Investigating the genetics and physiological basis of differences in cadmium and zinc concentrations in tubers of potato (solanum tuberosum L): implications for food safety and biofortification
Mengist, Molla Fentie
University College Cork
Because Cd is a non-essential element with toxic effects on humans, while the other Group 12 transition element, Zn, is an essential micronutrient for human health, simultaneous selection of reduced Cd and increased Zn, if possible, would be an important objective of potato breeding. To develop cultivars with greater Zn and less Cd concentrations in the tubers, it is important to understand the physiological and genetics mechanisms that gives rise to differences in tuber Cd and Zn concentrations. To achieve these objectives, two potato cultivars, which accumulate larger tuber Cd and Zn concentrations (Lady Rosetta) or smaller tuber Cd and Zn concentrations (Cara), were used in this study. The patterns of Cd concentration, Cd content and dry weight accumulation of the two cultivars were examined at different stages of plant growth. The data suggest that differences in total Cd uptake and in Cd partitioning among organs are the mechanisms governing differences in tuber -Cd accumulation in the two cultivars. The cultivar exhibiting less concentrations of tuber Cd exhibited less root-to-shoot and shoot-to-tuber translocation, driven by larger root and shoot biomass which retained more Cd in roots and shoots, respectively, reducing its movement to the tubers. Greater remobilization and more efficient tuber loading was observed in the cultivar which accumulated more tuber Cd, indicating that remobilization of Cd from leaves to tubers was a major factor, not only in tuber-Cd loading, but also in the establishment of differences in tuber-Cd. Regardless of cultivar differences, the concentration of Cd in the tuber was very low compared to that in other organs, suggesting that, despite its large phloem mobility, Cd tends to be sequestered in the shoots. Like Cd, the two cultivars differed in Zn concentration, Zn content and dry weight partitioning between organs. The differences in tuber Zn concentration between the two cultivars were associated with differences in internal Zn distribution between stem and tuber. Lady Rosetta translocated more Zn to the tubers while Cara accumulated relatively more Zn in the stems. The greater stem Zn concentration of Cara was associated with greater stem biomass rather than preferential allocation of Zn to the stem. Further examination of reciprocal grafted plants and selected F1 genotypes derived from a cross between the two cultivars showed that partitioning between tuber and shoot was an important variable for explaining differences in tuber Cd and Zn concentration. A segregating population, comprising 188 F1 progeny derived from a cross between the two cultivars, was genotyped using the SolCap 8303 SNP array, and evaluated for Cd, Zn and maturityrelated traits. Linkage and QTL mapping were performed using TetraploidSNPMap software, which incorporates all allele dosage information. The final genetic map comprised 3755 SNP markers with an average marker density of 2.94 per cM. Tuber Cd and Zn concentrations were measured in the segregating population over two years. QTL mapping identified four loci for tuber Cd concentration on chromosomes 3, 5, 6 and 7, which explained genetic variance ranging from 5% to 33%, and five loci for tuber Zn concentration on chromosome 1, 3, 5 and, 6, explaining from 5% to 38% of genetic variance. Among the QTLs identified for tuber Cd concentration, three loci coincided with tuber Zn concentrations. The major effect QTL for both tuber Cd and Zn concentration coincided with the maturity locus on chromosome 5, where earliness was associated with increased tuber concentration of both elements. Coincident minor-effect QTLs for Cd and Zn, sharing the same direction of effect, were also found on chromosomes 3 and 6, and these were unrelated to maturity. The results indicate partially overlapping genetic control of tuber Cd and Zn concentration in the cross, involving both maturity-related and non-maturity-related mechanisms.
Cadmium , Zinc , Quantitative trait loci , Uptake , Partitioning , Grafting
Mengist, M. F. 2018. Investigating the genetics and physiological basis of differences in cadmium and zinc concentrations in tubers of potato (solanum tuberosum L): implications for food safety and biofortification. PhD Thesis, University College Cork.