DISCLOSING THE ROLE OF ROOT EXUDATES IN MODULATING THE INTERACTION BETWEEN PLANT AND XENOBIOTIC DEGRADING BACTERIA

Positive interactions between plants and root-associated bacteria are fundamental to maintain the homeostasis of the holobiont, especially upon environmental stresses. This crosstalk is mediated by root exudates (REs), including plant secondary metabolites, that are released by the plant as a ‘cry-for-help’ to recruit beneficial bacteria. Plant-bacteria interactions are particularly relevant for rhizoremediation, a sustainable strategy for the clean-up of recalcitrant xenobiotics like polychlorinated biphenyls (PCBs), contaminants that induce severe phytotoxic effects, as well as posing serious threats to ecosystems and human health. The aim of this PhD thesis was to disclose the role of root exudates differentially released under PCB stress, in influencing bacterial rhizocompetence traits, necessary to guarantee efficient root colonization and induce PCB degradation. Among root exudates, flavonoids are well described as signaling molecules and were reported as inducers of the PCB degradation pathway. The first objective of this thesis was to test this capability using the PCB degrader Paraburkholderia xenovorans LB400, by assaying pure flavonoids and the complex blend of REs released by Arabidopsis thaliana. LB400-Arabidopsis interaction was monitored in presence and absence of PCB-18 using a fluorescence-tagged LB400 strain, to observe root colonization and plant-growth-promotion potential upon stress. Our findings indicated that flavonoids stimulated LB400 proliferation, induced PCB degradation potential and increased biofilm formation, swimming motility and chemoattraction, features overall implicated in the early root colonization. However, other non-flavonoids molecules seemed to be involved in later stages of colonization and supported the plant growth promotion effect induced by LB400 under phytotoxic growth conditions. The REs alteration caused by PCBs was therefore explored to identify the main players in the contaminant-driven ‘cry-for-help’. A metabolomic analysis was performed on the REs of Arabidopsis grown in control conditions and in presence of a phytotoxic concentration of PCB-18, to observe the differential abundancies of secreted compounds and assess the role of five identified metabolites on the metabolism of PCB-degrading bacteria. PCB-18 dramatically affected plant physiology and altered Arabidopsis exudation pattern. Remarkably, the secretion of scopoletin, a plant secondary metabolite with known antimicrobial characteristics, was inhibited in presence of PCB stress, thus putatively remodeling the rhizosphere ecological niche to provide more suitable conditions for PCB-degrading bacteria. Instead, over-exuded metabolites were utilized as carbon and nitrogen sources and affected different rhizocompetence features. Finally, a bacterial biosensor was developed to clarify the role of root exudation by using a more realistic plant-bacteria interaction experimental set-up. The use of Pseudomonas JAB1 biosensor strain allowed the prompt visualization of root colonization and of the spatial distribution of cells in which PCB degradation was triggered by the REs. Our findings substantiated the conclusions about a putative ‘cry-for-help’ of the plant exposed to PCB-18 and about the centrality of flavonoids in sustaining this beneficial interaction. To conclude, the results of this thesis unveiled the functioning of plant-bacteria associations upon PCB stress and confirmed the relevance of REs in assembling a rhizosphere microbiota that provides the plant with useful services to improve contaminant degradation and support the holobiont health. These findings will be crucial to develop improved rhizoremediation interventions in PCB contaminated soils.