Arabidopsis boron transporter for xylem loading

Abstract

Boron deficiency hampers the productivity of 132 crops in more than 80 countries1. Boron is essential in higher plants primarily for maintaining the integrity of cell walls2-4 and is also beneficial and might be essential in animals5 and in yeast6. Understanding the molecular mechanism(s) of boron transport is crucial for alleviating boron deficiency. Here we describe the molecular identification of boron transporters in biological systems. The Arabidopsis thaliana mutant borl-1 is sensitive to boron deficiency7,8. Uptake studies indicated that xylem loading is the key step for boron accumulation in shoots with a low external boron supply and that the borl-1 mutant is defective in this process. Positional cloning identified BOR1 as a membrane protein with homology to bicarbonate transporters in animals. Moreover, a fusion protein of BOR1 and green fluorescent protein (GFP) localized to the plasma membrane in transformed cells. The promoter of BOR1 drove GFP expression in root pericycle cells. When expressed in yeast, BOR1 decreased boron concentrations in cells. We show here that BOR1 is an efflux-type boron transporter for xylem loading and is essential for protecting shoots from boron deficiency.