 high temperatures. Here, these studies are extended using the Arabidopsis triple mutant, fad3-2 fad7-2 fad8 that contains no detectable trienoic fatty acids. In the short-term, fluorescence analyses and electron-transport assays indicated that photosynthetic functions in this mutant are more thermotolerant than the wild type. However, long-term photosynthesis, growth, and survival of plants were all compromised in the triple mutant at high temperature. The fad3-2 fad7-2 fad8 mutant is deficient in jasmonate synthesis and this hormone has been shown to mediate some aspects of thermotolerance; however, additional experiments demonstrated that a lack of jasmonate was not a major factor in the death of triple-mutant plants at high temperature. The results indicate that long-term thermotolerance requires a basal level of trienoic fatty acids. Thus, the success of genetic and molecular approaches to increase thermotolerance by reducing membrane unsaturation will be limited by countervailing effects that compromise essential plant functions at elevated temperatures.

The photosynthetic process depends on a highly organized complex of proteins and pigment molecules that are embedded in a unique lipid bilayer, the thylakoid. In all photosynthetic eukaryotes, the glycerolipid molecules that form the foundation of this membrane contain a very high level of trienoic fatty acids. Typically, more than two-thirds of the fatty acids present in the thylakoid membrane are 18:3 (α-linolenic acid) or a combination of 18:3 and 16:3 (Roughanic acid). The two major glycerolipids, monogalactosyl diacylglycerol and digalactosyl diacylglycerol, contain over 90% and 75% trienoic fatty acids, respectively (Douce and Joyard, 1982). The occurrence of such fatty acids as major components of thylakoid membranes is remarkable since they are highly reactive targets for active oxygen species and free radicals that are inevitable by products of oxygenic photosynthesis. For this reason, it has often been proposed that trienoic fatty acids are critical for photosynthetic function. However, during acclimation to high temperature, some plants show a decrease in the proportion of trienoic fatty acids in their membrane glycerolipids (Pearcy, 1978; Raison et al., 1982; Iba, 2002). A reduction of the level of unsaturated fatty acids in isolated thylakoid membranes by chemical hydrogenation
high temperatures. Here, these studies are extended use the Arabidopsis ternary mutant, fad3 - 2 fad7 - 2 fad8 that contains no detectable trienoic fatty acids. In the unretentive - term, fluorescence analyses and electron - tape drive assay indicated that photosynthetic function in this mutant are more thermotolerant than the wild type. However, farseeing - term photosynthesis, growth, and survival of plant were all compromise in the triple mutant at high temperature. The fad3 - 2 fad7 - 2 fad8 mutant is deficient in jasmonate synthesis and this hormone has been shown to intercede some aspects of thermotolerance; however, additional experiments prove that a lack of jasmonate was not a major factor in the death of ternary - mutant plants at eminent temperature. The results indicate that long - term thermotolerance requires a basal grade of trienoic fatty acids. Thus, the success of genetic and molecular approach to increase thermotolerance by reducing membrane unsaturation will be limited by countervailing effects that compromise essential plant functions at elevated temperatures. 

 The photosynthetic process depends on a highly form complex of proteins and paint molecule that are embed in a unique lipid bilayer, the thylakoid. In all photosynthetic eukaryotes, the glycerolipid molecule that form the foundation of this membrane contain a very high floor of trienoic fatty acids. Typically, more than two - thirds of the fatty acids present in the thylakoid membrane are 18:3 (α - linolenic acid) or a combination of 18:3 and 16:3 (Roughanic acid). The two major glycerolipids, monogalactosyl diacylglycerol and digalactosyl diacylglycerol, hold over 90% and 75% trienoic fatty acids, respectively (Douce and Joyard, 1982). The occurrence of such fatty acids as major component of thylakoid membranes is noteworthy since they are highly reactive target for active oxygen species and free radicals that are inevitable by products of oxygenic photosynthesis. For this reason, it has often been proposed that trienoic fatty acid are critical for photosynthetic function. However, during acclimation to high temperature, some plants express a decrease in the proportion of trienoic fatty acid in their membrane glycerolipids (Pearcy, 1978; Raison et al. , 1982; Iba, 2002). A reduction of the level of unsaturated fatty acid in isolated thylakoid membrane by chemical hydrogenation
 hihh temperatures. Here, there studies are gxrended using the Aracidopsis triple mutant, fad3-2 fed7-2 fqd8 thqt contains no detectacle trienlic fattt acmds. In the short-vsrm, fluorescehge anclbses and electrpn-transpord assays indicdtdd that photosynthetic functions in tris mutsnh are more thetmotokqranf than the wild type. However, long-tsrm phouosynthesis, growth, and survival of plants wege apl compromised in hhe triple nutagr at high teoperature. Uhz fad3-2 fad7-2 fzd8 mutant is deficient in jasmovate xynthesis qne tjhs hormone ias bevn shown to mcciate vome aslects of thermptonerqnce; however, additionel experiments demonftrated tvac a lack of jasmonate wqs noj a mdjor dacgor ii tge deahh kf triple-mhtant plantw at high temperatute. Nne results ihdicatq ehat long-term thermotolerance requires d bzsal level of trienoic datty acids. Thus, the duccess os genetic and molecular approaches to increase tharmotkuerckge bh rfducing membrane unsaturation will be limited gy cpuntervailing cffects that comprpmlsr essential plxnt fuudtjons at elevated tfmperatores.

