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References Publications referenced by this paper. Plant growth-promoting rhizobacteria and root system. Defining the core Arabidopsis thaliana root microbiome Derek S. Lundberg , Sarah L. Plant growth promoting rhizobacteria as biofertilizers J.


Kevin VesseyJ. Structure, variation, and assembly of the root-associated microbiomes of rice. Joseph Edwards , Cameron J. The use of pots comes with a range of disadvantages that may affect the study system, especially so in the field. First of all, studies often use sterilized soil or steamed potting soil, which excludes the interactions with resident soil organisms. Furthermore, pots not only impose a barrier to the root system, but also to the movement of the study organisms. Moreover, it prevents the influx of other soil organisms. Although pots may have the advantage of ensuring that the soil organisms are present at the root system, this methodology may be highly artificial compared to field plots.

The barrier also inherently limits plant growth i. Lastly, abiotic conditions in pots can be quite different from conditions in soil. Placing pots often of dark color, which absorbs more energy on top of the soil, may increase soil temperature in the pot under warm conditions. Moreover, they may cool down more rapidly under cold conditions.

We propose that pots can be extremely useful in studying soil organisms, both in laboratory and field conditions, but that they should be used with caution and that abiotic constraints should be countered as much as possible for example by burying the pots, using large enough pots and including live soils into the design. The use of pesticides in field experiments was a common approach in the early years of the development of this niche in ecology.

However, this also comes with many obvious disadvantages. Several studies have shown that, although the pesticides are often rather specific and indeed reduce target organisms, there are also undesirable side-effects that influence many other soil processes e. We propose that addition of soil organisms to field plots may be the best methodology, as this allows for interactions of both the added soil organisms and the plant with resident soil communities. From an applied perspective, results from soil organism addition studies are perhaps also the most useful as these scenarios are most comparable to application of soil organisms e.

However, it is very hard to standardize both the abiotic and biotic conditions of live field soils, and this can lead to considerable variation between or even within study sites. In this review we have explored the scientific literature that discusses the effect of biotic manipulations of the soil on aboveground plant-insect interactions in the field. First, we asked if there is a role for soil organisms in shaping aboveground plant-insect interactions under field conditions.

We searched the literature for studies that report on manipulations of the whole soil microbiome and how changes in soil community composition may affect aboveground insects in the field. It appears that there is ample evidence for effects of changes in whole soil communities on insect assemblages, but these findings are all correlative, not causative. To our knowledge, no studies thus far, have assessed these effects in a field setting. This is an important aspect of above-belowground ecology that deserves more attention in the future.

We argue that introducing the PSF concept as a fourth applicable field method to shift soil communities in a certain direction would be less disruptive than the commonly used methodologies and would incorporate more ecological realism. Our second question was whether the manipulation of specific taxa in the soil has the same effects on aboveground insects in the field as under more controlled conditions in greenhouses or growth chambers.

Our survey indicates that this is true for most taxa except for soil arthropods. Bacterial inoculation in the field generally promotes plant growth and depresses abundance and performance of insects in the field, as they do in laboratory studies e. For AMF, the effects observed in laboratory settings have been thoroughly reviewed Gehring and Bennett, ; Hartley and Gange, ; Koricheva et al. Field studies, we show, report similar patterns; AMF negatively influences generalist chewers, but positively affect specialist chewing insects. AMF also generally benefit sap-sucking insects, regardless of their specialization.

Under field conditions, nematodes affect chewing herbivores positively and sap suckers negatively and this is also in line with the general observations in laboratory studies Wondafrash et al. Patterns in the effects of soil arthropods are less straightforward. In the current review of field literature, we have not been able to observe a clear pattern.

One of the reasons for this could be the variation in abiotic and biotic conditions in the reported study systems. Furthermore, often only very few interactions are studied for each combination of taxa both below and aboveground. Therefore there is currently a lack of relevant data and this makes it hard to compare the different results more thoroughly, e.

