Overexpressing OsPIN2 enhances aluminium internalization by elevating vesicular trafficking in rice root apex

Aluminium (Al) sequestration is required for internal detoxification of Al in plant cells. In this study, it was found that the rice OsPIN2 overexpression line (OX1) had significantly reduced Al content in its cell wall and increased Al concentration in cell sap only in rice root tips relative to the wild-type (WT). In comparison with WT, OX1 reduced morin staining of cytosolic Al, enhanced FM 4-64 staining of membrane vesicular trafficking in root tip sections (0-1mm), and showed morin-FM 4-64 fluorescence overlap. Recovery treatment showed that cell-wall-bound Al was internalized into vacuoles via endocytic vesicular trafficking after removal of external Al. In this process, OX1 showed a higher rate of Al internalization than WT. Brefeldin A (BFA) interfered with vesicular trafficking and resulted in inhibition of Al internalization. This inhibitory effect could be alleviated when BFA was washed out, and the process of alleviation was slower in the cells of WT than in those of OX1. Microscopic observations revealed that, upon Al exposure, numerous multilamellar endosomes were detected between the cell wall and plasma membrane in the cells of OX1. Moreover, more vesicles enriched with Al complexes accumulated in the cells of OX1 than in those of WT, and these vesicles transformed into larger structures in the cells of OX1. Taken together, the data indicate that endocytic vesicular trafficking might contribute to Al internalization, and that overexpressing OsPIN2 enhances rice Al tolerance via elevated endocytic vesicular trafficking and Al internalization.