Effect of onion (Allium cepa) root exudates on the hyphal growth of Gigaspora margarita

 The effect of root exudates from onions differing in P status on spore germination and hyphal growth of arbuscular mycorrhizal fungi was investigated. Onion (Allium cepa) was grown in solution culture at different phosphorus concentrations (0, 0.1, 1.0, 8.0 and 24.0 mg P l^–1) and root exudates were collected. When spores of the arbuscular mycorrhizal fungus, Gigaspora margarita were incubated with these root exudates, spore germination was only slightly affected but hyphal growth was greatly affected, particularly with exudates from P-deficient plants. This suggests that the P nutrition of host plants influences the composition of root exudates and thereby the hyphal growth of arbuscular mycorrhizal fungi. Accepted: 25 June 1995     

Effect of root exudates and extracts on rhizosphere fungi

Summary The effects of root exudates and extracts of Cassia tora L. and Crotalaria medicaginea Lamk., on some dominant rhizosphere fungi isolated from the plants were studied. Root extracts induced a high degree of stimulation in rate of growth of a majority of the fungi tested. Root exudates caused only a marginal effect which was positive on all the fungi except Trichoderma lignorum which was inhibited by the root exudate of C. medicaginea.     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Effect of root exudates on the spore germiantion of rhizosphere and rhizoplane mycoflora of chilli (Capsicum annuum L.) cultivars

Summary From root exudates of three cultivars of chilli (Capsicum annuum L.) 12 amino acids and 7 sugars were detected. Methionine, d-1- phenylalanine, citrulline and d-xylose were detected only from the root exudates of resdistant cultivars. The root exudates of resistant variety inhibited spore germination of the pathogen (Fusarium oxysporum f. sp.capsici), but that of susceptible variety enhanced spore germiantion of the same. Spore germiantion of antagonistic fungi (Trichderma viride andAspergillus sydowi) was also influenced by the root exudates of resistant and susceptible varieties, but the influence was different.Spore germiantion of a number of rhizosphere fungi was studied and in general root exudate of susceptible cultivar enhanced spore germiantion of majority of fungi, but spore germination of antagonistic fungi against the pathogen was inhibited. However, root exudate of resistant cultivar stimulated spore germination of antagonistic fungi.     

Development of a model system to assess the impact of genetically modified corn and aubergine plants on arbuscular mycorrhizal fungi

We developed an experimental model system to monitor the impact of generically modified (GM) plants on arbuscular mycorrhizal (AM) fungi, a group of non-target soil microorganisms, fundamental for soil fertility and plant nutrition. The system allowed us to study the effects of root exudates of both commercial Bt corn and aubergine plants expressing Dm-AMP1 defensin on different stages of the life cycle of the AM fungal species G. mosseae. Root exudates of Bt 176 corn significantly reduced pre-symbiotic hyphal growth, compared to Bt 11 and non-transgenic plants. No differences were found in mycelial growth in the presence of Dm-AMP1 and control plant root exudates. Differential hyphal morphogenesis occurred irrespective of the plant line, suggesting that both exuded Bt toxin and defensin do not interfere with fungal host recognition mechanisms. Bt 176 affected the regular development of appressoria, 36% of which failed to produce viable infection pegs. Our experimental model system represents an easy assay for testing the impact of GM plants on non-target soil-borne AM fungi.     

Striga seed-germination activity of root exudates and compounds present in stems of Striga host and nonhost (trap crop) plants is reduced due to root colonization by arbuscular mycorrhizal fungi

Root colonization by arbuscular mycorrhizal (AM) fungi reduces stimulation of seed germination of the plant parasite Striga (Orobanchaceae). This reduction can affect not only host plants for Striga, resulting in a lower parasite incidence, but also false hosts or trap crops, which induce suicidal Striga seed germination, thereby diminishing their effectiveness. In order to better understand these AM-induced effects, we tested the influence of root colonization by different AM fungi on the seed-germination activity of root exudates of the Striga hermonthica nonhost plants cowpea and cotton on S. hermonthica. We also tested the effect of AM fungi on the seed-germination activity of the Striga gesnerioides host plant cowpea on S. gesnerioides. Moreover, we studied whether mycorrhization affects the transport of seed-germination activity to above-ground plant parts. Mycorrhization not only resulted in a lower seed germination of S. gesnerioides in the presence of root exudates of the S. gesnerioides host cowpea but also seed germination of S. hermonthica was also lower in the presence of root exudates of the S. hermonthica nonhosts cowpea and cotton. Downregulation of the Striga seed-germination activity occurs not only in root exudates upon root colonization by different AM fungi but also in the compounds produced by stems. The lowered seed-germination activity does not appear to depend on the presence of seed germination inhibitors in the root exudates of mycorrhizal plants. The implication for Striga control in the field is discussed.     

