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ARS Home » Pacific West Area » Wapato, Washington » Temperate Tree Fruit and Vegetable Research » Research » Research Project #425150

Research Project: Potato Germplasm Improvement for Disease Resistance and Superior Nutritional Content

Location: Temperate Tree Fruit and Vegetable Research

2015 Annual Report


Objectives
Objective 1: Apply genetic analyses, metabolic engineering, and targeted metabolic profiling to elucidate genetic, molecular, and biochemical factors governing host disease resistance and accumulation of select phytonutrients and vitamins in potatoes. Sub-objective 1.A. Characterize molecular and biochemical factors that modulate phytonutrient content. Sub-objective 1.B. Characterize molecular and biochemical mechanisms involved in disease/pest resistance. Objective 2: Evaluate, breed, and release potato germplasm with increased amounts of phytonutrients, which are suitable for the processing and fresh potato market, as well as for niche markets. Objective 3: Identify and release germplasm or varieties with improved resistance to powdery scab, black dot, Columbia root-knot nematode, zebra chip, potato mop top virus, potato cyst nematode, and examine the role of micronutrients in host resistance to Verticillium wilt. Sub-objective 3.A: Nematodes: Focus on identifying and developing germplasm, including trap crops, that can provide superior control options for Columbia root-knot nematode or Potato Cyst Nematode. Sub-objective 3B: Soil borne pathogens: Develop superior germplasm or management options for soil borne pathogens including powdery scab, potato mop top virus, black dot and Verticillium wilt. Objective 4: Determine available host-plant resistance and epidemiological parameters, and develop diagnostic tests for emerging pests and pathogens of potato such as zebra chip.


Approach
Objective 1: We will utilize molecular physiology approaches, including measuring gene expression, enzyme activity and metabolite pools by hyphenated techniques. Structural genes and regulatory genes will be assessed using transient assays or stable transgenics. The phenylpropanoid pathway will be a focus. HQT expression will be reduced using RNAi. LCMS will be used to assess differences in phenylpropanoids between wild type and silenced lines and the expression of at least 10-20 phenylpropanoid genes measured by qPCR. Another gene targeted for silencing will be dihydroflavonol-4-reductase (DFR). LCMS and GCMS analysis will be used to examine how phenylpropanoid and primary metabolism is reprogrammed in plants with altered DFR metabolism. MYB transcription factors will be identified in silico based on phylogenetic and protein similarity with known transcription factors. Function will be assessed in transient and stable assays. Compounds that cause the hatching of potato cyst nematode eggs will be partially purified from root extracts using chromatographic methods. Objective 2: Tuberling populations will be assembled and grown two successive seasons in the Klamath Basin of Oregon in unreplicated plots. Promising material will be analyzed for carotenoids, anthocyanins, antioxidants, and a range of other metabolites to select clones with high phytonutrient content. Statistically the data will be analyzed as a mixed model with locations, clones and interaction as fixed effects and reps within locations as random effects. We will use molecular markers to characterize hybrids and assure that we intercross only duplex Zep1 hybrids. Objective 3: We will combine PVY extreme resistance and CRKN resistant germplasm. The genetic nature will be explored by determining segregation ratios in reciprocal crosses. Mitochondrial fingerprinting will be expulsed as a diagnostic genetic marker of the restored phenotype. Crosses will be made to select a less spiny version of Solanum sisymbriifolium for use as a PCN trap crop. A. rhizogenes will be used to attempt to make a version of the plant with greater root mass. Hatching assays will be used to screen for other plants that may be a superior PCN trap crop. Crosses will be made to generate potatoes with resistance to Black dot and Powdery scab and evaluated in field trials with a randomized complete block design with four replications and ten plants per replication. The crown and root will be scored for degree of galling and sclerotia. The effect of micronutrient supplements on Verticillium wilt resistance will be assessed in field and greenhouse trials. Macro and micronutrients will be applied in-furrow. Objective 4: Psyllids collected during the survey and additional insects collected in the Pacific Northwest will be subjected to high resolution melt (HRM) analysis of the cytochrome oxidase gene in order to differentiate genetic variants of the psyllid. Extracts will be tested by PCR methods reported in the literature at dilutions up to 1,000 to determine level of sensitivity and reliability of the various methods on different host plant tissues.


