Spinach is an accumulator of cadmium (Cd) and an important crop in the California Salinas Valley, where some agricultural soils have naturally high levels of Cd. Soil Cd content is a major factor determining Cd uptake by crops, and recent studies highlight cases of “worrying Cd concentrations” in some leafy vegetables grown in high Cd soils. Cadmium is a rare but toxic element, and consumption of contaminated foods is a main source of chronic human exposure. The US does not set standards for Cd content in domestic foods; however, international standards limit total Cd content in spinach and other foods for international trade. Accordingly, strategies used by California farmers to reduce the Cd content of spinach are to not grow spinach on high Cd soils, and on soils with moderate levels of Cd, fertilizing with zinc which reduces Cd uptake. Another tool to help growers reduce Cd uptake by spinach is the use of cultivars that take up less Cd.
Breeding for low Cd accumulation is a promising approach since it has been successful in wheat, rice and other species. With support from the California Leafy Greens Research Board, our UC Davis spinach breeding program and collaborators aimed to 1) quantify the genetic variation in Cd accumulation among existing spinach genotypes, and 2) identify candidate genes that regulate Cd accumulation as a first step in the development of molecular markers for low Cd accumulation. Our long-term goal is to develop spinach cultivars with low Cd accumulation when grown in high Cd soils.
We developed an effective method to screen spinach for genetic variation in Cd accumulation in the greenhouse using low seed quantity, which is useful for early breeding cycles in cultivar development. Screenings were conducted using high Cd field soil (2.8 ppm Cd) and Cd content of harvested spinach was determined in the lab by Inductively Coupled Plasma Atomic Emission Spectrometry. Using this method, we screened wild genotypes, landraces, advanced breeding material, and named varieties obtained from international germplasm collections, covering a wide range of genetic diversity including 615 spinach accessions from 42 countries of origin. Observed Cd content varied widely among screened genotypes, ranging seven-fold from 3.4 to 24.2 ppm (on a dry weight basis). Some of the observed variation can be attributed to environmental variation. By adjusting to environmental differences and experimental blocks, the remaining variation in Cd content can be attributed to genetic variation among screened genotypes. Predicted genetic variation in Cd content ranged from 6.9 to 11.3 ppm, showing there is genetic potential to breed for low Cd accumulation in spinach. Low and high Cd accumulating genotypes with consistent performance were identified and will serve as a resource for the UC Davis spinach breeding program. Our next steps include field testing of low Cd accumulating genotypes and crossing into elite breeding lines.
Progress was also made identifying candidate genes in spinach for Cd accumulation and low Cd phenotype. We are developing marker assays for screening germplasm accessions to identify alleles in genes previously targeted for breeding low Cd wheat and rice. Initial testing showed alleles at multiple markers in two candidate genes may have an association with Cd content, suggesting that Cd accumulation may be partially controlled by these genes in spinach. Our study paves the way for introgression of the low Cd trait into elite spinach breeding material and provides candidate genes for further exploration of Cd regulation in spinach.