Cell-Type Specificity of the Expression of Os BOR1, a Rice Efflux Boron Transporter Gene, Is Regulated in Response to Boron Availability for Efficient Boron Uptake and Xylem Loading

Abstract

We describe a boron (B) transporter, Os BOR1, in rice (Oryza sativa). Os BOR1 is a plasma membrane-localized efflux transporter of B and is required for normal growth of rice plants under conditions of limited B supply (referred to as -B). Disruption of Os BOR1 reduced B uptake and xylem loading of B. The accumulation of Os BOR1 transcripts was higher in roots than that in shoots and was not affected by B deprivation; however, Os BOR1 was detected in the roots of wild-type plants under -B conditions, but not under normal conditions, suggesting regulation of protein accumulation in response to B nutrition. Interestingly, tissue specificity of Os BOR1 expression is affected by B treatment. Transgenic rice plants containing an Os BOR1 promoter-b-glucuronidase (GUS) fusion construct grown with a normal B supply showed the strongest GUS activity in the steles, whereas after 3 d of -B treatment, GUS activity was elevated in the exodermis. After 6 d of -B treatment, GUS activity was again strong in the stele. Our results demonstrate that Os BOR1 is required both for efficient B uptake and for xylem loading of B. Possible roles of the temporal changes in tissue-specific patterns of Os BOR1 expression in response to B condition are discussed.

Preferential Distribution of Boron to Developing Tissues Is Mediated by the Intrinsic Protein OsNIP3

“…A homolog of BOR1 in rice, OsBOR1, was reported as an efflux B transporter (Nakagawa et al, 2007), although it is not clear whether OsBOR1 is localized at the proximal side of exodermal and endodermal cells for cooperative transport with Lsi1. The accumulation of OsBOR1 transcripts was also not affected by B deprivation, but the protein seems to be enhanced (Nakagawa et al, 2007). Therefore, there is a possibility that the B uptake is regulated by OsBOR1 at posttranscriptional level.…”

“…In Arabidopsis, NIP5;1 and BOR1 are polarly localized at the distal and proximal side of the plasma membrane of the root cells, respectively (Takano et al, 2010), and their cooperative transport is required for efficient and directional uptake of B in the roots especially under a B-limited condition Reid 2014). Homologs of Arabidopsis NIP5;1 and BOR1 have been functionally characterized in other plant species such as rice (Oryza sativa), barley (Hordeum vulgare), rapeseed (Brassica napus), and maize (Zea mays; Nakagawa et al, 2007;Sutton et al, 2007;Chatterjee et al, 2014;Durbak et al, 2014;Hanaoka et al, 2014;Leonard et al, 2014;Zhang et al, 2017). Basically, they have a similar role in B uptake, although the expression patterns differ somewhat within plant species.…”

A Barley Efflux Transporter Operates in a Na+-Dependent Manner, as Revealed by a Multidisciplinary Platform

“…Despite a net boron concentration gradient directed into root cells, there is a favorable outwardly directed electrochemical gradient for borate anions that counteracts accumulation of boron in roots, thus reducing boron transport to the aboveground parts of the plant (Hayes and Reid, 2004). When At-Bor1, Os-Bor1, and Hv-Bot1 are expressed in Saccharomyces cerevisiae, they confer tolerance to high boron and reduce yeast intracellular boron concentrations (Takano et al, 2002;Nakagawa et al, 2007;Sutton et al, 2007). In barley roots, efflux of borate anions can be reduced by anion channel inhibitors and is pH dependent, where efflux no longer occurs at pH values approaching the pK a of boric acid (9.24) (Hayes and Reid, 2004).…”

“…For example, under conditions of limited boron supply to Arabidopsis thaliana, the At-Bor1 transporter is expressed specifically on the inner plasma membrane of pericycle and endodermal cells of the root, exporting boron to the xylem for transfer to shoots (Takano et al, 2010). Similarly, rice (Oryza sativa) Os-Bor1 is required for efficient xylem loading and shoot accumulation of boron (Nakagawa et al, 2007). In barley, the Bot1 transporter is highly expressed in roots of boron toxicitytolerant genotypes (Reid, 2007;Sutton et al, 2007) facilitating the efflux of borate anions from root cells.…”

“…Plant members of the AE family play important roles in tolerance to both high and low boron concentrations in soils (Takano et al, 2002;Nakagawa et al, 2007;Sutton et al, 2007;Pallotta et al, 2014). For example, under conditions of limited boron supply to Arabidopsis thaliana, the At-Bor1 transporter is expressed specifically on the inner plasma membrane of pericycle and endodermal cells of the root, exporting boron to the xylem for transfer to shoots (Takano et al, 2010).…”