Endocytosis and degradation of BOR1, a boron transporter ofArabidopsis thaliana, regulated by boron availability

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Abstract
Boron (B) is essential for plants but toxic when present in excess. Arabidopsis thaliana BOR1 is a B exporter for xylem loading and is essential for efficient B translocation from roots to shoots under B limitation. B translocation to shoots was enhanced under B limitation in WT but not in bor1-1 mutant plants. The enhanced translocation was suppressed upon resupply of high levels of B within several hours. Unlike a number of transporters for essential mineral nutrients, BOR1 mRNA accumulation was not strongly affected by B conditions. However, accumulation of a constitutively expressed BOR1-GFP fusion protein was elevated under conditions of limited B supply. Upon resupply of high levels of B, BOR1-GFP was degraded within several hours. These findings demonstrate that posttranscriptional mechanisms play a major role in regulation of BOR1 accumulation. Confocal laser scanning microscopy of root tip cells showed that BOR1-GFP is localized to the plasma membrane under B limitation. Shortly after B application, the protein was observed in dot-like structures in the cytoplasm before degradation. Colocalization studies of the fusion protein with an endocytic tracer FM4-64 and an endosomal Rab-GTPase Ara7 fused to monomeric red fluorescent protein suggested that BOR1 is transferred from the plasma membrane via the endosomes to the vacuole for degradation. These results establish that endocytosis and degradation of BOR1 are regulated by B availability, to avoid accumulation of toxic levels of B in shoots under high-B supply, while protecting the shoot from B deficiency under B limitation.