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.

Arbuscular mycorrhiza alter the concentration of essential oils in oregano (Origanum sp., Lamiaceae).

The effect of root colonization by Glomus mosseae on the qualitative and quantitative pattern of essential oils (EO) was determined in three oregano genotypes (Origanum sp.). To exclude a simple P-mediated effect through mycorrhization the effect of P application to plants on the EO accumulation was also tested. In two genotypes the leaf biomass was increased through mycorrhization. Root colonization by the arbuscular mycorrhizal fungus (AMF) did not have any significant effect on the EO composition in oregano; however, in two genotypes the EO concentration significantly increased. As EO levels in P-treated plants were not enhanced, we conclude that the EO increase observed in mycorrhizal oregano plants is not due to an improved P status in mycorrhizal plants, but depends directly on the AMF-oregano plant association.

Root exudates of mycorrhizal tomato plants exhibit a different effect on microconidia germination of Fusarium oxysporum f. sp. lycopersici than root exudates from non-mycorrhizal tomato plants.

The effect of root exudates from mycorrhizal and non-mycorrhizal tomato plants on microconidia germination of the tomato pathogen Fusarium oxysporum f. sp. lycopersici was tested. Microconidia germination was enhanced in the presence of root exudates from mycorrhizal tomato plants. The more tomato plants were colonized by the arbuscular mycorrhizal fungus Glomus mosseae, the more microconidia germination was increased, indicating that alterations of the exudation pattern depended on the degree of root AM colonization. Moreover, alterations of the exudation pattern of mycorrhizal plants are not only local, but also systemic. Testing the exudates from plants with a high and a low P level revealed that the alterations of the root exudates from mycorrhizal plants, resulting in a changed effect on microconidia germination, are not due to an improved P status of mycorrhizal plants.

Systemic inhibition of arbuscular mycorrhiza development by root exudates of cucumber plants colonized by Glomus mosseae.

The arbuscular mycorrhizal (AM) non-host plants mustard, sugar beet, lupin and the AM host plant cucumber were used as test plants. Cucumber plants were grown either in the absence of the AM fungus (AMF) Glomus mosseae or in a split-root system, with one side mycorrhizal and one side non-mycorrhizal. Root exudates of the AM non-host plants, the non-mycorrhizal cucumber plants and the mycorrhizal and the non-mycorrhizal side of the split-root system of mycorrhizal cucumber plants were collected and applied to cucumber plants inoculated with the AMF. Root exudates of non-mycorrhizal cucumber plants showed a significant stimulatory effect on root colonization, whereas root exudates from the mycorrhizal and the non-mycorrhizal sides of a split-root system of a mycorrhizal cucumber plant did not show this stimulatory effect and were even slightly inhibitory. Root exudates of the two AM non-host plants mustard and sugar beet significantly reduced root colonization in cucumber plants, whereas no such effect was observed when root exudates of the AM non-host plant lupin were applied.

Hyphal in vitro growth of the arbuscular mycorrhizal fungus Glomus mosseae is affected by chitinase but not by ß-1,3-glucanase.

Purified basic chitinase or ß-1,3-glucanase or a combination of the two enzymes were applied to hyphae of the arbuscular mycorrhizal fungus Glomus mosseae grown in vitro. Chitinase applied to the hyphal tip produced an inhibition of hyphal extension, lysis of the apex and alterations of the growth pattern of the fungus. No effect was observed, however, when chitinase was applied to subapical parts of the hyphae or when glucanase was applied to any part of the hyphae. Application of a combination of the two enzymes to the hyphal tip produced an effect similar to that of chitinase alone.

Differences in glucosinolate patterns and arbuscular mycorrhizal status of glucosinolate-containing plant species.

Under defined laboratory conditions it was shown that two glucosinolate-containing plant species, Tropaeolum majus and Carica papaya, were colonized by arbuscular mycorrhizal (AM) fungi, whereas it was not possible to detect AM fungal structures in other glucosinolate-containing plants (including several Brassicaceae). Benzylglucosinolate was present in all of the T. majus cultivars and in C. papaya it was the major glucosinolate. 2-Phenylethylglucosinolate was found in most of the non-host plants tested. Its absence in the AM host plants indicates a possible role for the isothiocyanate produced from its myrosinase-catalysed hydrolysis as a general AM inhibitory factor in non-host plants. The results suggest that some of the indole glucosinolates might also be involved in preventing AM formation in some of the species. In all plants tested, both AM hosts and non-hosts, the glucosinolate pattern was altered after inoculation with one of three different AM fungi (Glomus mosseae, Glomus intraradices and Gigaspora rosea), indicating signals between AM fungi and plants even before root colonization. The glucosinolate induction was not specifically dependent on the AM fungus. A time-course study in T. majus showed that glucosinolate induction was present during all stages of mycorrhizal colonization.

Ink and vinegar, a simple staining technique for arbuscular-mycorrhizal fungi

We developed a reliable, inexpensive, and simple method for staining arbuscular-mycorrhizal fungal colonizations in root tissues. Apart from applications in research, this nontoxic, high-quality staining method also could be of great utility in teaching exercises. After adequate clearing with KOH, an ink-vinegar solution successfully stained all fungal structures, rendering them clearly visible.

