plants-05-00016.pdf
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1、plantsArticlePaired Hierarchical Organization of 13-Lipoxygenasesin ArabidopsisAdeline Chauvin1,2,Aurore Lenglet2,Jean-Luc Wolfender1and Edward E.Farmer2,*1School of Pharmaceutical Sciences,University of Lausanne,University of Geneva,quai Ernest-Ansermet 30,CH-1211 Geneva 4,Switzerland;(A.C.);jean-l
2、uc.wolfenderunige.ch(J.-L.W.)2Department of Plant Molecular Biology,University of Lausanne,CH-1015 Lausanne,Switzerland;aurore.lengletunil.ch*Correspondence:edward.farmerunil.ch;Tel.:+41-21-692-4228These authors contributed equally to this work.Academic Editor:Eleftherios P.EleftheriouReceived:10 No
3、vember 2015;Accepted:18 March 2016;Published:24 March 2016Abstract:Embryophyte genomes typically encode multiple 13-lipoxygenases(13-LOXs)that initiatethe synthesis of wound-inducible mediators called jasmonates.Little is known about how theactivities of these different LOX genes are coordinated.We
4、found that the four 13-LOX genes inArabidopsis thaliana have different basal expression patterns.LOX2 expression was strong in softaerial tissues,but was excluded both within and proximal to maturing veins.LOX3 was expressedmost strongly in circumfasicular parenchyma.LOX4 was expressed in phloem-ass
5、ociated cells,incontrast to LOX6,which is expressed in xylem contact cells.To investigate how the activities of thesegenes are coordinated after wounding,we carried out gene expression analyses in 13-lox mutants.This revealed a two-tiered,paired hierarchy in which LOX6,and to a lesser extent LOX2,co
6、ntrolmost of the early-phase of jasmonate response gene expression.Jasmonates precursors producedby these two LOXs in wounded leaves are converted to active jasmonates that regulate LOX3 andLOX4 gene expression.Together with LOX2 and LOX6,and working downstream of them,LOX3 andLOX4 contribute to jas
7、monate synthesis that leads to the expression of the defense gene VEGETATIVESTORAGE PROTEIN2(VSP2).LOX3 and LOX4 were also found to contribute to defense againstthe generalist herbivore Spodoptera littoralis.Our results reveal that 13-LOX genes are organised in aregulatory network,and the data herei
8、n raise the possibility that other genomes may encode LOXsthat act as pairs.Keywords:jasmonic acid;jasmonate;oxylipin;eicosanoid;wounding;defense;herbivore1.IntroductionLipoxygenases(LOXs)function to produce lipid mediators that operate in a broad range ofprocesses,many of which are related to defen
9、se in animals 1 and in plants 2.The Arabidopsisthaliana genome encodes six LOXs of which four are 13-LOXs,where“13”refers to the carbon atomin polyunsaturated 18-carbon fatty acids that are preferentially oxygenated by the LOX.13-LOXsincorporate molecular oxygen into-linolenic acid to produce its 13
10、(S)-hydroperoxide 3,a moleculethat is transformed into jasmonates which regulate wound-induced defense gene expression 4,5.Tocomplete jasmonate synthesis,fatty acid hydroperoxides formed through LOX action are dehydratedand cyclized to form the intermediate 12-oxo-phytodienoic acid(OPDA)in reactions
11、 catalysed byallene oxide synthase(AOS)and auxiliary proteins 6,7.Further transformations of OPDA then resultin the production of jasmonic acid(JA)and its biologically active derivatives,chief among which isjasmonyl-isoleucine(JA-Ile)8.Plants 2016,5,16;doi:10.3390/ 2016,5,162 of 13Jasmonates(and/or
12、their immediate precursors)produced in response to wounding do notstay where they are formed and can be transported efficiently within tissues.Following wounding,jasmonates/jasmonate precursors produced via LOX6 action in the vasculature of Arabidopsis leavesare highly mobile and move radially outwa
