Role of overexpressed BOR4, a boron exporter, in tolerance to high level of boron in shoots

Abstract

High boron (B) tolerance is an important trait for crop production in high-B soils. We previously reported that overexpression of BOR4, an Arabidopsis thaliana B exporter, conferred high B tolerance in A. thaliana. This improvement appeared to be mainly due to the decreased B concentrations in roots through BOR4-mediated exclusion of B. In the present study, we describe a novel role of overexpressed BOR4 in shoots for high B tolerance. We compared shoot growth of wild type A. thaliana and transgenic plants moderately overexpressing BOR4 in relation to B concentration in shoots. It was found that growths and chlorophyll accumulation of shoots containing similar levels of B are better in the transgenic plants than the wild type. This finding established that BOR4-overexpressing plants are more capable of expanding leaves and accumulating chlorophyll in the presence of high B in shoot tissues. BOR4-GFP was found to be localized to plasma membrane in leaf epidermis cells. We suggest that overexpressed BOR4 alters B distribution in leaves by exporting B from the cytoplasm to the apoplast, leading to enhancement of high B tolerance in shoots.

Keywords:

• apoplast
• Arabidopsis thaliana
• boron exporter
• boron tolerance
• transgenic plant.

Acknowledgments

We thank Ms Mie Chiba for technical assistance and Dr Junpei Takano for critical reading. We appreciate the JSPS Research Fellowship for young scientists and the Hokkaido University leader development system in basic interdisciplinary research, granted to KM. This research was supported in part by a grant (Kiban S) from the Japanese Society for the Promotion of Science, and a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, from the Ministry of Agriculture, Forestry, and Fisheries of Japan (Genomics for Agricultural Innovation Grant IPG-0005) to TF.