Cadmium isotope fractionation and gene expression evidence for tracking sources of Cd in grains during grain filling in a soil-rice system

Grain filling is the key period that causes excess cadmium (Cd) accumulation in rice grains. Nevertheless, uncertainties remain in distinguishing the multiple sources of Cd enrichment in grains. To better understand the transport and redis-tribution of Cd to grains upon drainage and flooding during grain filling, Cd isotope ratios and Cd-related gene expres-sion were investigated in pot experiments. The results showed that the Cd isotopes in rice plants were much lighter than those in soil solutions (Delta 114/110Cdrice-soil solution = -0.36 to -0.63 96o) but moderately heavier than those in Fe plaques (Delta 114/110Cdrice-Fe plaque = 0.13 to 0.24 96o). Calculations revealed that Fe plaque might serve as the source of Cd in rice (69.2 % to 82.6 %), particularly upon flooding at the grain filling stage (82.6 %). Drainage at the grain filling stage yielded a larger extent of negative fractionation from node I to the flag leaves (Delta 114/110Cdflag leaves-node I =-0.82 +/- 0.03 96o), rachises (Delta 114/110Cdrachises-node I =-0.41 +/- 0.04 96o) and husks (Delta 114/110Cdrachises-node I =-0.30 +/- 0.02 96o), and significantly upregulated the OsLCT1 (phloem loading) and CAL1 (Cd-binding and xylem loading) genes in node I relative to that upon flooding. These results suggest that phloem loading of Cd into grains and transport of Cd-CAL1 complexes to flag leaves, rachises and husks were simultaneously facilitated. Upon flooding of grain filling, the positive fractionation from the leaves, rachises and husks to the grains (Delta 114/110Cdflag leaves/rachises/husks-node I = 0.21 to 0.29 96o) is less pronounced than those upon drainage (Delta 114/110Cdflag leaves/rachises/husks-node I = 0.27 to 0.80 96o). The CAL1 gene in flag leaves is down-regulated relative to that upon drainage. Thus, the supply of Cd from the leaves, rachises and husks to the grains is facilitated during flooding. These findings demonstrate that the excess Cd was purposefully transported to grain via xylem-to-phloem within nodes I upon the drainage during grain filling, and the expression of genes responsible for encoding ligands and transporters together with isotope fractionation could be used to tracking the source of Cd transported to rice grain.

Automated summary

The filling of rice grains is a critical period for the accumulation of excessive cadmium (Cd), but uncertainties still exist in identifying the sources of Cd enrichment in grains. In this study, pot experiments were conducted to investigate the transport and redistribution of Cd to grains during drainage and flooding. The results showed that Cd isotopes in rice plants were lighter than those in soil solutions but heavier than those in Fe plaques. Fe plaque was identified as a potential source of Cd in rice, particularly during flooding at the grain filling stage. Drainage during grain filling led to a negative fractionation of Cd from various parts of the rice plant, while flooding facilitated the transport of Cd from leaves, rachises, and husks to grains. These findings highlight the importance of ligands and transporters genes, along with isotope fractionation, in tracking the source of Cd transported to rice grains.