Nature is red in tooth and claw in plant leaves too, probably. In our lab at the University of Arizona, we are dissecting these interactions in a plant-microbe-insect community at the Rocky Mountain Biological Laboratory and throughout the west including near Greer, Arizona. Our goal is to determine how infection with endophytic (living within plant leaves) bacteria influences whether and how plants are susceptible to herbivory by arthropods. We believe that plant-bacterial interactions that play out in the leaves of plants may be under-appreciated symbioses that are not unlike human gut microbiomes. Ph.D. candidate Parris Humphrey is focusing on this research for his dissertation. With Dr. Pete Gauss, a colleague from Western State College, Parris discovered that co-occuring bacteria often produce bacteriocins that can selectively kill some, but not all, species. Trench warfare in a leaf?
We are also interested in understanding how bacterial communities on and in plant leaves assemble. Since they can disperse in the rain, snow, and clouds, these bacteria may simply not be limited by dispersal. Bill Hamilton and Tim Lenton wrote a paper on how bacteria may manipulate weather to their own advantage--a beautiful insight. The fact that ski hills used Pseudomonas syringae as ice-nucleators for snow-making until recently, suggests that this might not be so far-fetched as one might think at first glance (Bill thought of everything!). Such bacteria can increase the freezing temperature of water and often form the nuclei of snow crystals--they can also cause frost damage on plant leaves at temperatures higher than normal--allowing them access to the rich nutrients inside the leaves.
Below is a fluorescent image containing yellowish-green bacteria that are living in an Arabidopsis thaliana mustard leaf. The beautiful green orbs and disks are chloroplasts and thykaloids, respectively, from this genetic model plant (and weed). The bacteria (there are at least four that I can find, can you find them?), are in the center and one is to the left. A pair of bacteria has just undergone binary fission. They swim (yes, swim) into the leaf through stomata or wounds and then replicate within the apoplast, that region between (not within) plant cells. Are they pathogens? Mutualists? Commensals? It depends on the ecological context.
This world that normally remains unseen to our naked eyes is absolutely astonishing in its diversity, complexity and beauty. We are lucky to be able to study it--this will likely be a decades-long enterprise that will take us within the inner worlds of thousands of leaves. To give you an idea of the scale of the problem: there are typically between 10,000,000 and 100,000,000 bacterial cells per square centimeter of leaf tissue on every plant leaf, meaning that these cells outnumber the plant's own cells. Should the phyllosphere community and the plant together be thought of as a holobiont?
--Crested Butte, Colorado, July 22, 2012