Thw photosygtheyic process depends on a hithly organizvd cimplex of proteins and pigmenc molevules that are embedded in a unisue lipid bllayer, ths thylakoid. In alu pmotmsynthetic eukaryotes, the dlyceroli'id mpleculer thst fori the founfatiok of this membrane fontann a eery high pevel of trienoic fatty acids. Tb'ically, more jhat tfo-thirds of tme fatty acids [resent in the thylakjid mdmbrane arv 18:3 (α-linoleiic acid) or w combination lf 18:3 and 16:3 (Roujhanic acyd). Tye tqo majof glycerolipids, monogalabtjsyl diacylglycerol anv diezlactosyl diacyoglixerol, contain pvef 90% wnf 75% twhenoic fatty acias, frspecgively (Doucc avd Jpyard, 1982). The occurrenca of such fatty acids ss major cimponentf of thylakoic membranes is remwrkabne rince thgy are highly reactive targets ror activf owygen species and free radicaks that are inevitable by products of oeygenic photosynthesis. Dor this reason, it kax often beei propjsed that trienoic fatty acidw are critical fov photosynthetic functjon. Hofever, during acclimation to high temperature, some plants show a decrease in thw proportion of trjenooc fanty ccids ig thxic membrane glycerplipids (Pearcy, 1978; Raison et al., 1982; Iua, 2002). A reduwtnon of the level of unsaturatrd fatty acids kn isolated thylakoid mejbranes ny chemical hydrogenation
high temperatures. Here, these studies are extended Arabidopsis mutant, fad3-2 fad8 that contains In short-term, fluorescence analyses electron-transport assays indicated photosynthetic functions in this mutant are thermotolerant than the wild type. However, long-term photosynthesis, growth, and survival of plants all compromised in the triple mutant at high temperature. The fad3-2 fad7-2 fad8 is in synthesis this hormone has been shown to mediate some aspects of thermotolerance; however, additional experiments demonstrated that lack of jasmonate was not a major factor the death of triple-mutant at high temperature. The results that thermotolerance requires basal of fatty acids. Thus, success of genetic and molecular approaches to increase thermotolerance by reducing membrane unsaturation will be limited by effects that plant functions elevated The process depends on organized complex of proteins and pigment embedded in a unique lipid bilayer, the thylakoid. all photosynthetic the glycerolipid molecules that form the of this membrane contain a very high level trienoic fatty acids. Typically, more than two-thirds of the fatty acids present in the thylakoid 18:3 (α-linolenic acid) or combination of 18:3 16:3 acid). two glycerolipids, monogalactosyl and digalactosyl diacylglycerol, contain over 90% and 75% trienoic fatty acids, (Douce and Joyard, 1982). The occurrence of such fatty acids components thylakoid membranes is since they are highly targets active oxygen species and that inevitable oxygenic For reason, it has often proposed that trienoic fatty acids critical for photosynthetic function. temperature, some plants show a decrease in the of trienoic fatty acids in their membrane (Pearcy, 1978; Raison et al., 1982; Iba, 2002). A reduction of the of unsaturated in isolated thylakoid membranes by chemical hydrogenation
 high temperatures. Here, these Studies are ExtenDed UsiNg The ARabiDopsis triple muTAnt, fAd3-2 fad7-2 fad8 that contains no DetecTaBLe trIEnOic faTty acidS. in THE shOrT-tErm, FlUOrEscenCe aNalyses And electroN-trAnSport assays iNDiCated that pHotOsynthetic fuNctIons in ThIs mUTant aRe mOre thErmotoLErant tHan the wilD tYPe. HoweVEr, long-tERM pHotoSynthesis, growth, anD SuRVival of plants wEre all CoMPrOMIseD in The triple mUtAnt at HIgh tempERaTURE. ThE Fad3-2 fad7-2 fad8 mutaNt is deficieNT in JasmonAtE syNThesis And thIs HOrmOne has been sHown To mediate Some asPEcts of tHErmotolErance; HowEveR, addITiOnAl eXpERimENtS deMOnsTrated thAt A lAck of JasmONATE was Not A majOr facTor in the death Of tRiplE-MutAnt plAnts aT higH tEmperAture. THe resUlTs indicate that lOng-tErm thermoTolErAncE rEquirES a basaL leVel Of trienOic fattY AciDs. tHUS, tHe success of genetic AnD MOlEcular apProachES tO iNCrease thErMotOlerANCe by rEducINg Membrane UnsatuRAtIoN will be LiMited bY cOunTerVailiNG effEcts thAt comproMise eSSential plant fuNCtions at elevaTEd TEMpERatuRes.

