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Revision as of 04:04, 12 June 2015
OsNIP3;1, a rice boric acid channel, regulates boron distribution and is essential for growth under boron-deficient conditions. [1]
Contents
Annotated Information
Function
OsNIP3;1 is the boric acid channel gene, with similarities to both AtNIP5;1 and AtNIP6;1. This suggests that OsNIP3;1 functions both in roots and shoots for boron homeostasis, whereas AtNIP5;1 and AtNIP6;1 mediate boron transport in roots and shoots, respectively. The promoter–GUS staining analysis is indicative of a role for OsNIP3;1 in the vascular bundles of leaf sheaths and blades, as well as in root exodermis and stele. OsNIP3;1 regulates boron distribution among leaf tissues, similar to AtNIP6;1 in A. thaliana [2]. The RNAi plants of OsNIP3;1 showed significant disturbance in boron distribution between leaf blades and leaf sheaths and among leaf blades. Because boron transport in shoots is primarily transpiration-dependent, boron tends to accumulate in large, mature leaves with higher transpiration rates [3]. Boron is distributed to larger leaf blades rather than the leaf sheath in OsNIP3;1 RNAi plants. This suggests that OsNIP3;1 plays a role in regulating boron distribution against transpirational flow. Preferential boron distribution to young tissues is essential for their development under boron-deficient conditions, and AtNIP6;1 plays a central role in this process in A. thaliana [2]. Imbalance of boron distribution in RNAi plants was more apparent under boron-deficient conditions, which may result in growth defects in RNAi plants under boron-deficient conditions. In addition, OsNIP3;1 functions in boron distribution among leaves under boron-sufficient conditions. Rice has efficient boron transport and utilization mechanisms, such as boron translocation from mature leaves [4] and endosperm [5]. In broccoli (Brassica oleracea) and lupin (Lupinus albus), boron remobilization is mediated through the phloem [6]. OsNIP3;1 expression in the phloem tissues of leaf sheaths suggests that OsNIP3;1 plays a role in boron remobilization. Disruption of AtNIP5;1 impaired the growth of A. thaliana by resulting in insufficient boron uptake under boron-deficient conditions [7]. In rice, RNAi-mediated suppression of OsNIP3;1 expression also caused a severe growth deficit under the boron-deficient conditions, suggesting that OsNIP3;1 is involved in root boron uptake, similar to AtNIP5;1. Boron-dependent mRNA accumulation and enhanced expression of OsNIP3;1 in the exodermis under boron-deficient conditions supports this hypothesis, and similar results have been reported for AtNIP5;1 [7]. One of the two Casparian strips in rice roots is located in the exodermis, and a transporter expressed in this cell type in rice roots plays a major role in nutrient uptake [8][9][10]. Under boron-deficient conditions, OsBOR1 expression is induced in the exodermis, which is thought to enhance boron transport into shoots [9]. OsNIP3;1 expressed in the exodermis is likely to transport boron coordinately with OsBOR1, similar to AtBOR1 and AtNIP5;1 [11][12]. However, unlike nip5;1 mutants [7], disruption of OsNIP3;1 does not affect the total boron concentration or the boron distribution between shoots and roots, although boron contents in shoots were significantly lower in RNAi plants under boron-deficient conditions. These results suggest that OsNIP3;1 plays a role in facilitating boron uptake in roots, similar to AtNIP5;1, but the contribution of OsBOR1 and other transporters in boron uptake may be greater. In support of this hypothesis, the growth defect phenotype of osbor1 plants under boron-deficient conditions is considerably more severe [9] than that of OsNIP3;1 RNAi plants. OsBOR1 and OsNIP3;1 are expressed in the exodermis and endodermis under boron-deficient conditions [9]. The silicon uptake channel OsNIP2;1 (Lsi1), which is also expressed in the exodermis and endodermis [8], may mediate root boron uptake (Schnurbusch et al., 2010). In A. thaliana, AtNIP5;1 and AtBOR1 are expressed mainly in the epidermis and endodermis, respectively [12]. The distinct cell-type specificity of AtNIP5;1 compared with AtBOR1 highlights the major contribution of AtNIP5;1 to boron uptake by A. thaliana roots [7], whereas the redundant function and cell type-specific expression of OsNIP3;1 and other transporters in rice roots may have abrogated the effect of OsNIP3;1 disruption on boron uptake. It should be noted that the rice endosperm supplies sufficient boron to developing seedlings under boron-starved conditions [5]. This compensatory boron delivery to rice seedlings under boron-deficient conditions may also mask the reduction of boron uptake as the result of OsNIP3;1 repression. The paradox between unchanged boron uptake and significant growth inhibition of the OsNIP3;1 RNAi plants may also be explained by insufficient boron availability and/or improper distribution in the cells, similar to the ‘iron (Fe) deficiency paradox’ [13]. The Fe concentration in Fe-deficient leaves showing chlorosis is often comparable to that of Fe-sufficient leaves, and despite similar concentration to Fe-requiring enzymes in cells is believed to cause this phenomenon. Similar phenotypes have been observed in the mutants AtBOR2 and OsBOR4 [14][15]. These studies suggest that the lack of functional boron transporters impairs proper boron distribution in the cells and results in boron deficiency symptoms without affecting tissue boron concentrations. Given the cell type-specific expression patterns of OsNIP3;1, it is possible that disruption of OsNIP3;1 alters boron distribution to cells and/or cellular compartments with high boron demands, which may impair whole plant growth through homeostatic regulation.