The same problem arises when we attempt to elucidate patterns for less abundant feeding guilds such as leaf miners, gall makers or stem borers or natural enemies and pollinators. Very few studies, so far, have investigated the effects of soil organism manipulations in the field on these less apparent aboveground feeding guilds and this is an area that requires further attention in order to better understand patterns in soil arthropod-plant-insect interactions.

Although we observed similarities between field and laboratory studies, in the field, it is also important to note that a relatively large fraction of the studies that we detected reported neutral effects. We suggest that field methodology can drastically affect the outcome of above-belowground studies and that ecologists should be aware of this when designing experiments. Although there is a current lack of studies that compare the different field methodologies directly, the pattern is rather clear.

In the case of pot experiments and removal experiments in the field, the likelihood of observing a statistically significant effect of any kind, are twice as high as those in field addition experiments. However, we argue that the latter is, to date, by far the most realistic and useful methodology to understand ecological processes. Clearly, there are opportunities to explore alternative ways to manipulate soil organisms, or steer soil communities in specific directions. For example through manipulation of soil via plant-soil feedback mechanisms where soils are manipulated in the field by plant species with specific effects on soil communities, or by inoculation of plots with soils that have been conditioned by specific plant species.

Moreover, soil organisms can be manipulated via exclusion methods using variable mesh sizes that exclude certain soil taxa based on their sizes e. Four aspects of the field of above-belowground ecology deserve further development. First, the response of insect species from less apparent feeding guilds such as gall makers, stem borers, leaf miners and cell content feeders has often been overlooked so far.

In order to further elucidate patterns and more fully understand the ecological role of soil organisms in shaping plant-insect interactions, we need to use a more holistic approach that takes into account players from a broader range of guilds and trophic levels. Responses of natural enemies and pollinators aboveground have been studied infrequently, and are completely missing for certain types of soil manipulations, or soil taxa. The life history of the various natural enemies is quite diverse and their responses to soil biota-plant interactions may vary.

Parasitoids and other flying natural enemies may respond more quickly than wingless, cursorial predators like spiders. Furthermore, parasitoids are affected by changes in the quality of their herbivore hosts, as their life cycles intimately depend on host ecophysiology e. Moreover, when we searched for studies in the scientific literature, we could not detect any that focused on the effect of soil organisms, via plants, on interactions between plants and non-arthropod taxa, such as slugs, snails, but also higher vertebrates, such as grazers.

As plants are the primary producers that support food chains, it is likely that other organisms will also be affected by belowground organisms. Second, to increase our ecological understanding, it is important to also include more ecologically realistic model systems, as the current systems are often based on crops, as well as on insect species that are either crop pests or chosen for convenience, rather than based on ecological relevance Chen et al. This could be accomplished, for example, by using a range of wild plant species that vary in functional traits, which could give better insight into what traits may predict certain plant responses.

Studying their natural associated insect communities may also increase our understanding of which traits are important in mediating soil biota-plant-insect interactions. Future work could fill in these important gaps in our current knowledge. Third, more emphasis should be placed on the role of time and space in these aboveground-belowground interactions in the field. It is currently unknown whether performing manipulations with the same soil organisms at different locations e.

Future studies should also focus on the temporal aspects of above-belowground interactions in the field. As soil communities are dynamic and species-specific soil communities accumulate over time Diez et al. Various controlled studies have shown that the sequence of arrival of aboveground and belowground herbivores on the plant can greatly alter the outcome of soil biota-plant-insect interactions e.

In the field, insect communities also change throughout the season. How soil treatments affect insects early compared to late in the season, and to what extent this is due to changes in plant-soil interactions or changes in plant-insect interactions is not known. Fourth, most of the current research is focused on indirect effects that are mediated by shared host plants, but potential direct interactions should not be overlooked.

There are various organisms, such as entomopathogens in the soil that can have direct impacts on aboveground insect performance. For instance, infection by entomopathogenic fungi, such as Beauveria bassiana and Metarhizium anisoplae can result in the quick death of many insect species Meyling and Eilenberg, ; Vega et al. Interestingly, these fungi can also be endophytic in plants, and can influence both plant and herbivore performance Meyling and Eilenberg, ; Vega et al.

