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Hormone-Virus Interactions in PlantsPaula E. Jameson a & Sean F. Clarke ba Institute of Molecular BioSciences, Massey University, Private Bag 11222, Palmerston North,New Zealandb National Institute of Biological Standards and Control, Blanche Lane, South Mimms, PottersBar, Hertfordshire, EN6 3QG, United KingdomPublished online: 24 Jun 2010.

To cite this article: Paula E. Jameson & Sean F. Clarke (2002) Hormone-Virus Interactions in Plants, Critical Reviews in PlantSciences, 21:3, 205-228, DOI: 10.1080/0735-260291044241

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Critical Reviews in Plant Sciences, 21(3):205–228 (2002)

0735-2689/02/$.50© 2002 by CRC Press LLC

Hormone–Virus Interactions in Plants

Paula E. Jamesona* and Sean F. Clarke b

a Institute of Molecular BioSciences, Massey University, Private Bag 11222, Palmerston North, New Zealand;b

National Institute of Biological Standards and Control, Blanche Lane, South Mimms, Potters Bar, Hertfordshire,EN6 3QG, United Kingdom

* To whom correspondence should be addressed.

ABSTRACT : Symptoms of virus infection may be simplistically ascribed to a change in quantity of a particularplant hormone, and frequently virus-induced symptoms can be mimicked by application or removal of a planthormone. In this review, we look critically at the information available concerning changes in the biosynthesis andmetabolism of plant hormones following virus infection. In addition, we briefly review the effects of virus infectionon endogenous jasmonates and salicylic acid. We also briefly assess the involvement of the classical planthormones, jasmonic acid and salicylic acid in the induction of defense-related genes.

KEY WORDS: abscisic acid, cytokinin, auxin, gibberellin, ethylene, ACC, salicylic acid, jasmonic acid, defense-related genes, pathogenesis-related proteins.

I. INTRODUCTION

Virus infection can result in the alteration ofphysiological, biochemical, and metabolic pro-cesses within the plant (Fraser, 1987). These al-terations often lead to the appearance of symp-toms such as stunting of growth, mottling, and/orwrinkling of leaves, wilting of infected plants,appearance of chlorotic and/or necrotic lesions,abscission of leaves and fruit, and the develop-ment of growth forms such as galls (enations) andphylloidy (Fraser and Whenham, 1982).

There is a large variation in the response ofplants to virus infection, and the manner in whichplant growth is modified by such infection isunknown. Two general models have been pro-posed to explain these variations, a competitivemodel and a noncompetitive model (Fraser, 1987;Whenham and Fraser, 1990).

The competitive model allows that viral rep-lication is sufficiently active to create a sink foramino acids and nucleoside triphosphates. Somevirus infections result in the accumulation of verylarge amounts of virus nucleoprotein. For ex-ample, during tobacco mosaic virus (TMV) infec-tion of tobacco or tomato, viral protein and nucleic

acids account for over 1% of the leaf fresh weight(Fraser and Laughlin, 1980; Matthews, 1991).Furthermore, over 75% of the host’s capacity forRNA and protein synthesis is diverted into theproduction of virus nucleic acids and proteins(Fraser, 1973; Fraser and Gerwitz, 1980;Matthews, 1991). The use of plant amino acidsand nucleosides to form virus products may in-hibit host nucleic acid replication and proteinsynthesis competitively.

The noncompetitive model has been devel-oped because for many viruses the level of repli-cation is very low and is unlikely to inhibit plantgrowth through resource competition. In thesecases, the alterations in plant growth are believedto be due to the cytopathic effects of virus infec-tion (Whenham and Fraser, 1990), which may bemanifested by alterations in plant hormone me-tabolism.

In this article we review the effects of virusinfection on the biosynthesis and metabolism ofendogenous plant hormones and discuss whetherchanges in metabolism may account for alter-ations in host growth and metabolism. Further-more, we consider the possibility of changes inendogenous hormones being involved in the de-

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velopment of resistance or, conversely, the suc-cessful infection of the plant.

II. INTERACTIONS BETWEEN PLANTVIRUSES AND PLANT HORMONES

The effects of virus infection on host plantsare diverse. Many viruses spread systemically insome hosts but can be restricted to localized ne-crotic lesions in others. Some viruses such asbarley yellow dwarf virus are restricted to phloemand are unable to move into other tissues. With alarge diversity in virus form, replication, and patho-genic effect, it is likely that different viruses indifferent hosts have varied effects on plant hor-mone metabolism. Moreover, extreme severity ofsymptoms caused by many viruses could causechanges in hormone concentration that have noinvolvement in control of the host-virus interac-tion.

