BacterioFiles 340 - Plasmid Promotes Plant Pathogenesis

Pathogenic Rhodococcus causing
disfigurement
From: Savory et al, eLife 6:e30925

This episode: Some bacteria living around plants can become pathogenic just by gaining a few genes!

Thanks to Skylar Fuller for her contribution!

Download Episode (9.4 MB, 10.3 minutes)

Show notes:
Microbe of the episode: Alternanthera yellow vein betasatellite

News item

Journal Paper:
Savory EA, Fuller SL, Weisberg AJ, Thomas WJ, Gordon MI, Stevens DM, Creason AL, Belcher MS, Serdani M, Wiseman MS, Grünwald NJ, Putnam ML, Chang JH. 2017. Evolutionary transitions between beneficial and phytopathogenic Rhodococcus challenge disease management. eLife 6:e30925.

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Episode outline:

• Background: Plants interact with microbes as pathogens and mutualists
• Farmers work with microbes as helpful or oppose as harmful
• Gene transfer can change microbes to have other behavior
• Rhodococcus species: maybe some beneficial, but some cause disease in many types of plants
• Virulence genes understood and localized on plasmids
• Disfiguring growths of plant tissue
• Problematic for ornamentals
• What’s new: Now, Skylar Fuller and other first authors Elizabeth Savory and Alexandra Weisberg, along with coauthors William Thomas, Michael Gordon, Danielle Stevens, Allison Creason, Michael Belcher, Maryna Serdana, Michelle Wiseman, Nick Grunwald, Melodie Putnam, and Jeff Chang, publishing in eLife, have discovered that just a few genes can make the difference between a pathogen and a beneficial microbe!
• Methods: Got 60 Rhodococcus isolates from various sources
• Flowering plants with or without symptoms, irrigation equipment
• Some pathogens with related virulence genes
• Others lacking genes/linear virulence plasmid
• Tested effects of some isolates on plants
• Some with virulence genes, some without
• Inoculated on stems of Nicotiana benthamiana
• Those without genes did nothing, those with caused deformations
• On seedlings, prevented from growing much
• Though all reduced root growth at least a little
• But non-pathogenic strains in seedlings increased root hairs and lateral roots, possibly helpful
• Difference between types is just a few genes?
• Took plasmid from pathogenic strain, put into non-path strain
• Changed phenotype to become pathogenic
• Reverse also: removing plasmid from pathogen makes it like the non-paths
• Summary: To summarize, here’s one of the first authors, Skylar Fuller, describing her experience with the research: statement 
• Applications and implications: Epidemiology – not just tracing pathogens
• Also tracing genes that turn non-pathogens into pathogens
• Genes can be transferred sometimes easier than microbes, create pathogens in new places
• Despite care taken to prevent pathogens themselves from getting through
• Usu much more required than just a few genes though
• Like criminal in ski mask robs a bank, then changes into firefighter uniform and looks like helpful member of society
• So worthwhile to think about how to prevent gene flow sometimes, in specific ways
• But not when it's beneficial; tricky
• What do I think: Extra care necessary when interpreting metagenomic studies
• Can't just say "oh these species here, problematic" or "probably helpful"
• If one species could be pathogen or not just with or without a few genes
• Human takeaway: Just goes to show, some genes corrupt more than others

Author Transcript:
I started working on this project as an undergraduate, and continued to work on this project during my one year masters program. The original goal for my thesis was to describe the best approach for diagnosing pathogenic Rhodococcus. Something really short and sweet. But in the process, we realized that we had a much larger story to tell. It was more than just how to diagnose pathogenic Rhodococcus, or even how Rhodococcus causes disease. Because we realized that in agriculture, non-pathogenic Rhodococcus is ubiquitous. It's everywhere. So we started asking ourselves, why are they here, what are they doing, and could they be benefiting their host. We designed experiments to answer these questions. I'm most proud of the experiments that Elizabeth and I designed, to test if there are beneficial effects. And sure enough, these experiments showed that plants inoculated with non-pathogenic Rhodococcus had a proliferation of root hairs, which suggests they had an increase in the surface area of the root, which is key for obtaining important nutrients and water. This was a large and collaborative effort, and I can't talk about everything that was in the paper, but I will leave you with this: The thing I really appreciated while working on this project was getting the opportunity to participate in such a collaborative piece, and also see how my work in the lab has direct applications in agriculture.