Improved tolerance to boron deficiency by enhanced expression of the boron transporter BOR2

Abstract

<div><p></p><p>Boron (B) cross-links the pectin polysaccharide rhamnogalacturonan II (RG-II) and thus is important for cell wall structure in plants. B deficiency is an agricultural problem that causes significant losses of crop productivity worldwide. To address this, B deficiency-tolerant plants have been generated using B transporters. With the goal of further improving plant tolerance to low-B conditions, we generated transgenic <i>Arabidopsis thaliana</i> (L.) Heynh. with enhanced expression of BOR2, a B transporter that promotes cross-linking of RG-II and root elongation under low B supply. We generated a DNA construct containing the cauliflower mosaic virus 35S RNA promoter, a native promoter of <i>BOR2</i>, a <i>BOR2</i> gene and green fluorescent protein (GFP) (<i>Pro35S-BOR2:BOR2-GFP</i>), and obtained three independent transgenic lines with relatively high levels of BOR2-GFP expression. In the transgenic lines, BOR2-GFP was expressed mainly in the lateral root caps in the meristem zone and in the epidermal cells in the elongation zone, similar to its expression when driven only by its native promoter. In the <i>Pro35S-BOR2:BOR2-GFP</i> lines, BOR2-GFP was also expressed in various cells in the maturation zone of roots and epidermal cells of shoots, where the expression was hardly detectable in <i>ProBOR2:BOR2-GFP</i> lines. The transgenic lines were cultured under various B concentrations on solid media and it was found that root growth of three lines and shoot growth of two lines were enhanced compared to wild-type plants under low-B conditions. This finding established that enhanced expression of BOR2 leads to improved root growth under low-B conditions. We also examined growth of the transgenic lines under hydroponic conditions. One of the three lines showed better growth and fertility under a low-B condition, while the wild type did not set seeds under the same condition, suggesting the potential utility of BOR2 expression in agricultural applications.</p></div