A tissue atlas of cadmium accumulation and the correlation with thiol-containing chelates in zucchini provide insights into cadmium partitioning and food safety.

Widespread heavy metal pollution in soils has posed serious threat to vegetable production and food security, yet little is still known about heavy metal accumulation and distribution in the majority of vegetable crops. Here, we report the generation of a tissue atlas of cadmium accumulation in zucchini (Cucurbita pepo var. Giromontial), a globally important cucurbit crop, based on two-season experiment with six genotypes grown under cadmium contaminated soils. Plant growth and development as manifested by biomass, flowering time and plant architecture were unaffected by 10 mg/kg cadmium treatment, but high level of cadmium enrichment was detected in all genotypes. Roots accumulated the largest amount of cadmium, whereas the cadmium concentrations in fruits was also considerable. The exocarps of fruits possessed nearly half of the total cadmium in fruits, rendering it a "hotspot" of safety risk. Measurement of the thiol-containing chelates revealed that concentration of GSH but not PCs was correlated with the cadmium concentration in subdivided fruit tissues, suggesting a mechanism of phloem-specific transportation of cadmium in the form of Cd-GSH. Based on the collective data, a tentative model describing the relationship between long-distance phloem transport and cadmium distribution in sink organs is proposed. The implications for food safety are discussed.