Downy mildew-Associated Bacteria: Their growth and influence on reproduction of Hpa spores in Arabidopsis thaliana
Summary
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.