- Agronomic: potatoes
- Crop Production: application rate management
- Pest Management: genetic resistance
Bacterial soft rot, which is caused by Pectobacterium, is the most important bacterial disease of stored vegetables. It is a significant problem on potato and other stored vegetables and it affects profitability and represents a loss of resources since all of the inputs used to grow a crop are lost if it rots in storage. In this project, I will characterize resistance genes from two wild potato species and determine if they may be suitable for use in a potato breeding program. Cultivated potato is an out-crossing tetraploid, and as a result, potato breeding is complicated and inefficient. In addition, because potato is grown through vegetative propagation, seed production is also complicated and inefficient compared to production of other types of vegetables, including the closely related tomato. Therefore, through this project, I will also contribute to an effort to assess the potential of inbred diploid potato lines for more sustainable potato production.
Project objectives from proposal:
Crosses derived from two soft rot resistant accessions of potato obtained from the USDA National Plant Germplasm Bank in Sturgeon Bay, WI that are resistant to an aggressive strain of Pectobacterium carotovorum will be characterized in this project. Accession 500036 of Solanum microdontum is stem resistant and accession 258856 of Solanum violaceimarmoratum is leaf resistant. Stems of S. microdontum can be cut and placed into Pectobacterium suspensions and completely resist decay, whereas all cultivated varieties (20 tested to day) and nearly all wild accessions tested (150 to date) decay within two to three days. Although it is not stem resistant, unlike most potato species, leaves of S. violaceimarmoratum resist DspE-mediated killing. Therefore, these accessions have two different types of resistance to soft rot disease, both which will useful for vegetable and ornamental production. Both species are diploid and can be crossed with diploid Solanum tuberosum.
Objective 1: Characterization of soft rot resistance from S. microdontum and S. violaceimarmoratum.
Develop populations required to map soft rot resistant genes in potato. To develop these populations, I will cross the stem resistant accession 500036 with a S. tuberosum x S. chacoense hybrid line (523-3) that carries the Sli gene (10). Sli is a single dominant gene that confers selfing on potato, therefore, the progeny from this cross will be able to self-pollinate. Importantly, the stems of 523-3 are susceptible to soft rot decay. The F2 progeny seeds will be planted and tested for resistance by taking stem cuttings and placing them into test tubes containing Pectobacterium carotovorum cells. Under these conditions, stems from most potato species decay within a few days, but stems of S. microdontum remain healthy, even in the presence of very high bacterial concentrations, often forming roots. I will assess the bacterial concentration required to cause disease in stems and the time to symptom development for at least 100 progeny in the F2 population. I will also assess tuber resistance to determine if it correlates with stem resistance to soft rot. These data determine if the resistance trait is conferred by one or more genes and whether the gene(s) are dominant or recessive.Additional appropriate crosses will be made to further assess the gene(s) and these crosses will be designed based upon the results from this initial F2 population.
I will cross the leaf resistant accession 258856 with 523-3 and generate an F2 population from this cross. Leaves of 523-3 are susceptible to P. carotovorum. I hypothesize that S. violaceimarmoratum is resistant to P. carotovorum because it lacks an R protein that recognizes the effector protein DspE. If this is true, then resistance will appear as a single recessive gene in the F2 population. Additional appropriate crosses will be made to further assess the gene(s) and these crosses will be designed based upon the results from this initial F2 population.
Determine if S. microdontum and S. violaceimarmoratum are resistant to other strains of Pectobacterium
Pectobacterium is a diverse genus, with four species causing decay on potato, worldwide (4, 8). To determine if these wild Solanum species are resistant to multiple Pectobacterium strains, I will inoculate them withdifferent Pectobacterium strains. These strains are already available in my advisors strain collection. Based on preliminary data, I anticipate that both accessions will have resistance to all of the strains tested. If they are susceptible to a subset of Pectobacterium strains, the extensive genomic information available for Pectobacterium may aid us in identifying the mechanism for resistance in these accessions (1, 2).
Determine if P. carotovorum colonize S. microdontum. There is no known mechanism for soft rot resistance in potato. S. microdontum resists stem decay, but culture-based assays show that it doesn’t kill the bacteria in the inoculum nor do S. microdontum extracts inhibit P. carotovorum growth. To explore how this species defends against soft rot, I will inoculate stems of S. microdontum, S. tuberosum, the 523-3 hybrid, and representative stems from my F2 population with P. carotovorum cells carrying a plasmid that encodes the green fluorescent protein (GFP). These cells efficiently invade and colonize the vascular system of susceptible potato. I will use fluorescence microscopy to determine if the bacteria can invade the vascular system of resistant lines or if their invasion or growth is limited. If they are able to invade the vascular system, studies on virulence gene expression would be designed. If they are not invading, my studies would focus on mechanisms plants use to restrict bacterial invasion.
Determine if soft rot resistance from S. microdontum and S. violaceimarmoratum is observed under field conditions. Potatoes, as well as most other crops, are exposed to many environmental factors that influence plant growth and disease resistance. The F3 and/or F4 populations developed in this project will be planted and evaluated in the field in the summer of 2014 to evaluate whether these plants resist soft rot decay under field conditions. The plants will be assessed for wilted and macerated leaves and stems and soft rot decay at harvest and during storage.
Objective 2: Assess the performance of diploid and inbred diploid Solanum lines with organic production methods.
Evaluate inbred diploid lines in organic field plot. Our long term goal is to develop inbred diploid potato lines suitable for sustainable potato production. Inbred diploid lines offer advantages, including ease of seed production and simplified genetics that allow swifter integration of useful traits into commercial germplasm. Tomato, which is closely related to potato, is an example of a successful inbred diploid Solanum crop. My advisor’s research includes assessment of numerous heirloom and new potato lines grown under organic conditions at the West Madison Agricultural Research Station to identify potato lines suitable for low input and organic production. I will generate enough seed of ten of the most promising lines, based on greenhouse production of tubers, developed in my work and evaluate them in comparison to commercially available tetraploid and diploid potato varieties to assess their positive and negative traits, such as yield, resistance to foliar and tuber blights, and resistance to insect pests.