Proc Indian Natn Sci Acad 80 No. 5 December 2014 pp. 1013-1023  Printed in India. DOI: 10.16943/ptinsa/2014/v80i5/47970

Review Article

The Possible Overlap Between Salinity and Mechanical Challenges in Porteresia coarctata SONALI SENGUPTA Division of Biology, Bose Institute, P-1/12, CIT Scheme, VIIM, Kankurgachhi, Kolkata 700 054, India

(Received on 05 May 2014; Revised on 01 October 2014; Accepted on 30 October 2014)

In nature, certain abiotic stresses constitutively acting on often go unrecognized. Mechanical stress is one of such stresses, which is manifested on a plant as low-spectrum pressure on the cell membrane due to wind, soil hindrance, gravity or wave current. Although all plants face constant threats from mechanical stress, some ecosystems are more prone to receive such stress. Mangrove vegetation is daily inundated by saline tidal waves that pose a serious threat to the soil binding force of forest undergrowth. Porteresia coarctata is a mangrove , which being wild, is avaluable germplasm for bioprospecting of genes and proteins that may confer salt-tolerance to domestic rice. However, P. coarctata is important from ecological point of view as its salt and mechanical stress tolerance is an integral part of the mangrove ecosystem. It is not possible to understand the basic biology of P. coarctata without an ecological perspective. The subterranean part of Porteresia provides a high anchorage and binds soil, thus stabilizing the mangrove vegetation. The root system architecture of Porteresia is unique, with a rhizome and rhizoid-like rootlets. The root system interacts with both salinity and mechanical threat directly, and thus it is important to understand the molecular ecophysiology of Porteresia root and rhizomes to understand the nature of overlap between salinity and mechanical stress in a mangrove ecosystem, which is still elusive.

Key Words: Porteresia coarctata; Salinity; Mechanical Stress; Mangrove; Rice

Introduction stressors, termed as Interaction Tier B (Fig. 1). There is also a third type of reaction, wherein the Major environmental stresses that influence a plant’s molecular response to one stress may use same life often occur together in its natural habitat. Such signalling pathways or cascades of reactions for one co-occurrence of stresses may elicit similar defence or more biotic or abiotic stresses commonly known reactions or adaptations and may express similar as molecular cross talk and termed Interaction Tier group of transcription factors and shared group of C in Fig. 1. In an ecological niche, all such genes. Sometimes the stresses share physical or interactions shape a plant’s mode of survival and its chemical components as well. For convenience of position in the ecosystem. Moreover, anthropogenic discussion, stress overlap can be reduced into certain inputs, such as cultivation or eradication could play reaction types as shown in Fig. 1. For example, as an invasive pressure (Interaction Tier D in Fig. salinity and dehydration may have a common physical 1) at all levels of such interactions, and define element of osmotic shift; that may be termed as molecular ecophysiology of a plant. Interaction Tier A; sharing astress component. Salinity may co-occur with submergence in coastal This review will address all three tiers of ecosystem or industrial sewage washed cultivation interactions between salinity and mechanical stress system, which is co-occurrence of unrelated with special reference to , Porteresia

*Author for Correspondence: E-mail: [email protected] 1014 Sonali Sengupta

known to shift seamlessly between a saline and non- saline environment.They may also show certain energetically cheap adaptations compared to complex anatomical adaptations found in true mangroves, for example, vivipary and pneumatophores. Status of Porteresia coarctatais somewhat ambiguous in this regard. The Sunderban mangrove area is one of the largest mangrove forests washed with the distributaries of the Ganges, and forms the coast fringe of Bay of Bengal. The soil of estuarine area is highly saline and faces daily saline water submergence from high and low tides. The major vegetation of true mangrove trees and shrubs in Sundarban area are accompanied by a large proportion of the mangrove associates; which include some monocots belonging to Cyperaceae and

Fig. 1: Interaction types in molecular ecophysiology of a plant. For . details, refer the text Porteresia coarctata, a Salt-Tolerant Wild Rice

