ROOT–SOIL FUNGUS INTERACTIONS IN TOMATO

Plants are dynamic and highly sensitive organisms, which communicate with their environment in a complex fashion. Our understanding of plant-plant and plant-microbe interactions has increased to a great extent, yet the signal communication is still far from being satisfactorily understood. To add to the complexity, linkages between below- and aboveground physiological processes within a plant as well as facilitation and inhibition by adjacent plants are known to occur. Shortly after germination, the developing seedlings are colonized by a wide range of microorganisms that may cause either beneficial or antagonistic effects on their host. The importance of plant metabolites is apparent in these processes; they may significantly contribute to pathogen resistance and, in addition, comprise compounds determining the color, taste and odor of fruits. Moreover, some secondary metabolites that are especially accumulated in fruits may also act as antioxidants and thus exert beneficial effects to the health of the consumers by decreasing the risks of cancer and cardiovascular diseases. Consequently, they are also regarded as criteria for crop quality. We intend to elucidate the plant response in a biological system consisting of a crop plant and a beneficial and pathogenic soil fungus in monoculture and mixed cultivation. Out of the multitude of possible interactions we will focus on the chosen model system, which comprises tomato, the arbuscular mycorrhizal fungus Glomus mosseae and the soil borne tomato pathogen Fusarium oxysporum sp. lycopersici. The effects of single and combined inoculations as well as various combinations of co-culture of tomato with plants, which are either known to be stimulative or repressive for tomato, will be studied. We hypothesize that specific effects of (1) plant pathogenic and mycorrhizal fungi on metabolite profiles in the tissues and root exudates of their host plant, (2) root exudates, pathogenic and mycorrhizal fungi on the development of soil borne fungi in the rhizosphere, and (3) adjacent plants on the metabolite profiles of above- and belowground tomato organs as well as on the root exudates, which cause specific feedbacks on the development of fungal colonization, will occur. The proposed research will provide improved insights about metabolic dynamics in leaves, roots, and root exudates of tomato. The extent of correlation of any detected changes in the metabolite profiles in various organs of tomato to the inoculation with a specific mycorrhizal and pathogenic fungus, or a combination of both will be explored. Additionally, effects of intercropped plants, both with stimulating and inhibitory effects, on the variability of chemical profiles of the model plant will be characterized for the first time. Moreover, we will provide new information how much fruit quality (an aboveground character) fruit is affected by root colonization by pathogenic and symbiotic root fungi.

Arbuscular mycorrhizal fungi (AMF) are the most prevalent type of mycorrhizal fungi which form a mycorrhizal symbiosis with a wide range of herbaceous plants under natural conditions including many important crop species. The mycorrhizal symbiosis is a mutual association between plant and fungus where the plant delivers carbon to the obligate biotrophic fungus, while the fungal partner acquires phosphorus from the soil more efficiently than the plant and provides it to the plant in return. Apart from improved plant nutrition, AMF are also reputed to control a number of plant diseases, especially soil-borne diseases. This is of high significance in the field of sustainable agriculture, where the input of fertilizers and chemical plant protectants is reduced or even absent. Root exudates and their multiple components play an important role in plantmicrobe interactions in the rhizosphere (i.e. the soil zone that surrounds and is influenced by the roots of plants), especially, in the initial phase of these interactions. It is already known that root exudates influence the germination rate of the soil-borne tomato pathogen Fusarium oxysporum f. sp. lycopersici (Fol). However, the potential alteration in root exudation patterns due to AMF and its putative effect on Fol development are not satisfactorily understood. In this work the focus was laid on the role of AMF in the control of the soil-borne fungus Fol in tomato in different intercropping settings. Additionally, root exudation and its function in disease development was selected as one specific area of interaction and investigated in greater detail. For the intercropping studies an inoculum consisting of six different AMF species was tested against Fol with tomato intercropped with either leek, cucumber, basil, fennel or tomato itself. Arbuscular mycorrhizal root colonization of tomato was clearly affected by its intercropping partner. Furthermore, bioprotective effects of AMF resulting in the decrease of Fol disease severity and/or compensation of plant biomass were evident. However, these effects depended on the plant combination. Leek and basil proved to be beneficial intercropping partners of tomato. Additionally, alterations in root exudation of tomato mediated by a single AMF species and Fol were investigated. AMF inoculation increased the germination rate of Fol in total exudates, whereas, the simultaneous inoculation of AMF and Fol decreased the germination rate of Fol in total exudates. The subsequent chemical analyses revealed an AMF-dependent increase of sugars and decrease of organic acids, mainly glucose and malate. Furthermore, an increase of chlorogenic acid in root exudates of tomato plants inoculated with AMF and Fol was found, an effect, which could be shown for the first time. In conclusion, plant health can be influenced positively by the interactions of AMF and properly chosen intercropping partners such as leek. Apart from AMF and pathogens also other beneficial microorganisms are prevalent in the rhizosphere. The findings of this work about changes in root exudation dynamics due to AMF and Fol raise new questions about the effects of these dynamics on other rhizosphere inhabitants and a putative increase in biocontrol effects.