Breeding biofortified potatoes to relieve micronutrient malnutrition
CIP scientists are developing potatoes with high micronutrient content as part of a broader effort to address micronutrient deficiencies, which disproportionately affect the health of rural women and children. Those efforts have resulted in new potato breeding populations of nutrient-dense candidate varieties with up to 32–45 parts per million (ppm) dry weight iron and 22–37 ppm dry weight zinc, depending on the variety and location where the crop is grown. This represents an increase from baseline levels of 20 ppm iron and 16 ppm zinc in most potato varieties. Assuming bioavailability of 15%, eating 200 grams per day of a potato variety from the high range of these populations would provide 30% of the estimated average requirement of iron for a woman of child-bearing age, compared with just 12% from currently available potato varieties.
Andean-type, biofortified candidate varieties with outstanding culinary properties from these populations are in the advanced stages of evaluation in Rwanda, Ethiopia and Peru, whereas higher-yielding, commercial-type candidate varieties will be distributed for testing in September 2017. Merideth Bonierbale, leader of CIP’s Genetics, Genomics and Crop Improvement Division, explained that these candidate varieties are the products of an intricate process that began with the identification of Andean landraces with relatively high mineral concentrations. This breeding population then underwent recurrent selection to increase iron and zinc levels, followed by crossing with parental lines bred for other traits – but still relatively high in iron or zinc content – in order to produce high-yielding, disease-resistant and resilient biofortified potatoes.
To facilitate selection for micronutrients, CIP researchers adapted X-ray fluorescence technology (XRF) to develop a rapid and accurate assay for estimating iron and zinc contents in tuber samples, in collaboration with the CGIAR Research Program on Agriculture for Nutrition and Health (A4NH) and HarvestPlus. Calibrations and external validations showed that XRF could be used to estimate iron and zinc in solid or freeze-dried potato samples with high precision and reproducibility. CIP subsequently trained researchers from the Rwanda Agricultural Board (RAB) and the Ethiopian Institute of Agricultural Research in field sampling, sample preparation and XRF methods at CIP in Peru and RAB in Rwanda. By late 2016, approximately 20,000 potato samples had been analyzed.
At the same time, CIP is using genotyping by sequencing (GBS) to accelerate and improve the accuracy of biofortification as part of the transition to next-generation breeding. CIP researchers formed a panel of 170 landrace potatoes with genetic variation for zinc and iron content, which was genotyped using GBS and planted for field tests at two locations in the Peruvian Andes: one site with adequate soil zinc (Huancayo) and the other with zinc-deficient soil (Huancaní). The experiments were part of an effort to evaluate the suitability of genome-wide association studies (GWAS) for identifying genes or genomic regions associated with high iron, zinc and vitamin C content.
By late 2016, approximately 20,000 potato samples had been analyzed.
The GWAS analysis, using a mixed linear model, identified four markers significantly associated with iron concentration and seven with zinc concentration. At the site with adequate soil zinc, the iron and zinc content of varieties were positively correlated and a marker on chromosome 8 was significantly associated with both traits. However, at the zinc-deficient site, the correlation for these traits was non-significant. Moreover, GWAS did not detect the marker on chromosome 8, which highlights the importance of maintaining adequate amounts of available zinc and iron in the soil for such studies.
Genetic markers such as those associated with iron and zinc on chromosome 8, and zinc on chromosome 1, are located near annotated genes that encode metal transporters. Researchers will further study these loci and validate their utility as candidates for marker-assisted selection in 2017. Once these markers are validated, breeders can use them to accelerate the development of new biofortified potato varieties and increase the predictability of such breeding efforts. Ultimately, these high-throughput methods for accelerating genetic gain for iron and zinc content will contribute to the development of biofortified potato varieties with the traits that consumers want and the resilience that farmers need.
Photo: Farmers in Ethiopia cultivating biofortified potato. G. Wgiorgis/Ethiopian Institute of Agriculture Research