacterial populations to help the plant cope with subsequent pathogen attacks.     

Fertilization effects on fineroot biomass, rhizosphere microbes and respiratory fluxes in hardwood forest soils

Fertilizer-induced reductions in CO(2) flux from soil ((F)CO(2)) in forests have previously been attributed to decreased carbon allocation to roots, and decreased decomposition as a result of nitrogen suppression of fungal activity. Here, we present evidence that decreased microbial respiration in the rhizosphere may also contribute to (F)CO(2) reductions in fertilized forest soils. Fertilization reduced (F)CO(2) by 16-19% in 65-yr-old plantations of northern red oak (Quercus rubra) and sugar maple (Acer saccharum), and in a natural 85-yr-old yellow birch (Betula allegheniensis) stand. In oak plots, fertilization had no effects on fine root biomass but reduced mycorrhizal colonization by 18% and microbial respiration by 43%. In maple plots, fertilization reduced root biomass, mycorrhizal colonization and microbial respiration by 22, 16 and 46%, respectively. In birch plots, fertilization reduced microbial respiration by 36%, but had variable effects on root biomass and mycorrhizal colonization. In plots of all three species, fertilization effects on microbial respiration were greater in rhizosphere than in bulk soil, possibly as a result of decreased rhizosphere carbon flux from these species in fertile soils. Because rhizosphere processes may influence nutrient availability and carbon storage in forest ecosystems, future research is needed to better quantify rhizo-microbial contributions to (F)CO(2).     

The impact of Meu1-mediated resistance in tomato on longevity, fecundity and behavior of the potato aphid, Macrosiphum euphorbiae

The effect of the tomato resistance gene, Meu1, on feeding, longevity, fecundity and developmental rate of the pink biotype of the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera, Aphididae), was determined using nearly isogenic tomato (Lycopersicon esculentum Mill, Solanaceae) lines. Aphid mortality was significantly higher on resistant plants, with 60% of the aphids dying by the 4th day of exposure. By the 10th day, all the aphids on the resistant plants were dead whereas 100% of the aphids on susceptible plants were alive. Meu1-mediated resistance resulted in significantly decreased fecundity with a ten-fold decrease in the net fertility rate (4.5 and 45.7 progeny per aphid on resistant and susceptible tomato, respectively). A qualitative analysis showed that honeydew was produced by aphids on resistant and susceptible plants, suggesting that aphids initiate feeding on both plant types. However, significantly lower quantities of honeydew were present when aphids were caged on resistant plants. There were also significant differences in aphid location on resistant and susceptible leaves. Experiments evaluating behavior in less than 24 h showed that aphids left resistant leaves after relatively short exposure (3–6 h). Aphids transferred from resistant to susceptible tomato at intervals between 3 h and 24 h resumed feeding as evidenced by presence of honeydew. Although the mechanism by which Meu1-mediated resistance operates is not yet known, our data suggest that resistance factors act rapidly after initiation of feeding and that lower fecundity and longevity are related to reduction in aphid feeding.