Sheng Yang He
Professor, HHMI-GBMF Investigator
Department of Plant Biology
Department of Plant, Soil & Microbial Sciences
Department of Microbiology & Molecular Genetics
Office: (517) 353-9181
Lab: (517) 353-9182
MSU-DOE Plant Research Laboratory
Michigan State University
Plant Biology Laboratories
Room 4200 Molecular Plant Sciences Bldg.
East Lansing, MI 48824
“An Arabidopsis Gene Involved in the Conversion of Aldehydes to Alkanes in Arabidopsis Leaf Wax,” 2011, with W Zeng, H Gao, and Y Li. Patent Pending
“Broad-spectrum disease resistance conferred by expression of the Pseudomonas syringae hrmA gene in plants,” 2001, US 6342654 B1, with Qing Shun Li, Songhai Shen and Arthur Hunt.
“Elicitor of the hypersensitive response in plants,” 1998, CRF D-1172A, with Steve Beer et al. This patent has led to the development of the disease control product “MESSENGERTM” sold by Eden Bioscience Corp.
“Pseudomonas syringae pv. syringae HrpZ gene,” 1997, CRF D-1425, with Alan Collmer and Hsiou-chen Huang
Molecular Biology of Plant-Pathogen Interactions
Overview of Our Research
Despite the ability to mount myriad immune responses, every plant or animal is still highly susceptible to numerous pathogens. Why? Answering this question is of fundamental importance in medicine and agriculture, as it holds a key to globally understanding infectious diseases in plants and humans. Ultimately, our goal is to answer two major questions:
How do microbial pathogens manipulate plants to cause disease? How can we use pathogenesis as a probe for discovering fundamental cellular mechanisms in eukaryotic cells?
We use a model pathosystem consisting of the host Arabidopsis thaliana and the bacterial pathogen Pseudomonas syringae for our study. In this model interaction, both the host and the pathogen are genetically and genomically tractable, making it a powerful 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 dozens of “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.
1. Bacterial effector proteins: a central role in promoting disease susceptibility
Over the years, we have studied a number of different T3SS effectors, but our current efforts are mostly focused on two highly conserved effectors that are central to the ability of P. syringae and many other bacterial pathogens to cause disease in plants: HopM1 and AvrE. Our overarching goals in this research area are (1) to understand why HopM1 and AvrE make such a critical contribution to promoting disease susceptibility in diverse bacterial diseases, and (2) to inhibit these two effectors as a potential general 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 AtMIN7, a regulator of vesicle traffic (by activating the ARF family of GTPases) that 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 (Fig. 1). Our recent experiments show that both HopM1 and MIN7 are localized in trans-Golgi network (TGN)/endosome compartments (Fig. 1). Remarkably, HopM1 also interacts with Rad23 proteins, which deliver ubiquitinated proteins to the proteasome. This finding leads us to suggest that HopM1 effector may hijack a putative endosome ubiquitination/proteasome system to degrade MIN7. We are attempting to identify additional components of the putative endosome-associated degradation machinery using HopM1 and MIN7 as probes in protein complex trapping and purification.
Figure 1. A diagram illustrating that P. syringae secretes about 30 effectors (in red) into the plant cell. Effector HopM1 manipulates components of a putative TGN/endosome-associated proteasome degradation machinery (in blue: E3 ubiquitin ligase; Ub, ubiquitin; Rad23; and 26S proteasome) to remove the ARF-GEF protein MIN7, resulting in dysfunctional TGN/endosomes, immune suppression, and disease.
2. Plant stomata: the immune function against human and plant pathogens
Plant stomata are microscopic pores on the surface of all land plants; they are absolutely essential for exchange of CO2 gas and water vapor with the environment. As such, these pores are indispensable for plants to perform the most important function on earth: photosynthesis. 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 (Fig. 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 is particularly relevant to bacterial infections, as bacterial disease outbreaks often occur after rains and/or periods of high humidity. An exciting hypothesis is that stomatal immune response to pathogens may be compromised under disease-promoting weather conditions. Our ongoing experiments will test this hypothesis.
Figure 2. A diagram of a signaling cascade involved in stomatal response to bacteria (left). Dashed lines indicate many missing components. On the right is an artistic illustration of bacterial invasion through a stoma, based on microscopic images. Stomata are composed of pairs of kidney-shaped guard cells.
3. Coronatine: A remarkable molecular probe of the jasmonate receptor and signaling
For many years, we have been fascinated by a bacterial toxin called coronatine, because its chemical structure shares striking similarities to the plant hormone jasmonate. Jasmonate is a lipid-derived hormone that plays a crucial role in plant growth, development, and immunity. On this research topic, we collaborate with our colleague, Gregg Howe, who studies jasmonate signaling, especially in the context of wounding and insect interactions. We have used coronatine as a molecular probe in the identification of key regulators (e.g., JAZ repressors) and components of the jasmonate receptor complex. In these studies we also collaborate with the laboratories of John Browse at Washington State University and Ning Zheng at the University of Washington.
The identification of the JAZ repressor proteins (a total of twelve in Arabidopsis) is unleashing a wave of studies to define the entire JA regulon and many roles of JA signaling in development, growth, and immunity (Fig. 3). We will continue to focus our attention on the role of coronatine/jasmonate signaling in plant-pathogen interactions, because this topic has been traditionally understudied and has particularly high potential for new discoveries. 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 (Fig. 3).
Figure 3. A diagram showing that the 12 JAZ repressor proteins control a variety of downstream responses, including plant disease and immunity in Arabidopsis. The crystal structure of the jasmonate receptor complex is shown at the center of the JAZ circle on the left.
4. Application of our basic research: Inactivation of the T3SS and effectors
We are beginning to devote significant effort to aiming our basic research at disease control. 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. We reason that plants are particularly suited for finding such defenses because of the numerous secondary metabolites they produce as part of the immune response.
As an alternative strategy, we are taking a transgenic approach to block the virulence function of effectors. Specifically, we are interested in blocking the virulence action of HopM1 and AvrE, as these two broadly conserved effectors make a particularly crucial virulence contribution to diverse plant diseases. Overall, we are excited about the possibility that our basic research on the T3SS and bacterial effectors will lead to innovative strategies for bacterial disease control, solving one of the most important challenges in agriculture and medicine.