Multifunctionality and diversity of culturable bacterial communities strictly associated with spores of the plant beneficial symbiont Rhizophagus intraradices

Abstract

Arbuscular Mycorrhizal Fungi (AMF) live in symbiosis with most crop plants and represent essential elements of soil fertility and plant nutrition and productivity, facilitating soil mineral nutrient uptake and protecting plants from biotic and abiotic stresses. These beneficial services may be mediated by the dense and active spore-associated bacterial communities, which sustain diverse functions, such as the promotion of mycorrhizal activity, biological control of soilborne diseases, nitrogen fixation, and the supply of nutrients and growth factors. In this work, we utilised culture-dependent methods to isolate and functionally characterize the microbiota strictly associated to Rhizophagus intraradices spores, and molecularly identified the strains with best potential plant growth promoting (PGP) activities by 16S rDNA sequence analysis. We isolated in pure culture 374 bacterial strains belonging to different functional groups-actinobacteria, spore-forming, chitinolytic and N2-fixing bacteria-and screened 122 strains for their potential PGP activities. The most common PGP trait was represented by P solubilization from phytate (69.7%), followed by siderophore production (65.6%), mineral P solubilization (49.2%) and IAA production (42.6%). About 76% of actinobacteria and 65% of chitinolytic bacteria displayed multiple PGP activities. Nineteen strains with best potential PGP activities, assigned to Sinorhizobium meliloti, Streptomyces spp., Arthrobacter phenanthrenivorans, Nocardiodes albus, Bacillus sp. pumilus group, Fictibacillus barbaricus and Lysinibacillus fusiformis, showed the ability to produce IAA and siderophores and to solubilize P from mineral phosphate and phytate, representing suitable candidates as biocontrol agents, biofertilisers and bioenhancers, in the perspective of targeted management of beneficial symbionts and their associated bacteria in sustainable food production systems.

Facilitation of phosphorus uptake in maize plants by mycorrhizosphere bacteria

“…The differential effects of inoculated bacterial strains on plant performance were consistent with their previous functional characterization 43 . Hence, our findings that Streptomyces sp.…”

“…W43N and Streptomyces sp. W94 increased root P content of NM plants are in agreement with their functional traits, as they showed the largest phosphate solubilisation and a high phytate mineralization 43 . Indeed, a growing number of PGPB, including Bacillus and rhizobia, have been reported to mobilize soil P from insoluble to soluble forms through two main processes: the secretion of organic anions (such as gluconic acid) chelating cations bound to phosphate and the production of phytase/phospathase enzymes able to hydrolyse organic forms of phosphate compounds 47–50 .…”

“…Such results are consistent with recent reports on the in vitro production of IAA by Bacillus sp. CH10 43 . Indole acetic acid is an important phytohormone, playing a key role in plant growth by affecting many functional activities, such as cell division, elongation, root initiation and fruit development 61 .…”

“…The bacteria were previously isolated from R. irregularis IMA6 (formerly G. intraradices ) 43 spores and were selected for their plant growth promoting (PGP) properties, such as the production of indole acetic acid (IAA) and siderophores, phosphatase-solubilizing and phytase-mineralizing activities. Each bacterial strain was cultured and inoculum prepared as previously reported 72 .…”

“…Such beneficial bacteria showed key plant growth promoting (PGP) functions, encompassing nitrogen fixation, P solubilization, the production of indole acetic acid (IAA), siderophores and antibiotics, supplying fundamental nutrients and growth factors 37–42 . Recently, a culture-dependent approach allowed the isolation in pure culture of 374 bacterial strains strictly associated with Rhizophagus irregularis (formerly Glomus intraradices ) spores, and the selection of Actinobacteria, Bacillaceae and Sinorhizobium meliloti strains showing several PGP activities, including P mineralization from phytate and mineral P solubilization 43 .…”