Inhibition and Stimulation Effects in Communities of Wood Decay Fungi: Exudates from Colonized Wood Influence Growth by Other Species

The effects of exudates from uncolonized and from partly decayed beech wood on the extension rates of 16 later stage decay fungi were investigated. The partly decayed wood had been colonized by the pyrenomycete Eutypa spinosa, or the basidiomycetes Fomes fomentarius, Stereum hirsutum, and Trametes versicolor, all known as common early decay agents in European beech forests. Sterilized wood pieces were placed onto 0.5% malt agar, opposite to small agar plugs containing the test fungi. The latter showed very variable and species-specific growth responses to the various wood types. The presence of uncolonized wood stimulated extension rates in many species, whereas the four previously decayed wood types had variable stimulatory or inhibitory effects. Wood decayed by S. hirsutum resulted in reduced extension rate, delayed growth, or total inhibition in the majority of species, thus it is suggested that this species uses secondary metabolites in a defensive strategy. A single species was, however, stimulated in the presence of S. hirsutum-decayed wood. In contrast, the presence of wood decayed by F. fomentarius was stimulatory to 45% of the species. The other previously decayed wood types generally resulted in more variable responses, depending upon species. The results are discussed in an ecological context and it is suggested that the exudates from the partly decayed wood that are responsible for the reported effects may function as infochemicals, structuring microbial communities in wood.     

Effect of volatile substances released fromOriganum majorana andOcimum basilicum on the rhizosphere and phyllosphere fungi ofPhaseolus vulgaris

Differences were found in the counts and occurrence of fungi in the phyllosphere and thizosphere of two representatives of the Lamiacea family,Origanum majorana andOcimum basilicum, and in the phyllosphere and rhizosphere ofPhaseolus vulgaris growing separately or in coenosis withO. majorana orO. basilicum. Both the volatile substances released from ground leaves of the two latter pantl species and the root exudates affected considerably spore germination of isolated phylospheric and rhizospheric fungi. The results indicated a possible role of root exudates and volatile substances released from leaves in colonization of rhizosphere and/or phyllosphere by fungi, especially in associations of various plants.     

Biochemical Investigations of Sclerotial Exudates of Sclerotium rolfsii and their Antifungal Activity

Exudates from sclerotia of two Sclerotium rolfsii isolates (one causing collar rot in Cicer arietinum, isolate VC971, and the other leaf spots in Rauvolfia serpentina, isolate VL016) were assayed for their antifungal activity against 26 fungi consisting of plant parasites as well as saprophytes. Spore germination of all the test fungi was affected by the exudates reaching 100% in some cases. Foliar spray with exudates of isolate VL016 significantly reduced disease incidence of balsam (Impatiens balsamina) powdery mildew caused by Erysiphe cichoracearum and pea (Pisum sativum) powdery mildew caused by Erysiphe pisi, under field conditions. Characterization of exudates from 25 isolates of S. rolfsii revealed pH ranging from 3.8 to 5.3 and colour from light yellow to deep yellow. Among the phenolic acids found in the exudates were tannic, gallic, caffeic, vanillic, ferulic, chlorogenic and cinnamic acids. Oxalic acid was also found in varied amounts. Among the phenolic acids, ferulic acid was found to be present at high concentration in exudates of most isolates (3.9–153.4 μg/ml). The antioxidant properties of phenolics, which generally inhibit fungal morphogenesis including spore germination along with the antifungal nature of some phenolics are chiefly attributed to the inhibitory effect of sclerotial exudates of S. rolfsii. Additionally, both the isolates VC971 and VL016 showed almost similar antifungal activities despite they are of different origin and thereby demonstrate the antifungal nature of sclerotial exudates.     