Progress Report
Objective 1 focuses on improving the nutritional value and disease resistance of potatoes to increase sustainability, profitability and food security. Potatoes are the third largest supplier of health-promoting dietary phenylpropanoids. We showed that the biosynthetic pathway for chlorogenic acid, the most abundant polyphenol in tubers, is mediated by the gene HQT and answered the question of whether tubers would have a more diverse phenylpropanoid metabolome if the phenylpropanoid pathway was rerouted away from chlorogenic acid. Silencing HQT had complex effects, which interestingly differed depending on the organ. Increased amounts of some other phenylpropanoids were synthesized, but overall the phenylpropanoid pathway was down-regulated, showing that attempts to increase or decrease specific compounds in the pathway may not be straightforward. Phenylpropanoids are also involved in plant disease resistance, and chlorogenic acid is a resistance factor for some sucking insects, but our work showed that it had no effect on psyllids, the vector that transmit zebra chip disease. Tubers are known to have only relatively low amounts of dietarily desirable phenylpropanoids called flavonols, which suggested potatoes do not have machinery to synthesize large amounts of flavonols. However, we showed that other potato organs, especially flowers, have over a thousand-fold higher concentrations of flavonols. Therefore potatoes do have the machinery to produce large amounts of flavonols and it should be possible to markedly increase tuber flavonols through traditional and molecular breeding. Plants used conserved mechanisms to resist disease. We showed that free radicals including nitric oxide and reactive oxygen species induce systemic acquired resistance (SAR) in plants that protects the plant against a broad spectrum of infections. Free radicals cause the plant to synthesize fatty acid derived azelaic acid, which in turn induces glycerol-3-phosphate, an inducer of SAR. Plant galactolipids were found to have unique roles in triggering SAR, with one acting via nitric oxide, and another via azelaic acid. For objectives 2 and 3, we worked closely in multi-state trials with other members of the Tri-State Potato Breeding Program, including university and industry personnel, in evaluating and developing superior potato varieties. We carried out field trials in two different locations to test for resistance to Tobacco Rattle Virus and Potato Mop-Top Virus and identified clones that have shown high levels of resistance for more than one year. We tested 200 progeny in the field that had been previously characterized by molecular markers for resistance to Potato Virus Y (PVY) and Columbia root-knot nematode. We were testing the hypothesis that disease resistance, as judged by markers, impairs the overall performance of progeny in a breeding program. Our conclusion is that there is no consistent evidence for this. We were also testing to see if the markers associated with Rmc1(blb) a single dominant gene for resistance to reproduction on potato roots by Columbia Root Knot Nematode provide some aid in selecting tuber resistance. Our conclusion is that it does not. Therefore we will need to search for a new marker to label the gene responsible for tuber resistance. Our screening identified a white-flesh russet potato advanced breeding line with unusually high amounts of dietarily desirable polyphenols, higher than what we’ve found before after screening hundreds of different white-flesh genotypes. Our group has particular interest in “specialty potatoes” and a yellow-flesh breeding line developed at our Prosser worksite was found to have exceptionally high levels of antioxidants that rivaled the amounts found in purple- and red-flesh potatoes. White and yellow potatoes have greater consumer acceptance, so this is an important development because it shows that it is possible for white and yellow potatoes to have amounts that rival red- and purple-flesh potatoes, despite the lack of anthocyanins. Methyl Bromide fumigation has been the major method used to combat the Potato cyst nematode (PCN) infestation in quarantined infested fields in Eastern Idaho, but this has become increasingly contentious, further increasing the need for alternatives. Perhaps the most promising option is to use Litchi tomato as a trap crop. The Animal and Plant Health Inspection Service (APHIS) is interested in using this in field trials, but there is no commercial seed source available. We produced 150 kg of seed which has been planted in growers’ fields in Eastern Idaho. We have been making pollinations between diploid and tetraploid Litchi tomato to determine the ease with which we can produce triploid hybrids which would be sterile and not permit seed to be left in the field. We are pursuing several selection programs to increase the yield per plant of seed and to accelerate emergence of the crop. We showed that the fruits of Litchi tomato contain good amounts of various phytonutrients, which could provide further incentive for growers to deploy this as a trap crop and fumigation alternative.


