Fabio Battini

Diverse bacterial communities are recruited on spores of different arbuscular mycorrhizal fungal isolates

Arbuscular mycorrhizal fungi (AMF) establish mutualistic symbioses with the roots of most food crops, playing a key role in soil fertility and plant nutrition and health. The beneficial activity of AMF may be positively affected by bacterial communities living associated with mycorrhizal roots, spores and extraradical hyphae. Here, we investigated the diversity of bacterial communities associated with the spores of six AMF isolates, belonging to different genera and species and maintained for several generations in pot cultures with the same host plant, under the same environmental conditions and with the same soil. The occurrence of large bacterial communities intimately associated with spores of the AMF isolates was revealed by PCR denaturing gradient gel electrophoresis (DGGE) analysis and sequencing of DGGE bands. Cluster and canonical correspondence analysis showed that the six AMF isolates displayed diverse bacterial community profiles unrelated with their taxonomic position, suggesting that each AMF isolate recruits on its spores a different microbiota. The 48 sequenced fragments were affiliated with Actinomycetales, Bacillales, Pseudomonadales, Burkholderiales, Rhizobiales and with Mollicutes-related endobacteria (Mre). For the first time, we report the occurrence of Mre in Funneliformis coronatum and Rhizophagus intraradices and sequences related to endobacteria of Mortierella elongata in F. coronatum and Funneliformis mosseae. The bacterial species identified are known to possess diverse and specific physiological characteristics and may play multifunctional roles affecting the differential performance of AMF isolates, in terms of infectivity and efficiency.

Microbially-enhanced composting of olive mill solid waste (wet husk): Bacterial and fungal community dynamics at industrial pilot and farm level.

Bacterial and fungal community dynamics during microbially-enhanced composting of olive mill solid waste (wet husk), used as a sole raw material, were analysed in a process carried out at industrial pilot and at farm level by the PCR-DGGE profiling of the 16 and 26S rRNA genes. The use of microbial starters enhanced the biotransformation process leading to an earlier and increased level of bacterial diversity. The bacterial community showed a change within 15 days during the first phases of composting. Without microbial starters bacterial biodiversity increased within 60 days. Moreover, the thermophilic phase was characterized by the highest bacterial biodiversity. By contrast, the biodiversity of fungal communities in the piles composted with the starters decreased during the thermophilic phase. The biodiversity of the microbial populations, along with physico-chemical traits, evolved similarly at industrial pilot and farm level, showing different maturation times.

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

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

A major challenge for agriculture is to provide sufficient plant nutrients such as phosphorus (P) to meet the global food demand. The sufficiency of P is a concern because of it’s essential role in plant growth, the finite availability of P-rock for fertilizer production and the poor plant availability of soil P. This study investigated whether biofertilizers and bioenhancers, such as arbuscular mycorrhizal fungi (AMF) and their associated bacteria could enhance growth and P uptake in maize. Plants were grown with or without mycorrhizas in compartmented pots with radioactive P tracers and were inoculated with each of 10 selected bacteria isolated from AMF spores. Root colonization by AMF produced large plant growth responses, while seven bacterial strains further facilitated root growth and P uptake by promoting the development of AMF extraradical mycelium. Among the tested strains, Streptomyces sp. W94 produced the largest increases in uptake and translocation of 33P, while Streptomyces sp. W77 highly enhanced hyphal length specific uptake of 33P. The positive relationship between AMF-mediated P absorption and shoot P content was significantly influenced by the bacteria inoculants and such results emphasize the potential importance of managing both AMF and their microbiota for improving P acquisition by crops.

