Endocytosis and membrane trafficking are the major factors controlling the abundance of plasma membrane proteins, such as transporters and receptors. We have found that Arabidopsis borate transporter BOR1 is polarly localized to the inner (stele-facing) plasma membrane domain of various root cells under boron limitation, and when boron is supplied in excess, BOR1 is rapidly transferred to the vacuole for immediate degradation. The BOR1 polarity and degradation are controlled by membrane trafficking including endocytosis. In this chapter, we describe methods for observation of endocytic trafficking of BOR1, and detection of BOR1 ubiquitination that is required for vacuolar sorting for degradation. © 2014 Springer Science+Business Media New York.