2011 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.
Obj 1. We have introgressed resistance to late blight, early blight, soft rot, Verticillium wilt, potato virus Y, and common scab from Solanum species into cultivated potato. Fourteen Solanum species have been evaluated for resistance to Verticillium dahliae and for sequence variation in the Ve gene, which confers resistance to V. dahliae. Sequence data are being used to identify additional resistant species. We have created diploid and tetraploid populations segregating for a dominant gene conferring resistance to potato virus Y. A manuscript describing early blight resistant cultivated x wild species hybrids has been submitted for publication. We have further characterized the molecular mechanisms governing broad-spectrum late blight resistance mediated by the RB gene and have identified host-pathogen interactions between host and pathogen proteins that mediate the resistance response. We have also found that continued exposure of the late blight pathogen to RB plants does not select for more aggressive pathogen strains.
Obj 2. We have evaluated amylose content in tuber starch from 39 Solanum species and 349 varieties. Tuber samples from land race clones were produced and are undergoing biochemical evaluations of flavor components. We have continued sensory analyses in conjunction with the U.S. Potato Board and have added additional taste panel evaluations of potatoes grown on organic farms.
Obj 3. We continued phenotypic evaluations of cultivated x wild hybrids in greenhouse and field studies to establish a relationship between acid invertase activity and resistance to cold sweetening. Two populations derived from elite cultivars are being genotyped to identify genetic loci linked to processing quality traits and to develop molecular markers that can be used to increase the efficiency of potato breeding programs. We have released germplasm of five tetraploid clones with resistance to cold sweetening. We are completing evaluation of diploid clones with resistance to cold sweetening in anticipation of another germplasm release. Sugar composition, carbohydrate metabolism enzyme activity, and gene expression in cold-stored tubers of S. pinnatisectum have been quantified. These tubers have low acid invertase activity, and this is consistent with their observed low glucose content. The critical role that invertase plays in cold-induced sweetening was demonstrated in lines of five chipping cultivars that had the gene for acid invertase silenced using molecular approaches. In all highly silenced lines, dramatic improvements in processing quality were observed.
Obj 4. An experiment describing the detrimental effects of transient heat stress on sugar composition and defect formation in chip potatoes is being repeated. An evaluation of seed tuber storage temperature regimes and gibberellin treatment on seed tuber vigor as determined by rates of emergence, canopy closure timing, and tuber yield was conducted and is being repeated. Responses of potato tubers to ethylene in the storage environment are being evaluated in terms of respiration rates, chip color, tuber sugars, enzyme activities and maintenance of processing quality.
Resistance to cold-sweetening. Most potatoes in the U.S. are harvested from the northern tier of states in the fall. However, these tubers are consumed throughout the year. Consequently, many are placed into storage bins after harvest. Storage at low temperatures is beneficial because it reduces spoilage and delays sprouting. Unfortunately, low temperature storage results in an accumulation of tuber sugars, a process referred to as cold-induced sweetening. Tubers that have undergone cold-induced sweetening give rise to dark-colored potato chips and fries that are unacceptable to consumers. Genes for resistance to cold-induced sweetening have been transferred into potato from wild potato relatives by ARS scientists from Madison, WI, and progeny have been evaluated over many years. Five potato clones resulting from such crosses that are highly resistant to cold-induced sweetening have been described and were released for use in as parents in potato breeding programs to improve the quality of cold-stored tubers. These clones are currently being used by potato breeders in Michigan, Colorado, North Dakota, Minnesota, Maine, Idaho, Washington, Wisconsin, and Maryland.
Cold-induced sweetening. In a project aimed at reducing the effects of cold-induced sweetening, ARS scientists in Madison, WI have shown that inactivating the product of the gene for vacuolar acid invertase results in light colored chips and fries following cold storage of tubers. This enzyme catalyzes the most critical step in the pathway that produces reducing sugars in potato tubers. Tubers with high levels of reducing sugars produce dark products when fried. The effectiveness of this approach was demonstrated in five cultivars of potato, including four cultivars in commercial production. In each case, when acid invertase activity was greatly reduced using molecular tools, tuber quality after low temperature storage was dramatically improved. These data clearly define a specific genetic target for improvement of potatoes that will benefit consumers and producers by reducing spoilage and improving tuber quality and the healthiness of finished potato products.
Potato late blight. Potato late blight is a persistent and destructive pathogen that costs growers up to $3 billion per year in yield loss and disease control costs. Most potato breeders are interested in the identification and incorporation of simply inherited genes that confer robust resistance. However, introduction of these genes into cultivated varieties typically leads to changes in the pathogen that allow it to overcome resistance. One promising new source of resistance is the gene called RB, which is derived from a wild potato species. RB confers resistance to a wide range of different strains of the pathogen that causes late blight, but has not yet been introduced into potato varieties. In order to determine whether RB can be quickly overcome by the late blight pathogen, ARS scientists from Madison, WI, performed a long series of late blight inoculations using plants containing RB. It is estimated that our laboratory experiments are comparable to 3-5 field seasons. We found that continued exposure of the plants to late blight did not lead to a breakdown in resistance. This information will be important for breeders interested in late blight resistant germplasm, but are worried that development of a new variety will be inconsequential if new strains of the pathogen evolve to overcome resistance. Our results indicate that introduction of the RB gene will be an important and reliable method to control this disease. Four potato varieties containing the RB gene have been developed at the University of Wisconsin. A potato breeder at Michigan State University, is using the RB gene in the development of varieties with late blight resistance. Also, BASF Global is using the RB gene in transgenic lines to be released in the EU. Other foreign countries, such as India and Guatemala, have used the RB gene for the development of transgenic varieties.
Chen, Y., Halterman, D.A. 2011. Phenotypic characterization of potato late blight resistance mediated by the broad-spectrum resistance gene RB. Phytopathology. 101(2):263-270.
Rodriquez, F., Ghislain, M., Clausen, A., Jansky, S.H., Spooner, D.M. 2010. Hybrid origins of cultivated potatoes. Journal of Theoretical and Applied Genetics. 121:1187-1198.
Bhaskar, P.B., Wu, L., Bethke, P.C., Vaillancourt, B., Jansky, S.H., Busse, J.S., Buell, R., Hamernik, A.J., Jiang, J. 2010. Suppression of the vacuolar invertase gene prevents cold-induced sweetening in potato. Plant Physiology. Available: http://www.plantphysiol.org/content/154/2/939.