Barbara N. Kunkel

RPS2, an Arabidopsis disease resistance locus specifying recognition of Pseudomonas syringae strains expressing the avirulence gene avrRpt2

A molecular genetic approach was used to identify and characterize plant genes that control bacterial disease resistance in Arabidopsis. A screen for mutants with altered resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) expressing the avirulence gene avrRpt2 resulted in the isolation of four susceptible rps (resistance to P. syringae) mutants. The rps mutants lost resistance specifically to bacterial strains expressing avrRpt2 as they retained resistance to Pst strains expressing the avirulence genes avrB or avrRpm1. Genetic analysis indicated that in each of the four rps mutants, susceptibility was due to a single mutation mapping to the same locus on chromosome 4. Identification of a resistance locus with specificity for a single bacterial avirulence gene suggests that this locus, designated RPS2, controls specific recognition of bacteria expressing the avirulence gene avrRpt2. Ecotype Wü-0, a naturally occurring line that is susceptible to Pst strains expressing avrRpt2, appears to lack a functional allele at RPS2, demonstrating that there is natural variation at the RPS2 locus among wild populations of Arabidopsis.

Compartmentalized gene expression during sporulation in Bacillus subtilis

Two important features of endospore development in Bacillus subtilis--the compartmentalization of mother cell gene expression and the coordination of mother cell gene expression with forespore development--are governed by the highly regulated expression of the sigK gene, which encodes the mother cell-specific RNA polymerase sigma factor sigma K. Compartmentalized expression of sigK is associated both with a chromosomal DNA rearrangement and with the restriction of sigK transcription to the mother cell. A third mode of sigK regulation, which occurs at the level of activation of the sigK gene product by proteolytic processing, serves to couple gene expression between the mother cell and forespore compartments of the developing sporangium.

Identification of an Arabidopsis Locus that Governs Avirulence Gene-Specific Disease Resistance

We are using a molecular genetic approach to identify and characterize plant genes that control resistance in Arabidopsis thaliana to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst). A screen for Arabidopsis mutants that are altered in their ability to express resistance to Pst carrying the bacterial avirulence gene avrRpt2 has been initiated. Four fully susceptible and one partially susceptible mutants have been isolated to date and are now being further characterized. Most progress has been made in the characterization of the fully susceptible mutant D203. Mutant D203 is altered specifically in its ability to recognize bacteria expressing avrRpt2, as it retains resistance to bacteria carrying other avirulence genes. Susceptibility in mutant D203 is due to a defect at a single locus mapping to chromosome 4. Identification of a resistance locus in Arabidopsis with specificity for a single bacterial avirulence gene suggests that this locus, designated RPT2, may control the specific recognition of bacteria expressing avrRpt2. In a second approach we are taking advantage of the natural variation that exists among wild isolates (ecotypes) to study the genetic basis of disease resistance in Arabidopsis. Two ecotypes, Wii-0 and Po-1, that are susceptible to Pst strains expressing avrRpt2 have been identified. Genetic analysis of Wii-0 indicates that this ecotype lacks a functional allele of RPT2, demonstrating that there is natural variation at the RPT2 locus. Genetic analysis of ecotype Po-1 suggests that resistance to Pst strains expressing avrRpt2 may be controlled by more than one locus.