Context-dependency of microbe-mediated plant growth and defences

By 2050, the global population is expected to exceed 9.7 billion (a 26% increase), resulting in increasing demand for food and water. The ultimate goal of meeting global food needs should be to improve and preserve agricultural crop yield while minimising environmental impacts. Chemical fertilisers and pesticides improved crop output but constituted a significant hazard to human and environmental health, contradicting Sustainable Development Goals aimed at restoring biodiversity loss and achieving clean water. However, in the recent decade, attempts have been made to restore soil health by introducing natural agriculture practices such as the utilisation of soil beneficial bacteria and assuring healthy and sustainable crop production. There has been a surge in interest in the use of soil beneficial microorganisms as significant drivers of plant growth promotion and induced resistance against insect pests and diseases. Microbial inoculants have emerged as promising mitigators of our increased dependency on agrochemicals, declining global food production and the climate change crisis. Root colonizing fungal symbionts such as arbuscular mycorrhizal fungi (AMF) and endophytes can be used as alternate insect pest management and plant development strategies. However, the outcomes of plant-microbe interactions and the repercussions for the host plant are known to be context-dependent.

The context-dependency of such interactions has been recognized as a bottleneck for their integration in agricultural systems. Thus, it may hinder the wide scale success and acceptance of beneficial root symbionts in sustainable agriculture. The effects of interactions between plants and AMF or soil endophytes on insect herbivore resistance have been well studied under controlled conditions. However, implications of other environmental factors on plant-microbe-insect interactions have only recently started to receive attention. Using Solanum lycopersicum cultivar Moneymaker, I explore how light quality (manipulated directly or through inactivation of a plant phytochrome B photoreceptor), soil phosphorus application, drought and a hemibiotrophic pathogen, Pseudomonas syringae DC3000 alter plant-beneficial microbe interactions. I specifically look at the plant-microbe interaction with respect to plant growth and the plant’s response to herbivory by caterpillars of the polyphagous insect herbivores Chrysodeixis chalcites and Spodoptera exigua. Additionally, I looked at the implication of microbe-mediated effects on the third trophic levels and if insect natural enemies (parasitoids or predators) were also affected by the interplay between biotic and abiotic factors.

Experimental evidence has shown that conditions of shading by neighbouring plants lead to a decrease in the ratio between red and far-red light (R: FR), consequently activating the shade avoidance syndrome. However, this comes at the cost of increased plant susceptibility to pests and pathogens. Under these conditions, plants thus prioritize growth over defence. However, the application of a beneficial microbe may alleviate this light quality stress. The effect of low R: FR on beneficial microbes such as AMFs is not well known. First, the effects of ambient vs. low R: FR light on AMF (Rhizophagus irregulare) colonization of tomato plants, as well as the interactions between light quality and AMF on plant growth and the performance of an insect pest, and one of its natural enemies, Cotesia marginiventris was investigated. By looking at gene expression levels I also investigated the potential underlying mechanisms that modulate the relationships. We observed AMF counteracted the plant’s shade avoidance syndrome by reallocating biomass from shoot to root and only marginally improved the plant’s resistance to caterpillars with no strong beneficial effects on plant defences or gene expression level. Contrary to previous observations, induction of defensive gene expression in response to herbivory was only found in non-mycorrhizal plants under low R: FR light and defence induction was reduced by AMF under these light circumstances.

I then tested if the AMF-mediated effect on plant growth and resistance could be improved under conditions of phosphorus limitation. As the beneficial effects of AMFs are known to be most effective under P deficiency. I observed inactivation of phytochrome B or PhyB (shade perception) increased sensitivity to S. exigua. This was not observed for susceptibility to C. chalcites, indicating that this response differed between herbivore species. PhyB plants also produced fewer leaf volatiles (which may be implicated in indirect defence) during caterpillar attacks than Wildtype plants. This did not, however, diminish the attraction of the insect predator Macrolophus pygmaeus. The effects of the AMF inoculation on plant growth and defences were dependent on soil P level and plant line.

In a further experiment, I next evaluated a different abiotic factor. I studied if drought modulated the effects of a widespread endophytic fungus, Acremonium strictum, on plant resistance to a polyphagous insect herbivore and on plant volatile production, which may have implications for the attraction of the parasitoids C. marginiventris and Microplitis rufiventris. We found that drought and endophyte suppress plant leaf volatile production and additionally the interaction of drought and endophyte inoculation repelled the attraction of one of the parasitoids. Lastly, I studied the role of the hemibiotrophic pathogen Pseudomonas syringae in plant resistance, as well as in modulating the effects of AMF on plant growth and herbivore resistance. Previous research results suggest that pathogens can induce changes in the host plant that affect interactions with beneficial microbes and detrimental insects. Since Pseudomonas syringae and AMF induce antagonistic plant hormonal pathways we tested the hypothesis that prior establishment of pathogen will interfere with the mycorrhizae-induced resistance. I found that AMF-induced resistance to polyphagous herbivores is lost in the presence of the pathogen however, this effect was not supported by the expected molecular mechanisms.

This thesis demonstrates the potential of soil microbes, specifically arbuscular mycorrhizal fungi, as both plant biostimulants and bioprotectants, and the context-dependency of their growth-promoting and bioprotectant effects. I also show how an unspecialized, commonly found soil endophyte that has been documented to improve plant resistance to herbivores can significantly impede the plant’s interaction with the third trophic level, viz. beneficial insects. Thus, plants do not always benefit from symbiosis with the so-called “beneficial” microbes and microbe-mediated effects on plants depended on environmental conditions.