Evidence for leaf endophyte regulation of root symbionts: effect of <Emphasis Type="Italic">Neotyphodium</Emphasis> endophytes on the pre-infective state of mycorrhizal fungi

Neotyphodium endophytes and arbuscular mycorrhizal (AM) fungi are common constituents of natural grasslands. The plant–endophyte symbiosis can introduce changes in soil conditions that affect the density and activity of different functional groups of soil organisms. In the present work we performed in vitro assays to evaluate the effect of root and endophyte exudates on the pre-infective state of mycorrhizal fungi (Gigaspora margarita and G. rosea). Plant roots of Bromus setifolius from populations of Patagonia, and four strains of Neotyphodium were used to obtain the exudates. Root exudates of infected plants, at a high concentration, significantly increased AMF hyphal branches and length relative to exudates from naturally endophyte free plants. The effect of Neotyphodium endophyte exudates on AMF mycelial length varied depending on strain and the concentration used, suggesting a differential interaction between endophyte and AMF species. AMF hyphal branches were increased by Neotyphodium fungal exudates in both mycorrhizal species. A few previous studies have suggested that Neotyphodium endophytes can reduce mycorrhizal sporulation and colonization of host roots in commonly-cultivated agronomic hosts. In this study we report the opposite effect in B. setifolius. This study reports the direct and positive effect of root exudates from plants in symbiosis with Neotyphodium, on AMF pre-infective state. Further, identical effects were detected using exudates from Neotyphodium endophytes.     

Antimicrobial proteins from cowpea root exudates: inhibitory activity against Fusarium oxysporum and purification of a chitinase-like protein

Plants exude a variety of substances through their external surfaces and from germinating seeds, some of which have an inhibitory action against plant pathogens. The aim of this study was the investigation and characterization of defense proteins present in exudates from roots of cowpea seedlings (Vigna unguiculata (L.) Walp.). Root exudates were collected from seedlings that were grown hydroponically in three different media, including, 100 mM sodium acetate buffer pH 4.5, water pH 6.0 and 100 mM sodium phosphate buffer pH 7.5. The proteins from these exudates were analyzed by SDS–PAGE and SDS–Tricine–PAGE and the presence of antimicrobial proteins in the exudates was investigated by immunological and enzymatic assays. Results showed that roots from cowpea seedlings contained -1,3-Glucanases, chitinases and lipid transfer proteins (LTPs), all of which may potentially function as plant defense proteins. Immunolocalization of one of these proteins, chitinase, revealed its presence in the xylem cell wall vessel elements. These exudates also demonstrated an inhibitory effect on the growth of the fungus, Fusarium oxysporum, in vitro. The results suggest that plant roots may exude a variety of proteins that may function to repress the growth of root pathogenic fungi.     

Production of Strigolactones by Arabidopsis thaliana responsible for Orobanche aegyptiaca seed germination

The germination stimulants produced by Arabidopsis thaliana, a host of root parasitic plants Orobanche spp. but not of arbuscular mycorrhizal (AM) fungi were examined. Root exudates from the hydroponically grown A. thaliana plants were subjected to reverse phase high performance liquid chromatography (HPLC) and retention times of germination stimulants inducing O. aegyptiaca seed germination were compared with those of strigolactone standards. In addition, the root exudates were analyzed by using HPLC linked with tandem mass spectrometry (LC/MS/MS). A. thaliana was found to exude at least three different germination stimulants of which one was identified as orobanchol. This is the first report of strigolactone production by a non-mycotrophic plant. These results together with recent knowledge imply that strigolactones have other unrevealed functions in plant growth and development.     

Positive correlation between VAM-induced changes in root exudation and mycorrhizosphere mycoflora

The influence of vesicular-arbuscular mycorrhizal (VAM) fungi on rhizosphere mycoflora was studied together with the possible mechanism involved in this process. Six combinations of VAM fungi and phosphorus fertilizer treatments were applied to Leucaena leucocephala roots and quantitative and qualitative observations were made periodically of the rhizosphere mycoflora and constituents of root exudates. The results obtained indicate that the presence of specific mycoflora in the rhizosphere of mycrorrhizal roots is mediated through root exudates rather than being an outcome of improved P nutrition.     