Moreover, it has been shown for the fungus Metarhizium that it forms bridges between infected dead insects and plants, through which the fungus can provide the plant with extra nitrogen obtained from the insect bodies, which may also affect plant-insect interactions Wang and St Leger, ; Behie et al.

Little is known about the extent to which aboveground insects pick up soil microorganisms and how this may affect their fitness, either through pathogenicity, or perhaps mutualistic interactions e. We conclude that there is strong support for a significant role of soil organisms in shaping plant-insect interactions in the field.

With the exception of soil arthropods, we find that most field studies report effects that are similar to those of laboratory studies. We argue that future studies should be carefully planned, as the methodology applied in the field strongly affects the chance of finding robust results. Nonetheless, there are ample opportunities to develop this research field further, especially in terms of exploring alternative and more realistic methods to steer soil biomes into a targeted direction.

It should be emphasized that there is a large gap in our knowledge when it comes to less apparent insect herbivore taxa such as leaf miners, stem borers and others. There is virtually nothing known about the effects of soil organisms on a broad range of natural enemies predators and parasitoids. However, as there are consistent reports of effects of soil organism addition in the field on aboveground insects, this opens up opportunities for the exploration of soil organism manipulation in agriculture or ecosystem restoration e. Some groups of soil organisms may be promising agents for crop yield enhancement and protection.

A challenge is to disentangle the drivers of soil organism manipulation effects on insects in the field. This will be an important step toward understanding how belowground organisms drive aboveground insect abundance, diversity and impacts in the field. TB and RH conceived the idea for the literature review. RH performed the literature study. TB and AB provided several additional references. RH wrote the first version of the manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Alston, D. Response of Helicoverpa zea Lepidoptera: Noctuidae populations to canopy development in soybean as influenced by Heterodera glycines Nematoda: Heteroderidae and annual weed population densities. Bais, H. The role of root exudates in rhizosphere interactions with plants and other organisms.

Plant Biol. Barber, N.

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How do belowground organisms influence plant—pollinator interactions? Plant Ecol. Context-dependency of arbuscular mycorrhizal fungi on plant-insect interactions in an agroecosystem. Plant Sci. Root herbivory indirectly affects above-and below-ground community members and directly reduces plant performance.

Bardgett, R. Belowground biodiversity and ecosystem functioning. Nature , — Behie, S. Endophytic insect-parasitic fungi translocate nitrogen directly from insects to plants. Science , , — Bezemer, T. Interactions between above-and belowground insect herbivores as mediated by the plant defense system. Oikos , — Linking aboveground and belowground interactions via induced plant defenses. Trends Ecol. Interplay between Senecio jacobaea and plant, soil, and aboveground insect community composition. Ecology 87, — Above-and below-ground herbivory effects on below-ground plant—fungus interactions and plant—soil feedback responses.

Biere, A. Plant-mediated systemic interactions between pathogens, parasitic nematodes, and herbivores above-and belowground. Above-and belowground insect herbivory modifies the response of a grassland plant community to nitrogen eutrophication. Ecology 98, — Brunner, S. Impact of nitrogen fixing and plant growth-promoting bacteria on a phloem-feeding soybean herbivore.

Soil Ecol. Carter-Wientjes, C. Feeding and maturation by soybean looper Lepidoptera: Noctuidae larvae on soybean affected by weed, fungus, and nematode pests. Cahill, J. Disruption of a belowground mutualism alters interactions between plants and their floral visitors. Ecology 89, — Chen, Y. Crop domestication and its impact on naturally selected trophic interactions. Colella, T. Effect of irrigation regimes and artificial mycorrhization on insect pest infestations and yield in tomato crop. Phytoparasitica 42, — Commare, R. Pseudomonas fluorescens based bio-formulation for the management of sheath blight disease and leaffolder insect in rice.

Crop Protection 21, — Cornelissen, T. Size does matter: variation in herbivory between and within plants and the plant vigor hypothesis. Diez, J. Negative soil feedbacks accumulate over time for non-native plant species. Erb, M. Sequence of arrival determines plant-mediated interactions between herbivores. Evans, E. Experimental manipulation of herbivores in native tallgrass prairie: responses of aboveground arthropods.

