Evolutionary Ecology of Siderophores in Bacteria-Insect-Plant Interactions

Microbes produce a broad array of products which can often benefit plant and animal hosts. For example, microbes in the human gut are beneficial for human health and can protect against pathogens. Plant microbiomes can also protect plants from pests, and this is a promising direction for advances in biotechnology to improve plant health and agricultural production. However, our ability to apply microbes in biotechnology is often limited by our understanding of how microbial products function, both for microbes and for hosts. Bacterial siderophores, compounds that scavenge iron from the environment, are important for protecting plants from pathogens in below ground root microbiomes. However, how siderophores function in above ground microbiomes on leaves is poorly understood. On leaves, siderophores have been found to protect plants against pathogens and against herbivorous insect pests. This project will determine which siderophores are produced by bacteria on leaves and which environmental conditions lead to plant-protective siderophore production. This research will provide a foundation for understanding how probiotic bacteria could be used in plant microbiomes to provide the beneficial functions of siderophores, improving technology for sustainable and efficient agricultural production. The project will also train undergraduate and graduate students in the fields of microbiology, agroecology, and biotechnology to advance the future STEM workforce. The project advances NSF's priorities in Biotechnology. Above ground plant surfaces are specialized environments where bacteria encounter high environmental heterogeneity, and both biotic and abiotic stressors. Bacteria possess remarkable abilities to thrive under environmental stresses, but the precise genetics underlying such abilities are often unclear. A functional class of molecules implicated in surviving environmental challenges is siderophores, compounds used by bacteria to scavenge iron. Iron is a necessary cofactor for many metabolic processes but is biologically unavailable in most habitats. Siderophores can be secreted outside of cells to bind iron, sequestering it for future use by bacteria and increasing the biologically available pool of iron. Bacteria in the genus Pseudomonas (pseudomonads) produce several distinct siderophores and are also common members of plant microbiomes, but the function of different siderophores in this habitat is poorly understood. Siderophores can improve competitive abilities on plants, potentially allowing beneficial plant microbiome members to exclude competing pathogens. Siderophore production by pseudomonads also influences the feeding behavior of insect herbivores and can deter feeding pest aphids. However, the high redundancy of potential siderophores that pseudomonads possess, as well phenotypic variation in when they are produced, mean that we have a poor understanding of when siderophores may be important in multipartite interactions influencing plant health. The goals of this proposal are to understand how the plant environment selects for different siderophores, and how these influence plant microbiomes and interactions with herbivorous insects. First, this project will define the siderophore production potential of plant-associated pseudomonads and how production abilities are evolving. Second, using experimental and sequencing approaches the project will test various hypotheses focused on how abiotic and biotic factors may select for redundant siderophores in plant microbiomes. These include testing for benefits of varied siderophores under heterogeneous environmental conditions, such as temperature, water, or nutrient stress, or potential benefits of multiple siderophores due to competition with other microbes. Finally, this project will then test how the production of different siderophores ultimately influences interactions with herbivorous insects. The project will advance the NSF goal of understanding links between genotypes and phenotypes with outcomes integrated across biological scales and will inform applications of biotechnology using siderophore producing bacteria to protect plants from pathogens and insect pests. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.