Boron (B) is an essential micronutrient for plants, but a high concentration of B is toxic. Typical B toxicity symptoms include necrosis of marginal leaves and inhibition of root elongation. Reduction of crop productivity has been reported in high-B contaminated soils, present especially in semi-arid areas in the world. We have previously reported that overexpression of BOR4, an Arabidopsis thaliana (L.) Heynh. efflux-type B transporter, conferred high B tolerance in A. thaliana. In the present study, we characterized physiological roles and expression patterns of endogenous BOR4 in A. thaliana. Decreased shoot growths and increased B concentrations in shoots were found in A. thaliana T-DNA insertion mutants of BOR4 under toxic levels of B supply, compared to the wild type plants. β-glucuronidase (GUS) staining in the transgenic ProBOR4-GUS plants was predominately detected in root meristems and endodermis of mature portion of the roots. Furthermore, mRNA levels of BOR4 in roots were increased by 2-fold upon 3-day treatment of the high B condition and analysis of the transgenic ProBOR4-GUS plants showed that this high B-dependent induction is controlled by the 5' flanking sequences of BOR4 ORF. We concluded that endogenous BOR4 is a high-B inducible gene that functions in high-B tolerance.

Regulating the transport of mineral nutrients in plants is critical for maintaining homeostasis (Marschner, 2012). Plants control mineral transporter expression in response to mineral availability at various stages (Aibara and Miwa, 2014). Although transcriptional regulation of such transporters has been studied intensively, there are few reports of posttranscriptional regulation, especially translational regulation. Boron (B) is an essential micronutrient, as borate cross-links pectic polysaccharide in the primary cell wall (O'Neill et al., 2004; Marschner, 2012); however, it is toxic to plants when present in excess concentrations (Nable et al., 1997; Reid et al., 2004). B concentrations in soil solutions can be changed by natural events, such as leaching out after rainfall and accumulation after drying of topsoil water, and possibly via the decomposition of organic matter (Shorrocks, 1997; Argust, 1998; Park and Schlesinger, 2002). In soil solutions, B exists mainly as uncharged boric acid, which readily permeates cell membranes (Dordas et al., 2000). Under these circumstances, plants use different types of B transporters to cope with the narrow optimum range of B concentrations (Yoshinari and Takano, 2017). Under B deprivation, the NODULIN26-LIKE INTRINSIC PRO-TEIN5;1 (NIP5;1) and BOR1 genes are expressed in roots to support the effective translocation of B in Arabidopsis (Arabidopsis thaliana). NIP5;1 is a boric acid channel expressed mainly in epidermal cells that facilitates boric acid uptake from soil into root cells (Takano et al., 2006, 2010). BOR1 is an efflux borate transporter expressed in various cell types, including epidermis and endodermis, that exports borate out of cells toward the xylem and contributes to the translocation of

How do sessile plants cope with irregularities in soil nutrient availability? The uptake of essential minerals from the soil influences plant growth and development. However, most environments do not provide sufficient nutrients; rather nutrient distribution in the soil can be uneven and change temporally according to environmental factors. To maintain mineral nutrient homeostasis in their tissues, plants have evolved sophisticated systems for coping with spatial and temporal variability in soil nutrient concentrations. Among these are mechanisms for modulating root system architecture in response to nutrient availability. This review discusses recent advances in knowledge of the two important strategies for optimizing nutrient uptake and translocation in plants: root architecture modification and transporter expression control in response to nutrient availability. Recent studies have determined (i) nutrient-specific root patterns; (ii) their physiological consequences; and (iii) the molecular mechanisms underlying these modulation systems that operate to facilitate efficient nutrient acquisition. Another mechanism employed by plants in nutrient-heterogeneous soils involves modification of nutrient transport activities in a nutrient concentration-dependent manner. In recent years, considerable progress has been made in characterizing the diverse functions of transporters for specific nutrients; it is now clear that the expression and activities of nutrient transporters are finely regulated in multiple steps at both the transcriptional and post-transcriptional levels for adaptation to a wide range of nutrient conditions.

After the accident of the Fukushima 1 Nuclear Power Plant in March 2011, radioactive cesium was released and paddy fields in a wide area including Fukushima Prefecture were contaminated. To estimate the levels of radioactive Cs accumulation in rice produced in Fukushima, it is crucial to obtain the actual data of Cs accumulation levels in rice plants grown in the actual paddy field in Fukushima City. We herein conducted a two-year survey in 2011 and 2012 of radioactive and non-radioactive Cs accumulation in rice using a number of rice cultivars grown in the paddy field in Fukushima City. Our study demonstrated a substantial variation in Cs accumulation levels among the cultivars of rice.