Osmolytes: Proline Metabolism In Plants As Sensors Of .

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ANSFUNDATI OFONED ANDPLINAPEAL SCIENCURATJournal of Applied and Natural Science 9 (4): 2079 -2092 (2017)JANS2008Osmolytes: Proline metabolism in plants as sensors of abiotic stressAshu Singh*, Manoj Kumar Sharma and R. S. SengarDepartment of Agriculture Biotechnology, S. V. Patel University of Agriculture & Technology, Meerut-250110(U. P.), INDIA*Corresponding author. E-mail: ashubiot25@gmail.comReceived: February 19, 2016; Revised received: April 18, 2017; Accepted: September 28, 2017Abstract: Proline accumulation occurs in a large range of plant species in retaliation to the numerous abiotic stresses. An exclusive research pattern suggests there is a pragmatic relation between proline accumulation and plantstress tolerance. In this review, we will discuss the metabolism of proline accumulation and its role in stress tolerance in plants. Pertaining to the literature cited clearly indicates that not only does it acts as an osmolyte, it alsoplays important roles during stress as a metal chelator and an antioxidative defence molecule. Moreover, when applied exogenously at low concentrations, proline enhanced stress tolerance in plants. However, some reports pointout adverse effects of proline when applied at higher doses. Role of proline gene in seed germination, flowering and otherdevelopmental programmes; thus creation of transgene overexpressing this gene would provide better and robustplants. In this context this review gives a detailed account of different proline gene over-expressed in all the transgenic crops so far.Keywords: Abiotic stress, Osmoprotectant, Proline, ROS, TransgenicINTRODUCTIONPlants being exposed to different environmental stresses learn and or adopt to these stresses ina number ofways. Thus among these stresses, Osmolytesproduction stress, in particular that is caused due toabioteic stress such as drought and salinity is the mostcritical problem that limits plant growth and cropproductivity in agriculture (Boyer, 1982). Plantphysiological development and productivity is largelyaffected by many environmental stresses such asdrought, high salinity, and low temperature. Thesestresses trigger expression of an array of gene inresponse. Thus the expressed products of these genesfunction not only in stress response but also in stresstolerance. In the signal transduction network from onsive gene expression, many concernedtranscription factors and cis-acting elements in thestress-induced promoters function for plant mitigationto environmental stresses. Recent a lot of advanceshave been made in studying the complex cascades ofgene regulation in drought and cold stress responses,especially in identifying specificity and cross talk instress signaling. In this review article, we highlighttranscriptional regulation of gene expression inresponse to abiotic stresses, with particular emphasison the role of transcription factors and cis-actingelements in stress-inducible promoters. Genesexpressed during stress conditions function not only inprotecting cells from stress by producing importantmetabolic products/bio-molecules, but also in regulatinggenes for signal transduction in the stress response.Thus, these gene products are classified into twogroups (Fowler and Thomashow, 2002; Kreps et al.,2002; Seki et al., 2002). The primary group includesproteins that mainly function in stress subjection, suchas osmotin, antifreeze proteins, chaperones, LEA (lateembryogenesis abundant) proteins, RNA-bindingproteins, and it also includes important catalyzingbio-molecules i.e enzymes for osmolyte biosynthesissuch as water channel proteins, proline, sugar and proline transporters, The second group contains proteinfactors which play important role in regulation ofsignal transduction and gene expression machinery thatprobably works in response to stress reflex. Thisincluded an array of different transcription factors,suggesting that various transcriptional regulatorymechanisms function in the drought-, cold-, orhigh-salinity-stress signal transduction pathways (Sekiet al., 2003). Many works indicate that there are morethan 300 genes that have been identified as being stress-inducible (Kazuo and Kazuko, 2006). Among thesedifferent physiologically activated genes, more thanhalf of the drought induced genes are also induced byhigh salinity, indicating the existence of significantcross talk between the drought and high-salinityresponses. The section of small molecules known as"compatible osmolytes" includes certain amino acids(notably proline), quaternary ammonium compounds(e.g. glycinebetaine, proline betaine, β-alanine betaine,ISSN : 0974-9411 (Print), 2231-5209 (Online) All Rights Reserved Applied and Natural Science Foundation www.jans.ansfoundation.org

