In humans, changes in the gut-microbiome can have a significant impact on peripheral immune responses. Gut-microbiota imbalances are associated with health conditions such as irritable bowel syndrome, type 2 diabetes and eczema. There is emerging research suggesting that tryptophan metabolism may be linked to gut-microbiota interactions that maintain healthy immune responses in humans. Correspondingly, plants have evolved to possess a complex innate immune system characterized by “multikingdom microbial communities”. Plant roots are colonized by a diverse range of symbiotic microbes such as bacteria and fungi and together, they maintain host-microbe homeostasis and plant health. However, this can be compromised under pathological conditions, causing dysbiosis. A recent study published by Dr Katarzyna Wolinska et. al in Proceedings of the National Academy of Sciences has highlighted that tryptophan derivatives may play a role in the immune systems of plants, promoting survival under pathological conditions.
The authors studied Arabidopsis thaliana (A. Thaliana), known as the thale cress, as a model organism to study microbiota diversity in plant roots. Their initial experiments found that immunocompromised plants demonstrated dysbiosis in the bacterial and fungal composition of their roots, however, they could not explain the variability in plant growth and performance in these plants. In subsequent experiments, they discovered that the aggressiveness of fungal colonization in A. Thaliana roots is related to plant performance and growth. A high fungal load puts the plant at greater risk of pathogenesis than root-derived bacteria. Importantly, their results indicated that host (plant) TRP metabolism plays an integral role in driving microbe-mediated plant growth promotion as TRP derivates and bacterial root commensals are key factors in limiting fungal growth in A. thaliana roots. As such, the fungal composition and maintenance of fungal-host homeostasis in A. thaliana roots is directly related to plant health and can promote microbiota-induced plant growth, and TRP-derived metabolites can maintain optimal fungal-load in plant roots, thereby upholding the innate immunity system in A thaliana. Overall, it seems that TRP metabolism plays a role in both human and plant immune systems via host-microbiota interactions.