Flory, S. Pathogen accumulation and long-term dynamics of plant invasions. Gadhave, K. Plant growth-promoting Bacillus suppress Brevicoryne brassicae field infestation and trigger density-dependent and density-independent natural enemy responses. Pest Sci. Gange, A. Multitrophic links between arbuscular mycorrhizal fungi and insect parasitoids. Ecological specificity of arbuscular mycorrhizae: evidence from foliar-and seed-feeding insects.

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Ecology 86, — Dual colonization of Eucalyptus urophylla ST Blake by arbuscular and ectomycorrhizal fungi affects levels of insect herbivore attack. Arbuscular mycorrhizal fungi influence visitation rates of pollinating insects. Interactions between arbuscular mycorrhizal fungi and foliar-feeding insects in Plantago lanceolata L. New Phytol. Gehring, C.

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Mycorrhizal fungal-plant-insect interactions: the importance of a community approach. Mycorrhizal Ecol. Godschalx, A. Ants are less attracted to the extrafloral nectar of plants with symbiotic, nitrogen-fixing rhizobia. Ecology 96, — Guo, X. Independent role of belowground organisms and plant cultivar diversity in legume-grass communities. Root-knot nematodes Meloidogyne hapla can modify the positive plant intraspecific diversity-productivity effect on red clover in clover-grass communities.

Hartley, S. Impacts of plant symbiotic fungi on insect herbivores: mutualism in a multitrophic context. Harvey, J. Dynamic effects of parasitism by an endoparasitoid wasp on the development of two host species: implications for host quality and parasitoid fitness.

Development of the solitary endoparasitoid Microplitis demolitor : host quality does not increase with host age and size. Heeren, J. The interaction of soybean aphids and soybean cyst nematodes on selected resistant and susceptible soybean lines. Subsequently, these authors added 33 [P]-orthophosphate to the medium and confirmed that, indeed, the extraradical mycelium absorbs the H 2 PO 4 - ion and transfers it to the colonized roots.

After being absorbed, H 2 PO 4 - is accumulated in the hyphae in tubular vacuoles temporary storage and buffering of the H 2 PO 4 - concentration in the form of polyphosphate PolyP - , a linear chain of H 2 PO 4 - monomers, which can harbor thousands of orthophosphate ions , which is subsequently translocated along the hyphae Olsson et al.

The length of the PolyP - chain in the extraradical mycelium is greater than that in the intraradical one, suggesting that there is hydrolysis in the latter, which leads to a high concentration of H 2 PO 4 - , facilitating its efflux, only slightly increased due to C supply Solaiman et al. The GmPMA1 gene is expressed at high levels in the extraradical mycelium, especially during the nonsymbiotic growth phase, and at low levels during the symbiotic phase, with a reduction of about five-fold Requena et al.

The GmHA5 gene is little expressed during nonsymbiotic growth and is strongly induced in the symbiotic phase, 50 and 8-fold in the intra- and extraradical mycelia, respectively. A follow-up of the stages of intraradical mycelium development at 15, 20, 23, and 28 days post-inoculation, with the respective expression of GmPMA1 and GmHA5 , showed that, although few fungal structures were observed at 15 days - basically only multiple appressoria in the epidermis -, the expression of GmHA5 was already clearly detectable Requena et al.

According to these authors, as the infection progressed, the expression levels of this gene increased and became similar to those of GmPMA1. Next-generation sequencing has allowed the transcriptome analysis of the AMF genome and the quantification of transcript levels, which enables the confirmation or revision of previously obtained results, or even the targeting of new research. The genes MtHA1 of M. Arango et al.

Murphy et al. Gianinazzi-Pearson et al. The expression of the HA1 gene during the development of the arbuscular mycorrhiza promotes an adequate colonization of fungi, improves the absorption of H 2 PO 4 - by the plant, and energizes the periarbuscular membrane Wang et al. HA1 energizes the periarbuscular membrane of rice and M. The period, between 28 and 35 days after inoculation, in which the genes MtHA1 exclusively expressed in cells containing arbuscules and OsHA1 are strongly induced, is consistent with the development time of the arbuscules Wang et al.

