Genotype-dependent fortification of rice nodes dictates cadmium fate in grains: A synergistic barrier forged from cell wall and amino acid chelation

Abstract

Cadmium (Cd) accumulation in rice grains is critical for food safety, and nodes function as the central hub regulating Cd translocation to grains. However, the mechanisms for nodal Cd retention and the physiological basis for genotypic variation in this capacity remain unclear. This study investigated the nodal physicochemical characteristics of 11 genotypes (four inbred and seven hybrid) grown in Cd-contaminated fields. Nodes were identified as predominant organs of Cd sequestration in shoots, showing Cd concentrations 9.14–57.55 times higher than in internodes. Such a high retention is attributed to a "physicochemical synergistic barrier." Physically, Cd is sequestered in robust cell walls rich in pectin, hemicellulose, and dense functional groups. Chemically, nodes contain elevated levels of proteins and amino acids (notably aspartate and glutamate), forming an intracellular chemical barrier. Crucially, the efficacy of the barrier is genotype-dependent: in inbred rice, it functions as the rate-limiting step for Cd translocation, whereas in hybrid rice, the barrier is less restrictive despite substantial Cd sequestration. This study establishes the nodal physicochemical barrier as the core Cd retention mechanism, demonstrating that its genotype-dependent efficacy significantly contributes to the differences in grain Cd accumulation. Consequently, this research provides novel physiological targets for breeding low-Cd rice varieties.