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.
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.
The overall objectives of this project were to characterize soft rot resistance from wild species and to assess the performance of diploid lines on organic fields.
Many wild potato accessions and crosses from wild potato accessions were tested for Pectobacterium resistance. We found that the best family for this work was a set of lines from a S. tuberosum X S. chacoense that segregates for stem rot resistance and we completed multiple greenhouse, field, and lab disease assays with plants from this family.
We also analyzed soft rot data from our organic field plots. Finally, we planted diploid potatoes in an organically-managed field plot at the West Madison Agricultural Research Station.
The results were variable among replications of our virulence assays, but we were able to identify lines that were consistently highly resistant and highly susceptible. It was clear from this work that resistance is not due to a single gene and we were unable to use my results to map genes in this family. Jenna contributed to development of more accurate assays to improve our ability to map these resistance genes.
We analyzed results from several years of field trials and found that Pectobacterium soft rot was not a significant problem in our organic potato production plots. Diploid potatoes were generated from tetraploid potatoes and were planted in an organically-managed field plot at the West Madison Agricultural Research Station. These tubers will be planted in 2016 for further analysis and results from this trial are not yet ready to be analyzed.
A timelapse video is included with this report that shows a virulence assay with two susceptible plants and one resistant plant from the family that we assayed. The plants were inoculated with Pectobacterium and recorded for two days.
Educational & Outreach Activities
My work was used in outreach presentations by my lab group in Wisconsin, Colorado, Montana, Idaho, and Ontario from 2014-2016.
A publication to be submitted to a peer-reviewed journal is in preparation.
A poster was presented at the Potato Association Meeting in Portland Maine, July 19-23 “Charkowski, A. O., J. M. Lind, Y. Wang, and S. H. Jansky. Differential responses of resistant and susceptible diploid potato to Pectobacterium.” that described these data.
The most important result from this work is that potato lines suitable for genomic, proteomic, and metabolomic analysis of Pectobacterium stem rot resistance were identified. A new student has taken over this project and is collaborating with an expert on proteomic and metabolomic analysis to identify proteins and metabolites associated with resistance. A recombinant inbred line population was generated from this F2 family by a collaborating lab and this RIL population will be screened for Pectobacterium resistance in 2016.
Areas needing additional study
These plants are currently being studied with genomic, proteomic, and metabolomic methods to identify genes, compounds, or proteins that are correlated with resistance. This information will be used to develop markers linked to soft rot and stem rot resistance genes.