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United States Department of Agriculture

Agricultural Research Service


Location: Vegetable Crops Research Unit

2009 Annual Report

1a.Objectives (from AD-416)
Objective 1: Develop adapted potato clones with enhanced resistance to major potato diseases. Sub-objective 1.A. Characterize the molecular genetic basis for late blight (Phytophthora infestans) resistance in the diploid potato species Solanum bulbocastanum. Sub-objective 1.B. Identify resistance genes/factors present within late blight resistant accessions of the diploid wild potato species Solanum verrucosum. Sub-objective 1.C. Develop adapted potato germplasm with high levels of resistance to the fungal pathogen Verticillium dahliae and determine the genetic basis of resistance. Sub-objective 1.D. Identify sources of resistance to early blight (Alternaria solani), common scab (Streptomyces scabies), and soft rot (Erwinia spp., aka Pectobacterium spp.), and introgress them into S. tuberosum. Objective 2: Evaluate exotic potato germplasm for flavor and nutritional components, and introgress valuable genes into the cultivated potato. Sub-objective 2.A. Identify major components of flavor in potatoes and determine the range of variation for those traits in exotic potato germplasm. Relate biochemical variability to sensory analysis data. Sub-objective 2.B. Assess the genetic variability in wild Solanum species for nutritional quality traits including starch composition, antioxidant capacity, and vitamin and mineral levels. Where valuable variation exists, determine the genetic basis of the trait and begin studies to introgress useful genes into the cultivated potato. Objective 3: Examine exotic potato germplasm for resistance to low temperature sweetening and introgress valuable genes into the cultivated potato. Objective 4: Characterize molecular, physiological and environmental parameters that are determinants of potato quality, especially seed vigor and tuber processing quality. Sub-objective 4.A. Characterize molecular and physiological changes that occur in potato tubers that cause, or are tightly linked to, the accumulation of reducing sugars. Sub-objective 4.B. Determine the genetic and physiological basis of tuber vigor across storage times and environments. Sub-objective 4.C. Characterize water relations and respiration of stored potato tubers and determine the range of variation for these parameters in wild and cultivated potatoes.

1b.Approach (from AD-416)
This project focuses on utilizing wild potato germplasm as a source of genes for traits important to potato improvement, such as disease resistance, flavor, nutritional quality, and low temperature sweetening. We will identify novel resistance to major potato diseases in wild relatives of the potato and introgress that germplasm into the cultivated potato. We will also use molecular genetics to characterize resistance mechanisms in wild species and hybrids with the cultivated potato (Objective 1). In addition, we will screen wild and cultivated potato relatives for flavor and nutritional quality traits, and introgress genes for these traits into the cultivated potato. In parallel, we will determine the biochemical components that can be manipulated to improve flavor and nutrition (Objective 2). We will carry out genetic studies to determine the genetic basis of cold sweetening in wild and cultivated potatoes at the diploid level, and introgress selected germplasm into the cultivated potato (Objective 3). In addition, we will carry out studies to determine the physiological basis of tuber vigor and tuber processing quality (Objective 4). Together, the completion of these objectives will lead to the development of potato cultivars that contain increased genetic diversity, require less management and are highly marketable, leading to increased revenue for the U.S. potato industry.

3.Progress Report
The late blight resistance phenotype mediated by the RB resistance gene is being characterized through cell staining, and analysis of defense gene transcription. RB-mediated partial resistance differs slightly from resistance mediated by genes conferring immunity in that it allows the pathogen to spread slightly. New experiments are being developed to address how to increase the resistance level of RB-containing plants. Other proteins involved in RB-mediated resistance are being identified though interaction screening and identification of differentially accumulated proteins. Some candidate proteins have been identified and we are in the process of characterizing their role in late blight resistance. Interactions between RB and target pathogen molecules involved in pathogenicity are also underway. Late blight resistance gene orthologs from S. verrucosum have been identified and putative genes for resistance to other diseases are being identified using a Bacterial Artificial Chromosome (BAC) library and PCR-based approaches.

