Belowground plant-microbe interactions: effects of Fusarium culmorum and Serratia plymuthica volatiles on plant growth​

In terrestrial ecosystems fungi and bacteria live in complex multi-species networks. Among all these species, Fusarium culmorum is one of the most important fungal plant pathogens; it causes disease on a wide diversity of cereal and non-cereal crops. According to Schmidt et al. (2017), Fusarium culmorum volatile compounds induce the production of the secondary volatile metabolite Sodorifen in the bacterium Serratia plymuthica PRI-2C. Volatile organic compounds (VOCs) are low-molecular-weight molecules with a high vapor pressure that allows them to diffuse through the soil matrix, facilitating long-distance interactions among microorganisms and between microorganisms and plants. The aim of the experiments conducted for the present thesis was to test the effects of (i) Sodorifen pure compound and (ii) Fusarium culmorum / Serratia plymuthica volatiles on plant growth. The effects of Sodorifen pure compound on Solanum lycopersicum and Arabidopsis thaliana plants were tested. The determination of the growth parameters (such as fresh and dry biomass, root length and leaf-area) revealed that Sodorifen did not affect plant growth. The effects on the plants caused by the volatiles produced by Fusarium culmorum and Serratia plymuthica were studied in a field-like experiment. In particular, a pot-in-jar system was developed to expose Zea mays plants to volatiles. The plants were contained in pots, which were inserted in jars. At the bottom of the jars small Petri-dishes, containing different treatments, were inserted. Four different treatments were considered: (a) no fungus and no bacterium (control); (b) only the bacterium Serratia plymuthica (S); (c) only the fungus Fusarium culmorum (F); (d) Serratia plymuthica and Fusarium culmorum in combination (SF) (in this way Serratia, subjected to the VOCs emitted by the fungus, overproduces Sodorifen). VOCs emitted by the diverse microbe combinations differentially affected plant growth. The variation in root length and shoot/root biomass ratio indicated that the S-treatment impacted on the root development respect to the other treatments. In order to provide further insights on VOCs emitted by microbes on plant growth, the content of some mineral nutrients in shoot and root tissues was determined. Interestingly, depending on the combination of microorganisms considered, the nutrients content of both shoot and root tissues changed. This result suggested that belowground volatiles affected the content of some nutrients in plants (for instance, Mo-content was lower in F-treated samples in comparison with the control). The studies reported in the present thesis provided new insights into the role of Volatile Organic Compounds in the regulation of belowground plant-microbe interactions: the belowground volatiles emitted by microorganisms such as bacteria and fungi can influence the plant development also from the distance without a direct contact between plant and microbe.