J.A. Ocampo

Root colonization by arbuscular mycorrhizal fungi is affected by the salicylic acid content of the plant

Wild type, transgenic NahG tobacco plants with reduced levels of salicylic acid (SA) and transgenic CSA (constitutive SA biosynthesis) tobacco plants with enhanced SA levels were inoculated with the arbuscular mycorrhizal fungi (AMF) Glomus mosseae or Glomus intraradices. In a time course study the effect of SA content on root colonization by the fungal symbiont was determined. Throughout the experiment in NahG plants an enhanced root colonization level could be detected, whereas in CSA plants mycorrhization was reduced. At the end of the experiment with Glomus mosseae, root colonization was similar in wild type and in transgenic plants (NahG and CSA plants), indicating that enhanced SA levels in plants can have an effect on delay AMF root colonization, but do not affect the symbiotic potential of plants in terms of changes in maximal threshold of root colonization. Compared to non-mycorrhizal plants, in mycorrhizal wild type and NahG the SA concentration was reduced. The role of SA in the regulation of mycorrhization is discussed.

Induction of catalase and ascorbate peroxidase activities in tobacco roots inoculated with the arbuscular mycorrhizal Glomus mosseae

Catalase and ascorbate peroxidase enzymatic activities were examined during the interaction between Nicotiana tabacum and the arbuscular mycorrhizal Glomus mosseae. Transient enhancements of both enzymatic activities were detected in the inoculated plant roots coinciding in time with the stage of appressoria formation in the root surface. The analysis of free salicylic acid content in roots revealed that the increases in enzymatic activities were coincident in time with the accumulation of SA in inoculated roots. These data indicate that the first reaction of the root cells to the invasion of arbuscular mycorrhizal fungi is a defence response.

Interactions between Trichoderma koningii, Fusarium solani and Glomus mosseae: Effects on plant growth, arbuscular mycorrhizas and the saprophyte inoculants

Abstract The effect of inoculation with the saprophytic fungi Trichoderma koningii and Fusarium solani on maize ( Zea mays ) and lettuce ( Lactuca sativa ) with or without arbuscular mycorrhizal (AM) colonization by Glomus mosseae was studied in a greenhouse trial. Plant dry wt of non-AM inoculated maize and lettuce were unaffected by the presence of T. koningii and F. solani . In contrast, T. koningii decreased plant dry wt and AM colonization when inoculated into the rhizosphere before or at the same time as G. mosseae . In addition, the T. koningii population was considerably reduced when G. mosseae was inoculated 2 weeks before the saprophytic fungus. At this time T. koningii did not affect the proportion of the root length colonized by G. mosseae , but did adversely affect its metabolic activity assessed as succinate dehydrogenase activity. Although F. solani did not affect AM colonization of maize roots, its effect on AM colonization of lettuce roots was similar to that of T. koningii . The population of F. solani decreased significantly in the rhizosphere of both plants. The possible mechanisms of interaction between the saprophytic fungi and G. mosseae are discussed.

Ethylene-dependent⁄ethylene-independent ABA regulation of tomato plants colonized by arbuscular mycorrhiza fungi

We investigated the relationship between ABA and ethylene regulating the formation of the arbuscular mycorrhiza (AM) symbiosis in tomato (Solanum lycopersicum) plants and tried to define the specific roles played by each of these phytohormones in the mycorrhization process. We analysed the impact of ABA biosynthesis inhibition on mycorrhization by Glomus intraradices in transgenic tomato plants with an altered ethylene pathway. We also studied the effects on mycorrhization in sitiens plants treated with the aminoethoxyvinyl glycine hydrochloride (AVG) ethylene biosynthesis inhibitor and supplemented with ABA. In addition, the expression of plant and fungal genes involved in the mycorrhization process was studied. ABA biosynthesis inhibition qualitatively altered the parameters of mycorrhization in accordance with the plants ethylene perception and ethylene biosynthesis abilities. Inhibition of ABA biosynthesis in wild-type plants negatively affected all the mycorrhization parameters studied, while tomato mutants impaired in ethylene synthesis only showed a reduced arbuscular abundance in mycorrhizal roots. Inhibition of ethylene synthesis in ABA-deficient sitiens plants increased the intensity of mycorrhiza development, while ABA application rescued arbuscule abundance in the roots mycorrhizal zones. The results of our study show an antagonistic interaction between ABA and ethylene, and different roles of each of the two hormones during AM formation. This suggests that a dual ethylene-dependent/ethylene-independent mechanism is involved in ABA regulation of AM formation.