Mutation
RNAi-mediated OsNIP3;1 knockdown plants were established. The target sequence was selected from the C–terminal and 3′ UTR regions of OsNIP3;1 to avoid non-specific down-regulation of other OsNIP3 genes. The accumulation of OsNIP3;1 transcript was quantified in roots by RT-mediated real-time PCR. RNA samples were prepared from plants subjected to 1 day of −B treatment. Two independent lines of T2 progeny (L9 and L13) showed reduced OsNIP3;1 expression in both shoots and roots. In roots, the expression levels of OsNIP3;1 in lines L9 and L13 decreased to 3 and 30%, respectively, of the levels in the wild-type plants.
Expression
To investigate the subcellular localization of OsNIP3;1, GFP fused N–terminally to OsNIP3;1 (GFP–OsNIP3;1) was expressed transiently under the control of the CaMV 35S RNA promoter in tobacco (Nicotiana tabacum) BY2 cells. Fluorescence was observed mainly at the cell periphery, indicative of plasma membrane localization of the fusion protein, whereas free GFP localized to the cytoplasm and nucleus. This suggests that OsNIP3;1 is a plasma membrane-localized protein.
Evolution
Among 38 rice MIP genes listed in GenBank, 12 are known to belong to the OsNIP sub-family [16]. Based on phylogenetic analysis, the OsNIP sub-family may be divided into four groups: OsNIP1, OsNIP2, OsNIP3 and OsNIP4 [17]. Among 10 rice NIP genes and nine A. thaliana NIP genes, OsNIP3;1 (accession number AAG13499) is most similar to A. thaliana AtNIP5;1 [18]. Specific PCR primers were designed based on the predicted ORF of OsNIP3;1. Using cDNA prepared from plants grown under boron-deficient conditions as a template, a fragment corresponding to OsNIP3;1 ORF was isolated. The nucleotide sequence of the OsNIP3;1 cDNA was identical to the transcript sequence of LOC_Os10g36924.1. Phylogenetic analysis of the predicted amino acid sequences of all NIP genes in rice and A. thaliana revealed that OsNIP3;1 has high similarity to AtNIP5;1 (71.1% identity and 86.2% similarity) and AtNIP6;1 (60.7% identity and 80.0% similarity). There is no other paralog of OsNIP3;1 that is similar to AtNIP6;1, and OsNIP3;2–3;5 were more distantly related to OsNIP3;1. The substrate selectivity of aquaglycerol porins is determined based on the amino acid composition of the pore filter [18]. As mentioned above, NIPs may be classified into three sub-groups (NIP I, II and III) based on the sequences forming the pore [18][19]. OsNIP3;1 belongs to NIP sub-group II. All three A. thaliana NIPs from this group (AtNIP5;1, AtNIP6;1 and AtNIP7;1) possess transport activity for boric acid [7][11][20], suggesting that members of sub-group II have functions in boric acid transport. NIPs belonging to sub-group II were identified from the representative eudicots A. thaliana, Lotus japonicus and Vitis vinifera (grape) and the monocots rice, maize (Zea mays) and sorghum (Sorghum bicolor) based on genomic database analysis. Orthologs of AtNIP5;1 were identified from all tested plant species, including rice OsNIP3;1; however, orthologs of AtNIP6;1 were identified only in eudicots. OsNIP3;2–3;5 and their orthologs were unique to monocots, and were more distantly related to OsNIP3;1/AtNIP5;1 and AtNIP6;1.
Labs working on this gene
1. Biotechnology Research Center, University of Tokyo, Tokyo, Japan 2. Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo, Japan 3. Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University, Sapporo, Japan
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Hanaoka, H., Uraguchi, S., Takano, J., Tanaka, M. and Fujiwara, T. (2014), OsNIP3;1, a rice boric acid channel, regulates boron distribution and is essential for growth under boron-deficient conditions. The Plant Journal, 78: 890–902.
- ↑ 2.0 2.1 Tanaka, M., Wallace, I.S., Takano, J., Roberts, D.M. and Fujiwara, T. (2008) NIP6;1 is a boric acid channel for preferential transport of boron to growing shoot tissues in Arabidopsis. Plant Cell, 20, 2860–2875.
- ↑ Brown, P.H. and Shelp, B.J. (1997) Boron mobility in plants. Plant Soil, 193, 85–101.