Flavonoids and arbuscular-mycorrhizal fungi.

Arbuscular mycorrhizal fungi (AMF) are ancient Zygomycetes forming the most widespread plant-fungus symbiosis. The regulation of this association is still poorly understood in terms of the communication between the two partners. Compounds inside the root and released by the root, such as flavonoids, are hypothesized to play a role in this plant-fungus communication, as already demonstrated in other symbiotic associations (e.g. Rhizobium-leguminoseae). Here we give a general overview of the research concerning this question.

Nitrate depletion and pH changes induced by the extraradical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices grown in monoxenic culture.

The effect of the extraradical mycelium of the arbuscular mycorrhizal (AM) fungus Glomus intraradices Smith & Schenck on nitrate uptake and on the pH of the medium was studied in a monoxenic culture with tomato (Lycopersicon esculentum Mill. var. Vendor) roots obtained from root organ culture. The symbiosis was established in compartmented Petri dishes containing agar media amended with the pH indicator bromocresol purple. A pattern of pH changes was revealed as the symbiosis progressed in the media of the Petri dish compartments containing the dual, arbuscular-mycorrhizal fungi/root, culture as well as in the media of the hyphae, root-free compartments, in which the extraradical hyphae developed extensively, coming from the compartment containing the symbiosis. The colour changes in the media were measured spectrophotometrically, whilst maintaining the monoxenic conditions. The extraradical hyphae of G. intraradices strongly increased the pH of nutrient-free medium when supplied with nitrate, whereas the pH decreased m the absence of this N source. The hyphae developing from germinated spores and growing in axenic, nitrate-amended media did not induce any increase in pH. Nitrogen analysis revealed that a depletion of nitrate in the media accompanied increased pH. These results point towards an active uptake of nitrate by the extraradical mycelium of G. intraradices, probably coupled to a H+ -symport mechanism. The pH changes induced by AM fungal hyphae and the possible influence of the establishment of a functional symbiosis on these pH changes are discussed.

Rhizobial Nodulation Factors Stimulate Mycorrhizal Colonization of Nodulating and Nonnodulating Soybeans.

Legumes form tripartite symbiotic associations with noduleinducing rhizobia and vesicular-arbuscular mycorrhizal fungi. Co-inoculation of soybean (Glycine max [L.] Merr.) roots with Bradyrhizobium japonicum 61-A-101 considerably enhanced colonization by the mycorrhizal fungus Glomus mosseae. A similar stimulatory effect on mycorrhizal colonization was also observed in nonnodulating soybean mutants when inoculated with Bradyrhizobium japonicum and in wild-type soybean plants when inoculated with ineffective rhizobial strains, indicating that a functional rhizobial symbiosis is not necessary for enhanced mycorrhiza formation. Inoculation with the mutant Rhizobium sp. NGR[delta]nodABC, unable to produce nodulation (Nod) factors, did not show any effect on mycorrhiza. Highly purified Nod factors also increased the degree of mycorrhizal colonization. Nod factors from Rhizobium sp. NGR234 differed in their potential to promote fungal colonization. The acetylated factor NodNGR-V (MeFuc, Ac), added at concentrations as low as 10-9 M, was active, whereas the sulfated factor, NodNGR-V (MeFuc, S), was inactive. Several soybean flavonoids known to accumulate in response to the acetylated Nod factor showed a similar promoting effect on mycorrhiza. These results suggest that plant flavonoids mediate the Nod factor-induced stimulation of mycorrhizal colonization in soybean roots.

Colonization of Transgenic Tobacco Constitutively Expressing Pathogenesis-Related Proteins by the Vesicular-Arbuscular Mycorrhizal Fungus Glomus mosseae.

We studied the effect of constitutive expression of pathogenesis-related proteins (PRs) in tobacco plants on vesicular-arbuscular mycorrhiza. Tobacco lines genetically transformed to express various PRs constitutively under the control of the cauliflower mosaic virus 35S promoter of tobacco were examined. Immunoblot analysis and activity measurements demonstrated high levels of expression of the PRs in the root systems of the plants. Constitutive expression of the following acidic isoforms of tobacco PRs did not affect the time course or the final level of colonization by the vesicular-arbuscular mycorrhizal fungus Glomus mosseae: PR-1a, PR-3 (=PR-Q), PR-Q(prm1), PR-4, and PR-5. Similarly, constitutive expression of an acidic cucumber chitinase, of a basic tobacco chitinase with and without its vacuolar targeting peptide, of a basic (beta)-1,3-glucanase, and of combinations of PR-Q and PR-Q(prm1) or basic chitinase and basic (beta)-1,3-glucanase did not affect colonization by the mycorrhizal fungus. A delay of colonization by G. mosseae was observed in tobacco plants constitutively expressing the acidic isoform of tobacco PR-2, a protein with (beta)-1,3-glucanase activity.