Porteresia coarctata is a wild rice that has been coarctata. Porteresia, being a mangrove associate and receiving great attention for its unique salt tolerance a salt-tolerant bioprospecting model for rice, is quality and its close relation to rice. As proposed by extremely important in rice stress biology and Sengupta et al. (2010), Porteresia is a potent model biotechnological improvement of rice. for bioprospecting of genes and proteins for raising salt tolerant rice through biotechnological Environment-Genotype Interactions in Mangrove approaches. However, the domesticated rice Vegetation sativa and the wild rice Porteresia show significant Mangrove plants occupy a special niche in the differences. The native Porteresia vegetation ecosystem with plants of specialized adaptation. In a flourishes in a range of 100 to 500mM of NaCl, which mangrove habitat, the interactions among organism is comparable to seawater. Many native landraces of and environment are critical for optimal survival. The rice are also known to tolerate some extent of salinity, environmental forces operative on coastal or shoreline but never establish in a mangrove eco-system. mangrove forests are multifarious. They include Although suggested to be facultative, it has always salinity, flooding and anaerobic condition.Mangrove been difficult to establish a completely non-saline vegetation typically shows two types of plants, one formation of Porteresia and no such natural group represents the true mangroves, whereas the establishment has been observed. This prompted us other group consists of mangrove associates (Wang to term Porteresia as salt loving rice or halophilic et al., 2010).“Exclusive”, “obligate” or “true” rice, not a true facultative; and a fringe in mangrove species are not able to grow outside the context of mangrove-like property. We have observed mangrove environment whereas ‘nonexclusive’, that Porteresia grows better in a saline soil, shows facultative or mangrove associates may occupy any greater vegetative growth and propagation, has a terrestrial or aquatic habitat outside the mangrove higher biomass and a high photosynthetic rate, in ecosystem (Lacerda et al., 2002; Parani et al., 1998; comparison to control (non-saline)conditions Tomlinson, 1986). Differences between them are not (Sengupta and Majumder, 2009). It prefers vegetative well-defined and there are several fringe species with reproduction in the saline mode, and the occasional debated position. Non-exclusive mangroves are spikelets produce almost no viable seeds. In absence The Possible Overlap Between Salinity and Mechanical Challenges in Porteresia coarctata 1015 of salt in soil, it is possible to force Porteresia to ecosystems in India. The habitat of this plant in India enter in a sexual reproduction mode and to produce a has been categorized under ecologically sensitive few viable seeds occasionally (Sengupta et al., zone, and protected vide CRZ (Coastal Regulation unpublished) . In our previous observations,the Zone) Act of 1990, along with mangroves. However, establishment of an endophytic fungus was seen Porteresia habitat continue to be under constant favourable for the establishment of a Porteresia threat from ever increasing anthropogenic demands, vegetation. and hence warrants strict implementation of CRZ rule for their protection (Sengupta and Majumder, 2010). Ecological and Taxonomical Position of Porteresia Under such an ecological conditions, Porteresia faces Porteresia is a tetraploid monotypic (2n = 4x challenges from salt, physiological dehydration, = 48) (Sengupta and Majumder, 2010). The largest submergence and mechanical stress. Never continuous vegetation of Porteresia occurs at domesticated or cultivated, the anthropogenic input Sundarban delta that stretches along coastal West to the vegetation is unclear. In the coastline population Bengal and , covering about a million of Sundarbans, it is widely used as a fodder which is hector of land. The majority of the flora are trees, an important reason of its eradication. In the but the land is held by a large proportion of under coastlines of Sundarban area it is called Dhani ghas. growth of shrubs and herbs. Porteresia forms a vast This plant shows an elaborate rhizomatous system population in the coastline, binds the soil and prevents absent in other domestic rice species, but bears the coastline from erosion. Both the east and west similarity to the rhizomatous stem of a weedy rice, shore lines of India, including Orissa, Sundarban and Oryza longstaminata. Like Porteresia, weedy rice has Chennai coast forests are manifested with Porteresia a robust vegetative growth, but it prefers a non-saline coarctata. However, the vegetation is limited to the habitat and the seeds of weedy rice are not dehiscent. soil that gets inundated twice a day with saline river On the other hand, seeds of Porteresia are highly or seawater of 20 to 40 dSm-1 (Jagtap et al., 2006). dehiscent and not viable. It is clearly indicated that Porteresia is a valuable Oryza genome is made up of 24 species, of source of genes and proteins, that bear significant which only two are cultivated and domesticated homology to rice. Therefore, genomic, transcriptomic (Table 1). There are 10 genome types, of which and/or proteomic studies of Porteresia should offer diploids are AA through GG, and 4 are allote traploids. valuable bioresource for transgenic crop science. The speciality of Porteresia lies in the fact that the However, the ambiguity in its physiology very often HHKK genome type in Porteresia is special, as none contradicts part of its identity as a halophytic wild of the genome complements (HH or KK)are present (non-domesticated) model for rice which is curious. in the diploid genome groups of rice. The taxonomical Porteresia coarctata act as pioneer species in position of P. coarctata Tateoka (= O. coarctata) is the succession of mangrove formation along the discussed in detail by Sengupta and Majumder estuaries of India (Jagtap et al., 2006). Though of (2010). The inclusion of this plant in Oryza species great significance to estuarine and deltaic is highly debated; though based on the genetic, environments, it is poorly understood ecologically. anatomical and morphological data, several scientists The temporal and spatial patterns in the growth and have emphasized the positioning of Porteresia in the biomass production of P. coarctata were evaluated Oryza genus (Tateoka, 1965; Flowers et al., 1990; at selected localities along the banks of Mandovi Finch et al., 1997; Garcia, 1992; Latha et al., 1998; estuary, Goa, India (Jagtap et al., 2006; Sengupta and Ge et al., 1999). Though Ge et al. (1999) suggested a Majumder, 2010). Considering its ecological more ancient origin of Porteresia than O. sativa, the significance i.e. tolerance to wide salinity range and phylogenetic reconstruction of rice clearly shows that adaptability to sandy and muddy substrate, P. the closest ancestral species to KK (P. coarctata) coarctata is of a great value in protection, genome are DD and HH genomes. Comparison of conservation and restoration of estuarine and creek Monoculm1 (MOC1) genomic regions suggests that 1016 Sonali Sengupta