Ashu Singh et al. / J. Appl. & Nat. Sci. 9 (4): 2079 -2092 (2017)and choline-O-sulfate), and the tertiary sulfoniumcompound 3-dimethylsulfoniopropionate (DMSP).Throughout their life cycle, plants are subjected tovarious types of environmental stresses, water deficit,temperature extremes, toxic metal ion concentrationand UV radiations depending upon the severity ofstress. The environmental factors retard the growth andproductivity of plants to different degrees. In reflexresponse to different stresses plants accumulate largequantities of different types of compatible solutes(Serraj and Sinclair, 2002). Thus accumulation ofosmolyte compounds, usually called „osmoticadjustment‟ or „osmoregulation‟, is certainly a remedialmeasure to overcome the negative consequence ofwater deficite condition in plant‟s growth and survival.It has been proposed since long before as a remedialmechanism for drought and salt tolerance (Martin,1930; Bernstein, 1961), but it has gained fame duringthe last 20 years. Compatible solutes are low molecularweight, highly soluble organic compounds that areusually non-toxic at high cellular concentrations. These solutes provide immunity, to plants fromenvironmental-induced stress by regulating cellularosmotic adjustment, ROS detoxification, protection ofmembrane integrity and enzymes/protein stabilization(Ashraf and Foolad, 2005; Bohnert and Jensen, 1996;Yancey, 1994) These include proline, sucrose, polyols,trehalose and quaternary ammonium compounds(QACs) such as glycine betaine, alinine betaine, proline betaine and pipecolate betaine Exepidition of theaction of these solutes would providea clear cut evidence to combat environmental stresses, which is important as it gives direct hope to genetically manipulateplants to withstand this condition. There are manycellular mechanisms by which organisms mitigate theeffects of abiotic stresses; for instance, accumulationof compatible osmolytes such as proline is one suchphenomenon. The phenomenon of proline accumulationis known to occur under water deficit (Naidu et al.,1991), Salinity (Munns, 2005; Rhodes and Hanson1993), low temperature (Hare et al., 1998), heavy metal exposure (Bassi and Sharma, 1993a; Bassi and Sharma, 1993b; Schat et al., 1997; Sharma and Dietz,2006) and UV radiations, etc. Apart from acting asosmolyte for osmotic adjustment, proline contributesto stabilizing sub-cellular structures (e.g., membranesand proteins), scavenging free radicals and bufferingcellular redox potential under stress conditions (Ashrafet al., 2007). In many plant species, proline accumulationunder salt stress has been correlated with stress tolerance,and its concentration has been shown to be generallyhigher in salt tolerant than in salt sensitive plants(Fougère et al., 1991; Gangopadhyay et al., 1997;Madan et al., 1995; Petrusa and Winicov, 1997). Itmay also act as protein compatible hydrotrope(Strizhov et al., 1997), alleviating cytoplasmic acidosisand maintaining appropriate NADP /NADPH ratioscompatible with metabolism. Work relating to this i.eproline osmoprotectant is reviewed here. Somegeneralizations can be made: Firstly, the availability ofthe precursor to synthesize the osmoprotectants couldlimit the amount of osmoprotectant made in a transgenichost. Secondly, negative physiological consequencesof diverting the precursor to the osmoprotectants awayfrom primary metabolism should be considered (Suand Wu 2004). Thirdly, despite the availability ofphysiological data and techniques for assessing stresstolerance in plants, but still transgenic plants are rarelyhad been put to test their functioning which calls fortheir examination. Proline accumulation normallyoccurs in cytoplasm where it plays the role of molecularchaperons stabilizing/conditioning the structure ofproteins and its accumulation by buffering cytosolicpH and maintaining cell redox status. It has also beenproposed that its accumulation may be part of stresssignal cascade influencing adaptive responses thustaking in account this feature would be beneficial toincur stress tolerance via engineering transgenic overexpressing proline gene (Hoque et al., 2008).SENSORS OF ABIOTIC STRESS- OSMOLYTESThere are many mechanisms at cellular level throughwhich organisms ameliorate the effects of environmental stresses; for instance, accumulation of compatible osmolytes such as proline is one such phenomenon. Many plants, including halophytes, accumulatecompatible osmolytes, such as proline (Pro), glycinebetaine and sugar alcohols, when they are exposed todrought or salinity stress (Hellebust, 1976; Csonka,1989; McCue KF, Hanson, 1990; Delauney and Verma, 1993). The accumulation of Pro has been observednot only in plants but also in eubacteria, marine invertebrates, protozoa, and algae (Delauney and Verma,1993; Roosens et al., 2002). It has been suggested thatcompatible osmolytes do not interfere with normalbiochemical reactions and act as osmoprotectants during osmotic stress. Among known compatible solutes,Proline is probably the most widely distributed osmolytes. Results of investigations of the relationship between the expression of these genes and the accumulation of Proline under water stress indicate that the levelof Pro in plants is mainly regulated at transcriptionallevel during water stress. Moreover, the overproduction of Pro results in the increased tolerance of transgenic tobacco plants to osmotic stress. Thus tolerancetoabioticstress,especially to salt and improved plant growth, wasobserved in a variety of transgenics that wereengineered for overproduction of proline (Kavi Kishoret al., 1995; Bohnert and Shen, 1999; Kavi Kishor etal., 2005). Proline seems to have diverse roles underosmotic stress conditions, such as stabilization ofproteins, membranes and sub-cellular structures andprotecting cellular functions by scavenging reactive2080

