Radial transport difference mediated by root endodermal barriers contributes to differential cadmium accumulation between japonica and indica subspecies of rice (Oryza sativa L.)

Although Cd concentration of grains is generally lower in japonica than in indica subspecies, the effects of root endodermal barriers on the subspecific differences in Cd accumulation in rice (Oryza sativa L.) are poorly understood. Here, we characterized the differences in endodermal differentiation between japonica and indica subspecies and their effects on Cd radial transport. Casparian strips (CSs) and suberin lamellae (SL) in japonica subspecies were initiated at the 6%- 7% and 21%- 27% position from the root tip, respectively, which were 65% and 26% earlier than in indica subspecies, respectively. The lignin/suberin content in japonica subspecies was 47%/42% greater than that in indica subspecies because of the higher expression of lignin/suberin biosynthesis-related genes (OsCASP1, OsPAL, OsCYP86A1 and OsKCS20). Cd exposure induced endodermal plasticity in both subspecies, but the changes in japonica were greater than in indica subspecies. The earlier formation of CSs/SL in japonica subspecies significantly restricted the flow of radial transport tracer to reach the xylem and decreased Cd influx into roots, that is, endodermal barriers inhibited Cd radial transport via both apoplastic and cell-to-cell pathways, thus decreasing the root-to-shoot transport of Cd in japonica subspecies. Our findings are beneficial for the genetic modification of rice with low-Cd-accumulating ability.