Plants Tolerant of High Boron Levels

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Science  30 Nov 2007:
Vol. 318, Issue 5855, pp. 1417
DOI: 10.1126/science.1146634


Reduced crop productivity due to soils containing toxic levels of boron (B) is a worldwide problem in food production. It is estimated that up to 17% of the barley yield losses in southern Australia are caused by B toxicity. We found that the expression of AtBOR4, an Arabidopsis paralog of BOR1, the first identified boron transporter gene, generates plants that are tolerant of high B levels. BOR4 is a polarly localized borate exporter that enhances B efflux from roots. The present study is a foundation for the improvement of crop productivity in soils containing excess B, which are distributed in arid areas of the world.

Boron (B) is an essential nutrient for plants and animals, but in high concentrations it is toxic. Living organisms, including plants, must control the B distribution to maintain adequate levels of B in their cells. Reduced crop quality and yields in soils containing toxic levels of B are a worldwide problem in food production, especially in arid areas (1). By manipulating B transport, we have generated plants that are tolerant of high levels of B.

B accumulation occurs both naturally and through artificial means such as irrigation. About five million ha of soils containing greater than 15 mg kg–1 B, above the threshold for normal plant growth, exist in southern Australia, corresponding to 30% of the region ( Up to 17% of the barley yield loss in this area was estimated to be caused by B toxicity (2). B-tolerant cultivars with reduced B uptake are known for barley (3), but breeding programs have not yet produced a practical solution for B tolerance.

Arabidopsis thaliana BOR1, an efflux-type borate transporter, was the first B transporter identified in a biological system (4). BOR1 is required for the transport of B from roots to shoots under conditions of low B supply. BOR1 is capable of conferring high B tolerance to yeast by pumping boric acid out of the cell. However, in plants under high B conditions, BOR1 is degraded via endocytosis (5), and overexpression of BOR1 does not improve plant growth in the presence of toxic levels of B (6).

We focused on AtBOR4 (The Arabidopsis Information Resource code At1g15460 and GenBank code NM_101415), one of the six BOR1 paralogs present in the A. thaliana genome. B transport activities of BOR4 and BOR4–green fluorescent protein (GFP) fusion were confirmed in yeast (fig. S1). We generated seven independent transgenic A. thaliana lines producing the BOR4-GFP fusion under the control of cauliflower mosaic virus 35S RNA promoter.

Immunoblot analysis of a generated transgenic line showed that BOR4 accumulated in the presence of a high B supply (Fig. 1A), suggesting that BOR4 is exempt from the posttranslational BOR1 degradation system.

Fig. 1.

Improved growth in transgenic A. thaliana plants overproducing BOR4 under conditions of boron toxicity. (A) Immunoblot analysis of transgenic plant line 4 carrying Pro35S-BOR4-GFP. Plants were exposed to 0.1 μM (–B), 30 μM (+B), or 3 mM (++B) boric acid for 3 days, and root microsomal proteins were subjected to immunoblotting with an antibody against GFP. (B) Transgenic plants grown for 17 days on solid medium containing 10 mM boric acid. Scale bar indicates 10 mm. (Inset) An enlargement of the wild-type plant (scale bar, 1 mm). WT, wild-type Col-0; L4, L5, L12, transgenic lines 4, 5, and 12, respectively, carrying the Pro35S-BOR4-GFP construct. (C) Total B concentrations in shoots and roots. Plants were grown on solid medium containing 3 mM boric acid for 18 days. Means ± SD are shown (n = 5 to 6). Asterisks indicate significant difference (P < 0.001) by Student's t test. DW, dry weight. (D) GFP fluorescence in roots of transgenic plants carrying the ProBOR4-BOR4-GFP construct. Plants were grown on solid medium containing 30 μM boric acid for 10 days. Scale bars, 25 μm (left) and 100 μm (right). The left-hand image is a magnified view of the area inside the box in the right-hand image.

The supply of 10 mM boric acid was substantially lethal to wild-type plants, but much more vigorous root and shoot growth with varying degrees was observed in all the homozygous Pro35S-BOR4-GFP transgenic lines grown on solid medium containing 10 mM boric acid (Fig. 1B and fig. S2). Accumulation of BOR4-GFP and tolerance of B were positively correlated (fig. S2). The B concentrations in the roots and shoots of these transgenic plant lines were lower than that in the wild type in the presence of 3 mM boric acid (Fig. 1C). Overall tracer B uptake was also reduced in the transgenic line 4 (fig. S3). These results suggest that the overproduction of BOR4-GFP improved growth under conditions of B toxicity through B efflux.

Furthermore, GFP fluorescence derived from BOR4-GFP was strongly detected in the plasma membranes of the distal sides of epidermal cells in the elongation zone of roots of the transgenic lines carrying ProBOR4-BOR4-GFP (Fig. 1D). The distal localization of BOR4 is likely important for the directional export of B from the roots to the soil to prevent the accumulation of B in the xylem and growing cells. This enhanced B efflux from the roots of crop plants is expected to result in improved crop productivity in the B-toxic soils found in a number of regions of the world.

Supporting Online Material

Materials and Methods

Figs. S1 to S3


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