Using Wild Relatives of Potato to Illustrate Genetic Control Against the Pathogenic Bacteria Pectobacterium carotovorum

Bacterial soft rot, which is often caused by Pectobacterium, is the most important bacterial disease of stored vegetables. For this project, we used greenhouse, laboratory, and field-based trials of a diploid potato family that is segregating for Pectobacterium resistance. This family, which was developed by the Jansky lab, is an F2 family generated from two homozygous potatoes with sequenced genomes. I identified highly resistant and highly susceptible lines from this family. We then used a GFP-labeled P. carotovorum to monitor bacterial colonization of petioles from these plants. We found that the bacteria moved further distances in the xylem of susceptible plants than they did in resistant plants.

Introduction:

Pectobacterium is a gram-negative bacterial pathogen in the enterobacteriaceae bacterial family, which includes important plant and animal pathogens such as Erwinia, Escherichia, Salmonella, and Yersinia. Genetic, genomic, and biochemical studies demonstrate clearly that the main pathogenicity factor for Pectobacterium is the copious amounts of plant cell wall degrading enzymes secreted by these bacteria. These enzymes are required for the rotting and wilting symptoms caused by these pathogens. These pathogens also require many other virulence factors, including systems dedicated to iron acquisition, detoxification of antimicrobial compounds produced by plants, and the type III protein secretion system.

As with many other bacterial plant diseases, sanitation and exclusion are the only widely used and successful methods for control of bacterial soft rot. Use of virulence protein inhibitors has been proposed and some inhibitors have been identified, but they are not yet commercialized. Plant resistance is widely considered the best method for management of diseases caused by Pectobacterium, but resistance has not been widely employed even though Pectobacterium resistance has proven useful and durable in a some crops.

Plant resistance to Pectobacterium is likely dependent on multiple genes and both the presence/absence of genes and the timing of expression after bacterial infection likely affect resistance. In addition, multiple studies have shown little correlation between tuber and stem rot resistance, suggesting that different mechanisms provide resistance in different plant tissues. Soft rot resistance is heritable and thought to be controlled by both simply inherited and quantitative resistance loci. Data from multiple years must be evaluated to identify resistant potato lines because environment plays a large role in expression of resistance in potato. To date, we know little about mechanisms of soft rot resistance for any plant species.

            We identified several plant that appear to have different types of resistance to Pectobacterium, including accessions within S. chacoense, S. microdontum and S. violaceimarmoratum. Several crosses were made, but we had quite a bit of difficulty with sterility in the populations. We eventually put our effort entirely into an F2 population from and S. tuberosum X S. chacoense cross that is segregating for Pectobacterium stem rot resistance.