Table 1: Origin and genome types of Oryza genus analysed so far (Adapted and modified from Ge et al., 1999)

S.No. Species Genome Accession Habit Status Origin

1 P. coarctata (Syn. O. coarctata) HHKK (?) MSSR007 Halophyte Wild India 2 O. sativa AA IR64 Glycophyte Domesticated IRRI 3 O. glaberrima AA 100792 Glycophyte Domesticated Senegal 4 O. nivara AA 106185 Glycophyte Wild India 5 O. rufipogon AA 105908 Glycophyte Wild Thailand 6 O. longistaminata AA 103886 Glycophyte Co-domesticated Tanzania 7 O. punctata BB 100937 Glycophyte Wild Ghana 8 O. officinalis CC 101116 Glycophyte Wild Philippines 9 O. rhizomatis CC 105448 Glycophyte Wild SriLanka 10 O. minuta BBCC 100880 Glycophyte Wild Philippines 11 O. eichingeri CC 105408 Glycophyte Wild SriLanka 12 O. malampuzhaensis BBCC 105328 Glycophyte Wild India 13 O. alta CCDD 100025 Glycophyte Wild Surinam 14 O. grandiglumis CCDD 105156 Glycophyte Wild Brazil 15 O. latifolia CCDD 105139 Glycophyte Wild Guatemala 16 O. australiensis EE 105272 Glycophyte Wild Australia 17 O. brachyantha FF 101232 Glycophyte Wild Sierraleone 18 O. longiglumis HHJJ 105146 Glycophyte Wild Indonesia 19 O. ridleyi HHJJ 100820 Glycophyte Wild Thailand 20 O. granulata GG 101084 Glycophyte Wild Srilanka 21 O. meyeriana GG 106473 Glycophyte Wild Philippines 22 O. indandamanica unknown 105694 Glycophyte Wild India