Therefore, it is expected that the SlHA8 gene - an orthologous of MtHA1 and OsHA1 - also be strongly induced in cells containing arbuscules and inactivated in plants not colonized by mycorrhizal fungi and cultivated under normal growth or nutrient or salt stress conditions Liu et al. In fact, by reducing the levels of MtHA1 expression, a reduction in the intake of the symbiotic phosphate by mutant plants is observed Wang et al.

In addition, the mutants mtpt4 and mtha exhibit the same phenotype, especially a reduction in the level of colonization and a steep decline in the number of fully developed arbuscules Javot et al. Therefore, access to additional H 2 PO 4 - transported by the arbuscular mycorrhiza has a significant effect on plant growth and development Maldonado-Mendoza et al. Studies with radioisotopes have shown that the extraradical mycelium is responsible for the absorption of H 2 PO 4 - , which is subsequently translocated to the intraradical mycelium and then released to the plant Jakobsen et al.

GigmPT, for example, which acts as a high-affinity phosphorus transporter in the extraradical mycelium of G. At the amino acid level, GvPT shares GmosPT presented similar levels of expression in the extra- and intraradical mycelia; therefore, it has been suggested that it could control the efflux of H 2 PO 4 - in the periarbuscular space through the partial resorption of this nutrient Benedetto et al.

The resorption of H 2 PO 4 - in the periarbuscular space has also been attributed to GimPT, since its inactivation retards the growth of G. Glomus intraradices is able to perceive and respond to the levels of H 2 PO 4 - that surround its extraradical mycelium Maldonado-Mendoza et al. The GiPT gene is expressed in the extraradical mycelium in response to low-H 2 PO 4 - conditions in the environment that surround this mycelium and to the status of H 2 PO 4 - in the arbuscular mycorrhiza Maldonado-Mendoza et al.

These authors detected increases of GiPT transcripts, accompanied by a reduction in H 2 PO 4 - concentration, when the extraradical mycelium of the arbuscular mycorrhiza of G. In addition, they observed that, by providing 3. It should be pointed out that the molecular mechanisms that promote the efflux of H 2 PO 4 - in the intraradical mycelium, i.

At concentrations lower than 1. It should be highlighted that the high-affinity transport system of G. At concentrations higher than 1. AMF can absorb and transport large amounts of N to plants Jin et al. GintAMT3, located in the plasma and vacuolar membrane, is a low-affinity transporter that is more expressed in the intraradical mycelium than in the extraradical one Calabrese et al.

AMF NO 3 - transporters responsive to arbuscular mycorrhiza. The NO 3 - transporters NRTs of AMF have not been fully characterized, but transcriptome studies have shown the existence of several of these transporters in the spore and in the extra- and intraradical mycelia of the AMF G. A probable high-affinity transporter of this fungus, GiNT, is induced by the supply of NO 3 - in the extraradical mycelium Tian et al.

After absorption by the plant or fungus, NO 3 - is reduced by nitrate reductase to NO 2 -. Transcriptome studies have shown the existence of several NRTs in spores and in the extra- and intraradical mycelia of G. The genes GiNR1 and GiNR2 , which encode nitrate reductase, are induced in the extraradical mycelium in the arbuscular mycorrhiza of G.

Moench] under N deprivation or urea supplementation Koegel et al. These same genes, plus GiNR3 , are also induced in the intraradical mycelium when the extraradical one is supplied with glycine as the sole N source Tian et al. In plants not colonized by AMF, NO 3 - reduction occurs predominantly in leaves, and, in the colonized ones, in roots Kaldorf et al. A gene that encodes for nitrite reductase of the AMF G. The activity of nitrite reductase in ECM controls the expression of the gene that encodes the enzyme in the plant.