Effect of soil fungistasis on zoopathogenic fungi

The inhibiting action of west-Siberian soils on spore germination and the growth and development of zoopathogenic fungi such as Emmonsia (Chrysosporium) crescens, Trichophyton mentagrophytes, Beauveria bassiana, Metarrhizium anisopliae, Paecilomyces farinosus, P. fumoso-roseus and Chrysosporium keratinophilum have been studied by the authors. The influence of carbon sources and the root exudates of plants on fungistasis have also been studied.     

The effect of leaf exudates on the spore germination of phylloplane mycoflora of chilli (Capsicum annuum L.) cultivars

Summary Ninetten aminoacids, twelve sugars, eleven organic acids and ten phenols were detected in the leaf exudates of three cultivars of chilli. The number of aminoacids, sugars, organic acids and phenols increased as the plants grew older. More aminoacids and sugars were detected in the exudate from the susceptible cultivar (Malwa). More organic acids and phenols were detected from the resistant cultivar (Simla). The leaf exudate of the resistant cultivar (Simla) inhibited spore germination of the pathogen (Alternaria solani) while that of susceptible (Malwa) stimulated spore germination. The cultivar ‘Patna’ which is moderately resistant, occupied an intermediate position. Spore germination of the isolated fungi was enhanced in leaf exudate of susceptible cultivar (Malwa), while leaf exudates of the moderately resistant (Patna) and resistant (Simla) inhibited spore germination of the majority of fungi isolated. Most of the antagonistic fungi were not isolated from the susceptible cultivar and the percentage spore germination of these fungi was less in leaf exudate of the susceptible cultivar, while leaf exudates of resistant cultivars enhanced the percentage spore germination of antagonistic fungi,viz Aspergillus flavus, A. fumigatus, A. versicolor, Penicillium citrinum, P. restrictum andTrichoderma viride.     

Effect of seed and root exudates on the growth ofXanthomonas phaseoli var.fuscans

In a medium containing bean, barley and wheat seed exudates,Xanthomonas phaseoli var.fuscans (Burk.) Starr et Burk. grew substantially better than in that containing root exudates of these plants. When the bacteria were cultivated in a medium containing root exudates of bean plants deprived of cotyledons after eleven days of growth, growth was slower than in the presence of root exudates of control plants. On the other hand, the growth was stimulated in a medium containing root exudates of bean plants deprived of leaves. It was found that seed exudates of these plants contained biologically active peptides stimulating the growth of the microorganism. These peptides were not found in root exudates. These findings suggest a relationship between the survival ofXanthomonas phaseoli var.fuscans in the rhizosphere of bean and the exudation of biologically active peptides originating from the stock substances of seeds and cotyledons.     

Ectomycorrhizal exudates and pre-exposure to elevated CO2 affects soil bacterial growth and community structure

Ectomycorrhizal fungi produce low molecular weight organic compounds, supporting diverse microbial communities. To link mycorrhizal root exudation directly to bacterial responses, we used Scots pine exudates with (Suillus variegatus and Piloderma fallax) and without mycorrhiza as substrata for forest soil bacteria. Bacterial growth and vitality was monitored, and community composition determined using T-RFLP, cloning and sequencing. We investigated if the amount of organic acids in exudates explained bacterial growth, and whether bacterial communities were influenced by pre-exposure to elevated atmospheric CO₂. We demonstrated functional differences in bacterial growth rates related to CO₂. There was a shift in the bacterial community (e.g. Burkholderia sp. and gamma-proteobacteria) toward organisms better able to rapidly utilize exudates when pine microcosms were pre-exposed to elevated CO₂. Soil bacteria from all treatments tended to grow more abundantly and rapidly in exudates from Piloderma-colonized seedlings, suggesting that the organic acids and/or unidentified compounds present supported greater growth.     

