The presence of resident microbiota on and inside plants is hypothesized to influence many phenotypic attributes of the host. Likewise, host factors and microbe-microbe interactions are believed to influence microbial community assembly. Rigorous testing of these hypotheses necessitates the ability to grow plants in the absence or presence of resident or defined microbiota. To enable such experiments, we developed the scalable and inexpensive FlowPot growth platform. FlowPots have a sterile peat substrate amenable to colonization by microbiota, and the platform supports growth of the model plant Arabidopsis thaliana in the absence or presence of soil-derived microbial communities. Mechanically, the FlowPot system is unique in that it allows for total-saturation of the sterile substrate by "flushing" with water and/or nutrient solution via an irrigation port. The irrigation port also facilitates passive drainage of the substrate, preventing root anoxia. Materials to construct an individual FlowPot total ~$2.A simple experiment with 12 FlowPots requires ~4.5 h of labor following peat and seed sterilization. Plants are grown on FlowPots within a standard tissue culture microbox after inoculation, thus the Flowpot system is modular and does not require a sterile growth chamber. Here, we provide a detailed assembly and microbiota inoculation protocol for the FlowPot system. Collectively, this standardized suite of tools and colonization protocols empowers the plant microbiome research community to conduct harmonized experiments to elucidate the rules microbial community assembly, the impact of microbiota on host phenotypes, and mechanisms by which host factors influence the structure and function of plant microbiota.3

Understanding the mechanisms through which pathogens alter plant cell networks is essential for understanding plant-pathogen interactions and will inform efforts to reduce crop diseases. Oomycetes secrete diverse effector proteins into plant cells. The mechanisms through which these effectors promote virulence are largely unknown. We show that the HaRxL10 effector protein from the Arabidopsis thaliana pathogen Hyaloperonospora arabidopsidis (Hpa) targets a transcriptional repressor (JAZ3) involved in jasmonic acid (JA) signalling. This manipulation activates a regulatory cascade that inhibits salicylic acid (SA) signalling, which normally restricts Hpa infection. This virulence mechanism is functionally equivalent to but mechanistically distinct from activation of the antagonistic JA-SA hormone crosstalk by the bacterial JA-mimicking toxin coronatine and by bacterial Type III effectors. These results reveal a key role for JAZ3 in plant immunity and emphasize that JA-SA crosstalk is an Achilles’ heel in the plant immune system, vulnerable to manipulation by diverse microbes.

Bacterial phytopathogens deliver effector proteins into host cells as key virulence weapons to cause disease. Extensive studies revealed diverse functions and biochemical properties of different effector proteins from pathogens. In this study, we show that the Pseudomonas syringae effector AvrE, the founding member of a broadly conserved and pathologically important bacterial effector family, binds to phosphatidylinositides (PIPs) in vitro and shares some properties with eukaryotic PROPPINs (β-propellers that bind polyphosphoinositides). In planta pull down experiments with transgenic Arabidopsis plants expressing AvrE revealed that AvrE is associated with several plant proteins including plasma membrane lipid-raft proteins. These results shed new light on the properties of a bacterial effector that is crucial for bacterial virulence in plants.