Arbuscolar mycorrhizal fungal biodiversity in a low impact agrosystem

Symbiotic associations between arbuscular mycorrhizal fungi (AMF) and plant roots are widespread in natural environments and provide a range of benefits to the host plant. These include improved nutrition, enhanced resistance to soil-borne pests, diseases, and drought, as well as tolerance to heavy metals. In addition, the presence of a well developed AMF hyphal network improve the soil structure. As obligate mutualistic symbionts these fungi colonize the roots of many agricultural crops and it is often claimed that agricultural practices (use of fertilizers and biocides, tillage, dominance of monocultures and the growing of non-mycorrhizal crops) are detrimental to AMF. As a result, agrosystems impoverished in AMF may not get the fully expected range of benefits from these fungi. We selected two different areas, respectively representative of a low impact agrosystem (Azienda Agricola Manenti, Sostegno – Biella, mainly involved in horticultural production) and its surrounding grassland, for an arbuscular mycorrhizal fungi (AMF) community composition analysis. Roots of a crop plant (Allium fistulosum), roots of two spontaneous plants (Trifolium spp., Plantago spp.) and their rhizospheric soil were sampled. AMF in roots and soils were identified by cloning and sequencing a region of 550bp of the 18S rDNA (SSU) and 600bp of the 28S rDNA (LSU). A morphological investigation was carried out too. All plants were well colonized and showed all the typical infection structures (inter and intracellular hyphae, arbuscules, coils). More than 250 clones were screened for RFLP and a total number of 32 RFLP types was found. Sequencing and phylogenetic analysis on the rDNA SSU and LSU highlighted a more widely distributed AMF community for spontaneous plants than for the crop plant. In particular, analysis on the SSU characterized three RFLP types which clustered into Glomus group Aa and Ab in all selected areas, hinting at a lower specificity for the AM fungal species they belong. Seven RFLP types, according to the phylogenetic tree built on the LSU, did not cluster with any previously published AM sequence. Shannon indexes showed higher AMF biodiversity values for the natural ecosystem than for the low impact agrosystem. Both values appeared to be significantly higher than the average found in literature related to conventional high-impact agrosystems, putting forward low-impact agrosystems as semi-conservative of the indigenous AMF natural composition. The results show that low impact practice (no fertilizers, no pesticide, minimum tillage) can preserve and enhance the population of AM fungi suggesting how AM fungi could effectively act as natural fertilizers to obtain a good plant productivity. On this basis it appears important to evaluate commercial AMF inocula as alternative to fertilization in field or greenhouse where natural populations of this group of fungi are not always present. For this reason we undertook in parallel a molecular characterization analysis of some commercially available AMF inocula in order to verify their effectiveness on plant growth in nursery and in field.