Effect of Arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria on glomalin production

Abstract

There is accumulating evidence that arbuscular mycorrhizal fungi (AMF) produce a glycoprotein called glomalin, which has the potential to increase soil carbon (C) and nitrogen (N) storage, thereby reducing soil emissions of carbon dioxide (CO2) and nitrous oxide (N2O) into the atmosphere. However, other soil microorganisms such as plant growth-promoting rhizobacteria (PGPR) that interact with AMF could indirectly influence glomalin production. The objectives of this study were to determine the effects of AMF and PGPR interactions on glomalin production and identify possible combinations of these organisms that could enhance C and N storage in the rhizosphere. The effects of AMF and PGPR interactions on pea (Pisum sativum L.) growth and correlations between glomalin production and plant growth also were assessed.

A series of growth chamber and laboratory experiments were conducted to examine the effect of fungal and host plant species on glomalin production by comparing the amounts of glomalin produced by Glomus clarum, G. intraradices, and G. mosseae in association with corn (Zea mays L.), in addition to examining differences in the ability of corn, pea, and wheat (Triticum aestivum L.) to support glomalin production by G. intraradices. There were no significant differences in glomalin production [measured in the rhizosphere as Bradford-reactive soil protein (BRSP)] by the three AMF species, whereas host plant significantly affected glomalin production. Specifically, higher BRSP concentrations were found in the rhizosphere of corn as compared to pea and wheat.

Additionally, the effect of long-term storage on the growth promoting traits of the PGPR strains selected; namely, Pseudomonas cepacia R55 and R85, P. aeruginosa R75, P. putida R105, and P. fluorescence R111 were investigated. These bacterial strains previously had been identified as PGPR, but had since undergone approximately twenty years of storage at -80¢ªC; thus, it was necessary to confirm that these strains had retained their plant growth promoting characteristics. Apparently, long-term storage had no significant adverse effect on the PGPR strains as all strains increased the total biomass of wheat significantly and demonstrated antagonism against fungal pathogens.

The possibility that spore-associated bacteria (SAB) could influence AMF associations, thereby affecting glomalin production, and subsequent crop yield potential was assessed. This was achieved by first isolating bacteria from disinfested spores of the AMF species and determining their potential as PGPR for wheat. According to fatty acid methyl ester (FAME) profiles, four genera of bacteria were isolated from AMF spores namely; Arthrobacter, Bacillus, Micrococcus, and Paenibacillus, of which Bacillus species were the most common SAB. None of these isolates, however, showed growth promoting abilities on wheat.

Based on the preliminary findings, the combined effects of the three AMF species and the five PGPR strains were examined on plant growth and glomalin production under gnotobiotic conditions using pea as the host plant. Interactions between G. intraradices and R75, R85, or R105 resulted in increased BRSP concentration in the mycorrhizosphere of pea. Additionally, significant interactions were observed between the AMF species and PGPR strains on BRSP concentration in pea rhizosphere under non-sterile conditions. As observed under sterile conditions, the co-inoculation of pea with G. intraradices and R75 or R85 increased BRSP concentrations in the rhizosphere of pea grown in non-sterile soil, although interaction effects were not significantly different from the control or when G. intraradices was applied alone. Significant AMF and PGPR interactions were observed to affect AMF colonization; however, the combination of these organisms did not significantly affect pea growth, nutrient uptake, and C and N storage in the plant rhizosphere. No correlations were detected between glomalin-related soil protein (GRSP), pea growth, nutrient concentrations in the plant tissue, and soil organic C and N content. This study demonstrated that although the potential exists to manipulate certain AMF and PGPR to enhance glomalin production, co-inoculation of AMF and PGPR did not enhance plant growth or C and N storage beyond that achieved by inoculation of either organism.