Downy mildew-Associated Bacteria: Their growth and influence on reproduction of Hpa spores in Arabidopsis thaliana
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Downy mildews are host specific, obligate biotrophic oomycetes that cause disease in a plant’s foliage and lead to catastrophic agricultural crop loss annually. The role of phyllosphere microbes to protect a plant against Downy mildew infection is largely unknown, understood even though the phyllosphere is pivotal for the production of biocontrol agents that can prevent disease outbreaks in agriculture. A previous study has shown that leaf infection of the Downy mildew of Arabidopsis thaliana called Hyaloperonospora arabidopsidis (Hpa) strongly influenced the phyllosphere microbial community of a plant whereby specific bacteria (Hpa-associated bacteria) were enriched in multiple Hpa infected cultures. In this study, we looked at the influence a few selected Hpa-associated bacteria have on Hpa sporulation as well as how Hpa influences the growth of these single bacterial isolates. We did this in an axenic system using gnotobiotic Hpa (gnoHpa). Our results validated that the previously identified Hpa-associated bacteria also benefited from the presence of Hpa in a gnotobiotic growth system. Next to that, we observed that two Hpa-associated bacterial isolates from the genera Microbacterium sp. and Aeromicrobium sp. significantly reduced gnoHpa sporulation. The reduction in sporulation seen by Microbacterium sp. is likely by directly antagonizing Downy mildew. Together our results opened an interesting pathway where further research can be conducted to understand the fundamental interactions occurring in the phyllosphere of plants infected with Hpa. Understanding these interactions is of key importance to uncover novel mechanisms to deal with pathogen attacks through stimulation of the leaf microbiota.