Location: Vegetable Crops Research
2022 Annual Report
Objectives
Objective 1: Identify and maintain a set of wild potato plants, determine the DNA sequence of each, evaluate the distribution of genetic diversity among these wild potatoes, and use this information to guide breeders in developing improved potato germplasm.
Objective 2: Characterize the set of wild potato plants from Objective 1 for resistance to major potato diseases and pests, including late blight, early blight, Verticillium wilt, and Colorado potato beetle, and map these resistance traits to identify the genetic regions responsible for these traits.
Objective 3: Create hybrids between diploid cultivated potato and the set of wild potato plants from Objective 1, characterize these hybrids for plant and tuber traits, and provide the data to the breeding community to use in developing improved potato germplasm.
Approach
Objective 1: We have identified 10 wild diploid Solanum species with demonstrated utility in potato breeding. Within each species, we will choose 10 accessions for this project based on published resistance data, personal experience, and genebank passport data. Multiple individuals from each wild species will have their S-locus RNase alleles sequenced. Fertility of individuals will be assessed by assaying for pollen viability and production of berries with viable seeds. Disease and pest resistance screens will be carried out on multiple plants in each accession for which a specific resistance trait has been reported previously. Based on these data, twenty individuals from each species will be selected for SNP genotyping, detailed phenotyping and clonal maintenance.
Objective 2: Individual clones identified in Objective 1 will be characterized for resistance to major potato diseases and pests, including late blight, early blight, Verticillium wilt, and Colorado potato beetle. For each disease or pest, we will perform disease inoculations or beetle challenges that generate quantitative resistance scores using previously published methods. R-genes within each individual will be sequenced using RenSeq and the position of R-genes will be mapped to the potato genome.
Objective 3: We will create hybrids by crossing flowers of diploid cultivated potato with pollen from the 200 wild potato plants identified in Objective 1. Resulting hybrids will be characterized for plant growth and tuber traits including size, shape, color and yield. These phenotypic data will be shared with the potato breeding community to use in developing improved potato germplasm. Phenotypic data and genotypic data will be deposited into GRIN and the clones used for this research will be donated to the NRSP-6 potato genebank for use by others.
Progress Report
Objective 1: Multiple plants of 85 wild species accessions were screened for self-fertility and tuber appearance. Self-fertile plants and those with large tuber were retained.
Objective 1: Sequencing of the S-locus ribonuclease (RNase) gene in diverse wild species is underway. The S-locus RNase gene is a key determinant of self-fertility in potato. A gap in support personnel caused a delay in the progress of this project, but a new hire is making excellent progress on the project. An initial group of sequences has been collected and findings were used to refine the design of RNA capture probes. Additional sequence data are anticipated in FY 2022.
Objective 2. Tubers from wild species relatives of cultivated potato generated for us by the U.S. Potato Genebank were screened for soft rot resistance. These included 26 clones of Solanum brevicaule, 27 clones of Solanum candolleanum, 14 clones of Solanum microdontum, and 11 clones of Solanum verricosum. Multiple accessions per species exhibited soft rot resistance. Selected lines were rescreened to confirm resistance.
Objective 3: A total of 42 wild species clones were crossed to the cultivated diploid potato clone W4. Hybrid seed was successfully produced from each clone. Seed of each family are being retained to screen for fertility, disease resistance and tuber size.
Objective 3: Forty dihaploids, diploid clones produced from tetraploid starting material, were generated from potato varieties or evaluated for fertility. These dihaploids are potential parents for making additional hybrid families with wild species clones.
We found that many clones were infected with Potato virus X. We learned the method for removing viruses using heat and chemical treatments and are producing virus-free tissue culture plantlets of those clones.
Accomplishments
1. Screening of wild potatoes identifies new sources of late blight resistance. While most cultivated potato varieties are susceptible to the potato late blight, wild relatives of potato are an excellent source of resistance. Late blight, caused by the oomycete pathogen Phytophthora infestans, remains the most destructive disease of potato worldwide, causing annual losses estimated at up to six billion dollars. To incorporate resistance to late blight into new potato varieties most efficiently, sources of resistance must first be identified that confer single-gene resistance and are sexual compatibility with cultivated potato. ARS researchers in Madison, Wisconsin, received 384 accessions of 68 wild potato species from the U.S. Potato Genebank and screened them for resistance to the late blight pathogen Phytophthora infestans in a detached leaf assay. From this screen, 39 accessions were identified with strong disease resistance and another 33 accessions that contained a mixture of resistant and susceptible individuals. This was the first time that resistance to late blight was reported in Solanum albornozii, Solanum agrimoniifolium, Solanum chomatophilum, Solanum ehrenbergii, Solanum hypacrarthrum, Solanum iopetalum, Solanum palustre, Solanum piurae, Solanum morelliforme, Solanum neocardenasiI, Solanum trifidum and Solanum stipuloideum. These species will be a valuable source of novel late blight resistance for potato breeding.
Review Publications
Busse, J.S., Jansky, S.H., Agha, H., Schmitz Carley, C.A., Shannon, L.M., Bethke, P.C. 2021. A high throughput method for generating dihaploids from tetraploid potato. American Journal of Potato Research. 98:304–314. https://doi.org/10.1007/s12230-021-09844-1.
Vega Alfaro, A., Ramirez, C., Chavez, G., Lacayo, F., Bethke, P.C., Nienhuis, J. 2021. Flowering time and productivity of interspecific grafts between pepper species in contrasting high tunnel-sheltered and open-field production environments in Costa Rica. HortTechnology. 31(6). https://doi.org/10.21273/HORTTECH04904-21.
Vega Alfaro, A., Bethke, P.C., Nienhuis, J. 2021. Effects of interspecific grafting between Capsicum species on scion fruit quality characteristics. HortScience. 56(11):1347-1353. https://doi.org/10.21273/HORTSCI15948-21.
