Mechanisms of Salt Tolerance in Rice Plants: Cell Wall-Related Genes and Expansins

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(1)台灣農業研究 (J. Taiwan Agric. Res.) 66(2):87–93 (2017) DOI: 10.6156/JTAR/2017.06602.01. Review Article. Mechanisms of Salt Tolerance in Rice Plants: Cell Wall-Related Genes and Expansins Ching Huei Kao1,*. Abstract Kao, C. H. 2017. Mechanisms of salt tolerance in rice plants: Cell wall-related genes and expansins. J. Taiwan Agric. Res. 66(2):87–93.. Inhibition of crop productivity by salt stress is mainly due to its impact on crop growth. Cell wall is an important component of plant biomass. Salt stress inhibits growth through affecting cell wall loosening. Expansins are cell wall proteins that promote wall loosening. Thus, cell wall-related genes and expansins play roles in regulating salt stress tolerance. The present review summarizes the major research advances in elucidating the importance of cell wall-related genes and expansins in salt stress tolerance. Key words: Cell wall, Expansin, Rice, Salt stress.. INTRODUCTION Soil salinity is a severe abiotic stress in agriculture. About 20% of the world’s cultivated lands are influenced by salinity (Wu & Cheng 2014). Rice is an important staple food for most if not all Asian people. Rice plants encounter many abiotic stresses, while salt stress is an important constraint limiting rice yield. Generally, rice is sensitive to salt stress. Sensitivity of rice to salt stress varies with the growth stage, very sensitive at young seedling stages and less so at reproduction (Flowers & Yeo 1981; Lutts et al. 1995). In contrast, rice is considered to be relatively more salt tolerant at germination (Heenan et al. 1988; Khan et al. 1997). The problem of salt stress can be improved by irrigation management practices in salt-affected area. However, this approach is not easy to implement on a large scale. Introduction of salt-tolerant varieties in the saltaffected area is another alternative to improve. the problem of salt stress. It appears that development of salt-tolerant rice varieties is the most efficient approach to reduce yield caused by salt stress. When growing in saline soils, roots have to cope with two types of stresses. The first of these is an osmotic stress resulting from salt concentration in the soil that results in lowered water potential and a consequent loss of cell turgor in roots. The second is ionic stress induced by changes in the concentrations of Na +, Cl - or both in the root-growing medium and within root tissues. These stresses in turn cause a reduction in the water uptake and inhibition of plant growth (Lin & Kao 2001). The growth inhibition caused by salt stress of plants is due to the restriction of growth by retarding cell extension or expansion. The plant cell wall is a rigid wall responsible for the size and the shape of the cell. It is a polymeric network of crystalline cellulose microfibrils embedded in a hydrophilic matrix of hemicelluloses, pec-. Received: August 2, 2016; Accepted: October 3, 2016. * Corresponding author, e-mail: kaoch@ntu.edu.tw 1 Professor Emeritus, Department of Agronomy, National Taiwan University, Taipei, Taiwan, ROC.. 臺灣農業研究66(2) 01-高景輝.indd 87. 2017/6/12 下午 08:58:51.