O. coarctata (or P. coarctata) has a unique genome O. coarctata and O. ridleyi are likely to belong to type (Lu et al., 2009). Although most Oryza genome different genome types. To avoid confusion Lu et al. types were determined by traditional genome or (2009) even suggested O. coarctata should be molecular analysis (Li et al., 1964), O. coarctata was designated as KKLL (Sengupta and Majumder, designated as an HHKK genome type based solely 2010). on its phylogenetic position (Ge et al., 1999). When the HH subgenomes in O. coarctata (HHKK) and O. Morphology of the Plant ridleyi (HHJJ) were compared, no homology was observed in the intergenic regions. These findings To understand the relation of ecological status of contrast with other subgenome comparisons that show Porteresia to its genetic content, morphology of the homologous sequences and shared transposable plant is noteworthy. Discussed in greater detail in our elements in intergenic regions, such as the BB and earlier communication (Sengupta and Majumder, CC genome types (Lu et al. 2009). Moreover, the 2010), morphological uniqueness in Porteresia over gene sequence differences between the predicted HH other rice plants briefly include salt hairs, elaborate subgenome types in O. coarctata and O. ridleyi were rhizomatous system, rhizoid like rootlets and more different from AA and BB genome types. Both dehiscent seeds on scanty panicles. The morphology of these subgenomes were estimated to have diverged and physiology of Porteresia are discussed in detail from each other ~11 Ma. Hence, HH subgenomes in in Sengupta and Majumder (2010) (Fig. 3). The root The Possible Overlap Between Salinity and Mechanical Challenges in Porteresia coarctata 1017 and rhizome architecture of Porteresia remains Porteresia plants accumulate Na+ and Cl-ions in largely unaddressed in literature,however it probably leaves, but maintain a Na:K ratio as low as 0.7 even represents the most significant part of the physiology after 6 weeks of growth in 25% artificial saline water of Porteresia. (ASW) where the Na:K ratio was 34. This points towards an ability to leach out the salts as well as an The Subterranean System of Porteresia ability to take up saline solution through soil (Bal and Dutt, 1986). To achieve this, a very high root As mentioned previously, Porteresia has a distinctly pressure is required. To survive such high pressure, different underground system compared to that of root cells must have a much robust protection/ rice. The rhizomes are essentially runners (Fig. 3D), tolerance mechanism against salt. Unfortunately, root with high deposition of mechanical tissue. From the adaptation against salt is relatively less covered in nodes of the rhizome, leaf buds arise that are the previous research. progenitors of new leafy shoots (Fig. 3E). Scale leaves cover and protect the nodal meristematic regions from The Genomic and Proteomic Studies harsh environment as in salt-marsh grasses The genomic and proteomic studies so far done in (Cyperaceae). Thin rootlets emerge from nodes of Porteresia have been discussed by different workers rhizomes (Fig. 3) which may serve the purpose of (Sengupta and Majumder, 2009; Garg et al., 2013). absorption and feeble anchorage. The root system Porteresia has been vigorously used as a valuable hardly penetrates the soil deeper than a foot (roughly) bioresource for salt-tolerant gene pool in the last in Sundarban area, so we assume the roots may not decade (Majee et al., 2004; Sengupta et al., 2008). have a true root structure, and the rhizome has fullest The massive advancement of technology and the ability to absorb water and nutrients as well as integration of computation in biological sciences has providing anchorage to the substratum. The root changed our concept and modified our scope of system seems extremely important for the study of looking at the plant from a holistic viewpoint. The the biology of this mangrove associate. leaf proteome and EST profiles were analyzed in The plant exhibits a profound and special detail (Sengupta and Majumder, 2009). Their research system of salt exclusion from leaves, comparable to shows that the plant can keep a normal, or even salt marsh grasses. Originally described by Flowers vigorous growth profile under high salt stress. A et al. (1990) and later detailed by Sengupta and differential leaf proteomic profile was generated by Majumder (2009), two types of salt hairs are present Sengupta and Majumder (2009) in which a very small on Porteresia leaves with different mechanisms of subset of proteins related to salt tolerance in salt exclusion. Both are unicellular trichomes, the Porteresia were identified. Proteins identified were glands on the upper surface of leaf excrete salt, which, involved in several processes such as : protection of at high concentrations