The measurement of plant hormone concen-tration in plants is difficult. Apart from occurringat low concentrations, many exist in several formsand exist with substances that may interfere withthe final assay. The majority of studies investigat-ing effects of virus infection on plant hormonemetabolism relied on bioassay to quantify andidentify the hormone. However, these methodstend to be only semiquantitative, lack specificity,are prone to interference from nonrelated sub-stances, and do not give unequivocal identifica-tion of the hormone being measured. More re-cently, the development of immunologicaltechniques (enzyme-linked immunosorbent assay(ELISA) and radioimmunoassay (RIA)) coupledwith precise purification techniques, such asHPLC, have improved investigations into the roleand function of plant hormones. Unequivocal iden-tification of hormones is essential and is providedby mass spectrometry, coupled with adequate in-ternal standardization to correct for losses duringextraction and purification. Unfortunately, thesetechniques have been used rarely in the study ofhormone-virus interactions in plants.

The last comprehensive review on hormone-virus interactions in plants was by Fraser andWhenham (1982). A more recent review byWhenham and Fraser (1990) focused on TMV

infection of tobacco and tomato as the main modelsystem. Several brief reviews have been publishedby Pennazio covering ethylene (Pennazio, 1992),auxin (Pennazio and Roggero, 1996), and cytoki-nin (Pennazio and Roggero, 1998). Since thesereviews, detailed information about the plant hor-mones has continued to emerge. Many of theenzymes involved in biosynthesis and metabo-lism of hormones have been characterized and anumber of the genes responsible identified.Arabidopsis mutants deficient in the biosynthesis,sensitivity, or transport of specific hormones arenow being used to investigate hormone-virus in-teractions in plants. Transgenic plants over- orunderexpressing specific plant hormones are alsoproviding useful tools.

A review on hormone-virus interactions wouldbe incomplete without considering the link be-tween the classic plant hormones and the com-pounds now considered to play key roles in theinduction of defense genes, salicylic acid (SA),and the jasmonates (JA). While there are numer-ous recent reviews on signal transduction path-ways and plant disease, several of these refer tonecrogenic pathogens as a group and do not spe-cifically separate out the plant viruses (e.g.,McDowell and Dangl, 2000). Dong (1998) andFeys and Parker (2000) provide recent reviews ofSA, jasmonates, and ethylene in plant disease, butagain do not specifically refer to viruses. An ex-tensive review on SA and disease resistance inplants for this journal by Dempsey et al. (1999)emphasizes the increasing complexity of the sig-nal transduction pathways operating in plant de-fense. Critically, evidence is provided for a de-fense pathway unique to viruses in a recent reviewby Murphy et al. (1999).

A. Effects of Exogenous Application ofPlant Hormones on Virus Infection

In addition to studies on the influences ofvirus infection on plant hormone metabolism, therehave been attempts to induce resistance to, or tocontrol, virus infection in plants by the applica-tion of plant hormones or other chemicals, in-cluding JA and SA. Results have varied, eitherdue to the method and/or timing of exogenous

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application or concentration of chemical beingtested. Further, treatment of plant tissue with planthormones could also indirectly influence the in-fectivity of the virus produced, especially at highor phytotoxic concentrations. Therefore, reducedinfectivity in crude extracts might not necessarilyreflect a reduction in virus concentration. A physi-cal and/or immunological method of quantifyingvirus multiplication is desirable before claimingan effect of a plant hormone on replication.

Unfortunately, despite the advancement ofserological techniques, these techniques have beenused rarely when investigating the effect of planthormones on virus replication. Most investiga-tions have relied on local lesion assays, wherecrude infected sap from hormone-treated materialis inoculated onto a secondary host and locallesion production tabulated. This experimentalsystem yields data readily, but it is biologicallyextremely complex, and, consequently, the ef-fects reported have been varied and sometimescontradictory. For example, reports of the effectsof abscisic acid (ABA) application range fromABA causing an enhancement in replication,through no effect, to a decline in virus infectivity(Table 1). However, much of the earlier workutilized pharmacological levels of ABA, and fre-quently detached leaves (Table 1). When morephysiological levels were supplied via a wick tootherwise intact plants, ABA had no effect onreplication in either the inoculated leaf or sys-temically infected leaves (Clarke et al., 1998).