Ashu Singh et al. / J. Appl. & Nat. Sci. 9 (4): 2079 -2092 (2017)oxygen species (Sasaki et al., 2005).Salinity is detrimental to the various processes of cropssuch as seed germination, seedling growth and vigor,vegetative growth, flowering and fruit set andultimately it causes diminished economic yield andalso quality of produce (Stewart and Larher, 1980).Rice crop is important not only as food crop but alsodue to its medicinal value (Bajaj and Mohanty, 2005),due to virtue of it, it acts as a model monocot systemfor various biotechnological, metabolic, geneticengineering and functional genomics developmentstudies worldwide (Munns and Tester, 2008).However, the yield of rice, especially Asian rice, isseverely susceptible to salinity (Sairam et al., 2005). InIndia and especially the rainfed rice is hindered bythree major abiotic stresses namely drought,submergence and Salinity (Rice KnowledgeManagement Portal, 2011).PROLINE AND ITS FUNCTION IN OSMOREGULATIONProline plays versatile functions in plants. As aminoacid it is a one of the building blocks of proteinstructure, but it also plays a major role in of stressosmolytes solute under environmental stress conditions.Proline synthesis has been associated with tissuesundergoing rapid cell divisions, such as shoot apicalmeristems, and appears to be involved in floral transitionand embryo development. Lofty levels of proline canbe found in pollen and seeds, where it serves ascompatible solute, where it acts as dehydrationprotector of cellular structures during plantdevelopment. The agglomeration of proline at variousterrain such as cells, tissues and other vital organs suchas vascular bundles are controlled by reciprocity ofbiosynthesis, degradation, and cellular transport arcade.Thus, both the uniques properties of proline and itsvariegated action through two most widely studiedtransporter, both general amino acid permeases andselective compatible solute transporters indicates itsprime position to be use in production of abiotic resilientplants engineered through manipulating it genes(Armengaud et al., 2004).All the mechanisms encompassing the proline actionbe it accumulation or degradation shows that mechanismsregulating proline differ substantially from other aminoacids (Yu et al., 1983). Proline accumulation is acommon metabolic riposte, of higher plants to waterdeficits, and salinity stress, and has been the subject ofnumerous reviews over the last 20 years (Stewart andLarher, 1980; Thompson, 1980; Stewart, 1981; Hansonand Hitz, 1982; Samaras et al., 1995; Taylor, 1996;Rhodes et al., 1999). This versatile amino acid hashighest water solubility and is accumulated by leavesof many halophytic higher plant species grown insaline environments (Stewart and Lee, 1974; Briensand Larher, 1982; Treichel, 1986), in leaf tissues andshoot apical meristems of plants undergoing waterstress (Barnett and Naylor, 1966; Boggess et al., 1978;Jones et al., 1980) in desiccating pollen (Hongqi et al.,1982), in root apical regions growing at low waterpotentials (Voetberg and Sharp, 1991), and insuspension cultured plant cells reorganized to waterstress (Tal and Katz, 1980; Rhodes, 1987), or NaClstress (Tal and Katz, 1980; Rhodes and Handa, 1989;Thomas et al., 1992). Proline shields membranes andproteins against the adverse effects of elevatedconcentrations of inorganic ions and extremetemperature (Pollard et al., 1979; Paleg et al., 1981;Nash et al., 1982). Proline may also function as aprotein-compatible hydrotrope (Srinivas and Balasubramaniam, 1995) and as a hydroxyl radical scavenger (Smirnoff and Cumbes, 1989). The proline gathered in response to water stress or salinity stress inplants is primarily confined in the cytosol (Leigh et al.,1981; Pahlich et al., 1981; Ketchum et al., 1991). Inaddition, the transient accumulation of proline, mightserve as a safety valve to calibrate cellular redox stateduring stress (Shen et al., 1999; Kuznetsov andShevyakova, 1999).PROLINE METABOLISM AND ITS IMPLICATIONS FOR PLANT-ENVIRONMENT INTERACTIONVery high agglomeration of cellular proline (upto 80%of the amino acid pool under stress and 5% under normal condition) has been documented in many plantspecies (Choudhary et al., 2005; Widodo et al., 2009).The increase in osmoprotectants is achieved either byamendment of metabolism (increasing biosynthesisand/or decreasing degradation) or by transport(increase uptake and/or decrease export) which alsodepends upon the type of stress and the type of speciesunder consideration. Unlike other amino acids, prolinehas cyclized amino nitrogen that has significantinfluence on the conformation as well as uniqueness ofpolypeptides. Proline is also a prime component ofstructural proteins in animals and plants besides beinga known osmoprotectants capable of mitigating thefootprint of drought, salt, and temperature stress inplants (Rodriguez and Redman, 2005).Proline, and its metabolism, is eminent from other amino acids in several ways. In plants proline issynthesized from glutamate as well from arginine/ornithine. The most fundamental is that proline is theonly one of the proteogenic amino acids where thea-amino group is present as a secondary amine. Whilethis may seem like a distinction more important tochemists than plant biologists, the unique properties ofproline are highly relevant to understanding its role inplants. Second important feature of proline has beenstudied a lot and recorded that its accumulation iscaused by most of stresses relating to environmentalstresses (Hare et al., 1998). The role of proline and2081