Flavonoids exhibit fungal species and genus specific effects on the presymbiotic growth of Gigaspora and Glomus.

The effect of the flavonoids chrysin, isorhamnetin, kaempferol, luteolin, morin and rutin on pre-symbiotic growth, such as spore germination, hyphal length, hyphal branching and the formation of auxiliary cells and secondary spores, of the arbuscular mycorrhizal fungi Gigaspora rosea, G. margarita, Glomus mosseae and G. intraradices was studied. According to the effect on each fungal growth parameter, the tested compounds could be classified to be genus and/or species specific or specific, for a certain developmental stage of pre-symbiotic growth. A clear arbuscular mycorrhizal genus specific, and even species specific, effect of some flavonoids was observed. However, this specificity of a flavonoid could not be generalized but differs depending on the pre-symbiotic stage of the AM fungus. Moreover, our show that for a better understanding of the role of compounds in the AM symbiosis, studies should not be conducted only with one AM fungus looking at one fungal growth parameter such as spore germination or hyphal length, but should be wider, including several growth parameters and several AM fungi.

Arbuscular mycorrhizal colonization and growth of soybean (Glycine max ) and lettuce (Lactuc sativa ) and phytotoxic effects of olive mill residues

We studied the influence of olive mill dry residue (DOR) on growth of soybean (Glycine max ) and lettuce (Lactuca sativa ) colonized with the arbuscular fungi Glomus mosseae or G. deserticola. The DOR has 6 mg g 21 of soluble phenolic compounds and the application of 2.5 g kg 21 of DOR (15 mg kg 21 of phenolic content) to soil decreased the growth of plants colonized with the AM fungi. The application to the soil of 10 g kg 21 of DOR (60 mg kg 21 of phenolic content) decreased the dry weight of nonarbuscular mycorrhizal (AM) plants. The application of DOR decreased the percentage of AM colonization of plants except those were inoculated with G. mosseae or G. deserticola 4 weeks before the application of DOR. No effect of the residue on the colonization of these previously AM inoculated plants was observed. DOR had no effect neither on the fungal succinate dehydrogenase activity nor on the most probable number of AM propagules in all treated soils. The obtained results showed than AM fungi increased the phytotoxicity of DOR in lettuce and soybean. q 2002 Elsevier Science Ltd. All rights reserved.

Interactions between Glomus mosseae and arbuscular mycorrhizal sporocarp-associated saprophytic fungi

The saprophytic fungi Wardomyces inflatus (Marchal) Hennebert, Paecilomyces farinosus (Holm & Gray) A. H. S. Brown & G. Sm., Gliocladium roseum Bain., sterile dark mycelium (SDM-54), Trichoderma pseudokoningii Rifai and Trichoderma harzianum Rifai were isolated from sporocarps of Glomus mosseae. The effect of saprophytic fungi on G. mosseae spore germination was tested on water agar. Wardomyces inflatus decreased the percent germination of G. mosseae spores; G. roseum, T. pseudokoningii and T. harzianum had no effect on germination; and P. farinosus and SDM-54 increased the percentage of spore germination of G. mosseae after 4 d. Wardomyces inflatus significantly decreased hyphal length of spores which germinated, but no other saprophytic fungi affected hyphal growth. Trichoderma pseudokoningii, T. harzianum, P. farinosus and SDM-54 increased the number of auxiliary cells formed by G. mosseae. The effect of saprophytic fungi on arbuscular mycorrhizal colonization of soybean was studied in a greenhouse trial. The percentage of soybean root length colonized was decreased by W. inflatus, unaffected by SDM-54 and T. harzianum, and increased by P. farinosus. Gliocladium roseum decreased root length colonized when plants were 12 wk old, and T. pseudokoningii increased colonization of roots when plants were 4 wk old. Antagonistic, synergistic and neutral actions of G. mosseae upon the saprophytic fungi were observed. The population of T. harzianum decreased and the populations of T. pseudokoningii and SDM-54 increased in the presence of G. mosseae. Our results indicate a complex interaction between G. mosseae and associated saprophytic fungi.