- ↑ Bellaloui, N., Yadavc, R.C., Chern, M.S., Hu, H., Gillen, A.M., Greve, C., Dandekar, A.M., Ronald, P.C. and Brown, P.H. (2003) Transgenically enhanced sorbitol synthesis facilitates phloem-boron mobility in rice. Physiol. Plant. 117, 79–84.
- ↑ 5.0 5.1 Uraguchi, S. and Fujiwara, T. (2011) Significant contribution of boron stored in seeds to initial growth of rice seedlings. Plant Soil, 340, 435–442.
- ↑ Shelp, B.J., Kitheka, A.M., Vanderpool, R.A., Van Cauwenberghe, O.R. and Spiers, G.A. (1998) Xylem-to-phloem transfer of boron in broccoli and lupin during early reproductive growth. Physiol. Plant. 104, 533–540.
- ↑ 7.0 7.1 7.2 7.3 7.4 Takano, J., Wada, M., Ludewig, U., Schaaf, G., von Wiren, N. and Fujiwara, T. (2006) The Arabidopsis major intrinsic protein NIP5;1 is essential for efficient boron uptake and plant development under boron limitation. Plant Cell, 18, 1498–1509.
- ↑ 8.0 8.1 Ma, J.F., Tamai, K., Yamaji, N., Mitani, N., Konishi, S., Katsuhara, M., Ishiguro, M., Murata, Y. and Yano, M. (2006) A silicon transporter in rice. Nature, 440, 688–691.
- ↑ 9.0 9.1 9.2 9.3 Nakagawa, Y., Hanaoka, H., Kobayashi, M., Miyoshi, K., Miwa, K. and Fujiwara, T. (2007) Cell-type specificity of the expression of OsBOR1, a rice efflux boron transporter gene, is regulated in response to boron availability for efficient boron uptake and xylem loading. Plant Cell, 19, 2624–2635.
- ↑ Sasaki, A., Yamaji, N., Yokosho, K. and Ma, J.F. (2012) Nramp5 is a major transporter responsible for manganese and cadmium uptake in rice. Plant Cell, 24, 2155–2167.
- ↑ 11.0 11.1 Takano, J., Miwa, K. and Fujiwara, T. (2008) Boron transport mechanisms: collaboration of channels and transporters. Trends Plant Sci. 13, 451–457.
- ↑ 12.0 12.1 Takano, J., Tanaka, M., Toyoda, A., Miwa, K., Kasai, K., Fuji, K., Onouchi, H., Naito, S. and Fujiwara, T. (2010) Polar localization and degradation of Arabidopsis boron transporters through distinct trafficking pathways. Proc. Natl Acad. Sci. USA, 107, 5220–5225.
- ↑ Römheld, V. (2000) The chlorosis paradox: Fe inactivation as a secondary event in chlorotic leaves of grapevine. J. Plant Nutr. 23, 1629–1643.
- ↑ Miwa, K., Wakuta, S., Takada, S., Ide, K., Takano, J., Naito, S., Omori, H., Matsunaga, T. and Fujiwara, T. (2013) Roles of BOR2, a boron exporter, in cross linking of rhamnogalacturonan II and root elongation under boron limitation in Arabidopsis. Plant Physiol. 163, 1699–1709.
- ↑ Tanaka, N., Uraguchi, S., Saito, A., Kajikawa, M., Kasai, K., Sato, Y., Nagamura, Y. and Fujiwara, T. (2013) Roles of pollen-specific boron efflux transporter, OsBOR4, in the rice fertilization process. Plant Cell Physiol. 54, 2011–2019.
- ↑ Bansal, A. and Sankararamakrishnan, R. (2007) Homology modeling of major intrinsic proteins in rice, maize and A. thaliana: comparative analysis of transmembrane helix association and aromatic/arginine selectivity filters. BMC Struct. Biol. 7, 27.
- ↑ Sakurai, J., Ishikawa, F., Yamaguchi, T., Uemura, M. and Maeshima, M. (2005) Identification of 33 rice aquaporin genes and analysis of their expression and function. Plant Cell Physiol. 46, 1568–1577.
- ↑ 18.0 18.1 18.2 Wallace, I.S., Choi, W.G. and Roberts, D.M. (2006) The structure, function and regulation of the nodulin 26-like intrinsic protein family of plant aquaglyceroporins. Biochim. Biophys. Acta, 1758, 1165–1175.
- ↑ Mitani-Ueno, N., Yamaji, N., Zhao, F.-J. and Ma, J.F. (2011) The aromatic/arginine selectivity filter of NIP aquaporins plays a critical role in substrate selectivity for silicon, boron, and arsenic. J. Exp. Bot. 62, 4391–4398.
- ↑ Li, T., Choi, W.-G., Wallace, I.S., Baudry, J. and Roberts, D.M. (2011) Arabidopsis thaliana NIP7;1: an anther-specific boric acid transporter of the aquaporin superfamily regulated by an unusual tyrosine in helix 2 of the transport pore. Biochemistry, 50, 6633–6641.
