Division of Plant Sciences
Genetic and molecular analysis of plant disease resistance.
Plants are continuously exposed to potential pathogens, yet most plants are resistant to most pathogens. Plants have evolved a surveillance system with similarities to the innate immune system of animals that detects invading pathogens. Specifically, plants express a large number of resistance genes that, directly or indirectly, interact with effector proteins of pathogens and trigger a plant disease resistance response. Pathogens in turn evolve to overcome this resistance, either by deleting or altering the detected effector proteins or directly counteracting the plant response. The co-evolution of host and pathogen is evident in the large number of pathogen strains within a given species expressing specific effector proteins, and in the large number of plant disease resistance genes. The genome sequence of Arabidopsis contains about 150 genes with homology to known disease resistance genes.
Research in my lab is aimed at understanding the function of this important class of plant genes. We are focusing on the Arabidopsis RPS4 gene specifying resistance to Pseudomonas syringae expressing the cognate avirulence gene avrRps4. RPS4 belongs to the TIR-NBS-LRR class of plant disease resistance genes. This class shows homology at the N-terminus of the predicted protein to known proteins involved in the animal innate immune response. Since avrRps4 constitutes the only cloned avirulence gene for an Arabidopsis TIR-NBS-LRR resistance gene, this system will allow us a detailed analysis of the TIR-NBS-LRR gene-dependent disease resistance signaling pathway. This involves structure/function analyses of the RPS4 gene product as well as genetic screens of randomly mutagenized Arabidopsis plants for mutants in the RPS4 signaling pathway. For example, we have isolated mutants in the naturally avrRps4-susceptible Arabidopsis accession RLD that are now specifically resistant to bacteria expressing avrRps4, but do not show general enhanced resistance. These mutants may define novel loci in the RPS4 signaling pathway that would not be identified in loss-of-resistance screens. Further, we are studying the Arabidopsis disease resistance response using electrophysiological techniques. We have identified a pattern of membrane depolarization that precedes plant defense-related cell death. These results will be a basis to understand the physiology of the plant disease resistance response in real-time.
Current trace recorded from an oocyte expressing AtOPT6. The response to increasing concentrations of extracellular glutathione is shown. Downward deflections of the current trace denote uptake of net positive charge. OPT transporters function as proton-coupled transporters.
My lab is also interested in the function of membrane transporters in various aspects of plant physiology other than disease resistance. In collaboration with the lab of Gary Stacey, we are characterizing the function of nine Arabidopsis oligopeptide transporters of the OPT class by functional expression in Xenopus laevis oocytes and two-electrode voltage clamping. This class of peptide transporters is hypothesized to function in the uptake of small peptides consisting of 4-5 amino acids. To date, we have succeeded in characterizing thetransport function of yeast ScOPT1 and two Arabidopsis OPTs, AtOPT4 and AtOPT6. Taken together, our results demonstrate that OPTs are proton-coupled oligopeptide transporters with broad, but distinct substrate specificities and affinities. Our work is the first electrophysiological characterization of this class of transporters from any organism.Identification of possible substrates for AtOPTs will allow us to develop hypotheses regarding the biological function of this class of transporters.