Location: Temperate Tree Fruit and Vegetable Research
2018 Annual Report
Objectives
Develop or identify new breeding lines, germplasm and named cultivars with superior quality, disease and pest resistance, and nutritional value. This will involve collaborative and independent work by our three-person team using our respective expertise in potato breeding, molecular physiology and plant pathology. The three objectives below undertake complimentary approaches to germplasm improvement. Objective 1 involves largely breeding for targeted traits. Objective 2 seeks to determine basic mechanisms that govern trait expression. Objective 3 will develop new or improved methods to evaluate breeding lines and germplasm. We will work closely with the TriState Breeding Program, as we have for over 20 years.
Objective 1: Evaluate, identify, breed, and release potato germplasm with improved traits of interest, especially improved disease and pest resistance, and increased amounts of phytonutrients.
Subobjective 1A. Develop breeding lines, cultivars or identify germplasm with enhanced amounts of phytonutrients and visual appeal.
Subobjective 1B. Develop breeding lines, cultivars or identify germplasm with superior disease resistance with a focus on soil-borne diseases.
Objective 2: Characterize genetic, environmental, molecular, physiological, and biochemical factors that control accumulation of potato phytonutrients and mechanisms that lead to plant disease resistance, and use this knowledge to produce new superior potato cultivars.
Subobjective 2A: Determine mechanisms that mediate tuber phytonutrient expression.
Subobjective 2B: Increase information and develop methods with potential to be used for control of Potato Cyst Nematode (PCN) and for improved disease resistance.
Objective 3: Develop improved pathogen diagnostic techniques and phenotyping approaches that can be used for potato germplasm evaluation, development of host-resistance, and identification of emerging potato diseases.
Subobjective 3A. Identify and characterize emerging and evolving pathogens and pests in the Pacific Northwest.
Subobjective 3B: Characterize Tobacco rattle virus (TRV)-potato interactions to develop better detection methods and determine the relationship between viral titer, cultivar, symptoms and resistance.
Approach
1A. Germplasm will be intercrossed and progeny evaluated in the field. Replicated plots will be grown in successive years across multiple locations. Lines will be analyzed for carotenoids, anthocyanins, antioxidants, total protein, potassium and iron. Molecular markers will be used to characterize high carotenoid lines. Liquid chromatography mass spectrometry (LC-MS) will be used to quantitate phytonutrients. If germplasm does not provide the desired traits, we will import additional germplasm.
1B. Resistance to nematodes, viruses and fungi will be developed using resistant lines to make crosses and evaluating progeny in field trials. Selected clones will be evaluated under high disease pressure and molecular markers used for Meloidogyne chitwoodi breeding. If progeny have lower selection rates than expected the size of the initial population will be increased.
2A. Expression of structural genes and transcription factors in potatoes or organs that have low or high amounts of phenolics, are cold-treated, or wounded will be analyzed using reverse transcription quantitative polymerase chain reaction (RT-qPCR) and LCMS. We will use ribonucleic acid sequencing (RNA-seq) to generate transcriptomic data. Effect of environment on glycoalkaloids will be assessed by growing 13 genotypes in six locations and methanolic extracts from freeze-dried tubers analyzed by LCMS. If key genes are identified, resources will be redirected to apply this knowledge through precision breeding efforts.
2B. Potato cyst nematode (PCN) trap crop seed will be produced by sowing true seed directly into the soil at ¼ inch depth. Hatching factor purification will be tested on diverse High-performance liquid chromatography (HPLC) columns and fractions tested for activity. If a hatching factor is identified and quantitated by LCMS, increased resources will be directed.
3A. Samples from symptomatic plants will be collected. Grafting experiments will evaluate transmissibility. Established molecular tools will be used to detect any pathogens present. If targeting known pathogens does not identify a biological agent, primers that target unknown pathogens will be used. Psyllid involvement in beet-leafhopper transmitted virescence agent (BLTVA) will be tested using field and cage experiments. Development of improved diagnostic tools for BLTVA and Candidatus Liberibacter solanacearum (Lso) will be assessed using a single-tube nested PCR technique, RT-qPCR or Kompetitive allele-specific PCR. If unable to identify any known pathogen in a sample, next generation sequencing platforms will be used.
3B. Tobacco Rattle Virus (TRV) sampling methods will be evaluated for efficacy. Lines will be evaluated for resistance in field trials. PCR will be used to compare viral titer with symptom severity. Varieties will be exposed to TRV and differences in resistance/insensitivity and susceptibility compared. Daughter tuber symptoms and viral titer will be compared to mother tuber symptoms, viral titer, plant emergence, and daughter tuber yield. If TRV infection becomes sporadic, we will focus on the genotypes that were subject to sufficient disease pressure.
Progress Report
This report is for a new project which began in March of 2018 and builds upon discoveries from our previous Project, 2092-21220-001-00D, "Potato Germplasm Improvement for Disease Resistance and Superior Nutritional Content".
