Golgi-localized membrane protein AtTMN1/EMP12 functions in the deposition of rhamnogalacturonan II and I for cell growth in Arabidopsis.

Appropriate pectin deposition in cell walls is important for cell growth in plants. Rhamnogalacturonan II (RG-II) is a portion of pectic polysaccharides; its borate crosslinking is essential for maintenance of pectic networks. However, the overall process of RG-II synthesis is not fully understood. To identify a novel factor for RG-II deposition or dimerization in cell walls, we screened Arabidopsis mutants with altered boron (B)-dependent growth. The mutants exhibited alleviated disorders of primary root and stem elongation, and fertility under low B, but reduced primary root lengths under sufficient B conditions. Altered primary root elongation was associated with cell elongation changes caused by loss of function in AtTMN1 (Transmembrane Nine 1)/EMP12, which encodes a Golgi-localized membrane protein of unknown function that is conserved among eukaryotes. Mutant leaf and root dry weights were lower than those of wild-type plants, regardless of B conditions. In cell walls, AtTMN1 mutations reduced concentrations of B, RG-II specific 2-keto-3-deoxy monosaccharides, and rhamnose largely derived from rhamnogalacturonan I (RG-I), suggesting reduced RG-II and RG-I. Together, our findings demonstrate that AtTMN1 is required for the deposition of RG-II and RG-I for cell growth and suggest that pectin modulates plant growth under low B conditions.

Regulation, Diversity and Evolution of Boron Transporters in Plants.

Boron (B) is an essential trace element in plants, and borate cross-linking of pectic polysaccharide rhamnogalacturonan-II (RG-II) in cell walls is required for normal cell growth. High concentrations of B are toxic to cells. Therefore, plants need to control B transport to respond to B conditions in the environment. Over the past two decades, genetic analyses of Arabidopsis thaliana have revealed that B transport is governed by two types of membrane transport molecules: NIPs (nodulin-26-like intrinsic proteins), which facilitate boric acid permeation, and BORs, which export borate from cells. In this article, we review recent findings on the (i) regulation at the cell level, (ii) diversity among plant species and (iii) evolution of these B transporters in plants. We first describe the systems regulating these B transporters at the cell level, focusing on the molecular mechanisms underlying the polar localization of proteins and B-dependent expression, as well as their physiological significance in A. thaliana. Then, we examine the presence of homologous genes and characterize the functions of NIPs and BORs in B homeostasis, in a wide range of plant species, including Brassica napus, Oryza sativa and Zea mays. Finally, we discuss the evolutionary aspects of NIPs and BORs as B transporters, and the possible relationship between the diversification of B transport and the occurrence of RG-II in plants. This review considers the sophisticated systems of B transport that are conserved among various plant species, which were established to meet mineral nutrient requirements.

Roles of BOR2, a boron exporter, in cross linking of rhamnogalacturonan II and root elongation under boron limitation in Arabidopsis.

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.

Molecular mechanisms of boron transport in plants: involvement of Arabidopsis NIP5;1 and NIP6;1.

Understanding of the molecular mechanisms of boron (B) transport has been greatly advanced in the last decade. BOR1, the first B transporter in living systems, was identified by forward genetics using Arabidopsis mutants. Genes similar to BOR1 have been reported to share different physiological roles in plants. NIPS;1, a member of aquaporins in Arabidopsis, was then identified as a boric acid channel gene responsible for the B uptake into roots. NIP6;1, the most similar gene to NIPS;1, encodes a B channel essential for B distribution to young leaves. In the present chapter, recent advancement of the understanding of molecular mechanisms of B transport and roles of NIP genes are discussed.

Isolation of Arabidopsis thaliana cDNAs that confer yeast boric acid tolerance.

An Arabidopsis thaliana cDNA library was introduced into a Saccharomyces cerevisiae mutant that lacks ScBOR1 (YNL275W), a boron (B) efflux transporter. Five cDNAs were identified that confer tolerance to high boric acid. The nucleotide sequence analysis identified the clones as a polyadenylate-binding protein, AtPAB2; a ribosomal small subunit protein, AtRPS20B; an RNA-binding protein, AtRBP47c'; and two Myb transcription factors, AtMYB13 and AtMYB68. The expression of these five genes also conferred boric acid tolerance on wild-type yeast. Two yeast genes, ScRPS20 and ScHRB1, that are similar to the isolated clones, were necessary for this boric acid tolerance. The possible roles of these A. thaliana and S. cerevisiae genes in boric acid tolerance are discussed.