(2) 88. 台灣農業研究第 66 卷第 2 期. tins and proteins (Varner & Lin 1989; Cho et al. 2009). The plant cell wall has high tensile strength and must be loosened to enable the cell to grow (or enlarge irreversibly) (Cosgrove 2005). Cell extension is driven by turgor generated by water uptake. In other words, wall expansion requires cell turgor pressure to stretch the cell wall and put the network of interlinked cellulose microfibrils under tension. Salt stress may restrict cell expansion by affecting the uptake rates of water, turgor generation and/or cell wall properties. It is now clear that changes in wall properties indeed occur during salt stress. Thus, salt stress may inhibit growth through affecting cell wall loosening. Alternatively, growth inhibition caused by salinity may be associated with cell wall stiffening (Neumann et al. 1994). Salt stress has been shown to regulate cell wall-related genes (Fan et al. 2013; Zagorchev et al. 2014). It is believed that the cell wall may play an important role in salt tolerance. Previously, we have described the regulatory role of compatible solutes, aquaporings, Na + transporters, and reactive oxygen species-scavenging systems in salt tolerance of rice plants (Kao 2015a, 2015b, 2017). Here, we briefly describe the possible role of cell wall-related genes in salt tolerance of rice plants. Expansins are proteins that promote wall lossening. Thus, the functional importance of expansns in salt tolerance of rice plants is also discussed in this review.. CELL WALL-RELATED GENES AND RICE SALT TOLERANCE To understand the processes of cell wall biosynthesis, modification and degradation, Sharma et al. (2011) assessed the transcriptional dynamics during enzymatic removal and regeneration of rice cell walls in suspension cells over time. They demonstrated that rice lines carrying Tos17 mutations in genes upregulated during cell wall removal exhibit dwarf phenotype. The observed dwarf phenotypes suggest that these genes might be involved in cell wall-related function.. 臺灣農業研究66(2) 01-高景輝.indd 88. Porteresia coarctata is a halophyte which can grow in salt conditions corresponding to 500–650 mM NaCl (Sengupta et al. 2008). It is a close wild relative of rice and represents an important source of genes related to salt tolerance (Latha et al. 2004; Sengupta et al. 2008). There are reports showing that engineering of some genes related to inositol metabolic pathway from Porteresia can confer salt tolerance in tobacco (Majee et al. 2004) and in cultivated rice (Sengupta et al. 2008). In order to identify the candidate genes involved in salt tolerance, Garg et al. (2014) analyzed gene expression of Porteresia under salt stress conditions and demonstrated that one of the key metabolic pathways involved in salt tolerance is cell wall structures. These results suggest that genes related to cell wall structures are important for designing salt tolerance rice plants. The knowledge of drought stress transcriptomes in rice are mostly relying on the comparative studies of diverse genetic background under drought conditions. A more reliable approach is to use near-isogenic lines (NILs) with a common genetic background but contrasting levels of tolerance to drought stress under water deficit. Moumeni et al. (2011) examined two pairs of NILs in I R64 background with contrasting drought tolerance. They found that genes, such as cellulose synthase-like family, are highly activated under drought stress in two tolerant NILs. Their data suggest that these genes in root tips of tolerant rice under drought stress resulted in enhanced root growth and elongation. All the experiments discussed above seem to strengthen the idea that cell wall plays a central role in enhancing salt tolerance in rice plants.. EXPANSINS Discovery Growing plant cells characteristically exhibit acid growth. Isolated cell walls also. 2017/6/12 下午 08:58:52.

