“A leaf is not a petri dish” (a lesson from Steven Lindow)

Earlier this quarter I had the fortune to attend a guest lecture by Steven Lindow of UC Berkeley. Lindow is a renowned expert in plant-microbiology who has worked on some pretty cool projects over the years (he was one of the first to work with ice-causing bacteria).

His lecture to us focused on his research with bacterial communities on leaves. One of the most surprising messages of the presentation was how inhospitable leaves are to microbes. Unlike microbes grown in labs that live on moist, nutrient-rich petri dishes, microbes living on exposed leaves face unique environmental challenges, most prominently the lack of water and the exposure to sunlight.

It turns out that to a bacterium, a leaf is much like the Saharan desert, mostly uninhabitable, except for small “oases” around which they cluster in aggregates and produce an extracellular, polysaccharide matrix (i.e. they make sugar slime) to protect themselves from the sun and dehydration. Because of these challenges, bacteria living on leaves tend to gather around the edges, veins and pores of the leaf where nutrient and water concentrations are highest.


A microscopic view of a leaf and stoma (pore). Bacteria often build up around pores and other moist and nutrient-rich areas on the surface of leaves.
Source: http://en.wikipedia.org/wiki/File:Tomato_leaf_stomate_1-color.jpg

In many cases, competition with local well-adapted wildlife can discourage a newly introduced species from establishing itself. But paradoxically if you’re a bacterium falling through the air onto a leaf, your best bet is actually to move into a crowded neighborhood. Sure, you might have to fight for a meal but at least with other bacteria around you’ll have a better chance against the environment.

So if your goal is to reduce the number of human pathogens on leaf surfaces (a big problem in the fresh fruit and vegetable industry), you might actually want to try targeting the non-pathogenic bacteria as well as the pathogenic ones, because without the work and special adaptations of native plant bacteria, there is very little chance that a pathogen that is well-adapted to the human gut will survive the dry environment of a leaf.

All of this discussion of bacterial leaf communities left me pondering my own project which will involve studying the microbes on an aquatic plant system. After hearing about the moisture-based limitations of land plant leaves I wondered: if water is the limiting factor in land-plant bacterial growth, what is the limiting factor in a water-dwelling plant? I asked Professor Lindow and he thought that an aquatic environment might face pressure in the form of predation from larger planktonic organisms. Although by the end of my project, we’ll still be far away from having enough data to answer such questions, it’s an interesting area of exploration to save for later.

About Hannah Holland-Moritz

Hannah Holland-Moritz is a graduate student working in Noah Fierer’s lab. She graduated from UC Davis in June 2014 with a major in Biochemistry and Molecular Biology and minor in Bioinformatics and most recently spend a gap year working in Jonathan Eisen’s lab on the microbiome of seagrasses. Interested in Evolution, Ecology, Bioinformatics and all things microbial, she plans to pursue a career in research.
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