The expression of nitrite reductase in the plant is repressed when its roots are colonized by the wild-type fungus, but increases when they are colonized by the fungus with this defective enzyme Bailly et al. In fact, in the roots of plants colonized by mycorrhizal fungi, the levels of nitrite reductase transcripts are lower than those in the ones of the uncolonized control Kaldorf et al. Regardless of the N source, GiGluS is more expressed in the extraradical mycelium than in the intraradical one Tian et al. Glutamine plays a central role in the metabolism of N, by donating this nutrient, a precursor to many essential metabolites, such as nucleic acids and amino acids histidine, tyrosine, and asparagine, for example , and by regulating the involved genes Marzluf, ; Javelle et al.

Due to these important functions, the levels of free glutamine in AMF are tightly controlled Gachomo et al. The N absorbed by the extraradical mycelium is quickly transformed into amino acids, particularly arginine, which is accumulated at high concentrations in this mycelium Govindarajulu et al. The assimilation of NO 3 - Govindarajulu et al. Higher concentrations of arginine were observed in the extraradical mycelium than in the tissues of roots both colonized and not colonized with AMF Jin et al. In addition, arginine levels above nmol L -1 mg -1 dry weight were reported in this mycelium Jin et al.

Therefore, the amino acids of colonized-root proteins were not detectably labelled with 13 C even when large amounts of N were transferred from the fungus to the plant, which shows that the amino acids were translocated to the intraradical mycelium and hydrolyzed, and that inorganic N was transferred to the periarbuscular space Govindarajulu et al. Moreover, the strong labeling by 15 N in free amino acids and in those obtained by the hydrolysis of colonized-root proteins shows the translocation and transfer of N from the extraradical mycelium to the cells of the host plant Govindarajulu et al.

Gene expression studies are consistent regarding the biosynthesis of arginine in the extraradical mycelium. Soon after the supply of NO 3 - or other N sources, the transcript levels of carbamoyl phosphate synthetase CPS , argininosuccinate synthase ASS , and argininosuccinate lyase AL are induced in this mycelium Tian et al.

PLB112- Plant Rhizosphere Interaction

CPS catalyzes the formation of carbamoyl phosphate from CO 2 , adenosine triphosphate ATP , and NH 3 , which, together with ornithine synthesized from glutamate , are converted to citrulline and H 2 PO 4 - by ornithine transcarbamylase Cruz et al. ASS converts citrulline and aspartate into argininosuccinate, and AL into fumarate and arginine Jennings, ; Tian et al.

The biosynthesis of arginine in the extraradical mycelium and the subsequent hydrolysis of this amino acid in the intraradical mycelium are spatially separated, but synchronized processes that occur in AMF through the anabolic and catabolic pathways of the urea cycle, respectively Cruz et al. The synchronization of these processes suggests that arginine plays an important role in the translocation of N from the extraradical mycelium to the intraradical one Govindarajulu et al. An increase in C supply by the plant leads to a greater induction of the genes involved in N assimilation e.

This shows that the host plant is able to regulate the gene expression of the fungus with the provision of C and to stimulate the transport of N towards the periarbuscular space Fellbaum et al. Figure 3 shows a model for the absorption of N and P by AMF, including their transport and release into the periarbuscular space and their association with C. N is assimilated and concentrated mainly in the form of arginine via GS-GOGAT, asparagine synthetase, and the anabolic pathway of the urea cycle, whereas H 2 PO 4 - is converted into PolyP - in the extraradical mycelium.

In this mycelium, arginine is transported to the fungal vacuole, where it binds to PolyP - , which, together with the amino acid, is transported to the intraradical mycelium. The absorbed P and N stimulate photosynthesis and the release of sucrose towards the periarbuscular space, where the hydrolysis of sucrose occurs through the acidic invertases of the host plant and the absorption of hexoses by the monosaccharide transporters located in the plasma membrane of the intraradical mycelium.

Figure 3. Model of N and P transport by the arbuscular mycorrhizal fungus uptake pathway. Larsen Guether et al. Four members NPF2. Since GmAMT4. Recently, two AMTs of M. According to these authors, arbuscules are prematurely degraded in mtpt4 mutants, where PT4 - a mycorrhiza-inducible transporter of plant H 2 PO 4 - - is not expressed, which is critical for the absorption of H 2 PO 4 - in the apoplastic interface. When the plant is grown under N stress, this premature degradation of arbuscules is suppressed by the expression of AMT2;3, but not by that of AMT2;4.

