Pseudomonas spp. as models for plant-microbe interactions

As is the case with other multicellular eukaryotes, plants are colonized by large numbers of unicellular microorganisms. They may be free-living commensals, epiphytes, symbionts (endophytes), or obligate parasites. The plant holobiont is in effect an ecosystem, and it is of interest to know how this assemblage is established and maintained, and reacts to both biotic and abiotic cues. The current view, initially elaborated in the context of coral-dwelling microbial communites, is that the multicellular organism is more inclusively described by the term “holobiont” that includes associated microbiota, and is a valid unit of natural selection (Rosenberg et al., 2007). The holobiont then, is often dependent on its microbiota for crucial functions, drastic imbalances in which, termed dysbiosis, are thought to lead to compromised or deficient functioning. The association of plants with microbes is phylogenetically ancient, going back to the macroalgae (Marshall et al., 2006). The role of the microbiota of plants, collectively termed the “phytomicrobiome,” in their overall life cycle is now under investigation, close on the heels of more extensive studies on animal, especially human, microbiota. The development of Arabidopsis thaliana (thale cress) and Brachypodium distachyon (purple false brome) as model systems for dicotyledonous and monocotyledonous plants respectively, and the availability of genome databases for Pseudomonads (Winsor et al., 2011) and plants (Duvick et al., 2008) indicate that the potential for both hypothesis-based and discovery science are indeed great. The assembly, development and maintenance of the plant holobiont is not possible without an exchange and sensing of, and responses to, biomolecular cues between its constituents. Within this overall theme, we focus on a few recently discovered, novel interand intra-species interactions of some Pseudomonas spp., indicating their utility as model systems, and highlighting some previously unforeseen mechanisms that could have a bearing on plant-phytomicrobiome interactions. Note that, for purposes of this article, we use the word “signaling” to refer generically to the sensing and response of organisms to environmental cues of both biotic and abiotic origins.