Immunity is a double-edged sword: without it, the organism will succumb to infection; too much of it is detrimental to the growth and survival of the plant. The situation is even more perplexing in plants, as they do not have specialized immune cells. To protect the whole organism, immune responses have to be balanced with other biological functions in each cell. My main research goal is to understand the mechanisms of plant immune responses and the dynamics of their regulation. In this research, my laboratory uses modern genetic, genomic, and molecular approaches to study plant defense in Arabidopsis thaliana.
Compared to the sophisticated immunity in jawed vertebrates, the immune system of plants appears far less complex. However, using distinctly different defense strategies, plants are capable of mounting highly specific defense responses. Plants exposed to pathogens can develop a memory that lasts for weeks or longer. This memory enables the plants to mount a more robust response to subsequent stresses.
How do plants achieve these immune properties? At the cell surface, microbe-associated molecular patterns (MAMPs) are detected by pattern recognition receptors (PRRs) leading to MAMP-triggered immunity (MTI). Adaptive pathogens can normally overcome MTI by delivering effectors directly into plant cells. Even though these effectors are highly polymorphic among pathogen strains, they target a few critical host proteins to suppress MTI and enhance virulence. Perturbations of the host target caused by some effectors can be detected by the plant resistance (R) proteins, resulting in effector-triggered immunity (ETI). ETI, the major immune mechanism in plants, is characterized by programmed cell death at the site of attempted infection. Each plant genome encodes hundreds of such R proteins to "guard" these cellular (i.e., self) targets and to provide immunity against a wide range of effector specificities.
Activation of local ETI can often lead to systemic acquired resistance (SAR), which is a long-lasting broad-spectrum resistance. The plant hormone salicylic acid (SA) is a necessary and sufficient signal for the development of SAR. An increase in plant SA levels, either by de novo biosynthesis or by exogenous application, can trigger major transcriptional reprogramming and secretion of a large array of antimicrobial pathogenesis-related (PR) proteins.
The Molecular Mechanism of SAR and the Function of the Master Immune regulator, NPR1
The research of my laboratory has been focused on SAR. Through a genetic screen, we identified the mutant npr1(nonexpresser of PR genes 1), which fails to respond to SA and is compromised in SAR. Like NFκB/IκB in the mammalian immune system, NPR1 is a master regulator of SAR in plants. We first found that nuclear translocation is a key regulatory step for this protein. In the absence of pathogen challenge, NPR1 is retained in the cytoplasm as an oligomer through redox-sensitive intermolecular disulfide bonds.
Upon activation by SA, the NPR1 monomer is released to enter the nucleus. My lab recently discovered that nitric oxide and thioredoxin are the redox mediators of NPR1 translocation. Discovery of this novel regulatory mechanism provided insights into how pathogen-induced cellular redox changes lead to induction of immune responses in plants. In the nucleus, NPR1 serves as a cofactor to transcription factors, such as TGAs, to induce PR genes encoding antimicrobial proteins and endoplasmic reticulum (ER)-resident genes to ensure proper secretion of the PR proteins.
The presence of a BTB-domain in NPR1 suggested that NPR1 might interact with Cullin-3 E3 ligase and mediate substrate degradation. However, our research led to the surprising finding that the NPR1 protein itself is degraded by the proteasome. Before induction, unmodified NPR1 is degraded in the nucleus to dampen basal gene expression. After induction, NPR1 is phosphorylated at the IκB-like phosphodegron motif, ubiquitinylated, and degraded to stimulate target gene expression through accelerated recycling of the transcription initiation complex. Such a mechanism serves as a molecular switch for coactivator function.
Our future work on NPR1 will be focused on its nuclear function and regulation. We will characterize the NPR1 nuclear complex and determine how the stability and activity of this complex is dynamically regulated to control the transcriptional cascade during SAR.
The Interplay Between the DNA Repair Response, Cell Cycle Control, Ploidy, and Plant Immunity
To identify additional signaling components in SAR, we performed two rounds of suppressor screens. The first one identified sni1 as a suppressor of npr1. In the subsequent screen for suppressors of sni1, we unexpectedly found multiple components of the DNA damage response machinery, such as BRCA2 and RAD51. In humans, mutations in these genes are either lethal or lead to a predisposition to cancer. For example, BRCA2 is a mediator of RAD51 in pairing of homologous DNA. Mutations inBRCA2 result in a predisposition to breast and ovarian cancers. It is puzzling how a defect in a mechanism as general as DNA homologous recombination can lead to these specific cancers. In Arabidopsis, treatment with the DNA damage agent bleomycin primes the defense genes for induction by SA. Both brca2 and rad51 are hypersusceptible not only to genotoxic substances but also to pathogen infection. Strikingly, chromatin immunoprecipitation demonstrated that RAD51 is specifically recruited to the promoters of defense genes during SAR in an SA- and BRCA2-dependent manner.
The synergism between the plant DNA damage response and the plant immune response is surprising, but the implications are compelling. DNA damage response is the most fundamental and evolutionarily ancient stress response upon which other defense mechanisms could be built. It is well known that both biotic and abiotic stresses can trigger the release of reactive oxygen species (ROS). ROS, while serving as signals, can also cause collateral damage to the host genome. Based on this knowledge, we would like to test the following hypotheses: (1) DNA damage repair proteins are recruited to the defense gene promoters to induce chromatin remodeling and to safeguard against transcription-associated DNA instability. (2) Pathogen-induced DNA recombination facilitates generation of new R genes through DNA rearrangement in the R gene clusters, serving as a long-term survival strategy.
The Mechanisms by Which Plants Use the Circadian Clock to Regulate Plant Immune Responses
Recently my laboratory's research has taken on another new direction, resulting from a functional study of genes required for resistance against the downy mildew pathogen,Hyaloperonospora arabidopsidis (Hpa). Bioinformatic analysis of the promoters of the defense genes revealed a significant enrichment of the "evening element," which is regulated by the circadian clock component, CCA1. The involvement of the circadian clock was confirmed by the responses of various clock mutants to Hpa challenge. The infection was carried out at dawn, the time when Hpa spores are normally disseminated in nature. The cca1 mutant showed compromised resistance to Hpa, whereas a CCA1-overexpression line (CCA1OE) showed enhanced resistance.
We propose that regulation of the defense genes by the circadian clock allows plants to "anticipate" infection when the pathogen is expected and to minimize collateral damage to self. In future research, we would like to understand the regulatory circuitry between the circadian clock and immune regulators and to determine whether the clock plays a wide role in plant immunity.
This work is partially supported by grants from the National Institutes of Health and the National Science Foundation.
As of May 30, 2012