Boron deficiency hampers the productivity of 132 crops in more than 80 countries. Boron is essential in higher plants primarily for maintaining the integrity of cell walls and is also beneficial and might be essential in animals and in yeast. 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 bor1-1 is sensitive to boron deficiency. Uptake studies indicated that xylem loading is the key step for boron accumulation in shoots with a low external boron supply and that the bor1-1 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.

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(T.I., . Sciences, Kannondai, Tsukuba-city, borl-1 (high bomn requiring), an Arabidopsis thaliana mutant that requires a high level of B, was isolated. When the B concentration in the medium was reduced to 3 p~, the expansion of rosette leaves was severely affected in borl-l but not in wild-type plants. In a medium containing 30 p~ B the mutant grew normally but showed female sterility, whereas the wild type was able to set seeds.These defects of the borl-7 mutant were not detected with supplementation of 100 p M B. In vivo concentrations of B in borl-l mutants were lower than those of the wild type, especially in the inflorescence stems. Tracer experiments using "B suggested that the mutant has defects in uptake and/or translocation of B. The mutation was mapped on the lower arm of chromosome 2.Although B was established as an essential element in higher plants more than 70 years ago (for review, see Loomis and Durst, 1992), its mechanism of action is still poorly understood. B-deficient plants exhibit various visible symptoms and disorders. Effects of B deficiency in maize seedlings first appear as cessation of root growth followed by the collapse of meristematic regions, suggesting that B plays a key role in cell division (Kouchi and Kumazawa, 1976). A number of studies have been conducted on the physiological effects of B deficiency. These studies established that B deficiency causes various changes in properties such as membrane integrity and permeability, auxin metabolism, sugar transport, lignification in the cell wall, carbohydrate metabolism and transport, respiration (for review, see Loomis and Durst, 1992), and reduced fertility (Marschner, 1995). However, the primary defects caused by B deficiency in plants are still unclear.B is predominantly localized in the cell walls in tobacco and squash (Matoh et al., 1992;Hu and Brown, 1994).Recent findings showing that B in cell walls is bound to rhamnogalacturonan I1 (Ishii and Matsunaga, 1996;Kobayashi et al., 1996; ONeill et al., 1996) and that this complex is present in a wide range of plant species (Matoh et al., This work was supported in part by grants-in-aid from the Ministry of Education, Science, Sports, and Culture of Japan (no. 08760055 to T.F. and no. 06278102 to S.N.).* Corresponding author; e-mail atorufuah0ngo.ecc.u-tokyo.ac.jp; fax 81-3-5689-7226. S.N.); and National lnstitute of Agro-Environmental lbaraki 305, Japan (T.M., H.O.) 1996) suggest that it exerts its effects, at least partially, in cell walls. However, for B to be transported from roots into aerial portions of the plant through the xylem, it must cross the plasma membrane at or near the casparian strips. Studies at the cellular level established that the uptake of B occurs mainly by passive diffusion, although other mechanisms may also be involved (Raven, 1980;Brown and Hu, 1994). Translocation of B is also known to occur through transpiration streams, although severa1 observations suggest that B can also be mobilized through phloem (Brown and Hu, 1996). The detailed mechanism...

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