Flavonoids induce germination of basidiospores of the ectomycorrhizal fungus <Emphasis Type="Italic">Suillus bovinus</Emphasis>

Under laboratory conditions, spores of ectomycorrhizal fungi usually germinate very poorly or not at all. In a previous study, we showed that spores of the ectomycorrhizal fungus Suillus bovinus germinated through the combination of activated charcoal treatment of media and co-culture with seedlings of Pinus densiflora, which suggested that some substances contained in root exudates induced the germination. Among the compounds reported from root exudates, flavonoids have been elucidated to play various and substantial roles in plant–microbe interactions; we therefore investigated the effects of flavonoids on basidiospore germination of S. bovinus by the diffusion gradient assay on water agar plates pretreated with charcoal powder. Seven out of the 11 flavonoids tested, hesperidin, morin, rutin, quercitrin, naringenin, genistein, and chrysin, had greater effects than controls, whereas flavone, biochanin A, luteolin, and quercetin showed no positive effects. The effective concentration presumably corresponded to several micromolar levels, which was equivalent to those effective for pollen development, nod gene induction, and spore germination of F. solani f. sp. pisi and AM fungi. The results suggest that flavonoids play a role as signaling molecules in symbiotic relationships between woody plants and ectomycorrhizal fungi.     

Alterations in Root Exudation of Intercropped Tomato Mediated by the Arbuscular Mycorrhizal Fungus Glomus mosseae and the Soilborne Pathogen Fusarium oxysporum f.sp. lycopersici

Arbuscular mycorrhizal fungi (AMF) can control soilborne diseases such as Fusarium oxysporum f.sp. lycopersici (Fol). Root exudates play an important role in plant–microbe interactions in the rhizosphere, especially, in the initial phase of these interactions. In this work, we focus on (i) elucidating dynamics in root exudation of Solanum lycopersicum L. in an intercropping system due to AMF and/or Fol; (ii) its effect on Fol development in vitro; and (iii) the testing of the root exudate compounds identified in the chromatographic analyses in terms of effects on fungal growth in in vitro assays. GC‐MS analyses revealed an AMF‐dependent increase in sugars and decrease in organic acids, mainly glucose and malate. In the HPLC analyses, an increase in chlorogenic acid was evident in the combined treatment of AMF and Fol, which is to our knowledge the first report about an increase in chlorogenic acid in root exudates of AM plants challenged with Fol compared with plants inoculated with AMF only, clearly indicating changes in root exudation due to AMF and Fol. Root exudates of AMF tomato plants stimulate the germination rate of Fol, whereas the co‐inoculation of AMF and Fol leads to a reduction in spore germination. In the in vitro assays, citrate and chlorogenic acid could be identified as possible candidates for the reduction in Fol germination rate in the root exudates of the AMF+Fol treatment because they proved inhibition at concentrations naturally occurring in the rhizosphere.     

The effect of tree-root exudates on the growth rate of ectomycorrhizal and saprotrophic fungi

Summary The object of this investigation was the promotion by root exudates of the growth rate of ectomycorrhizal fungi, discovered by Elias Melin in 1954. Eight ectomycorrhizal and ten non-mycorrhizal species were used as test fungi in the experiments. Different species often reacted differently: none of the eight isolated strains of Suillus luteus were promoted by pineroot exudate, whereas the growth rates of all seven strains of S. granulatus were increased. Among the other ectomycorrhizal species, S. variegatus, Laccaria bicolor, Pisolithus tinctorius and Thelephora terrestris, each represented by only one, two or three strains, usually reacted to the pine root exudate with an increased growth rate; S. bovinus and Paxillus involutus did not respond at all. Hitherto, studies of root-exudate effects on fungi have been based exclusively on the responses of ectomycorrhizal species; in the present study saprotrophic fungi were also used as test organisms. Seven out of ten saprotrophic species reacted with markedly accelerated growth when exposed to a pineroot exudate. Melin's assumption that a constituent of the root exudate, the M-factor, could replace the exudate growth-promoting activity was verified. By means of TLC fraction it was found that the fatty acid palmitic acid alone caused an increase in growth rate equal to that of the pine-root exudate. In line with previously published data by Gogala (1970), we also showed that certain cytokinins, especially isopentenylaminopurine, could act as substitutes for the total root exudate. Thus both palmitic acid and isopentenylaminopurine are able to function as M-factors equivalent to a root exudate.