Accomplishments
1. Identification of germplasm with multiple resistances to virus. Three viruses common in potato, Potato Virus Y, Potato Mop-Top Virus and Tobacco Rattle Virus, cause necrotic symptoms in the tuber flesh rendering the product unusable. We made crosses incorporating various genetic sources and achieved a potentially commercializable long russet skin clone with resistance to all three viruses. This was done with the collaboration of scientists at the University of Idaho, Washington State University and Oregon State University. This clone would suffer less loss to these viruses and save growers money.

2. Phytonutrient analysis of Litchi tomato, a potato cyst nematode (PCN) trap crop. This quarantine pest is difficult to control with current methods including fumigation. The use of Litchi tomato as a trap crop could increase if it had additional value as a food crop. ARS researchers at Prosser, Washington, in collaboration with scientists at the University of Idaho, showed Litchi tomato contains good amounts of polyphenols, vitamin C and carotenoids. These findings could increase the use of this environmentally friendly option to manage PCN.

3. Identification of potato germplasm with high iron reductase activity in the roots. Iron is one of the micronutrients identified as a target for increase for one billion people who are at risk of iron deficiency anemia. ARS scientists in Prosser, Washington, measured the iron reductase in thirty clones. Interestingly, red-skinned potatoes clearly had higher iron reductase than white skinned potatoes. This work was done at the Prosser worksite in collaboration with the Mongolian Agricultural University by means of a visiting scientist. Utilization of high iron potato could lower the frequency of iron deficiency anemia among the world’s poorest people.

4. Rerouting phenylpropanoid flux in potatoes by suppressing chlorogenic acid biosynthesis. Phenylpropanoids have important roles in plant abiotic stress resistance, pathogen resistance and also in human health. Increased understanding of their regulation in potatoes can lead to plants with improved growth and increased dietary value. ARS scientists at Prosser, Washington, along with collaborators at Washington State University and the University of Georgia, suppressed chlorogenic acid biosynthesis in potatoes, which revealed complex effects on phenylpropanoid metabolism. This demonstrated how this important compound is synthesized by potatoes and identified key regulatory mechanisms that control the pathway. These discoveries create new ways to produce potatoes with optimal phenylpropanoid profiles for both plant and human health.


Review Publications
Brown, C.R. 2015. Russet Burbank, no ordinary potato. HortScience. 50:157-160.
Payyavula, R., Navarre, D.A., Shakya, R., Sengoda, V., Munyaneza, J.E., Swamy, P. 2015. Synthesis and regulation of chlorogenic acid in potato: Rerouting phenylpropanoid flux in HQT silenced lines. Plant Biotechnology Journal. 13:551-564.
Dinh, P., Zhang, L., Brown, C.R., Elling, A. 2014. Plant-mediated RNA interference of effector gene Mc16D10L confers resistance against Meloidogyne chitwoodi in diverse genetic backgrounds of potato and reduces pathogenicity of nematode offspring. Nematology. 6:669-682.
Bamberg, J., Moehninsi, M., Navarre, R., Suriano, J. 2015. Variation for tuber greening in the diploid wild potato Solanum microdontum. American Journal of Potato Research. 92(3):435-443.
Dinh, P., Brown, C.R., Elling, A.A. 2014. RNA interference of effector gene Mc16D10L confers resistance against Meloidogyne chitwoodi in Arabidopsis and potato. Phytopathology. 94:1098-1106.