Studies On The Nature Of Resistance To Fusarium Wilt Of Cotton:

Mohamed Shams El-din Mohamed Khashaba

reacti.on•’the res1lJ.t8obte.1n8d cQU1.dbe szedin 10118foU.O’Wi,ng :,_ ~ ••WIlberof fUllgaJ. iBol.e.t••••showed e. ,,14. epect~ ofvirul-encelevelS. It waSnot possi.~· to d•••1•&11&’·such iSolates ••••dUferent races bSCalJlleof the l.-lEof differentiaJ. lin ••••with known geneUc compaeiUS.2- CCOIlPIlX’W resiSt””,t and sU8cepti~e cotton vsri.e1il”in both the ”hesl;tb;J” and ”diSeaSed” C(Dd1ti.OIllI111.respect to lIlidn0 acl4 compollition indj.c.ted that Ie.)At early at,.gee ot plant grOWth. :zoot canteD’ll oflIIIlinoacids willialJDOIItidenticaJ. in both r881B-and susceptibl.8 healthy p~an:tsl then clecrea8e4’With age and to a greater extent in the 8U8~roots. As a. reeul t of infection, higher levelSof free amino acids were experienced with 1’OOt8 ofthe susceptible variety and continued to increaeein rel.atio~ to healtb1 tiSsues as the disea.ee p~ogressed.Although some degree of reduction i.n ejhinoacids occurred in the roots of infeoted resistan~variety at early stages, their !evel maintained a60 % lllcrease over the healthy tiSsues at l.ater stag •• •The most consistent change was ’the increwa8 in theooncentration of cystine in the resistant varietywhich ’WIIS reported to affect the production of~aric acid. Someother amino a.cids changed inconcentration as a result of in:fection; however,theJ.r ro:le 1.amel-ear exc~pt for tryptophan whichcou1d have some bearing on III synthesiSb- J1,2aOet .iaiJ,.a;r reaul.ts. witll some di:ffe:renQes, wereobserved for the aminoaoid contents of stems· TotalaI3 well. as certain amino acids increased as a resultof infection. The degree of variation was :lesspronounced for leaf tiS sues. Few aJDino acidsincreaeed in leaves of infected suso.epti~8 variety,whereas aU. but tryptophan decrea.aed in the resiStantTariety.It appears difficuJ.t to rel.ate the type of p~antreaction to any given aJDi,no acid in the heal-thYtissues • ~e ohange in certain aJIlino acids triggeredby in:fection o.o111.dbe one of the keydetet’!l1inante of the outcome of hoet-P8%’aS1te interaction.In other words, resistance mq not be apreformed but rather an induced state.,_ With respect to sugars t different organs ot differentvarieties exhib1 ted both Q.ualitative and quantitativedifferences in BUBarcontent. Sucrose, gaLactoee andglucose were pr08ent in aU. organs of both susceptibleand resistant varieties, whereas maltose was only f0UD4J~ -~----_.~.- .. ’in stem tissue,.( c:n the other hand, fructos e was onl.7found in different organs of the susceptible variety •.Asa result,.’ of 1n:fection, sugar contenta deer.as edIin roots of both v~i.et1est with galactose + glucoseremaini ng at almost the SaJIle concentration in thesuscept1b~e variety; however, they inereseed in theresistant one. On the other hand, sucrose decreasedin the susceptible and increased in the resistantvariety when comparedwith the respective contltols.fluch change8 in sU«ar levels could have aOlfe hea~on the nutri tiou of the invading fungus either dir.c~on indirectly and indicate certain metabolie changesin carbohydrate metabolism due to infect1c.n.Other p~ant organs responded somewhat diftereatlyfrom roots wJ:tl:l respeot to certain sugars. As to thestems, sucrose allowed a.much more increase in theresi.stant variety than in the suscept1b~e one. On thecantrary, galactose + gJ.UOOS8 increased first and th8i1decrea#3edupon infection of the ausceptib1e variety,whereas they were always Leas in the resistant varietysubjected to ~ect:lon whencomparedwith the respectivehealthy oontrols. As to leaf tissue, no change occurredin the level of sucrose 1n the resistant variety,whereas it increased in the ausoeptib.1.e one. On thecontrary, a