Auxin-associated apoplastic barrier development influences cadmium uptake in a high cadmium-accumulating rice (Oryza sativa L.) line

Cadmium (Cd) contamination poses a serious risk to both rice production and human health. As a primary phytohormone governing plant development, auxin (IAA) plays important roles in regulating plant responses to abiotic stress, including Cd toxicity and uptake. However, the underlying regulatory mechanisms in rice genotypes with contrasting Cd accumulation remain unclear. In this study, we performed integrated transcriptomic, metabolomic, and anatomical analyses on a high Cd-accumulating rice line (Lu527-8) and a normal Cd-accumulating line (Lu527-4) under Cd stress. Exogenous IAA application significantly reduced whole-plant Cd accumulation by 27.63% in Lu527-8 and 8.09% in Lu527-4. In Lu527-8, the pronounced reduction in Cd uptake was associated with both enhanced development of apoplastic barriers (including Casparian strips and suberin lamellae, CSs and SL) and downregulation of the expression of key Cd transporter genes (OsNRAMP5, OsNRAMP1, OsIRT1, and OsCd1). In Lu527-4, downregulation of transporter gene expression was the dominant factor. Exogenous IAA increased root IAA concentration by 31.03% in Lu527-8 and 22.93% in Lu527-4. This difference was accompanied by divergent development of apoplastic barriers and a larger reduction in Cd accumulation in Lu527-8. Overall, these findings highlight genotype-dependent differences in IAA-mediated regulation of Cd uptake in rice and provide valuable insights into hormonal strategies for reducing Cd accumulation in rice.