A crossing block was successfully implemented during the spring of 2018 by ARS scientists in Prosser, Washington, and thousands of seeds were collected for future field trials. Parental material for these crosses were chosen with an eye towards developing new potato varieties with improved resistance to the nematode Meloidogyne chitwoodi, and resistance to Potato virus Y (PVY), Tobacco rattle virus (TRV) and Potao mop top virus (PMTV). An additional emphasis was to make crosses designed to generate progeny with increased phytonutrient amounts and high tuber set or fingerling shape.
Work is underway to examine in-depth the factors that may predispose a tuber to be more or less susceptible to greening, which is becoming a trade issue in some markets. Tubers from different cultivars were light-treated, and biochemical and molecular markers are being analyzed in freeze-dried tuber tissue.
Work is ongoing to identify key regulatory elements of phenylpropanoid synthesis in potatoes. Potatoes can contain high amounts of these compounds, which have a wide-range of health-promoting effects in humans, and diverse roles in the plant including stress resistance. Phenylpropanoid metabolism and gene expression is being characterized using liquid chromotography-mass spectrometry (LCMS) and qRT-polymerase chain reactions (PCR) in different potato cultivars and tissues. Phenylpropanoids can also be involved in tuber discoloration, so an in-depth study is underway examining to what extent different types of internal tuber discolorations correlate with each other and various factors such as phenylpropanoids.
During the 2017 growing season in the Columbia Basin of Washington and Oregon, potato plants were identified with symptoms of crinkled, distorted leaves, purpling terminal leaves, and bumps or necrotic lesions along the stems. These symptoms were often seen in as many as 90 percent of the potato plants in a field and were present in varying degrees in all different potato cultivars. ARS scientists in Prosser, Washington, have been attempting to identify a pathogenic cause. Leaf terminals isolated from symptomatic field samples (48 samples from 7 different commercial or research plots) were grafted to healthy plants in the greenhouse to determine if systemic infection could occur. Two of these samples developed symptoms in the recipient plants, confirmed positive as Beet Leafhopper transmitted virescence agent (BLTVA) phytoplasma, the causal agent of purple top disease in the Columbia Basin. No other samples with purpling terminals, leaf distortion, or stem bumps showed transmission of symptoms to the recipient plant. Molecular analysis in the laboratory using conventional methods did not identify any pathogen directly associated with these symptoms. Overall, these results suggest that the cause of the 2017 symptoms in the Columbia Basin was not pathogenic in nature.
Potato tubers originating from Oregon were received from Animal Plant Health Inspection Service (APHIS) for the validation of Candidatus Liberibacter (Ca. L.) solanacearum infection, the causal agent of zebra chip disease. A single tuber showed classic zebra chip striped symptoms and tested positive by molecular analysis for Ca. L. solanacearum. Further analysis of three different Ca. L. solanacearum genes identified the bacterium as the Ca. L. solanacearum species, but did not identify the strain as haplotype A or B, the two haplotypes currently identified in solanaceous crops (ie. potatoes) in the U.S. Additionally, sequences of the three genes did not match up with the other three Ca. L. solanacearum haplotypes (C – E) that infect umbelliferous crops in Europe and Africa. The sequence and phylogenetic analyses of this Ca. L. solanacearum sample point to the identification of a new Ca. L. solanacearum haplotype, designated F, that infects potato. This finding suggests that three different Ca. L. solanacearum haplotypes may have similar hosts range, including potato, Solanum tuberosum.
Progress was made in determining what role, if any, the potato psyllid may have in the epidemiology of the potato purple top disease caused by BLTVA phytoplasma. Two separate greenhouse trials were conducted to determine if acquisition of BLTVA by the potato psyllid was occurring, and subsequently, if the potato psyllid could transmit the pathogen to a healthy host plant. In addition to this, wild-caught psyllids from the Columbia Basin in 2016 were extracted either individually or in bulk and tested for the presence of BLTVA. These experiments will determine if the potato psyllid could be a key component in the spread of phytoplasma in the Pacific Northwest.
Accomplishments
1. PMTV in certified seed potatoes. The potato mop-top virus (PMTV) is an emerging problem that causes internal tuber discoloration and impacts exports. ARS researchers at Prosser, Washington, extended previous findings by screening tubers from numerous certified seedlot trials planted by Washington State University scientists. Molecular analysis showed that approximately six percent of the tested seed lots contained PMTV, many of which were asymptomatic. These results suggest previously clean fields can become infected with PMTV originating from certified seed that is not currently required to be tested for the presence of this virus. PMTV can be extremely difficult to eradicate if introduced in a field containing the vector. These results emphasize the need for resistant cultivars and improved testing requirements for seed certification.
2. Development of a new potato cultivar. To protect grower profitability and satisfy consumer preferences, new potato varieties are needed with superior traits, including disease and pest resistance. Castle Russet is a newly released dual purpose potato cultivar with resistance to Potato virus Y, potato mop-top virus and corky ringspot. The initial cross was made by ARS scientists in Prosser, Washington, and the line was developed with collaborators in the Northwest Potato Variety Development program at Oregon State University, the University of Idaho and Washington State University. Castle Russet has greater resistance to Fusarium dry rot, common scab, and less internal and external defects than Russet Burbank. It has high late season yields with good appearance and culinary qualities and is suitable for both processing and fresh-market production. This new variety can reduce losses to pathogens, enhance yields and provide consumers with a high-quality potato.
