Roles of BOR2, a Boron Exporter, in Cross Linking of Rhamnogalacturonan II and Root Elongation under Boron Limitation in Arabidopsis

• Published in: Plant physiology (Bethesda) 2013-12, Vol.163 (4), p.1699-1709
• Main Authors: Miwa, Kyoko, Wakuta, Shinji, Takada, Shigeki, Ide, Koji, Takano, Junpei, Naito, Satoshi, Omori, Hiroyuki, Matsunaga, Toshiro, Fujiwara, Toru
• Format: Article
• Language: English
• Subjects: Anion Transport Proteins - metabolism; Arabidopsis - drug effects; Arabidopsis - genetics; Arabidopsis - growth & development; Arabidopsis - metabolism; Arabidopsis Proteins - metabolism; Biological and medical sciences; Biological Transport - drug effects; Borates; Boron; Boron - pharmacology; Cell growth; Cell membranes; Cell Size - drug effects; Cell walls; Dimerization; Dimers; DNA, Bacterial - genetics; Fundamental and applied biological sciences. Psychology; MEMBRANES, TRANSPORT, AND BIOENERGETICS; Models, Biological; Molecular Sequence Data; Mutagenesis, Insertional - genetics; Mutation - genetics; Organ Specificity - drug effects; Pectins - metabolism; Plant Epidermis - cytology; Plant Epidermis - drug effects; Plant physiology and development; Plant roots; Plant Roots - drug effects; Plant Roots - growth & development; Plant Roots - metabolism; Plants; Plants, Genetically Modified; Root growth; Saccharomyces cerevisiae - drug effects; Saccharomyces cerevisiae - metabolism; Subcellular Fractions - drug effects; Subcellular Fractions - metabolism; Transgenic plants
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.113.225995

Boron (B) is required for cross linking of the pectic polysaccharide rhamnogalacturonan II (RG-II) and is consequently essential for the maintenance of cell wall structure. Arabidopsis (Arabidopsis thaliana) BOR1 is an efflux B transporter for xylem loading of B. Here, we describe the roles of BOR2, the most similar paralog of BOR1. BOR2 encodes an efflux B transporter localized in plasma membrane and is strongly expressed in lateral root caps and epidermis of elongation zones of roots. Transfer DNA insertion of BOR2 reduced root elongation by 68%, whereas the mutation in BOR1 reduced it by 32% under low B availability (0.1 μM), but the reduction in shoot growth was not as obvious as that in the BOR1 mutant. A double mutant of BOR1 and BOR2 exhibited much more severe growth defects in both roots and shoots under B-limited conditions than the corresponding single mutants. All single and double mutants grew normally under B-sufficient conditions. These results suggest that both BOR1 and BOR2 are required under B limitation and that their roles are, at least in part, different. The total B concentrations in roots of BOR2 mutants were not significantly different from those in wild-type plants, but the proportion of cross-linked RG-II was reduced under low B availability. Such a reduction in RG-II cross linking was not evident in roots of the BOR1 mutant. Thus, we propose that under B-limited conditions, transport of boric acid/borate by BOR2 from symplast to apoplast is required for effective cross linking of RG-II in cell wall and root cell elongation.