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The arbuscular mycorrhizal (AM) symbiosis is functionally important for the nutrition and growth of most terrestrial plants. Nearly all phytohormones are employed by plants to regulate the symbiosis with AM fungi, but the regulatory role of cytokinin (CK) is not well understood. Here, we used transgenic tobacco ( Nicotiana tabacum) with a root-specific or constitutive expression of CK-degrading CKX genes and the corresponding wild-type to investigate whether a lowered content of CK in roots or in both roots and shoots influences the interaction with the AM fungus Rhizophagus irregularis. Our data indicates that shoot CK has a positive impact on AM fungal development in roots and on the root transcript level of an AM-responsive phosphate transporter gene ( NtPT4). A reduced CK content in roots caused shoot and root growth depression following AM colonization, while neither the uptake of phosphorus or nitrogen nor the root transcript levels of NtPT4 were significantly affected. This suggests that root CK may restrict the C availability from the roots to the fungus thus averting parasitism by AM fungi. Taken together, our study indicates that shoot- and root-borne CK have distinct roles in AM symbiosis. We propose a model illustrating how plants may employ CK to regulate nutrient exchange with the ubiquitous AM fungi. [ABSTRACT FROM AUTHOR]

Aims: Root herbivory incurs fitness costs, but herbivory by nematodes can additionally increase nutrient availability mediated by enhanced root exudation and soil microbial activity and change plant susceptibility to aboveground herbivores due to systemic changes in plant defence. We hypothesized that such positive indirect effects may outweigh the negative direct effects of root herbivory by nematodes on plant performance. Methods: We tested the effects of transient (3 weeks) and continuous (11 weeks) belowground herbivory by root-knot nematodes ( Me loidogyne incognita) on the biomass, nitrogen levels, and reproductive output of Nicotiana attenuata, and its interaction with a specialist aboveground herbivore ( Manduca sexta) in a greenhouse study. Results: Continuous nematode herbivory caused an increase in shoot biomass and enhanced nitrogen levels in roots, shoots and seeds, but reduced the reproductive output of N. attenuata. Short-term, transient nematode herbivory had no effects. Feeding by the aboveground herbivore reduced reproductive output and increased seed nitrogen content. Nicotine levels in leaves and the plant interaction with M. sexta were not influenced by nematode herbivory. Conclusion: In summary, only continuous nematode herbivory indirectly increased nitrogen availability and caused resource sequestration to the shoots; however, this effect was not strong enough to outweigh the direct fitness costs of nematode herbivory. [ABSTRACT FROM AUTHOR]

Moringa is a mycorrhizal crop cultivated in the tropics and subtropics and appreciated for its nutritive and health-promoting value. As well as improving plant mineral nutrition, arbuscular mycorrhizal fungi (AMF) can affect plant synthesis of compounds bioactive against chronic diseases in humans. Rhizophagus intraradices and Funneliformis mosseae were used in a full factorial experiment to investigate the impact of AMF on the accumulation of glucosinolates, flavonoids, phenolic acids, carotenoids, and mineral elements in moringa leaves. Levels of glucosinolates were enhanced, flavonoids and phenolic acids were not affected, levels of carotenoids (including provitamin A) were species-specifically reduced, and mineral elements were affected differently, with only Cu and Zn being increased by the AMF. This study presents novel results on AMF effects on glucosinolates in leaves and supports conclusions that the impacts of these fungi on microelement concentrations in edible plants are species dependent. The nonspecific positive effects on glucosinolates and the species-specific negative effects on carotenoids encourage research on other AMF species to achieve general benefits on bioactive compounds in moringa. [ABSTRACT FROM AUTHOR]

Plant-soil feedback responses for native and invasive plant species are well documented, but little is known about how feedback effects from the soil biota community affect plant interactions with herbivores. Here we examine whether changes of the soil biota community by the successful invader Solidago canadensis influence growth and herbivore susceptibility of two coexisting native plant species ( Tanacetum vulgare, Melilotus albus). Root zone soil from two different habitat types ('urban' and 'suburban') was collected and used as inocula in a plant-soil feedback study. Each plant species was grown either in its own soil biota community or with the community with a history from the competitive invasive or native plant species. To identify potential drivers of responses to the different soil biota communities, we analyzed root colonization by arbuscular mycorrhizal fungi and dark-septate endophytes (DSE), and the community composition of soil inhabiting nematodes at the end of our experiment. Results show that S. canadensis and M. albus were not affected by soil history. In contrast, T. vulgare showed increased plant growth in 'foreign' soil derived from S. canadensis root zone compared with its 'home' soil suggesting a growth promotion by the soil biota community of S. canadensis. From the examined drivers, the abundance of DSE explained the growth response of T. vulgare to the S. canadensis soil biota community best. However, shoot herbivory by banded snails ( Cepaea nemoralis, C. hortensis) was not affected by soil history, but by the habitat type where the soil inocula originated. Our study shows that a native plant species may profit from the presence of an invasive competitor mediated by changes in the soil biota community. [ABSTRACT FROM AUTHOR]