A similar situation appears to exist for theapplication of cytokinins. The majority of reportswould favor a negative effect of cytokinins onvirus replication if the cytokinin were applied/supplied prior to inoculation with virus (Table 2).Such inhibition was recorded to occur at the levelof the subgenomic double-stranded RNAs, aneffect shown also following application of SAand JA (Clarke et al., 2000a). There are alsoreports that suggest that the eradication of virusvia tissue culture methods may be related to thepresence of cytokinin in the medium (e.g.,Johnstone and Wade, 1974). However, cytokininsupplied after inoculation appears to enhance le-sion number and virus infectivity (Table 2).

The effects of auxins remain to be elucidated.There appears to be little consistency both be-

tween and within reports or between differentauxin analogues (e.g., enhanced replication/viruscontent being reported by Ben-Tal and Marco,1980; Schuster, 1975; Dhingra et al., 1991;Sindelarova et al., 2000), and reduced replicationbeing reported by Simons et al., 1972; Schuster,1975). Dose-dependency is also reported (vanLoon, 1979; Clarke et al., 1998) but confoundedby the fact that at high concentrations auxinspromote ethylene formation. For example, bothvan Loon (1979) and Clarke et al. (1998) suggestthat the action of higher concentrations of indole-acetic acid (IAA) on the inhibition of virus maybe due to auxin-induced ethylene production.However, it is worth noting that biologically inac-tive analogues of auxin have little affect (vanLoon, 1979).

Early work on the gibberellins (GAs) focusedmore on attempts to reverse the stunting effect ofvirus (Maramorosch, 1954; Bailiss, 1968; Russelland Kimmens, 1971; Aharoni et al., 1977). Wherevirus replication was assessed, most reports con-cur that exogenous GA3 reduced/inhibited virusreplication (Boiko et al., 1988; Cheema et al.,1991; Bukovac and Yuda, 1991). However, Clarkeet al. (1998) showed no effect of GA3 on virusreplication in the inoculated bean leaf using whatmight be described as more physiologically rel-evant levels of GA3 than those used by manyprevious workers. However, they did show a re-duction in virus content in the systemically in-fected leaves (Clarke et al., 1998), which could berelated more to GA3-enhanced growth than to anyeffect on replication or systemic spread.

Low concentrations of JA (0.25 to 25 nM)were effective in inhibiting the replication ofWClMV in P. vulgaris at the subgenomic dsRNAlevel and also in preventing systemic spread(Clarke et al., 1998, 2000a). Earlier, Petrovic andRavnikar (1995) reported that 100 nM JA consis-tently inhibited replication of potato virus Y, buthigher supra-optimal concentrations did not.

Exogenous application of SA is usually re-ported to induce resistance to viruses, character-ized by a decrease in virus titer and a delay in theonset of disease symptoms (see references inMurphy et al., 1999; Clarke et al., 1998, 2000a),although exceptions to this have been noted(Pennazio et al., 1985; Roggero and Pennazio,

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1991; Raiola and Faccioli, 1997; Kachroo et al.,2000).

Substantially more consistent data might beobtained from the types of experiments describedin this section if care is taken to use physiologi-cally relevant concentrations of hormone and toapply the hormone to plants that are effectivelyintact. Serologically based detection methodsshould now be adopted as a matter of course.

B. Plant-Virus Interactions with AbscisicAcid

A number of bioassay-based reports suggestedthat increases in inhibitory compounds occurredafter virus infection (e.g., van Steveninck, 1959;Rao and Narasimham, 1974a; Mohanty et al.,1979). However, the reported effects of virus in-fection on ABA concentration itself have varied.A decrease in ABA concentration in TMV-in-fected tobacco in the early stages of virus infec-tion was reported by Rajagopal (1977), whereasKeller and Lüttge (1991) demonstrated (usingELISA) that infected plants susceptible toRhizomania had a higher ABA content than toler-ant plants grown in contaminated soil. BecauseRhizomania-infected plants suffer water stress,but leaf turgor was not lost, the authors suggestedthat the ABA was being synthesized in the rootsand transported to the leaves.

Zhang et al. (1997) monitored ABA levels byELISA in leaves of banana plants inoculated withbanana bunchy top virus. ABA levels peaked inleaves at the time of symptom development, somedays after peak virus levels. This was one of thefew groups to monitor several hormones simulta-neously. They also showed a decrease in GA andin isopentenyladenine, and concluded that symp-toms were related to an imbalance in plant growthsubstances, and only indirectly to virus move-ment.

