Despite the ability to mount a myriad of immune responses, every plant or animal is still highly susceptible to numerous pathogens. Why? Answering this question is of fundamental importance in agriculture and medicine, as it is key to a global understanding of infectious diseases in plants and humans. Our long-term goals are (1) to elucidate how microbial pathogens manipulate plants to cause disease and (2) to use pathogenesis as a probe for discovering fundamental cellular mechanisms in eukaryotic cells.
Currently, we use a model pathosystem consisting of the host Arabidopsis thaliana and the bacterial pathogen Pseudomonas syringae for our study. Both the host and the pathogen in this model are genetically and genomically tractable, making it an excellent system in which to elucidate many of the basic principles that govern pathogenesis in eukaryotic hosts. To cause disease, P. syringae bacteria produce a variety of virulence factors, including numerous "effector" proteins that are secreted through the type III protein secretion system (T3SS), and the phytotoxin coronatine, which functions as a molecular mimic of the plant hormone jasmonate. We have made steady progress in the understanding of how these virulence factors manipulate host innate immunity, vesicle trafficking, jasmonate signaling, and stomatal functions.
Bacterial T3SS and the Molecular Action of Effector Proteins
The bacterial T3SS delivers effector proteins into plant and mammalian cells to promote disease. Our early work revealed the secretion function and part of the supramolecular structure of the T3SS of P. syringae. More recently, we have shown that a key function of P. syringae effectors is to suppress plant basal immune responses. Our effort is now directed at (1) understanding how various effector proteins suppress host immune responses and (2) inhibiting the T3SS and effectors as a novel strategy for bacterial disease control.
We have identified the host targets of HopM1 in Arabidopsis. In particular, we have shown that HopM1 binds to the Arabidopsis ARF-GEF protein MIN7, a regulator of vesicle traffic (by activating the ARF family of GTPases). MIN7 is required for plant immune response. The physical interaction of HopM1 with MIN7 triggers the ubiquitination and subsequent degradation of MIN7 through the host proteasome (Figure 1). Our recent experiments show that both HopM1 and MIN7 are localized in trans-Golgi network (TGN)/endosome compartments. HopM1 also interacts with Rad23 proteins, which deliver ubiquitinated proteins to the proteasome. This finding suggests that HopM1 may hijack a putative host endosome ubiquitination/proteasome system to degrade MIN7. To identify additional components of the putative endosome-associated degradation machinery, we are using HopM1 and MIN7 as probes in protein complex trapping and purification.
Because of the central role of the T3SS in causing bacterial infections in plants and humans, there have been various efforts to inactivate this system as a broadly applicable strategy for bacterial disease control. We are looking into natural host defenses that could be aimed at the T3SS. As an alternative strategy, we are taking a transgenic approach to block the virulence function of effectors. This basic research on the T3SS and bacterial effectors may lead to innovative strategies for bacterial disease control.
The Innate Immune Function of Plant Stomata
Plant stomata are microscopic pores on the surface of all land plants; they are essential for exchange of CO2 gas and water vapor with the environment. As such, these pores are indispensable for plants to perform photosynthesis, the most important function of plants on earth. In the plant pathology discipline, it has long been assumed that stomata serve as passive portals of entry for plant pathogens, particularly bacterial pathogens. However, our recent work shows that plant stomata have an important immune function. Specifically, stomata close in response to plant and human pathogenic bacteria. Stomatal guard cells could perceive bacteria and pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors, such as flagellin receptor FLS2, activating a signaling cascade that requires the plant stress hormones salicylic acid and abscisic acid.
A newly discovered immune response, the signal transduction pathway underlying stomatal closure to pathogens, is poorly characterized. We are taking several approaches to increase our understanding in this area. First, we are investigating the epistatic relationships between various signaling pathways in the stomatal guard cell (Figure 2). Second, we are isolating Arabidopsis mutants, based on compromised stomatal response to P. syringae bacteria, to identify new signaling components involved in the pathogen-triggered guard cell immune response. Third, because stomatal opening and closing are also regulated by abiotic signals, such as humidity, temperature, and CO2 concentration, we are studying potential cross-talk between stomatal responses to abiotic and biotic signals. This may be particularly relevant to bacterial infections, as bacterial disease outbreaks often occur after rains and/or periods of high humidity. An interesting possibility is that stomatal immune response to pathogens may be compromised under disease-promoting weather conditions. Our ongoing experiments, in collaboration with Maeli Melotto (University of Texas at Arlington), will test this hypothesis.
Coronatine and Jasmonate Signaling in Disease
For many years, we have been interested in identifying the host target of coronatine, a P. syringae toxin. Coronatine shares striking structural similarities to the plant hormone jasmonate, which plays an important role in plant growth, development, and immunity. We have used coronatine as a molecular probe in the identification of key regulators (e.g., JAZ repressors) of jasmonate signaling and components of the jasmonate receptor complex. In these studies, we collaborate with Gregg Howe (Michigan State University), John Browse (Washington State University), and Ning Zheng (HHMI, University of Washington).
The identification of the JAZ repressor proteins (a total of 12 in Arabidopsis) is unleashing a wave of studies to define the entire jasmonate regulon and many roles of jasmonate signaling in development, growth, and immunity (Figure 3). We will continue to focus on the role of coronatine/jasmonate signaling in plant-pathogen interactions. Jasmonate signaling has opposite effects on pathogens of different lifestyles: it promotes infection of biotrophic pathogens (which multiply in living host tissues) but inhibits infection of necrotrophic pathogens (which live in dead host tissues). The molecular bases of these opposing effects are not known and remain a fundamental question. Our overall hypothesis is that different JAZ repressors interact with common as well as unique downstream transcription factors to specify common and varied downstream outputs based on specific external stimuli (Figure 3).
Grants from the National Institutes of Health and the Department of Energy provided partial support for these projects.
As of May 30, 2012