Plants have evolved to form symbiotic relationships with a variety of microbes that can colonize their root system. This symbiotic relationship affects plant performance as well as its defence by altering its physiology and biochemistry. Microbe-induced resistance is any form of defence induction triggered by inoculation with beneficial microbes and it can be either local or systemic. It
can be efficient against a broad variety of attackers including herbivores and plant-pathogens. The defence response can directly change the transcriptome and metabolome or prime the plant, which will cause a stronger/faster response against future attacker. However, the effects of inoculation with beneficial microbes may vary depending on various biotic and abiotic factors.
This thesis aims to investigate context-dependency of microbe-induced resistance and specifically if root-associated entomopathogenic fungi can induce the resistance of tomato plants, how this induced resistance is affected by the presence of other fungal microbes either beneficial or pathogenic.
In the 1st Manuscript, we studied whether photosynthetic efficiency of tomato leaflets is suppressed by short term herbivory by Spodoptera exigua larvae. The undamaged leaflet area activated a compensatory mechanism, which resulted in upregulated photosynthetic efficiency to make up for damaged lost tissue.
In the 2nd Manuscript, we tested the effect of root-inoculation with the entomopathogenic fungi Metarhizium brunneum and Beauveria bassiana on the photosynthetic efficiency of tomato leaflets and how inoculation can modulate the photosynthetic response to short term herbivory by Spodoptera exigua larvae. Metarhizium brunneum had a negative effect on the photosynthetic efficiency of tomato plants and Beauveria bassiana did not alter the photosynthetic performance of tomato. After short-term herbivory both isolates and the non-inoculated plants, activated the same compensatory photosynthetic mechanism by increasing the fraction of absorbed light energy used for photochemistry.
In the 3rd Manuscript, we evaluated the effect of root-associated entomopathogenic fungi on tomato performance and defence against S. exigua larvae and investigated if co-inoculation with the arbuscular mycorrhizal fungus, Rhizophagus irregularis acts synergistically or antagonistically with them. The three isolates tested (Metarhizium brunneum, Metarhizium robertsii and Beauveria bassiana) increased the shoot biomass on their own and when co-inoculated with Rhizophagus irregularis. Entomopathogenic fungi did not alter the production of defence compounds, α-tomatine and dehydrotomatine, whereas AMF inoculation upregulated their production Entomopathogenic fungi in combination with arbuscular mycorrhizal fungi in this study exhibited complementary roles by enhancing plant growth and plant protection respectively.
In the 4th Manuscript, we studied if two entomopathogenic fungi can prime tomato plants against herbivory and if prior infection with necrotrophic fungus can hinder this microbe-induced resistance or it can act as an additive priming factor. Metarhizium brunneum and Beauveria bassiana primed the defences of tomato plants, by up-regulating marker genes of hormonal defence, when the plants were challenged with herbivory by, S. exigua larvae. When plants were
previously infected with a pathogen there was no evidence of priming in the inoculated plants compared to non-inoculated plants. Inoculation with Metarhizium brunneum and Beuaveria bassiana did not induce the total biomass of tomato plants. When plants were challenged with Botrytis cinerea, Metarhizium brunneum-inoculated plants had higher shoot biomass but when plants were challenged with both a necrotrophic fungus and a chewing herbivore, B. bassianainoculated plants had higher shoot biomass. Moreover, the two isolates had isolate-specific effects on the fruit production in various ways depending on the conditions (herbivory and pathogen infection).
In conclusion, the research presented in this thesis supports the ability of root-associated entomopathogenic fungi to prime plants defence against herbivores and their potential role in future more sustainable herbivore managements in agriculture. At the same time, we want to emphasize the high context-dependent nature of inoculation with root-associated entomopathogenic fungi on plant performance, and the need for further research in order to evaluate the optimal root-associated entomopathogenic fungi isolate for a specific crop system.