Dual roles of suberin deposition at the endodermal Casparian strip in manganese uptake of rice

Rice roots are characterized by having two Casparian strips (CSs) at the exodermis and endodermis, where transporters for mineral nutrients are expressed. However, the exact role of the CS in expression of the transporters and subsequent nutrient uptake is poorly understood. Here, we first investigated the role of the CS in manganese (Mn) uptake by using a rice mutant (oscasp1) defective in formation of the endodermal CS. Knockout of OsCASP1 resulted in decreased Mn uptake under limited Mn conditions, but increased Mn uptake at high Mn concentration. Immunostaining revealed that knockout of OsCASP1 did not affect the cell specificity of localization of two transporters (OsNramp5 and OsMTP9) required for Mn uptake, but decreased the protein abundance of these transporters at the endodermis regardless of Mn concentrations tested. Furthermore, we found that overaccumulation of suberin at the endodermis of the mutants suppressed the expression of two transporters; the expression of the two transporters was only observed in the endodermal cells without suberin deposition, but not in the cells with suberin deposition. Taken together, our results indicate that there are two roles for the CS in Mn uptake; maintaining normal expression of the transporters at limited Mn concentration and preventing Mn diffusion to the stele at high Mn concentration.