Symbionts-Plant-Pathogen Interactions

As a response to the increasing demand from society for agriculture to minimize the negative impacts on environment and landscape values, farmed legumes play a key role. Among them, field pea (Pisum sativum L.) is one of the oldest domesticated crops worldwide. It provides flexibility and many benefits to growers and consumers. However, P. sativum diseases, Mycosphaerella pinodes can limit delivering its services and its productivity. M. pinodes may cause severe yield losses when lesions reduce green photosynthetic tissue of the plant. Although significant effects of belowground tripartite symbiosis (legume roots + Rhizobium + mycorrhizal fungi) on legume growth and quality were measured, studies of the symbionts effects on plant health (fitness and its defense response against aboveground plant pathogenic fungi) and the reciprocal suppressions have not yet been well addressed and understood. Nonetheless, these interactions need to be studied case by case and data results from such kinds of studies fill databases with a wealth of information. The objectives of the project are: to elucidate the host-mediated effects of above ground M. pinodes on performances of belowground symbionts (Rhizobium and/or mycorrhizal fungi) to reveal the effects of belowground tripartite symbiosis and aboveground pathogenic fungi interactions on the metabolic dynamics and physiology of the host plant to elucidate the hostmediated effects of belowground symbionts on above ground M. pinodes severity to P. sativum A randomized three factorial design with the root treatments as one, genotype as the second and foliar pathogen as the third factors will be established. To understand deeply such complex interactions and identify diverse metabolic changes rapidly and accurately, recent technological advances allowed analyzing complex metabolite and protein mixtures using mass spectrometry based methods. Additionally, microbial infectivity and effectiveness, diseases severity, photosynthesis rate, nutrient composition and balance and yield structures will be assessed.

Pisum sativum is the main host of the Ascochyta blight disease and significant yield losses are recorded globally every year. The causing pathogen in this fungi complex is known to be Didymella pinodes. The current knowledge about the biochemical stress response of the plant is still not fully understood. Like the multitude of effects a pathogen has on its host, are the great influences of beneficial microbes on the plant metabolism. In case of legumes it is the essential relation with rhizobia, but also mycorrhiza. This relation is much specialized as rhizobia can positively influence the plants immune system. This comprehensive project focused on components of the host plant, interacting with pathogens and beneficial microbes. By applying high-throughput methods, systems biology provides an overall picture of the metabolic impact of the symbionts on the host, as well as of the stress response against D. pinodes. A susceptible pea was compared with a more tolerant cultivar. The scientists found that microsymbionts have a strong influence on the metabolism and hence the phenotype of the host plant. However, mode of action and effectiveness depend on symbiont (rhizobia or mycorrhiza) as well as pea cultivar. Although pathogen repressed/reduced formation of symbioses and thus symbiotic efficiency, Rhizobium-Symbiosis inhibited infestation and impact of the pathogen on the plant through a so-called Induced Systemic Resistance (ISR). Through this, specific defense mechanisms of the plant are accelerated and/or enhanced respectively. Hence, despite pathogen-attack, Rhizobium-inoculation lead to a better plant growth and yield performance than nitrogen fertilization. This effect was even more significant in the more susceptible pea cultivar. In contrast, co-symbiosis of rhizobia and mycorrhiza had an unfavorable influence on the plants growth. In this experiment, microsymbionts seemed to repress each other. This project has substantially contributed to a comprehensive insight into the different pathogen defense strategies of pea plants and the important potential of microsymbionts for improved agricultural bio-control. Hence, future breeding strategies in legumes should increase their attention towards possible Rhizobium application.