Ashu Singh et al. / J. Appl. & Nat. Sci. 9 (4): 2079 -2092 (2017)Fig. 1. An overview of salinity stress.sulphur metabolism during osmotic stress tolerance inplants has been emphasized recently (Verma, 1999).Gene involved in biosynthesis of enzymes forbiosynthesis and degradation of Proline is very welldocumented. Results of exploration of the relationshipbetween the expression of these genes and theagglomeration of proline under deficit water stressindicate that the level of proline in plants is mainlyregulated at micro-cellular transcriptional level duringwater stress. Moreover in this context, the overproductionof proline results in the increased tolerance oftransgenic tobacco plants to osmotic stress was reportedby Dobra et al. (2011).Fig. 2. Potential roles of proline during abiotic stress(Made by the author).The basic of proline metabolism involves two enzymescatalyzing proline synthesis from glutamate in thecytoplasm or chloroplast, two enzymes catalyzing proline catabolism back to glutamate in the mitochondria, aswell as an alternative pathway of proline synthesis viaornithine (Fig. 3). The inter conversion of proline andglutamate is sometimes referred to as the “proline cycle”. The transcriptional up regulation of proline synthesis from glutamate and down regulation of prolinecatabolism during strain condition provides a ency to this pattern has been observed (Stineset al., 1999). This is not the only side of the storyhowever, as posttranslational modulation, of theseenzymes has been little inspected and the role ofornithine as a proline precursor remains obscure(Phang, 1985). Likewise, the proline cycle may at firstseem to be an ineffectual cycle; however, apprehendingthe intrinsic corporative modulation of this cycle andmetabolic flux is the clue to understanding the prolinemetabolism biochemistry.Proline not only being n important constituent ofprotein it is also a very versatile molecule playing aimportant part in osmoprotectant, cellular signalmolecule during stress condition. After plant exposedto salt stress in Arabidopsis it accounted for 20% of theamino acid pool (Verbruggen et al., 1996). There aretwo different pathways in proline biosynthesis in higher plants: the ornithine and the glutamatepathways. The plant glutamate pathway is quitedifferent from those in microbes and human. Inbacteria and human, the conversion of glutamate toglutamate-5-semialdehyde (GSA) is catalyzed by twoenzymes via two successive reactions, whereas, inhigher angiosperms the conversion is catalyzed by a bi-functional enzyme in a single step reaction (Hu et al.,1992). This brings us to the conclusion that manyresearch activities are being drawn specificallystudying salinity and drought tolerance induced byproline (Williamson and Slocum, 1992).The pathway for the biosynthesis of proline in plantswas elucidated by reference to the pathway inEscherichia coli (Leisinger, 1987). Fig. 3 shows theproline biosynthesis and metabolism pathway inplants. The pathway in bacteria begins with theATP-dependent phosphorylation of the γ-carboxygroup of L-glutamic acid (L-G1U) by γ-glutamyl kinase(γ-GK). The product of γ-GK is reduced to glutamic-γsemialdehyde (GSA) by GSA dehydrogenase(GSADH), with which γ-glutamyl kinase forms anobligatory enzyme complex. GSA cyclizes spontaneouslyto form 1-pyrroline-5-carboxylate (P5C), which isfinally reduced to proline by P5C reductase (P5CR). Ithas been suggested that, in plants, proline issynthesized either from Glu or from ornithine and thatthe pathway from Glu is the primary route for thesynthesis of Pro under conditions of osmotic stress and2082

Ashu Singh et al. / J. Appl. & Nat. Sci. 9 (4): 2079 -2092 (2017)Table 1. Tra

O U N D A T I O ANSF N Journal of . (Bassi and Sharma, 1993a; Bassi and Shar-ma, 1993b; Schat et al., 1997; Sharma and Dietz, 2006) tion of Proline under water stress indicate that the level and UV radiations, etc. Apart from acting as osmolyte for osmotic adjustment, proline contributes to stabilizing sub-cellular structures (e.g., membranes and proteins), scavenging free radicals and .

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