Microbial effector recognition in soybean
Direct or indirect interactions between host encoded resistance (R) protein and pathogen encoded avirulence (Avr) proteins induce signaling resulting in species-specific resistance termed effector triggered immunity (ETI). The avirulence proteins also enhance pathogen virulence on hosts lacking the cognate R protein. We hypothesize that the plant immune response is triggered upon sensing the consequence of effector-derived perturbations of a common set of key cellular proteins. The goal is to identify such key targets because their manipulation will enable the preferential induction of ETI irrespective of the host genotype (i.e. in hosts lacking specific R proteins). We utilize the soybean-Pseudomonas syringae pv. glycinea (Psg), soybean-soybean mosaic virus, and the soybean-Phytophhora sojae pathosystems to study these mechanisms. For instance, we have proteins that interact with Psg effectors and not only regulate resistance derived from cognate R loci, but also the virulence activities of those bacterial effectors in susceptible plants. Current efforts are directed towards understanding how pathogen effectors alter host immune responses to bacteria and viruses.
Functional genomics: soybean fatty acid/triacyl glyceolipid biosynthesis
Increasing evidence suggests a role for components of primary metabolism in regulating plant defense as well as microbial pathogenesis. It is quite likely that primary metabolic pathways in both plant and pathogen interface with disease-related signaling, since the pathogen relies upon host nutrients for survival and thereby successful pathogenicity.
Our studies in Arabidopsis have shown that the fatty acid oleic acid (18:1) is an important regulator of defense signaling. We have shown that reduced levels of 18:1 induce the expression of multiple R genes and result in the constitutive activation of defense signaling in Arabidopsis, soybean, and common bean. In addition, glycerol-3-phosphate (G3P) an obligatory precursor of all glycerolipid biosynthesis is also an essential regulator of plant defense. We showed that the ability to rapidly accumulate G3P is important for induced resistance against the hemibiotrophic anthracnose fungus, Colletotrichum higginsianum. Preliminary studies indicate that G3P regulates basal defense to several other pathogens in both Arabidopsis and soybean. We also showed that an unidentified derivative of G3P along with the lipid transfer protein DIR1 comprises a crucial mobile inducer of systemic immunity in Arabidopsis and soybean alike. We have characterized the defense-related functions of soybean genes encoding stearoyl acyl carrier protein desaturase, omega-3-fatty acid desaturases, as well as several enzymes involved in G3P metabolism. To fully characterize the connection between glycerolipid metabolism and defense signaling, we are involved in the functional characterization of metabolic activities regulating glycerolipid biosynthesis.
Plant symbiotic interactions
We are also studying the roles of defense components is mediating interactions with beneficial organisms such as rhizobacteria. In spite of being one of the more abundant elements on earth, nitrogen is often a limiting factor for plant growth. In order to be usable by plants, atmospheric nitrogen must be fixed to ammonia or nitrate. Biological nitrogen fixation is achieved by soil microorganisms that form symbiotic relationships with plants and is advantageous because it reduces the need for mineral fertilization. Soybeans (Glycine max [L.] Merr.), which can obtain 70-80% of their total nitrogen requirement from biological nitrogen fixation, have been reported to fix up to 337 kg N/ha in association with Bradyrhizobium japonicum. However, successful nodulation and biological nitrogen fixation depends on several factors including the genetic backgrounds of the plant host and the symbiotic microorganism. We find that conserved defense components regulate the specificity of soybean interaction with symbiotic rhizobacterial strains. Silencing genes encoding these components can eliminate specificity in nodulation such that these plants can be nodulated by rhizobacterial strains that are normally incompatible on that cultivar. The underlying processes are being studied.
Systemic acquired resistance
Systemic acquired resistance (SAR) is a unique form of induced immunity in plants that provides protection against a broad spectrum of secondary pathogens. SAR involves the generation of mobile signals at the site of primary infection, which then travel to systemic portions of the plant and prepare them for subsequent infections by related or unrelated pathogens. We are interested in understanding the mechanism of mobile signal generation and in particular the role of fatty acids and lipids in this pathway. We are also studying the role of the plant cuticle in the “perception” of the mobile SAR signal.