Wild relatives of potato are being evaluated for disease resistance and quality traits, and selected plants are maintained as clones. They are being crossed to diploid cultivated potato and the hybrids are being evaluated for traits contributed by wild species, as well as adaptation, fertility, tuber type, and 2n gamete production. Crosses are being carried out to transfer desirable traits to the tetraploid level, where selected clones can be used by breeders as parents. A bridge cross method has been developed and is being used to incorporate genes from wild species that can not be directly hybridized to the cultivated potato. These species often contain exceptional disease resistance, nutritional quality, and processing quality traits. Inheritance and mapping studies are underway to determine the genetic basis of resistance to common scab and early blight. New disease resistance assays are being developed for Verticillium wilt and early blight.

The effects of heat and drought stress on processing quality of potatoes are being determined. Quality defects being targeted are sugar end defect in processing russets and stem end chip defect in chip stock. Measured responses include tuber water status, sugar content, and osmotic adjustment. Research on the effects of harvest date, vine desiccants and storage temperature profiles is being conducted to establish how these practices affect the vigor of potato seed and the respiration rate of stored potato tubers. Time to emergence and canopy closure has been found to vary with seed tuber harvest date and storage temperature. Respiration rates varied with time after harvest. Persistent effects of vine desiccation on post-harvest quality were observed. A research project was initiated to determine if ethylene gas accumulates in potato storages to the extent that it might degrade tuber processing quality. Data reporting gene expression changes in tubers stored at low temperatures are being analyzed.

1. Sugar end defect of potato tubers. Environmental stress during the growing season can produce quality defects in potato tubers that decrease the values of the harvested crop. Tuber responses to transitory heat stress, mild drought, or a combination of the two were examined. Stress at early tuber bulking was found to be much more detrimental to tuber quality than stress later in the growing season. Early water stress resulted in elevated tuber sucrose contents at the end of the stress period, and elevated, undesirable sugar contents at the stem end of tubers at harvest. Tubers did not show significant acclimation to water stress, and as a consequence even moderate water stress resulted in a loss of tuber growth-promoting pressure potential. These data provide guidance for growers of processing russets and suggest that careful attention to soil moisture status during early tuber growth is essential for the production of high quality tubers with minimal sugar end defect rates. The methods used are suitable for evaluating breeding lines and new cultivars for consistent tuber quality.

2. Evaluation of wild relatives of potato for tubers that produce light colored fried chips after cold storage. Cold storage of potato is highly beneficial because it reduces sprouting and minimizes spoilage. Commercial cultivars destined for potato chip production are not stored cold, however, because they accumulate sugars at low temperatures that cause dark, bitter tasting chips. Tubers from a diverse range of wild potato relatives were evaluated for chip color and sugar content. A small number of species were identified that would be beneficial for use in developing advanced lines for breeding chipping potatoes. Statistical analysis of these species demonstrated that substantial genetic variability existed within each species, and subsequent breeding efforts need to identify individuals within each species that have superior genetic characteristics. This research has the potential to improve the quality and value of potato tubers used for potato chip production.

3. Some wild relatives of potato are exceptional sources of disease resistance, nutritional quality, and processing quality traits, but they can not be crossed to the cultivated potato. We have developed a bridge crossing method in which these species are crossed to the wild species S. verrucosum. Then, the hybrids can be crossed to the cultivated potato. We have demonstrated the feasibility of the system, identified effective S. verrucosum parents, and determined which wild species are most amenable to the bridge crossing method. Now, we are crossing S. verrucosum to wild species clones selected for disease resistance, resistance to cold sweetening, and high amylose content in tuber starch. Hybrids will be selected for these traits and then crossed to the cultivated potato.

4. The potato late blight resistance gene RB is distinct from most plant R genes in that it confers resistance to a broad spectrum of pathogen isolates. It also confers partial resistance instead of immunity to late blight. We have used cell biology and molecular techniques to better describe the resistance phenotype mediated by RB. Through comparison with the isolate-specific R gene R9, we have found that RB mediates the induction of host responses similar to isolate-specific genes but that the partial resistance phenotype is likely due to the timing and intensity of these responses. While RB-containing plants are still considered resistant, they allow the pathogen to spread beyond the initial site of inoculation. We have also found that the pathogen molecule recognized by RB to elicit resistance is wide-spread among all P. infestans isolates tested and is highly conserved in structure. Therefore, the RB gene should be quite effective against most P. infestans isolates.

Review Publications
Hamernik, A.J., Hanneman Jr, R.E., Jansky, S.H. 2009. Introgression of Wild Species Germplasm with Resistance to Cold Sweetening into the Cultivated Potato. Crop Science. 49:529-542.

Last Modified: 4/19/2014
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