marked decrease in the level of gaJ-actose+ gJ.ucose followed the infection of the suscep’tibl.evariety and the appoait8 was true for the resistantvariety.4- With respect to phenol.ic compounds, three major groupsi.e. mono-, d1- and poly-pheno~s were found in bothvarieties. The di-and pol.yphenol.s were present incomparable concentrations in the healthY org~ of bothresistant 8lld susceptibl.e var1e’t1t1Js. Uponinfection,these two groups of phenolS increased in orgaJl8 of thesUBcept1bl.e variety, whereas the opposite vas true, inmost cases, With the resistant one. .Aa to the memo...•phenoJ.a, the two varieties exhibited comparable concentrationsin roots and 1eaves at early stages and inst8J18 at relatively later stag.. Following infection,the susceptible variety alwa;ya exhibited a DROP in themonophenoJ. ~evel compared with the healthy tissues,whereas the opposite was mostly the case for the ~reaiatantvariety - Such interconve1.”8iona of phenolic SUbstancescouJ.d be accomplished by the eneymes ot the pathogen and/or the host in . response to infection. and the 1I:ype ofphenolic compotmdsthspredominate: could play a roJ.e indetermining the type of host reaction as these compOt1ndsdiffer in their toxicity.5- With respect to certain hydrolytic en~eaJ 1_.1.:. pecticand celluJ.o1ytic enzymes, comparative studies shovedthat:a) Healthy tissues of both 8usoeptible ana. resistantvarieties containe4 some activity of PMB’but noneof the pectin degrading ~,m.ee. Upon infection,80me increase in PU activity occurred; however,to greater extent in the susoeptible than in theresistant variety. On the other hand, the pectindegrading enzymes were not found except in theinfected susoeptible tissues_b) Very low activity of cell~ases vas found :in thehealthy t1eeue8 ot both rea’istant and SUBcepti.bJ.e-varieties. In:fection was usu.a1].y followed by someinorease in acti.v1ty i however, to a greater extent ionthe susceptible variety. The magnitude of increasewas relatively low to warrant major role ot theseenzymes in pathogenesis.6- Oertain oxidative enz.ymes,i...!- po~yphenol oxidase,peroxidase, catalaSe, and ascorbic acid oxidase werestudied in various host-parasite ccabinations• ResUl-tsobtained coul.d be sYDJrnar1zed as followS :-a) With respect to PFO, comparab1.eactivi.ty was found,in most casee, in the healthY susceptible andresistant varieties. As a reauJ.t of inf action, theenzYDl9activity increa.8ed in root tiSsues of bothvarieties; however, to a greater extent in the8uscept1bl.e one. At later stages, the activity inthe two varieties w~ not too muohdifferent. In.1I.om -tiSsues, siJDilar situation was found, exceptthat the acti.vity in the resiStant variety maintaineda somewhathigher levu than ”the control at laterstages- The Emhancedactivity of PPOin the rootsof infected susceptible variety could account forthe oxidation of phenoliCS to non-toxic polymericsubstanoes ~ead;ng to the production of the brownishp1£sD81lts characteriStic of vascular diScoloration.On the other hand. the J.:!Jnj.ted1.ncreelie 1.nPFOfC)Ulld1.nresistant tiSSUes IIl8Y result 1.n a ~owQld.dation products such 8IJ I.”unotUIS”,bich arehighlY reactive end more toxic. SUChproducts coulda1Jlo ,nbibit the enzym•••••thus pertiaUY accountiD.!lfor the lOWactivity level. obServed· ~herefore. 