Distribution and Speciation of Cu in the Root Border Cells of Rice by STXM Combined with NEXAFS.

Root border cells (RBCs) serve plants in their initial line of defense against stress from the presence of heavy metals in the soil. In this research, light microscopy and synchrotron-based scanning transmission X-ray microscopy (STXM) combined with near edge X-ray absorption fine structure spectroscopy (NEXAFS) with a nanoscale spatial resolution were used to investigate the effects of copper (Cu) upon the RBCs, as well as its distribution and speciation within the RBCs of rice (Oryza sativa L.) under aeroponic culture. The results indicated that with increasing exposure time and concentration, the attached RBCs were surrounded by a thick mucilage layer which changed in form from an ellipse into a strip in response to Cu ion stress. Copper was present as Cu(II), which accumulated not only in the cell wall but also in the cytoplasm. To our knowledge, this is the first time that STXM has been used in combination with NEXAFS to provide new insight into the distribution and speciation of metal elements in isolated plant cells.