However, only AMT2;4 was a functional transporter, as it facilitated the growth of mutant yeast in the yeast complementation test Breuillin-Sessoms et al. It has been suggested that some mycorrhiza-inducible nutrient transporters located in the periarbuscular membrane could also act as transceptors Xie et al. Arbuscular mycorrhiza induce the expression of some plant transporters such as MtPT4 Javot et al. Xie et al. LjTP4 is essential for the development of functional symbiosis and facilitates the transfer of phosphate from the fungus to the plant Volpe et al.

Cells containing arbuscules need LjTP4 to signal an adequate formation of the arbuscule in the fungus and to improve the absorption of phosphate by the plant Volpe et al.

Plant growth-promoting rhizobacteria (PGPR): their potential as antagonists and biocontrol agents

When this transporter is expressed at low levels, the structures of the fungus show an abnormal phenotype, with small arbuscules and a swollen and scarcely branched main trunk Javot et al. According to these authors, as to the concentration of phosphate in the root, the PT4i strain had lower P content than the GUSi one. Therefore, when detecting the concentration of H 2 PO 4 - in the soil, LjPT4 could activate the transcription factors involved in the formation of lateral roots Volpe et al.

The greater branching of the root system under low H 2 PO 4 - concentrations would eventually increase the chances of identifying AMF Volpe et al. From localization experiments, with the fusion between the gene promoter and the codifying region of the GUS reporter protein and with real-time polymerase chain reaction, it was possible to conclude that LjPT4 and MtPT4 are also expressed in the apex of the roots, besides in the periarbuscular membrane; the transcript levels of these carriers were also dependent on phosphate levels Volpe et al.

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The colonization of the host plant is controlled by N and P feedback mechanisms; therefore, these two nutrients are crucial for arbuscular mycorrhiza symbiosis Kiers et al. Under this type of stress, the plant decreases the expression levels of defense genes and increases those of the genes involved in the biosynthesis of strigolactone Bonneau et al. In most cases, the high availability of P reduces the colonization of the plant by AMF, but this inhibitory effect is reversed by N deficiency, which triggers the signals that promote this colonization Nouri et al.

In addition, under N deficiency or low N conditions, respectively, the degeneration of AMF arbuscules is suppressed in mtpt4 mutants and the expression of P transporters induced by arbuscular mycorrhiza is not critical to this mycorrhiza Javot et al. The negative, neutral, or positive effects of arbuscular mycorrhiza on N nutrition have been reported George et al. When the extraradical mycelium of the AMF G.

However, these authors concluded that N supply by the hyphae was not sufficient to ensure an adequate nutrition of the host pant under limiting N conditions. It should be pointed out that the ability of AMF to enhance the N nutrition of the host plant was relatively dispersed within the Glomeromycota phylum and that, due to high intraspecific diversity, the high symbiotic performance of each isolate is independent of the fungus species to which it belongs to Mensah et al.

Also according to these authors, among 31 fungal isolates tested, only 6 were able to increase alfalfa Medicago sativa L. This shows that the uptake of N by the extraradical mycelium and its translocation to the colonized roots occur regardless of whether the roots of the host plant are under N limiting conditions or not Govindarajulu et al. Saia et al. The endophytic fungus P. It belongs to the Sebacinaceae family Sebacinales order and colonizes roots of several plant species, promoting their growth Varma et al. In fact, Cruz et al. Likewise, Sherameti et al.

These effects were attributed to a higher NO 3 - uptake and expression of the genes Nia2 and SEX1 encoding NO 3 - reductase and glucan-water dikinase, respectively, involved in the starch degradation process. The expression of these genes was observed in the roots and shoots of seedlings inoculated with P.

Trichoderma spp. These fungi enhance plant growth and development through the exudation of auxins or other metabolites Contreras-Cornejo et al. Therefore, the inoculated plants show a better performance in several physiological processes and growth indicators, which translates into greater productivity.

