• Arbuscular mycorrhizal fungi
• Maize inbred lines
• Extraradical mycelium
• Phosphate transporter gene expression
• Endophytic bacterial communities

Arbuscular mycorrhizal fungi (AMF) are a key group of beneficial obligate biotrophs, that establish mutualistic symbiosis with the roots of 80% of terrestrial plant taxa, including the most important food crops. AMF symbionts facilitate the absorption and transfer of mineral nutrients, such as phosphorus (P), nitrogen (N), sulfur (S), potassium (K), calcium (Ca), copper (Cu) and zinc (Zn), from the soil to the host plants by means of the extraradical mycelium (ERM) extending from colonized roots into the soil. In return, they obtain carbon compounds which they are unable to synthesize or to feed off as saprotrophic organisms (Smith and Read, 2008). Symbiosis with AMF improves carbon sequestration and soil aggregation, plant tolerance to biotic and abiotic stresses and an increase in the content of beneficial secondary metabolites (Gianinazzi et al., 2010; Avio et al., 2018). Several studies showed that the multiple services provided by AMF are the result of the synergistic activity of diverse bacterial communities living in the mycorrhizosphere, strictly associated with mycorrhizal roots, spores, sporocarps, and extraradical mycelium (Rambelli et al., 1973). The mycorrhizospheric microbiota showed different functional activities, ranging from the role of “mycorrhiza helper” (MH) (Frey-Klett et al., 2007) to that of “plant growth promoters” (PGPB; Mayo et al., 1986; Mugnier and Mosse, 1987; Tylka et al., 1991; Carpenter-Boggs et al., 1995). The aim of this research project was to understand the role played by AMF and PGPB in plant nutrition. Specifically, given that among the most important benefits that AMF produce in plants there is the improvement of P nutrition, mainly in soils with low P content (Smith and Read 2008) and given that P is one of the most important nutrients for plant growth and development, as a first aim the diversity of Phosphate Transporters 1 (PT1) gene among AMF species and isolates was analysed. As a second aim, fungal and plant PTs expression was evaluated in four different maize inbred lines (Oh40B, Mo17, Oh43 and B73) in symbiosis with Rhizoglomus irregulare (isolate IMA6) in an in vivo experimental system. Finally, endophytic bacterial communities living in the roots of the same four maize inbred lines were characterized. Specifically, it was assessed whether the endophytic microbiota of maize roots could be modulated by two different AMF inocula (R. irregulare isolate IMA6 and Funneliformis mosseae isolate IMA1) and different maize inbred lines.
In the first experiment of my Ph.D. project, the genetic diversity of PT1 gene sequences and sequences of the taxonomically relevant SSU-ITS-LSU region were investigated in two isolates of the species Funneliformis coronatus, three isolates of the species F. mosseae, and two species of the genus Rhizoglomus from geographically distant areas. The results showed that the partial PT1 sequences not only successfully differentiated AMF genera and species, as well as the ribosomal gene sequences, but also revealed intraspecific diversity among F. mosseae and F. coronatus isolates. The study of functional genes related to mineral nutrient uptake by AMF is a key step in selecting efficient isolates to be used as inocula in sustainable agriculture. In the second experiment, with the aim of gaining further insight into how plant genotype influences a set of plant and fungal variables involved in the acquisition of low external P, four different maize inbred lines (Oh40B, Mo17, Oh43, and B73), grown in symbiosis with R. irregulare (isolate IMA6) in an in vivo experimental system, were assessed. To this aim, I evaluated (i) expression patterns of fungal and plant PTs in ERM, IRM, and mycorrhizal roots, (ii) ERM and plant phenotypic traits, related to host growth and P acquisition and (iii) ERM extension and structure. The results of this study demonstrated that, at low phosphorus availability, the mycorrhizal maize inbred lines differed in their fungal/plant PT patterns of transcript accumulation and ERM phenotypic traits, and in the relationships between molecular and phenotypic traits with mycorrhizal plant benefits, suggesting that mycorrhizal plant responsiveness and fungal performance are the result of fine-tuning of cost-benefit balance, specific to each host-AMF combination. In particular, the divergent behavior of maize lines B73 and Mo17 suggested the presence of complex and different response strategies to low phosphorus availability. Indeed, B73 line showed larger hyphal ERM density and interconnection, lower host P concentration and higher mycorrhizal P increase, while Mo17 line showed higher levels of plant ZmPHT1s expression, larger host P concentration and lower mycorrhizal P increase. In the third experiment, the sequencing the V3-V4 hypervariable region of 16S rDNA using the Illumina MiSeq platform, allowed the characterization of the endophytic bacterial communities living in maize roots. Specifically, four maize inbred lines (B73, Mo17, Oh43, and Oh40B) were inoculated with two isolates of AMF, F. mosseae IMA1 and R. irregulare IMA6, in microcosm systems. Results showed that a different endophytic bacterial biota was shaped by the two different inocula in maize roots. In plants inoculated with IMA1 and IMA6, more OTUs were detected in the roots of maize lines than in non-inoculated maize plants (control plants). Specifically, some genera known to include PGP strains, such as Streptomyces, Pantoea, and Erwinia were abundant in the roots of IMA6-colonized maize, whereas Enterobacter, Roseateles, and Lactobacillus were recovered in IMA1 ones. In addition, endophytic communities were also influenced by inbred lines, showing a different modulation of the microbiota in roots. Our results may represent a starting point for the exploitation of beneficial endophytes in maize roots and may open new perspectives on the role of AMF in shaping the plant microbiome.