The most comprehensive study of the effectsof virus infection on ABA is by Whenham’s groupwho based their work on tobacco plants systemi-cally infected with TMV (reviewed in Whenhamand Fraser, 1990). The plants developed a distinctmosaic of light green and dark green areas in theleaves that become infected at an early stage of

development. High concentrations of virus accu-mulated in the light green tissues; the dark greenareas contained little virus. Infection also inhibitedgrowth. Whenham et al. (1986) proposed that ABAplayed a role in the development of mosaic symp-toms with the greater increase in ABA, in both freeand bound forms, in the dark green compared withthe light green areas. In healthy leaves the ABAanion is sequestered in the chloroplast, while thebound form (probably inactive glucosyl ester) islocated in the nonchloroplastic fraction. Whenhamand Fraser (1981) and Fraser et al. (1986) reportedthat the increased free ABA was located predomi-nantly outside the chloroplast. As the concentra-tion of bound ABA increased in both the chloro-plastic and nonchloroplastic fractions, theyconcluded that the increases were probably theresult of de novo synthesis and not the result ofrelease from a “bound” form. Furthermore, theyshowed that alteration in cellular pH followingvirus infection was not sufficient to enable releaseof ABA from the chloroplast.

The metabolism of ABA following virus in-fection was investigated further by Fraser andWhenham (1989) in both tobacco and tomato.Both ABA and phaseic acid (PA), an inactivecatabolite of ABA, increased in TMV-infectedtobacco, indicating an increased flux through theABA catabolism pathway. However, TMV infec-tion had little effect on leaf content or catabolismof ABA in tomato. In addition, tomato plantscontaining the Tm-1 gene for resistance wereshown to have higher endogenous ABA than sus-ceptible plants. Fraser and Whenham (1989) con-cluded that the Tm-1 gene might influence ABAmetabolism and suppress symptom development,possibly by preventing cytoplasmic accumulationof free ABA. However, after assessing a numberof plant varieties and virus strains, Whenham etal. (1985) and Fraser et al. (1986) postulated thatwhile changes in ABA concentration alone couldaccount for the correlation between symptom se-verity and growth in tobacco, they could not intomato, leading these authors to suggest that intomato the inhibition of growth may be through adifferent mechanism, involving plant hormonesother than ABA.

It is apparent that intracellular localization ofABA is important in the development of symp-

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toms following virus infection. Whether the in-creased cytoplasmic ABA is due to virus-induceddamage to the chloroplastic envelop and/or ABAsynthesis external to the chloroplast is not known(Whenham and Fraser, 1990). In some instances,reduced growth may be a direct consequence ofcompetition for assimilates; in others it appears tobe a consequence of virus-induced increases inABA biosynthesis.

C. Plant-Virus Interactions withCytokinins

There were a number of early investigationsin which attempts were made to quantify thechanges in endogenous cytokinins following vi-rus infection. Kuriger and Agrios (1977) demon-strated a decline in cytokinin activity in root exu-dates of cowpea infected with tobacco ringspotvirus (TRSV) and attributed this to compoundswith chromatographic properties similar to zeatin (Z)and/or zeatin riboside (ZR). Tavantzis et al. (1979)also attributed the decline in cytokinin activity intobacco leaves following TRSV infection to adecline in Z and/or ZR. Thompson et al. (1983)noted that during the acute phase of disease causedby tomato spotted wilt virus (TSWV), which ad-versely affects the water content of aerial parts ofthe plant, there was a decrease in the cytokinincontent of the aerial parts. The decline was attrib-uted to Z/ZR-like compounds. Furthermore, theynoted that TSWV-infected roots contained morecytokinin-like activity than healthy roots, andconcluded that the main effect of the disease wasto inhibit translocation of cytokinin from roots byinducing water stress.

However, Sridhar et al. (1978), working withTungro virus, which causes greening in suscep-tible rice leaves, noted that infected tissue con-tained higher levels of cytokinin-like activity. deFazio (1981), also working with a virus that causesretarded leaf senescence as well as the develop-ment of lateral buds, reported higher cytokinincontent in beans infected with bean golden mo-saic virus.