of substrate salinity forms the photosystems from oxidative/hyperionic damage crystals on the upper surface. On the other hand, hairs and maintaining the ETS function; enhancing on the lower surface are prone of shedding themselves available catalytic sites of the main carbon- off at higher salt concentration, and regenerate at low assimilating enzyme Rubis CO under low stromal salt concentration. This is a typical salt-inundated CO2 concentration; shunting some active oxygen estuarine adaptation, where saline water level species to photorespiratory carbon oxidation (PCO) constantly fluctuates. To keep at continuum with the cycle; savings of overall energy costs by favouring diurnal variation of salt concentration in the medium, low-energy pathways; synthesizing osmotically active the mechanism of such shedding of glands and re- compounds; detoxifying the system by removing growing must be a very successful strategy. The stress-generated alcohols; controlling the cellular sodium ion concentration and sodium: transcriptional regulatory network through stress- potassium ratio in leaf remains low (Sengupta and induced transcription factors; enhanced synthesis of Majumder, 2009). According to Flowers et al. (1990), chaperones to uphold normal protein structure that 1018 Sonali Sengupta can be altered during high-stress regime; maintaining cellular integrity through supplying high amount of cell wall components and thus retaining normal to robust growth under stress (Sengupta and Majumder 2009). The identified proteins can well be functionally related to the physiology of Porteresia under stress (Fig. 2A, Sengupta and Majumder, 2009). In a subtractive cDNA profiling, a vast metabolic alteration in Porteresia leaves in the presence of salt stress was indicated (Fig. 2B, unpublished data from the laboratory). More recent next-generation transcriptome enrichment (Garg et al., 2013) indicates a close functional overlap of submergence and salinity tolerance trait in Porteresia, as is expected of mangrove habitat plants. However, Garg et al. (2013) and Sengupta and Majumder (2009) made an observation that the similarity of Porteresia transcripts to rice transcripts was not very high. Candidate gene based studies showed that a large number of Porteresia genes are similar to Rice genes. On the other hand, a more comprehensive scenario obtained from proteomic and genomic studies highlights the fact that many of the salt-stress induced transcripts in Porteresia do not have any homologues or orthologues in rice or wild rice; and they are not even anotated (Sengupta et al., 2009; Garg et al., 2013). It has also been shown by Sengupta et al. 2008 Fig. 2: A. A summary of proteomic responses of Porteresia coarctata (adapted from Sengupta and Majumder, 2009). that genes like Inositol methyl Transferase, which has Diagrammatic representation of the salt-induced molecular not been reported to be present in rice, are present in functions and proteins identified through proteomic Porteresia. Many hypothetical proteins are identified analysis: in Porteresia coarctata to be upregulated under salinity stress. The orange boxes and shapes represent the in Porteresia that may represent a completely proteins and functions identified. Major abbreviations used different cluster of genes absent in rice and also are: UDPG UDP-glucose, sus: sucrose synthase, CS1: important in salt-tolerance physiology of the plant. cellulose synthase 1, GS1: glutamine synthase, SU IV subunit We assume that these genes probably contribute IV, MIPS L-myo-inositol-1-P synthase, ADH: alcoholdehydrogenase, HSP heat shock protein, FNR profoundly to the special physiological traits of the ferredoxin-NADP oxidoreductase, FD ferredoxin, cp plant, including high mechanical strength. The chloroplast. B. Functional grouping of salt-induced ESTs biological processes and the anatomical specialties from Porteresia coarctata are closely related to the restricted habitat of the plant; and one may conclude that the unknown or is repressed under submergence stress (Garg et al., hypothetical proteins/transcripts belong to the KK 2013). Among the salinity upregulated transcription genome complement of Porteresia, that is of untraced factors, NAC, MYB and WRKY are indicated, origin within rice genotypes. whereas among the submergence-upregulated The extended families of salt-induced transcription factors, bZIP, bHLH, HSF and AP2- transcription factors reported by Garg et al., (2013) EREBP families may play a major role. Incidentally are rather significant. Most of the TFs are upregulated NAC and MYB are the most important TFs associated during salinity stress and the transcriptional activity with lignin deposition in plants and so are bZIP and The Possible Overlap Between Salinity and Mechanical Challenges in Porteresia coarctata 1019