(3) Cell Wall-Related Genes, Expansions, and Salt Stress. exhibit this phenomenon. At neutral pH, the cell walls soon stop extension, but they rapidly extend when pH is lowered (Cosgrove 2000). This acid-induced extension is not merely a physical property of wall polysaccharides, but requires active wall proteins (Cosgrove 1989). When the acid-growth behavior of isolated walls is eliminated by heat or protease treatment, it can be almost fully restored by the addition of purified wall proteins (McQueen-Mason et al. 1992). The active wall proteins were named as expansins, which are hypothesized to stimulate cell wall extension by disrupting non-covalent interactions between hemicellloses and crystalline cellulose microfibrils (Cosgrove 2005).. Nomenclature Expansins were first identified from cucumber hypocotyl walls (McQueen-Mapson et al. 1992). The availability of the fullgenome sequences of Arabidopsis and rice permits the genome-wide search of expansin gene sequences. The results of genome-wide search have revealed that expansin genes contain four subfamilies (Choi et al. 2006). They are α-expansin (EXPA), β-expansin (EXPB), expansin-like A (EXLA), and expansin-like B (EXLB). In rice, there are 34 OsEXPA, 18 OsEXPB, 4 OsEXLA, and 1 OsEXLB (AbuQamar 2014). It is now clear that expansin-like genes evolved from the common ancestor with EXPA and EXPB. However, we do not know the actual function of expansin-like genes. Recently, Boron et al. (2015) demonstrated that AtEXLA2 may function as a positive regulator of cell elongation in the dark-grown hypocotyl of Arabidopsis by possible interference with cellulose metabolism.. Role in growth of rice plants Rapid intermodal elongation of deepwater rice occurs upon submergence (Kende 1998). The expression of α- and β-expansin genes is correlated with internodal growth (Cho & Kende 1997; Huang et al. 2000; Lee & Kende 2001). Rice coleoptile is also able to elongate. 臺灣農業研究66(2) 01-高景輝.indd 89. 89. under submergence. Expression of OsEXPA4 and OsEXPA2 in rice coleoptile is greatly induced by submergence (Huang et al. 2000). Choi et al. (2003) generated transgenic rice plants that expressed sense and antisense expansin gene OsEXPA4. They found that the overexpression and antisense of OsEXPA4 enhance and decrease growth of rice seedlings, respectively. OsEXPA8 is a root-specific α-expansin gene. Ma et al. (2013) provided direct evidence that overexpression of this rootspecific expansin gene increases plant height, enhances leaf number, enlarges leaf size, and improves root system architecture (longer primary roots, more lateral roots and root hairs) (Ma et al. 2013). Atomic force microscopy (AFM) technology is now able to measure living cell wall stiffness alternation. They clearly demonstrated that overexpression of OsEXPA8 enhances cell wall extensibility of suspension rice cells. In another experiment of the same group, the importance of OsEXPA3 gene in root system development in rice was examined via RNAi strategy (Qiu et al. 2014). They provided strong evidence showing that RNAi rice plants exhibit root growth inhibition, which is mostly due to the poorer root system architecture. Recently, Zou et al. (2015) identified a rice β-expansin gene, OsEXPB2, which is predominately in roots of rice. Based on the results of subcellular localization assay, they found that OsEXPB2 is localized in the cell wall. In order to know the functions of OsEXPB2, RNA interference (RNAi) was used to inhibit gene expression. They clearly showed that RNAi transgenic lines had less-developed root systems. In other words, OsEXPB2 plays key role in regulation of root system architecture and has the potential to be utilized in transgenic root breeding to improve abiotic stress tolerance. It appears that expansins are indispensable for the growing tissues of rice seedlings.. Improvement of rice plants to salt stress Halophytes are land plants that are able to grow under high salt conditions. Hwang et al.. 2017/6/12 下午 08:58:52.