Rhizophagus intraradices or its associated bacteria affect gene expression of key enzymes involved in the rosmarinic acid biosynthetic pathway of basil

In recent years, arbuscular mycorrhizal fungi (AMF) have been reported to enhance plant biosynthesis of secondary metabolites with health-promoting activities, such as polyphenols, carotenoids, vitamins, anthocyanins, flavonoids and lycopene. In addition, plant growth-promoting (PGP) bacteria were shown to modulate the concentration of nutraceutical compounds in different plant species. This study investigated for the first time whether genes encoding key enzymes of the biochemical pathways leading to the production of rosmarinic acid (RA), a bioactive compound showing antioxidant, antibacterial, antiviral and anti-inflammatory properties, were differentially expressed in Ocimum basilicum (sweet basil) inoculated with AMF or selected PGP bacteria, by using quantitative real-time reverse transcription PCR. O. basilicum plants were inoculated with either the AMF species Rhizophagus intraradices or a combination of two PGP bacteria isolated from its sporosphere, Sinorhizobium meliloti TSA41 and Streptomyces sp. W43N. Present data show that the selected PGP bacteria were able to trigger the overexpression of tyrosine amino-transferase (TAT), hydroxyphenylpyruvate reductase (HPPR) and p-coumaroyl shikimate 3′-hydroxylase isoform 1 (CS3′H iso1) genes, 5.7-fold, 2-fold and 2.4-fold, respectively, in O. basilicum leaves. By contrast, inoculation with R. intraradices triggered TAT upregulation and HPPR and CS3′H iso1 downregulation. Our data suggest that inoculation with the two selected strains of PGP bacteria utilised here could represent a suitable biotechnological tool to be implemented for the production of O. basilicum plants with increased levels of key enzymes for the biosynthesis of RA, a compound showing important functional properties as related to human health.

Suppression of the activity of arbuscular mycorrhizal fungi by the soil microbiota

Arbuscular mycorrhizal fungi (AMF) colonise roots of most plants; their extra-radical mycelium (ERM) extends into the soil and acquires nutrients for the plant. The ERM coexists with soil microbial communities and it is unresolved whether these communities stimulate or suppress the ERM activity. This work studied the prevalence of suppressed ERM activity and identified main components behind the suppression. ERM activity was determined by quantifying ERM-mediated P uptake from radioisotope-labelled unsterile soil into plants, and compared to soil physicochemical characteristics and soil microbiome composition. ERM activity varied considerably and was greatly suppressed in 4 of 21 soils. Suppression was mitigated by soil pasteurisation and had a dominating biotic component. AMF-suppressive soils had high abundances of Acidobacteria, and other bacterial taxa being putative fungal antagonists. Suppression was also associated with low soil pH, but this effect was likely indirect, as the relative abundance of, e.g., Acidobacteria decreased after liming. Suppression could not be transferred by adding small amounts of suppressive soil to conducive soil, and thus appeared to involve the common action of several taxa. The presence of AMF antagonists resembles the phenomenon of disease-suppressive soils and implies that ecosystem services of AMF will depend strongly on the specific soil microbiome.

Quorum sensing in rhizobia isolated from the spores of the mycorrhizal symbiont Rhizophagus intraradices

Most beneficial services provided by arbuscular mycorrhizal fungi (AMF), encompassing improved crop performance and soil resource availability, are mediated by AMF-associated bacteria, showing key-plant growth-promoting (PGP) traits, i.e., the production of indole acetic acid, siderophores and antibiotics, and activities increasing the availability of plant nutrients by nitrogen fixation and phosphate mobilization. Such functions may be affected by the ability of AMF-associated bacteria to communicate through the production and secretion of extracellular small diffusible chemical signals, N-acyl homoserine lactone signal molecules (AHLs), that regulate bacterial behavior at the community level (quorum sensing, QS). This work investigated the occurrence and extent of QS among rhizobia isolated from AMF spores, using two different QS reporter strains, Agrobacterium tumefaciens NTL4 pZRL4 and Chromobacterium violaceum CV026. We also assessed the quorum quenching (QQ) activity among Bacillus isolated from the same AMF spores. Most rhizobia were found to be quorum-signaling positive, including six isolates producing very high levels of AHLs. The results were confirmed by microtiter plate assay, which detected 65% of the tested bacteria as medium/high AHL producers. A 16S rDNA sequence analysis grouped the rhizobia into two clusters, consistent with the QS phenotype. None of the tested bacteria showed QQ activity able to disrupt the QS signaling, suggesting the absence of antagonism among bacteria living in AMF sporosphere. Our results provide the first evidence of the ability of AMF-associated rhizobia to communicate through QS, suggesting further studies on the potential importance of such a behavior in association with key-plant growth-promoting functions.