faced was that of osmotic stress. In a mangrove vegetation, the impact of tide is ecologically enormous on the pioneering undergrowth species, which remain under noticed. Species like Porteresia, forms a dense rhizomatous mat and gives support to soil, which helps in preventing soil erosion during high and low tides of salt-water. Its rhizoid-like roots are able to invade through comparatively restricted air spaces present amongst the soil particles in estuarine clay. They do not penetrate further in soil A B as sea tides are the major source of water. Robust vegetative propagation and high mechanical strength of rhizomes is a prerequisite for such pioneering species. Also, the subterranean part acclimatizes to the interplay of salt, mechanical and submergence stress, which seems to be Porteresia’s natural C D E ecological adaptation. Despite its high vegetative growth ability, it has never been reported from a non- saline ecosystem, unless artificially introduced. This suggests that Porteresiais a natural halophile, H although not an obligate. Its unique rhizomatous system may account both for its salinity and mechanical stress tolerance, probably more than the F G I J K aerial parts. Unfortunately, the ecology of root and Fig. 3: A-K A. Porteresia plant; B. IR-64 Rice potted plant; C. rhizome system architecture was never studied with Plantlets of Porteresia emerging from underground rhizomes respect to genetic basis in Porteresia coarctata, as D. Plantlets maintained in culture. Black arrow shows the rhizome. E. Multiple buds emerging from rhizome that has been studied for the shoot system (Sengupta and matures into shoots-shown by a black circle. F. Rice root Majumder, 2009). system G. Porteresia roots. H. Leaf transverse section of Porteresia (figure not to scale) I, J. Light microscopic view Overlap Between Salinity and Mechanical Stress of salt hairs (figure not to scale), K. Scanning Electron Microscope view of the salt hairs Cellular dehydration and osmotic adjustment brings a decrease in ambient osmotic potential. This also brings in mechanical stress, degree of which changes bHLH (Dharmawardhana et al., 2010). A close link with the volume of cell cytoplasm. Salt treated plant can be assumed between the mechanical stress cells show reorganization of cytoskeleton and changes tolerance, and saline submergence tolerance in this in abundance of several cytoskeletal and plant which is discussed in the next section. Table 2 cytoskeleton-associated proteins. Such structural presents all the genes identified from Porteresia till proteins also play a major role in maintaining date, most of which are reported to be upregulated mechanical strength of plant cells. Proteins having during salt stress. such shared role are actin, tubulin; profilin—an actin- A Hitherto Unexplored Relation to Mechanical binding protein involved in polymerization and depolymerization of actin filaments; kinesin—a Stress microtubule motor involved in microtubule- Mechanical stress is considered a complex form of dependent transport processes, especially during cell pressure-induced stress. It has often been suggested cycle and cytokinesis (Pang et al., 2010; Askari et that the first mechanical stress early microorganisms al., 2006; Wang et al., 2009; Dooki et al., 2006; 1020 Sonali Sengupta

