• Zinc (Zn)-deficient soils constrain rice (Oryza sativa) production and cause Zn malnutrition. The identification of Zn-deficiency-tolerant rice lines indicates that breeding might overcome these constraints. Here, we seek to identify processes underlying Zn-deficiency tolerance in rice at the physiological and transcriptional levels.
• A Zn-deficiency-tolerant line RIL46 acquires Zn more efficiently and produces more biomass than its nontolerant maternal line (IR74) at low [Zn]ext under field conditions. We tested if this was the result of increased expression of Zn2+ transporters; increased root exudation of deoxymugineic acid (DMA) or low-molecularweight organic acids (LMWOAs); and⁄or increased root production. Experiments were performed in field and controlled environment conditions.
• There was little genotypic variation in transcript abundance of Zn-responsive root Zn2+-transporters between the RIL46 and IR74. However, root exudation of DMA and LMWOA was greater in RIL46, coinciding with increased root expression of putative ligand-efflux genes. Adventitious root production was maintained in RIL46 at low [Zn]ext, correlating with altered expression of root-specific auxinresponsive genes.
• Zinc-deficiency tolerance in RIL46 is most likely the result of maintenance of root growth, increased efflux of Zn ligands, and increased uptake of Zn-ligand complexes at low [Zn]ext; these traits are potential breeding targets.