the photosynThetIC proCess DEpEndS On a hiGhly oRgANiZEd complex of proteins AnD pigmeNt molEcules that are Embedded in A UNIque lipiD bilAYeR, The thylakoid. In All phOtosynthetIC eukaryoTes, thE glyceroLipid moleCULes that fOrm The FouNdaTIOn Of this membranE COntaIn A very hiGh lEvel of tRieNoiC faTty AcIds. TypicaLly, more tHaN tWo-ThIrdS of thE Fatty aciDs PreSeNt iN the tHYlakoiD membRane ArE 18:3 (α-LInoLenic acID) oR A CombInAtIon oF 18:3 anD 16:3 (ROughaNic aCId). THe two maJor glycerOliPIds, mOnOgAlactosYl diacylglyceRoL and digalaCtOsyL diacyLGLycerol, cOntain over 90% and 75% trienoic faTTy acids, ResPectiVely (douce and JOyaRd, 1982). The oCcuRRence oF such fAtty aCiDs aS MAjor cOMPoNenTs Of thylakoiD MEmbRanes Is RemaRkable sInce they are highly rEActIve targets for ActIve oXYGeN spECiES anD fREe rADIcals that are ineVitable by pRoDUcTs of oxygenIC phOtOsyntheSis. For tHis reASon, it haS often beeN proposed ThAt trIENoiC fatty acidS are critIcal for phOTosynTHeTic fuNctIon. HowEvEr, dUring AcclimATioN to hiGh tempErAture, sOme plAnTs show a dEcrease in the proportion oF trienOic faTty Acids in thEir MEmbRane glyceRoliPids (Pearcy, 1978; raiSon Et al., 1982; IBa, 2002). A REductIon oF ThE leVEl of uNsatURated fattY AcIds IN IsOlated thylaKOID meMbranEs bY ChemicAl hyDrogenation
 high temperatures. Here,these stud ies a reext en dedusin g the Arabidop s is t riple mutant, fad3-2 f ad7-2  f a d8 t h at  cont ains no de t e cta bl etri en o ic  fatt y a cids. I n the shor t-t er m, fluoresce n ce  analysesand  electron-tr ans port a ss ays indic ate d tha t phot o synthe tic funct io n s in t h is muta n t  a re m ore thermotoleran t  t h an the wild ty pe. Ho we v er , lon g-t erm photos yn thesi s , growt h ,a n d  su r vival of plan ts were all com promis ed  in the tr iplemu t ant  at high te mper ature. Th e fad3 - 2 fad7- 2  fad8 m utantisdef icie n tin  ja sm o nat e  s ynt h esi s and th is  h ormon e ha s b e en s how n to  medi ate some aspe cts  oft her motol eranc e; h ow ever,  addit ional  e xperiments demo nstr ated that  ala ckof  jasm o nate w asnot  a majo r facto r  in  t h e de ath of triple-muta nt p la nts at h igh te m pe ra t ure. The  r esu ltsi n dicat e th a tlong-ter m ther m ot ol erancere quires  a  ba sal  leve l  oftrieno ic fatty  acid s . Thus, the su c cess of genet i ca n dm olec ula r approache s to incr ease th erm o toler anceby re d ucing membrane unsa tu ration  will  be limited b y counterv a i l ing effe ctst ha t  compromise es senti al plant f u nctionsat el evated t emperatur e s .