Effect of carbamate herbicides on VA mycorrhizal infection and plant growth

SummaryThe effect of the carbamate herbicides Chlorpropham, Sulfallate and Phenmedipham, which are believed to inhibit photosynthesis, on VA mycorrhizal infection and on plant growth, were examined. Foliar spraying of Phenmedipham decreased the root concentration of total and reducing sugars and the fungal metabolism (using a staining reaction for succinate dehydrogenase as indicator) 48 h after application. However, all three carbamate herbicides tested, whether applied by foliar spray or directly to soil, did not affect the amount of VA mycorrhizal infection present at the end of the experiment. These herbicides decreased plant growth when they were applied to soil. But when the herbicides were sprayed only Phenmedipham, applied at high concentrations, decreased plant growth. Moreover, our results show that VA mycorrhizas may help plants recover from the deleterious effect of Phenmedipham.

The Jasmonic Acid Signalling Pathway Restricts the Development of the Arbuscular Mycorrhizal Association in Tomato

The role of the jasmonate signalling pathway in modulating the establishment of the arbuscular mycorrhiza (AM) symbiosis between tomato plants and Glomus intraradices fungus was studied. The consequences of AM formation due to the blockage of the jasmonate signalling pathway were studied in experiments with plant mutants impaired in JA perception. The tomato jai-1 mutant (jasmonic acid insensitive 1) failed to regulate colonization and was more susceptible to fungal infection, showing accelerated colonization. The frequency and the intensity of fungal colonization were greatly increased in the jai-1 insensitive mutant plants. In parallel, the systemic effects on mycorrhization due to the activation of the jasmonate signalling pathway by foliar application of MeJA were evaluated and histochemical and molecular parameters of mycorrhizal intensity and efficiency were measured. Histochemical determination of fungal infectivity and fungal alkaline phosphatase activity reveal that the systemic application of MeJA was effective in reducing mycorrhization and mainly affected fungal phosphate metabolism and arbuscule formation, analyzed by the expression of GiALP and the AM-specific gene LePT4, respectively. The results of the present study clearly show that JA participates in the susceptibility of tomato to infection by arbuscular mycorrhizal fungi, and it seems that arbuscular colonization in tomato is tightly controlled by the jasmonate signalling pathway.

Interactions between Trichoderma pseudokoningii strains and the arbuscular mycorrhizal fungi Glomus mosseae and Gigaspora rosea.

The interaction between Trichoderma pseudokoningii (Rifai) 511, 2212, 741A, 741B and 453 and the arbuscular mycorrhizal fungi Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe BEG12 and Gigaspora rosea Nicolson & Schenck BEG9 were studied in vitro and in greenhouse experiments. All T. pseudokoningii strains inhibited the germination of G. mosseae and Gi. rosea except the strain 453, which did not affect the germination of Gi. rosea. Soluble exudates and volatile substances produced by all T. pseudokoningii strains inhibited the spore germination of G. mosseae. The germination of Gi. rosea spores was inhibited by the soluble exudates produced by T. pseudokoningii 2212 and 511, whereas T. pseudokoningii 714A and 714B inhibited the germination of Gi. rosea spores by the production of volatile substances. The strains of T. pseudokoningii did not affect dry matter and percentage of root length colonization of soybean inoculated with G. mosseae, except T. pseudokoningii 2212, which inhibited both parameters. However, all T. pseudokoningii strains decreased the shoot dry matter and the percentage of AM root length colonization of soybean inoculated with Gi. rosea. The saprotrophic fungi tested seem to affect AM colonization of root by effects on the presymbiotic phase of the AM fungi. No influence of AM fungi on the number of CFUs of T. pseudokoningii was found. The effect of saprotrophic fungi on AM fungal development and function varied with the strain of the saprotrophic species tested.