Earthworms and arbuscular mycorrhizal fungi (AMF) have profound impacts on plant performance. However, it is largely unknown if and how earthworms and AMF may affect plant succession. We planted mesocosms with an early-mid successional and a mid-late successional grassland plant community and added endogeic earthworms and commercial AMF in a full-factorial way to natural background soil. Earthworms had a positive effect on the total root and shoot biomass of both plant communities, with the effect on the shoots being slightly enhanced by co-inoculation with AMF. Surprisingly, the earthworm effect on the mid successional plant species depended on the successional stage of the plant community. Earthworms had a positive effect on the mid successional plant species when they were growing in the mid-late successional plant community, but no effect when the same plant species were growing in the early-mid successional plant community. Addition of AMF alone tended to reduce the shoot biomass of the early successional plant species, while the addition of earthworms in the presence or absence of AMF increased their shoot biomass. We conclude that the impacts of earthworms on plant species may depend on the successional stage of the plant community, while the effect of AMF addition depends on the successional stage of the plant community and may be changed by the presence of earthworms. Earthworms and AMF addition affect plants and plant communities of different successional stages differently with potential effects on plant succession. [ABSTRACT FROM AUTHOR]

Root-feeding insects are important drivers in ecosystems, and links between aboveground oviposition preference and belowground larval performance have been suggested. The root-colonizing arbuscular mycorrhizal fungi (AMF) play a central role in plant nutrition and are known to change host quality for root-feeding insects. However, it is not known if and how AMF affect the aboveground oviposition of insects whose offspring feed on roots. According to the preference-performance hypothesis, insect herbivores oviposit on plants that will maximize offspring performance. In a greenhouse experiment with rice ( Oryza sativa), we investigated the effects of AMF ( Glomus intraradices) on aboveground oviposition of rice water weevil ( Lissorhoptrus oryzophilus), the larvae of which feed belowground on the roots. Oviposition (i.e., the numbers of eggs laid by weevil females in leaf sheaths) was enhanced when the plants were colonized by AMF. However, the leaf area consumed by adult weevils was not affected. Although AMF reduced plant biomass, it increased nitrogen (N) and phosphorus concentrations in leaves and N in roots. The results suggest that rice water weevil females are able to discriminate plants for oviposition depending on their mycorrhizal status. The discrimination is probably related to AMF-mediated changes in plant quality, i.e., the females choose to oviposit more on plants with higher nutrient concentrations to potentially optimize offspring performance. AMF-mediated change in plant host choice for chewing insect oviposition is a novel aspect of below- and aboveground interactions. [ABSTRACT FROM AUTHOR]

ivated carbon (AC) is widely used in ecological studies to elucidate the role of allelopathic substances in interspecific plant competition. However, by adsorbing chemical signalling compounds AC may also have negative effects on plants with symbiosis partners such as arbuscular mycorrhizal fungi and rhizobia. Here we test whether addition of AC has detrimental effects on the mycorrhizal root colonization of the native forb Plantago lanceolata and the exotic legume Lupinus polyphyllus, the nodulation of L. polyphyllus, and the nutrient uptake and growth of the plants growing in intra- and interspecific competition. Allelopathic effects probably occurred in the germination and seedling establishment phase when P. lanceolata suffered from the presence of L. polyphyllus. However, this negative effect of L. polyphyllus on P. lanceolata was not ameliorated by AC addition. AC negatively affected L. polyphyllus root biomass in week 4, and root and shoot biomass of P. lanceolata in week 9 of the experiment; both effects were independent of the presence and absence of the competing plant species. Mycorrhizal root colonization of both plant species was reduced in the presence of AC, although the effect tended to be stronger for L. polyphyllus. No significant effect of AC on the nodulation of L. polyphyllus was detected. P. lanceolata was the superior competitor and led to reduced biomasses of L. polyphyllus in interspecific competition. We conclude that AC can reduce the mycorrhization and performance of plants which may lead to changes in interspecific competition without the involvement of allelopathy. Contrary to former studies the AC used in our study did not enhance the nutrient availability for the plants, but reduced plant growth and mycorrhization. We suggest that the nutrient properties of the used AC are of crucial importance for the direction and the mechanisms of the effects and should always be reported. [ABSTRACT FROM AUTHOR]