13、rds from veins into the mesophyll 9.In additionto jasmonate mobility,many 13-LOXs are themselves jasmonate-inducible.Therefore,in theory,jasmonates produced via the activity of any 13-LOX could be dispersed to different cell types capableof expressing other LOXs that also make jasmonate precursors.T
14、his raises an obvious question:howis the activity of the four 13-LOX genes in Arabidopsis coordinated?The roles of 13-LOXs in jasmonate-controlled defense responses have been studied in numerousplants,including,but not restricted to potato 10,wild tobacco 11,tomato 12,rice 13,andmaize 14,as well as
15、in Arabidopsis,a plant in which systematic LOX gene mutagenesis has beenemployed 15.Intriguingly,while all four 13-LOXs encoded in the A.thaliana genome contribute to thesynthesis of jasmonic acid 15,they each appear to have somewhat different functions in physicallydamaged leavesthe subject of the
16、present work.For example,in addition to the initiation of JAsynthesis in wounded leaves 16,17,LOX2 also plays a minor role in JA synthesis in undamagedleaves distal to wounds 18.Furthermore,close to the site of damage,LOX2-derived hydroperoxidesare also channelled into the synthesis of arabidopsides
17、,galactolipids that carry one or more esterifiedOPDA or dinor-OPDA molecules 17,1921.Consistent with arabidopsides being defensive secondarymetabolites,plants lacking LOX2 were more susceptible to the lepidopteran herbivore Spodopteralittoralis than is the wild type 17.LOX6 also plays a role in leaf
18、 defense.The experiments that revealed this role involved thegenetic removal of each of the three other 13-LOXs through producing a lox2 lox3 lox4 triple mutant.In this plant,LOX6 functioning alone was capable of maintaining the defense of emerging leavesand shoot apical tissues in Arabidopsis roset
19、tes 15.Interestingly,the relative impact of LOX6 in earlywound-stimulated jasmonate production in leaves increases with the distance from damage sites.Thatis,LOX6 was necessary for most of the rapid distal expression of the regulatory gene JASMONATEZIM-DOMAIN 10(JAZ10)when the rosette was wounded 15
20、,making this LOX of particular relevancein studies of long distance wound signalling.Finally,the LOX3/LOX4 pair contributes approximately 20%of the total JA pool that accumulatesin leaves in the first three minutes after wounding 15,however,no roles for these two LOXs in leavesare known.Here,we inve
21、stigated the relative contributions of LOX2,LOX3,LOX4,and LOX6 to eachothers expression,as well as to the expression of a defense gene.We then used herbivory assays toinvestigate LOX3 and LOX4 function in leaves.Our results revealed a lipoxygenase network thatoperates to coordinate jasmonate synthes
22、is and defense responses in wounded leaves.2.Results and Discussion2.1.13-LOX Expression Patterns in Unwounded RosettesThe expression patterns of Arabidopsis LOXs have been examined at the seedling stage 22,butequivalent data for leaves were lacking.Is each 13-LOX expressed in a different leaf cell
23、type?Tocharacterize basal 13-LOX gene activity in unwounded leaves,each promoter was fused to a secretable-glucuronidase(GUS)reporter gene.LOX6,principally expressed in xylem contact cells 9,15,servedas a comparison with other 13-LOXs,as shown in Figure 1.GUS staining in younger leaves wasstronger t
24、han in older leaves for all 13-LOX promoters,and sections of younger leaves were compared.LOX2 had the only promoter among the four that was widely active in most tissues except in andnear maturing veins.Because of this,transversal sections for visualizing LOX2 reporter expressionwere cut nearer the
25、 leaf tip than for the other reporters(red bars in Figure 1).LOX2 protein is readilydetectable in leaves 23 and LOX2 expression was strong in mesophyll cells,bundle sheaths,andleaf-tip vasculaturebut only at a distance from maturing veins.LOX3 activity was perivascular andstrongest in the xylem regi
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