Lateral root and root hair-mediated uptake of lead and cobalt in rice

Agricultural soil lead (Pb) and cobalt (Co) contamination threatens human health. As an important food for a large portion of the global population, rice ( Oryza sativa ) can act as a major dietary source of these toxic metals. Yet, the mechanisms governing Pb and Co acquisition by rice roots remain poorly characterized. Here, the contribution of lateral roots and root hairs to Pb and Co uptake was investigated using two rice mutants, including osiaa11 , which is defective in lateral root formation, and osrhl1 , which lacks functional root hairs. When grown hydroponically, osiaa11 exhibited markedly lower Pb and Co accumulation compared with the wild type (WT, cv. Kasalath), whereas osrhl1 showed no significant difference from WT. Reduced Pb and Co contents were also observed in osiaa11 shoots under soil-grown conditions. Expression of OsNramp5 , which serves as the principal transporter for Pb and Co uptake, was significantly down-regulated in osiaa11 but not in osrhl1 under both Pb and Co treatments. Because OsNramp5 is localized primarily to lateral roots rather than root hairs, transgenic rice lines expressing OsNramp5 under the root-hair-specific OsCSLD1 promoter were engineered. These transgenic lines displayed substantially enhanced Pb and Co uptake compared with non-transgenic controls. Collectively, these findings demonstrate that lateral roots, but not root hairs, are pivotal for Pb and Co acquisition in rice.