Table 2. Genes/Loci so far reported from Porteresia coarctata in NCBI

Gene/Locus Accession no. Reference (Article/Gene) Oryza coarctata Na+/H+ antiporter (NHX1) JQ782416 Kizhakkedath P et al, 2003) Porteresia coarctata serine-rich protein (PcSrp) AF110148 Mahalakshmi S et al., (2006) Planta 224(2), 347-359 Porteresia coarctata V-ATPase subunit c (PVA1) AF286464 Unpublished (Senthilkumar P et al., 2000) Porteresia coarctata translational initiation factor eIF1 AF380357 Unpublished (Rangan L et al., 2001) Porteresia coarctata inositol 1-phosphate synthase (PINO1) AF412340 Majee et al., (2004) J. Biol. Chem. 279 (27), 28539-28552 Porteresia coarctata histone H3 AF109910 Senthilkumar P et al., (1999) Plant Physiol. 119(2), 806 Porteresia coarctata homeobox protein AF384375 Rangan L et al., 2001 Porteresia coarctata metallothionein AF257465 Padmanaban S et al., 2000 Porteresia coarctata alcoholdehydrogenase I (Adh1) AF148593 Ge et al., (1999) 96 (25), 14400-14405 Porteresia coarctata alcohol dehydrogenase II (Adh2) AF148628 Ge et al., (1999) 96 (25), 14400-14405 Porteresia coarctata maturase (matK) gene AF148669 Ge et al., (1999) 96 (25), 14400-14405 Porteresia coarctata fructose-1,6-bisphosphatase (PcCFR) AF218845 Chatterjee J et al., (2013) PCTOC Porteresia coarctata catA AB014455 Iwamoto M et al., (1999) Theor. Appl. Genet. 98, 853- 861 Oryza coarctata phosphoenolpyruvate carboxylase (PEPC) EU371116 Goswami L. et al., 2008 Oryza coarctata metallothionein type 3 (MT3) EU121847 Usha B et al., 2007 Oryza coarctata tRNA-Leu (trnL) AY792522 Guo YL and Ge, S Am. J. Bot. 92(9), 1548-1558 (2005) Oryza coarctata NADH dehydrogenase subunit 1 (nad1) AY507935 Guo YL. and Ge, S. (2003) Oryza coarctata G protein alpha subunit (GPA1) AY792544, Guo YL and Ge, S. Am. J. Bot. 92(9), 1548-1558 AY792545 (2005) Oryza coarctata Rp1-like protein pseudogene (OcRp1) GU733154 Luo S et al., Mol. Biol. Evol. (2011) 28(1), 313-325

Porteresia coarctata inositol methyl transferase (PcIMT) EU240449 Sengupta et al., 2008 Oryza coarctata ubiquitin 2 HQ340170 Philip A et al., (2013) Plant Cell Rep. 32 (8), 1199- 1210 Oryza coarctata iron deficiency-responsive cis-acting JN615010 Purohit D et al., (2011) element-binding factor 1 (IDEF1) Oryza coarctata inositol-1-phosphate synthase (INO1-1) FJ237299 Ray S et al., (2010) Planta 231 (5), 1211-1227 Oryza coarctata inositol-1-phosphate synthase (INO1-2) FJ237300 Ray S et al., (2010) Planta 231 (5), 1211-1227 Oryza coarctata triose phosphateisomerase EU371994 Sengupta S et al., (2008) Oryza coarctata plastid NADH dehydrogenase (ndhF) HE577878 Aliscioni S et al., (2012) New Phytol. 193 (2), 304-312 Oryza coarctata plastid ribulose bisphosphate carboxylase HE577876 Aliscioni S et al., (2012) New Phytol. 193 (2), (rbcL) 304-312 Oryza coarctata Ycf3 protein (ycf3) FJ908683 Tang L et al., (2010) Mol. Phylogenet. Evol. 54(1), 266-277 Oryza coarctata tRNA-Gly (trnG) FJ908510 Tang L et al., (2010) Mol. Phylogenet. Evol. 54 (1), 266-277 Oryza coarctata PSII 10kDa phosphoprotein(psbH) FJ908378 Tang L et al., (2010) Mol. Phylogenet. Evol. 54 (1), 266-277 Oryza coarctata ATP synthase beta chain(atpB) gene FJ908106 Tang L et al., (2010) Mol. Phylogenet. Evol. 54 (1), 266-277 Monoculm1 locus (Clone a0295K14 Monoculm1, FJ032636 Lua F et al., PNAS (2009) 106 (6) 2071–2076 Mlo family protein, aspartic proteinase nepenthesin-1 precursor, microtubule-associated protein MAP65-1a, IQ calmodulin-binding motif family protein,EMB2261 putative, polygalacturonase precursor, exopolygalac- turonase precursor, and putative RNA polymerase A(I) large subunit genes) The Possible Overlap Between Salinity and Mechanical Challenges in Porteresia coarctata 1021

