Location: Vegetable Crops Research2010 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
Progress was made on all four objectives. In Objective 1, we have identified resistance in wild Solanum species to late blight, early blight, soft rot, Verticillium wilt, potato virus Y, and common scab. Selected clones have been introgressed into the cultivated potato and being screened for disease resistance and parental quality. A Verticillium wilt molecular marker is being evaluated for its effectiveness in a wide array of germplasm. This marker resides within a putative resistance gene and we are isolating and testing this gene using transgenic plants. Work is also underway to develop a molecular marker for potato virus Y resistance. We have further characterized the molecular mechanisms governing broad-spectrum late blight resistance mediated by the RB gene and have identified interactions between host and pathogen proteins that mediate the resistance response. In Objective 2, we have evaluated wild Solanum relatives for amylose content in tuber starch. We have identified clones that range in amylose content between 18% and 37%, compared to about 25% in the cultivated potato. Genetic studies are underway. Collaborations to carry out the biochemical evaluations of flavor components are being explored. We have expanded our sensory analyses to include potato chips, in conjunction with the U.S. Potato Board. In Objective 3, we have created seven populations of cultivated x wild hybrids and are using them for genetic studies as well as evaluations of the relationship between acid invertase activity and resistance to cold sweetening. We demonstrated the central role of vacuolar acid invertase in cold-induced sweetening resistance using plants with very low rates of invertase gene transcription that were generated by either molecular biology or conventional breeding approaches. A manuscript describing this work was submitted for publication in summer 2010. We have also prepared a germplasm release of five tetraploid clones with resistance to cold sweetening. An evaluation of wild Solanum species for resistance to cold-induced sweetening was finalized and a manuscript describing that work submitted. Species enriched for individuals with desirable chip quality were identified and relationships between reducing sugars and chip color presented. Sugar composition of S. pinnatisectum (pnt) tubers in low temperature storage was found to be atypical and we have shown that pnt responds to low temperature storage by accumulating sucrose but not glucose. Finally, in Objective 4, we have used microarray expression data to develop a model for transcriptional changes that occur during cold-induced sweetening and which are likely to contribute to sucrose and reducing sugar accumulation. The effects of transient water and heat stress on sugar-end defect formation were documented in a published manuscript. A manuscript describing how vine-kill treatment and time of tuber harvest affect post-harvest tuber respiration rates and tuber processing quality was published. An evaluation of seed tuber storage temperature regimes on rates of emergence, canopy closure timing, and tuber yield was conducted and is being repeated.
1. Resistance to potato late blight disease. 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. ARS researchers in Madison, Wisconsin have determined that the partial resistance phenotype is due in part to suppression of RB resistance by effector proteins from the pathogen that interact directly with the RB resistance protein within the plant cell. Specific amino acid mutations within the pathogen effector protein determine whether they interact with the RB protein and elicit or suppress resistance. An increase in diversity of effector proteins that target RB is also correlated with increased aggressiveness of the pathogen. An understanding of the nature of the host-pathogen interaction in the development of late blight disease will allow plant breeders to more effectively develop potato cultivars with durable resistance to the disease.
2. Distribution of soft rot resistance genes. A widely accepted belief is that traits are associated with related organisms and that nearby populations of the same species are likely to be similar to each other. Consequently, taxonomic relationships and biogeographical data are commonly believed to have the power to predict the distribution of disease resistance genes among plant species. ARS scientists in Madison, Wisconsin tested predictivity claims for soft rot resistance in a group of widely distributed wild potato species. There was no clear association between resistance and taxonomic relationships. However, associations were identified between resistance and environmental data. In addition, resistance was mostly found in species with high levels of phenotypic plasticity. This research will guide breeders and geneticists when searching for useful genetic diversity in the potato gene bank.
3. Germplasm resistant to cold-induced sweetening. Potato tubers stored at temperatures less than 10°C accumulate glucose and fructose in a process referred to as cold-induced sweetening. Such tubers produce dark colored, bitter tasting chips and fries that have unacceptable amounts of acrylamide. One way to prevent these problems is to breed new cultivars with resistance to cold-induced sweetening. ARS researchers in Madison, Wisconsin have conducted an extensive evaluation of wild relatives of cultivated potato to identify species containing individuals with extreme resistance to cold-induced sweetening. We have also quantified sugar composition and asparagine content in tubers of these wild species. Because glucose, fructose and asparagine are substrates for the pigments that contribute to fried chip color and to the formation of acrylamide, these data provide timely guidance to potato breeders who want to minimize these problems by incorporating germplasm resistant to cold-induced sweetening into cultivated potato, and to food scientists seeking materials that will allow them to better model the contribution of each substance to chip color and acrylamide formation.
Halterman, D.A., Liu, Z. 2009. Analysis Of Proteins Differentially Accumulated During Potato Late Blight Resistance Mediated by the RB Resistance Gene. Physiological and Molecular Plant Pathology. 74(2):151-160.