(4) 90. 台灣農業研究第 66 卷第 2 期. (2013) isolated three α-expansin genes (SjEXPAs) from Suaeda japonica, a halophyte. They demonstrated that expression of SjEXPAs does not show any remarkable change at different NaCl concentrations, suggesting that EjEXPAs may play a role in the unaffected seedling growth of S. japonica in a high-salt environment. There are only a few reports have evaluated the effect of salt stress on the expression of expansin genes and the abundance of expansin proteins. From the two-dimensional proteome analysis, Pitann et al. (2009) have shown that a decrease in β-expansin protein in a salt-sensitive maize cultivar, whereas β-expansin level in a resistant cultivar is less affected by salt stress. Similarly, Geilfus et al. (2010) demonstrated that reduced expression β-expansin protein correlates positively with reduced shoot growth of salt-sensitive maize cultivar under salt stress. The decrease of this expansin protein is due to the down-regulation of ZmEXPB2, ZmEXPB6, and ZmEXPB8 expression. In contrast, the maintenance of shoot growth in salt-tolerant maize cultivar is related to an unaffected β-expansin protein in the shoot. Improvement of salt tolerance has been shown in overexpression of a wheat expansin gene, TaEXPB23, and a rose expansin gene, RhEXPA4, in tobacco and Arabidopsis, respectively (Han et al. 2012; Lu et al. 2013). Recently, Qin et al. (2014) demonstrated that the expression of OsEXPA3 is upregulated by NaCl stress, suggesting that OsEXPA3 might have great potential in improving salt tolerance of rice plants. Further work will be required to examine the response of OsEXPA3 or other expansin genes by transgenic and genetic studies.. is the cell wall. The role of the plant cell wall in growth is emerging in recent years. Endler et al. (2015) identified a protein family whose members are components of the cellulose synthesizing machinery in Arabidopsis. Their results revealed that mutation of these proteins led to salt-sensitive phenotypes and proposed a mechanism for how plants maintain the biomass producing capacity under salt stress conditions. It is probably that investigations into the role of sustained cellulose synthesis in rice plants during salt stress will be initiated. There is no doubt that expansins have the potential to be utilized as targets for improving stress tolerance of rice plants. However, much remains to be discovered about the role of expansins in salt stress tolerance of rice plants. Future research needs to emphasize on the genetic modification of expansin expression in rice plants.. CONCLUSIONS AND PERSPECTIVES. Choi, D., Y. Lee, H. T Cho, and H. Kende. 2003. Regulation of expransin gene expression affects growth and development in transgenic rice plants. Plant Cell 15:1386–1398.. The adverse effect of Na + and/or Cl - on plants is called salt stress. Crop yield is severely affected by salt stress. This occurs directly due to the impact of salt stress on crop growth. An important element to plant biomass. 臺灣農業研究66(2) 01-高景輝.indd 90. REFERENCES AbuQamar, S. 2014. Expansins: Cell wall remodeling proteins with a potential function in plant defense. J. Plant Biochem. Physiol. 2:e118. Boron, A. K., B. Van Loock, D. Suslov, M. N. Markakis, J. P. Verhelen, and K. Vissenberg. 2015. Overexpression of AtEXLA2 alters etiolated Arabiopsis hypocotyl growth. Ann. Bot. 115:67–80. Cho, T. and H. Kende. 1997. Expression of expansin genes is correlated with growth in deepwater rice. Plant Cell 9:1661–1671. Cho, W. K., X. Y. Chen, H. Chu, Y. Rim, S. Kim, S. T. Kim, S. W. Kim, Z. Y. Park, and J. Y. Kim. 2009. Proteomic analysis of the secetome of rice calli. Physiol. Plant. 135:3313–3341. Choi, D., H. T. Cho, and Y. Lee. 2006. Expansins: Expanding importance in plant growth and development. Physiol. Plant. 126:511–518.. Cosgrove, D. J. 1989. Characterization of long-term extension of isolated cell walls from growing cucumber hypocotyls. Planta 177:121–130. Cosgrove, D. J. 2000. Expansive growth of plant cell walls. Plant Physiol. Biochem. 38:109–124.. 2017/6/12 下午 08:58:52.

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(7) Cell Wall-Related Genes, Expansions, and Salt Stress. 93. 水稻耐鹽之機制：細胞壁相關基因與擴展蛋白高景輝 1,* 摘要高景輝。2017。水稻耐鹽之機制：細胞壁相關基因與擴展蛋白。台灣農業研究 66(2):87– 93。鹽分逆境影響作物生長進而降低產量。細胞壁是生質量 (biomass) 之主要成分。鹽分逆境經由影響細胞壁鬆軟而抑制生長。擴展蛋白 (expansins) 是鬆軟細胞壁之蛋白。因此，細胞壁相關基因與擴展蛋白對作物調控鹽分逆境耐性扮演重要功用。本文說明細胞壁相關基因與擴展蛋白在水稻耐鹽重要性之主要研究進展。關鍵詞：細胞壁、擴展蛋白、水稻、鹽分逆境。. * 1. 投稿日期：2016 年 8 月 2 日；接受日期：2016 年 10 月 3 日。通訊作者：kaoch@ntu.edu.tw 國立台灣大學農藝學系名譽教授。台灣台北市。. 臺灣農業研究66(2) 01-高景輝.indd 93. 2017/6/12 下午 08:58:52.

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