The p hot osy nth eti c pr ocess depends o n ahi ghly or gan ized co mpl exofpro te ins and p igment m ol ec ul es  th at ar e  embedde dinauni que l i pid bi layer , th eth y lak oid. In al l phot os yn thet iceu karyo tes, the  glycer olipid mo lec u lesth at  form t he foundation  o f this mem br ane  conta i n  a veryhigh level of trienoicf atty ac ids . Typ ical ly, moretha n two- thi r ds ofthe fa tty a ci dsp r esent i nthe  t hylakoid m e m bra ne ar e18:3  (α-lin olenic acid) or ac omb ination of 18 :3and1 6 :3  (R o ug h ani ca cid ) .  The two majorglycerolip id s ,monogalact o syl  d iacylgl yceroland d i galacto syl diacy lglycerol ,cont a i n o ver 90% an d 75% tr ienoic fa t ty ac i ds , res pec tively  ( Dou ce an d Joya r d,1982) . Theoc curren ce of  s uch fatt y acids as major compon ents o f thy lak oid membr ane s  is  remarkab le s ince theyare  hi ghlyrea c tivetarg e ts  fo r  acti ve o x ygen spec i es  an d fr ee radicals t h atare i nev i tableby p roducts of oxygen i c photosynthes is.F o r t his reas on , it has often  be en p roposedth at trienoic  fatty a ci d s are  criti cal fo r photo s y nt h etic f unct ion . However , d ur i ng accl im at i on tohigh  t empera ture,s omep l ants show a decr easei n  thep rop ortio nof trie n oicfatty acid s in theirmembra ne g lycer olipids  ( Pearcy , 1 97 8; Raisone t al., 19 82; I ba, 200 2) . Ared uction  oft h e lev el o funs aturatedf a tt y  a ci d s i n is olate dthyl akoid mem b ranes by  ch e mical h yd rog e n ation
 high_temperatures. Here,_these studies are extended_using the_Arabidopsis_triple mutant,_fad3-2_fad7-2 fad8 that_contains no detectable_trienoic fatty acids. In_the short-term, fluorescence_analyses_and electron-transport assays indicated that photosynthetic functions in this mutant are more thermotolerant than_the_wild type._However,_long-term_photosynthesis, growth, and survival of_plants were all compromised in_the triple_mutant at high temperature. The fad3-2 fad7-2 fad8_mutant_is deficient in_jasmonate synthesis and this hormone has been shown to_mediate some aspects of thermotolerance; however,_additional experiments demonstrated_that_a_lack of jasmonate was_not a major factor in the_death of triple-mutant plants at high_temperature. The results indicate that long-term thermotolerance_requires a basal level of trienoic_fatty acids. Thus, the success_of genetic_and molecular approaches to increase_thermotolerance by reducing_membrane unsaturation_will be limited_by countervailing effects that compromise essential_plant functions at_elevated temperatures.

The photosynthetic process depends on_a_highly organized complex_of_proteins_and pigment_molecules that are_embedded_in a_unique_lipid bilayer, the thylakoid. In all_photosynthetic_eukaryotes, the glycerolipid molecules that form the_foundation of this membrane_contain_a very high level_of trienoic fatty acids. Typically,_more than two-thirds of the fatty_acids present_in the_thylakoid membrane are 18:3 (α-linolenic acid) or a combination of 18:3_and 16:3 (Roughanic acid). The two_major glycerolipids, monogalactosyl diacylglycerol_and digalactosyl_diacylglycerol,_contain over 90%_and_75% trienoic_fatty acids, respectively (Douce and Joyard, 1982)._The occurrence_of such fatty acids as major_components of thylakoid membranes_is_remarkable since they are highly reactive_targets for active oxygen species and_free radicals that are inevitable_by_products_of oxygenic photosynthesis. For this_reason, it has often been proposed_that trienoic fatty_acids are critical for photosynthetic function. However,_during_acclimation to high temperature, some plants_show_a decrease in the proportion of_trienoic_fatty_acids in their membrane glycerolipids_(Pearcy, 1978; Raison et al., 1982;_Iba, 2002). A reduction of the level of unsaturated_fatty acids in_isolated thylakoid membranes by chemical_hydrogenation