The barrier to radial oxygen loss impedes the apoplastic entry of iron into the roots of Urochloa humidicola

Lack of O-2 and high concentrations of iron (Fe) and manganese (Mn) commonly occur in waterlogged soils. The development of a barrier to impede radial O-2 loss (ROL) is a key trait improving internal O-2 transport and waterlogging tolerance in plants. We evaluated the ability of the barrier to ROL to impede the entry of excess Fe into the roots of the waterlogging-tolerant grass Urochloa humidicola. Plants were grown in aerated or stagnant deoxygenated nutrient solution with 5 mu M or 900 mu M Fe. Quantitative X-ray microanalysis was used to determine cell-specific Fe concentrations at two positions behind the root apex in relation to ROL and the formation of apoplastic barriers. At a mature zone of the root, Fe was 'excluded' at the exodermis where a suberized lamella was evident, a feature also associated with a strong barrier to ROL. In contrast, the potassium (K) concentration was similar in all root cells, indicating that K uptake was not affected by apoplastic barriers. The hypothesis that the formation of a tight barrier to ROL impedes the apoplastic entry of toxic concentrations of Fe into the mature zones of roots was supported by the significantly higher accumulation of Fe on the outer side of the exodermis.

Automated summary

The study indicates that establishing a barrier to ROL can effectively prevent Fe from entering the roots, thereby enhancing the plant's tolerance to waterlogging. The exclusion of Fe at the exodermis with a strong barrier to ROL, while K uptake remains unaffected.