TOL proteins mediate vacuolar sorting of the borate transporter BOR1 in Arabidopsis thaliana

ABSTRACTBoron (B) is an essential micronutrient for plants; however, it shows cytotoxicity at high concentrations. A borate transporter BOR1 is required for efficient transport of B toward the root stele in Arabidopsis thaliana. BOR1 shows polar localization in the plasma membrane of various root cells toward the stele-side under B limitation. To avoid over-accumulation of B, BOR1 in the plasma membrane is rapidly internalized and transported into the vacuole for proteolysis after high-B supply in an ubiquitination-dependent manner. Although BOR1 has been predicted to be transported into multi-vesicular bodies/late endosomes (MVB/LEs) via the endosomal sorting complex required for transport (ESCRT) machinery, experimental evidence was absent so far. In this study, we investigated the intracellular localization of BOR1 by visualizing endomembrane compartments, and tested the involvement of ESCRT-0-like proteins TOM1-LIKEs (TOLs) in the vacuolar sorting of BOR1. Under low-B conditions, a large portion of cy...