Fatehi et al., 2012; Sobhanian et al., 2010; Du et al., In a saline-water-washed ecosystem, however, 2010). In salt stressed rice roots, a plant specific bending and escaping is not a feasible route. It is Myosin VIII heavy chain is upregulated that links necessary for the subterranean cells to maintain their cytoskeleton to cell wall linker and also activates osmotic pressure. The plant has to survive salinity callose synthase complexes in plasma membrane and the pressure exerted by high and low tides, and (Cheng et al., 2009). Remorin, is another plant- thus, the rhizome cells must show osmotic tolerance, specific plasma membrane/lipid raft-associated and must be mechanically very strong. There are filamentous protein which might play an important needs of lignification and secondary tissue role in cytoskeleton reorganization under salt and development in the root. The future research needs mechanical stress (Cheng et al., 2009). A common to address the rhizome biology of Porteresia; from a adaptation towards salinity is enhanced cellulose genomic, transcriptomic and proteomic aspect and synthase found in salt-treated Porteresia. This also needs to correlate that with the physiology. indicates a requirement of plasticity to adapt to an Porteresia in its native habitat, i.e. growing as a enhanced osmotic pressure (Sengupta and Majumder, mangrove undergrowth, covers all the interaction 2009) Similarly, an increased level of β-d- types discussed in Fig. 1. High anthropogenic input glucanexohydrolase was found in creeping bentgrass without attempts to domesticate renders Porteresia (Xu et al., 2010). Cell-wall associated glycine-rich as ecologically vulnerable species. Being the only proteins (GRP) reveal both mechanical and defence salt-tolerant wild rice, Porteresia represents a properties (Dooki et al., 2006; Du et al., 2010). valuable source of genes that may confer stability Changes in cytoskeletal as well as plasma-membrane and tolerance to high-yield sensitive rice and increase associated proteins with mechanical functions the viable agricultural area in the coasts. In our recent indicate profound alterations in both intracellular and work, we observed that the bending and directional cell wall architecture of plant cells facing the impacts growth of rice roots is similar in salt and mechanical of an osmotic stress. It is expected that at high tide, stress, with a large set of co-expressing transcripts Porteresia rhizome and roots would display an array (Unpublished data from laboratory, Adak et al.). It of proteins that will increase the mechanical strength will be of great interest to know what is the bending of the subterranean part alongwith providing an and structural alteration pattern that Porteresia root osmotic protection. No studies till date has been done and rhizomes may exhibit in a mangrove ecosystem with Porteresia growing in its native habitat, in high during high and low tides. It will be of further interest and low tidal conditions. No effort has been made to to determine what are the common molecular identify the effect of mechanical stress on this wild determinants that dictate the specific physiological rice. It is important to start such studies and look for characteristics and help the plant to survive the mechanical sensors and shared response pathways combined salinity, submergence and mechanical for coupled salt and mechanical stress in Porteresia challenges in nature. Mechanical stress and salinity, coarctata. The knowledge thus obtained can be probably has more shared characteristic than transferred to cultivated rice for agricultural benefits. acknowledged so far. A mangrove associate like Porteresia present us with ample scope to evaluate Conclusion such hypothesis. In our earlier work (Sengupta and Under high salinity, or mechanical obstruction, rice Majumder, 2009), we dissected the molecular root bends away from the saline zone and scans the physiology of this plant. Molecular ecophysiology rhizosphere for a stress-free area (Unpublished data of Porteresia coarctata will provide a more rational from the laboratory). Regulated by combined function insight into the biology of the system and help of ethylene and auxin on root growth, such bending understand the interplay of mechanical and salinity is a characteristic of the root system of a plant, and stress. can be used as a tolerance index (unpublished data). 1022 Sonali Sengupta

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