Plant secondary metabolites play an important role in constitutive and inducible direct defense of plants against their natural enemies. While induction of defense by aboveground pathogens and herbivores is well-studied, induction by belowground organisms is less explored. Here, we examine whether soil microorganisms and nematodes can induce changes in levels of the secondary metabolites aucubin and catalpol (iridoid glycosides, IG) in roots and root exudates of two full-sib families of Plantago lanceolata originating from lines selected for low and high constitutive levels of IG in leaves. Addition of soil microorganisms enhanced the shoot and root biomass, and the concentration of aucubin in roots of both Plantago lines without affecting IG levels in the rhizosphere. By contrast, nematode addition tended to reduce the root biomass and enhanced the stalk biomass, and increased the levels of aucubin and catalpol in root exudates of both Plantago lines, without affecting root IG concentrations. The Plantago lines did not differ in constitutive levels of aucubin and total IG in roots, while the concentration of catalpol was slightly higher in roots of plants originally selected for low constitutive levels of IG in leaves. Root exudates of “high IG line” plants contained significantly higher levels of aucubin, which might be explained by their higher root biomass. We conclude that soil microorganisms can induce an increase of aucubin concentrations in the roots, whereas nematodes (probably plant feeders) lead to an enhancement of aucubin and catalpol levels in root exudates of P. lanceolata. A potential involvement of secondary metabolites in belowground interactions between plants and soil organisms is discussed. [ABSTRACT FROM AUTHOR]

Belowground mechanisms involved in plant competition are still poorly understood. Since plant species are differently affected by soil organisms, changes in soil community composition might affect interspecific competition with consequences for plant community structure. We studied whether soil community composition affects competition between the grass Holcus lanatus L. and the legume Lotus corniculatus L. We established three different soil communities by adding no soil organisms (control), microorganisms <30 μm, and a soil suspension including microorganisms >30 μm, AMF and nematodes to gamma-sterilized soil. Nodulation and aboveground biomass of Lotus was decreased in the sterilized control soil and in the presence of Holcus. Contrastingly, the grass grew better in the presence of the legume than in monoculture and was not affected by soil community composition. Legume monocultures tended to produce the greatest aboveground biomass of the plant combinations when soil microorganisms were present, while the root biomass in legume monocultures was the lowest. Then, in a second experiment, we used natural (not sterilized) soil and added activated carbon to test whether the reduced nodulation of Lotus in interspecific competition is caused by allelopathic compounds of Holcus. In the natural soil, nodulation and flowering of Lotus was reduced, but the aboveground biomass was not affected by the competition with Holcus. Contrary to our expectations, activated carbon had a strong negative effect on the nodulation, growth and flowering of Lotus and shifted the interspecific competition in favour of Holcus. Probably, activated carbon impeded the nodulation by disrupting the communication between the legume and N2-fixing bacteria. We suggest that interruption of plant-microbe communications by activated carbon might be widespread and will confound interpretations on the role of allelopathy. Generally, we observed that the symbiosis of the legume with N2-fixing bacteria plays a crucial role in the grass-legume competition. When the symbiosis was deterred, the legume was outcompeted by the grass. [ABSTRACT FROM AUTHOR]

Plants compete for limited resources. Although nutrient availability for plants is affected by resource distribution and soil organisms, surprisingly few studies investigate their combined effects on plant growth and competition. Effects of endogeic earthworms (Aporrectodea jassyensis), root-knot nematodes (Meloidogyne incognita) and the spatial distribution of15N labelled grass litter on the competition between a grass (Lolium perenne), a forb (Plantago lanceolata) and a legume (Trifolium repens) were investigated in the greenhouse. Earthworms promoted N uptake and growth ofL. perenne. Contrastingly, shoot biomass and N uptake ofT. repensdecreased in the presence of earthworms.P. lanceolatawas not affected by the earthworms. We suggest that earthworms enhanced the competitive ability ofL. perenneagainstT. repens. Nematodes increased the proportion of litter N in each of the plant species. Litter distribution (homogeneous vs. patch) did not affect the biomass of any plant species. However,P. lanceolatatook up more15N, when the litter was homogeneously mixed into the soil. The results suggest that endogeic earthworms may affect plant competition by promoting individual plant species. More studies including decomposers are necessary to understand their role in determining plant community structure. [ABSTRACT FROM AUTHOR]

Human management practices and large detritivores such as earthworms incorporate plant litter into the soil, thereby forming a heterogeneous soil environment from which plant roots extract nutrients. In a greenhouse experiment we investigated effects of earthworms and spatial distribution of 15N-labelled grass litter on plants of different functional groups [Lolium perenne (grass), Plantago lanceolata (forb), Trifolium repens (legume)]. Earthworms enhanced shoot and root growth in L. perenne and P. lanceolata and N uptake from organic litter and soil in all plant species. Litter concentrated in a patch (compared with litter mixed homogeneously into the soil) increased shoot biomass and 15N uptake from the litter in L. perenne and enhanced root proliferation in P. lanceolata when earthworms were present. Growth of clover (T. repens) was rather independent of the presence of earthworms and organic litter distribution: nevertheless, clover took up more nitrogen in the presence of earthworms and exploited more 15N from the added litter than the other plant species. The magnitude of the effects of earthworms and organic litter distribution differed between the plant species, indicating different responses of plants with contrasting root morphology. Aphid (Myzus persicae) reproduction was reduced on P. lanceolata in the presence of earthworms. We suggest that earthworm activity may indirectly alter plant chemistry and hence defence mechanisms against herbivores. [ABSTRACT FROM AUTHOR]