In fact, in a commercial corn crop in the USA, the plants inoculated with the biocontrol agent Trichoderma harzianum T Ascomycete , at different doses of N 20, 40, 80, , and kg ha -1 , responded more quickly to doses less than or equal to kg ha -1 , being greener and larger at the bolting phase and showing greater silage and grain yields at maturation, in comparison with the noninoculated control Harman, In addition, rice plants inoculated with Trichoderma spp.

Figure 4. Hypothetical root colonized by a dark septate endophytic DSE fungus: A, branching, towards the root system, of the mycelium present in the disk of the culture medium; B, penetration of the hyphae through the root hair, accessing the cells of the cortex; and C, formation of various structures of the DSE fungus in cortex cells, showing the intracellular melanized septate hypha hsmi , melanized microsclerotia mm , stained microsclerotia at the young stage mj , intracellular hyaline septate hypha hh , and intercellular melanized septate hypha hsm.

The DSE fungi makeup a paraphyletic group Yuan et al. The mechanism by which DSE fungi establish associations with the host plant is not yet fully understood. However, studies indicate that growth promotion may occur indirectly or directly. The first way would be through plant protection from abiotic stresses, such as drought Santos et al. In Brassica campestris L. The best responses in growth promotion by DSE are observed when organic sources of N are used Newsham, ; Qin et al. The same authors, while inoculating this fungus in B.

Similar results have been reported in the literature Diene et al. In contrast, in cucumber Cucumis sativus L. However, in rice plants inoculated with DSE fungi, grown under controlled conditions and supplemented with NH 4 NO 3 , significant increments were observed for plant dry matter, N amino acid, soluble sugars, number of tillers, besides changes in NO 3 - absorption kinetic parameters K m and V max Vergara et al. These authors, based on Zhang et al. The green revolution was essential to solve the hunger problems that humanity faced shortly after World War II Chardon et al.

These effects have gradually awakened humanity to the use of less inputs in agriculture, which would also decrease the high farmer expenditures due to the excessive use of fertilizers Chardon et al. Furthermore, the non-renewable resources phosphate rocks and phosphate deposits used to manufacture phosphate fertilizers are running out; some analyzes even indicate that, if the current rate of consumption continues, world reserves should only last for approximately years Gilbert, Although the production of a quality inoculum, i.

These attributes of AMF can be even more important to ensure an adequate food supply to developing countries, where fertilizer costs are very high, especially for small-scale farmers. Although AMF are obligate biotrophs, they present a genome, already sequenced and characterized, with an enzymatic repertoire involved in N and P uptake and metabolism, showing the specialization of this group of fungi to extract mineral nutrients from the soil.

This has led to increasing researches on other facultative biotrophic fungi, such as P. While the N absorbed by the extraradical mycelium of AMF is assimilated and concentrated into arginine via the glutamine synthetase-glutamate synthase pathway, asparagine synthetase, and the anabolic pathway of the urea cycle, H 2 PO 4 - is converted to polyphosphate. Although further studies are necessary, for instance, to characterize NO 3 - transporters in the extradical mycelium of AMF and in the host plant, it is clear that this group of fungi can affect plant growth by the absorption of N and P, changing the agronomic characteristics of a given crop.

However, there is a need to investigate which agrosystem management could maximize the beneficial effects of these fungi. In addition, these parameters can help plant breeders to select genotypes with more sensitive kinetic parameters for detecting nutrients present in the soil, which could improve the recovery efficiency of applied fertilizers, reduce fertilizer expenses, and increase plant dependence on fungal symbionts. The knowledge generated from studies on arbuscular mycorrhizal symbiosis may help to understand other interactions between plants and other fungi, such as P.

It may also allow of comparisons between these different symbioses, in order to evaluate which of them is more feasible in agronomic terms, and even complement the benefits of each one through the coinoculation of different fungi in a single plant. Cruz et al. Application of Trichoderma harzianum T22 as a biofertilizer potential in maize growth. Journal of Plant Nutrition, v.