1’1’0could have a varied effsot 1.nthe host-parasiteoODlb>nationdependmg on the extent of <>xidatiCRleffected.b- AS to the peroxidas •••• , it was fC)Ulldthat their activity”,as enhanced fol.l-owing:lJ1fection in roots ofboth susoeptibl.e and resistant varieties. Later.such activity DROPpedin the former bUt continuedto increase in the ~atter. SUChcontinued enhancedactivity of peroxidas ••••in the resiStant varietycould be one of the determinanta of such. r~le;hOWever.it is dUficult to pinPoint the affectedprocesses sinoe peroxide/ites have quite a variedeffect on different vi.taL processes.c) AS to catale/ites. it ”’e/itfound that infection resW.tedin an mereslle in their aotiv1.ty m both varieti”.with the magnitude of increase being h#gher in theres1Stan:’G variety.d- l!’or ascorbic ac1d ox1daIOe, the increase in a.ctivityin response to infection wall higher 1n roots of thesusoeptible variety, whereaf;l the rlifVerse was truefor stems. ’rh1a enhanced activity in ”the stems ofresistant tissues coUld exert a drainage effect onthe concentration of ascorbic acid, thUS l.oweringthe reducing power and ~dering the phenoli.o compoundSmaintain their oxidi.zed toxic form. Furthermore,such an en’!;Y’JDSeIld other ox1da.tive BIl’!;Y’JDmBs;y8p:J..eya roJ.e in the reoxidation of NAID?Hproduced throughthe modified metaboJ..iSmariSing in response toinfection.7- Jbpl.oying the eJ.eotrophoretic techniQ.ues in polyacry-:l.am1degeJ..s, certain enzymeswere studied in extractsof healthy and diSeased plants aI3 well as in extractsof fw,gel iso).ates. ’rhe following results wereobtained:a) NOdifference was obServed beiiween the healthy sUScept1bJ.e and resiStant varieties as to the peroxidaseactiv1ty or the isozyme patte:rn- AJ3 a resultof infection, the l.evel. Of activity inereased ’e)as evidenced by band width and staining density.The magni-tude of increase was greater in the ausceptib~e than in the resistant variety. Anactivityband corresponding to ”that found for the fungalperoxidae e appeared in the d1Seased extraots. Themultiplicity of peroxidase iso~es was not evidentwith pyrogal.lol as the substrate, unless ’the activitywas too low, thus preventing the coalescence of bands.b} With respect to esterases, ten healthy cottonvarieties showeda high degree of similarity 1ntheir isozyme pattern. Stems were more active thanroots. SimUarly J nine f’lmgal isolates were aimUarin their esterase pattern, regardless of their virulence.Whena susceptible and a resistant varie”ty subjectedto ~ection vere oompared, it was found that healtbYroots of both varieties exhibited similar patterns;however, the level of acti.vity DROPped as the plantsgrew older. Oomparing diseased with healthy extraots,the former was alw8¥s poorer in activity as judgedby the band staining density. AJ3 to the stem tissues,healthy extracts showed a high degree of simllar:1tyin both resistant and susceptible varieties; however,there was an indication of an additional band inthe former. No consistent trend of change wasfound in reepons e to infection except that thediseased susoeptible tissues maintained theuo seteraaeactivity at later stages whlle the correspondingheal.tby tissues were almost devoid of such an activity.The role of esterase in disease physiology isunclear at present; however, it appears to have aval-us aI3 a diagnostic too~ in view of the high degreeof s~arity in isozurme pattern amongciifferentcotton varieties and a,lso aJIlong different fungalisolatese8- Certain groupS of growth regUlating substances werestudied in both healthy and diseased susceptible cottonvarietiese The results are summarized as follows :-a) With respect to auxin activity, as judged by theextent of root fOl.1llation in cowpea seedlingS, itwas :found that infection resulted in a state ofhyperaUXinJ’i however, to a greater degree in thesuscept1bl.e variety part1eu1.arly at the early stagesof disease deveJ.opment.b) With respect to gibberellinS, 1t was found thatinfection led to a reduction in the activity ofth1s group of compounds;however, the magnitud6 ofreduction waa greater in the infeoted susceptib~evariety than 1n the infected resistant onee Thiscould Part1aJ.1Y.,tccount for the stunting Sympt~..---- __oharaoteriziDB;the wllt disease.