Sziraki et al. (1980) reported that the systemicinfection by CMV caused an increase in the cyto-kinin content in leaves of tobacco and induced

resistance to TMV. They suggested that resistancewas dependent on the accumulation of cytokinins.Balazs et al. (1977) also followed the changes inendogenous cytokinin-like activity in noninoculatedupper leaves of Xanthi tobacco that were systemi-cally resistant to challenge inoculation by TMV.The peak concentration of cytokinin (Z/ZR andisopentenyladenine/isopentenyladenosine, iP/iPA)occurred at the height of systemic acquired resis-tance (SAR) in the upper leaves.

Faccioli et al. (1984) monitored cytokinin ac-tivity in inoculated leaves as well as in upperuninoculated leaves of Chenopodium amaranticolorinfected with tobacco necrosis virus. In agreementwith several reports, they showed an increase incytokinin-like activity (Z/ZR, iP/iPA, and un-knowns) in the systemically resistant leaves. How-ever, they also reported an increase in cytokininactivity in the inoculated leaves. In contrast, Gowduand Nayudu (1989) showed a decrease in cytokininlevels in both leaves and roots of groundnut 16days after inoculation with groundnut chloroticspot virus at the time when symptoms were severe.

Owing to the use of low-resolution chroma-tography and bioassays, various cytokinin metabo-lites were either overlooked or underrepresented inthe above analyses. More recently, physical andimmunological methods have been employed toanalyze the cytokinin content of virus-infectedleaves. Whenham (1989) and Whenham and Fraser(1990) showed that tobacco infected with TMVhad reduced Z levels in the dark green islands ofsystemically infected plants, but significantly in-creased levels of both the storage O-glucosides andthe inactive 7-glucoside of zeatin, leading them tosuggest that alterations in metabolism had occurredfollowing infection in favor of a reduction in activeforms of cytokinin and an increase in conjugation.Indeed, Whenham and Fraser (1990) suggestedthat some of the chromatographic systems em-ployed by earlier researchers would not have dis-tinguished between bases and ribosides and theircorresponding O-glucosides. Further, as the O-glu-cosides are active in callus bioassays, they sug-gested that reports of enhanced activity might havebeen due to O-glucosylation and not due to in-creases in free bases and/or ribosides at all. Inter-estingly, Whenham (1989) also showed that spraysof ABA could induce similar metabolic changes in

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the cytokinins as did TMV infection. Whenhamand Fraser (1990) present suggestions that relatethe ABA-induced changes in cytokinin metabo-lism to reductions in cell division, the subsequentreduction in leaf growth, and to the formation ofgreen islands.

Changes in cytokinin metabolism were alsoexplored by Dermastia et al. (1995). They alsoreported a shift from biologically active free basesand ribosides toward inactive conjugates (the9-glucosides) in roots of potato plants infectedwith potato virus Y (PVY). They also investi-gated changes in potato plants infected in vitro(Dermastia and Ravinkar, 1996), but in this caseshowed an increase in both iP-9-G and ZR. How-ever, the methods they employed would have leadto hydrolysis of the cytokinin nucleotides, whichcould then have contributed to the amount of ZRobserved.

Zhang et al. (1997), using ELISA, reported asignificant decrease in iP by 14 days postinfectionof banana with banana bunchy top virus. Infec-tion levels were high after 21 days with symp-toms displayed in the top leaves by 35 days. iPremained at low levels during the infection pe-riod.

Clarke et al. (1999) reported a detailed inves-tigation into cytokinin metabolism following vi-rus infection. Bean plants infected with whiteclover mosaic virus (WClMV) showed markedalterations in cytokinin metabolism, indicatingvirus influences on biosynthesis, catabolism, and/orconjugation. Free bases and storage O-glucosidesdeclined within 48 h following infection, whilenucleotides and N-glucosides increased. Nucle-otides are the most likely biosynthetic precursorsof active cytokinins, while the O-glucosides rep-resent a storage pool of cytokinins and are consid-ered to be physiologically inactive but readilyconverted to the active free bases. The N-gluco-sides are considered to be metabolically inactiveand normally represent irreversible deactivationof active cytokinins (Jameson, 1994). The in-crease in nucleotides following virus infectionmay represent an inhibition in the biosyntheticpathway through an inhibition of the turnover ofnucleotides to active forms, or increased activityof enzymes converting the free bases and ribosidesback to their nucleotide forms. Decreases in Z

indicated that active cytokinins were being re-moved from the cytokinin pool, either by catabo-lism by cytokinin oxidase or increased activity ofN-glucosyltransferases. Furthermore, the obser-vation that virus titer did not increase until active-cytokinin concentration had been reduced raisedthe possibility that cytokinins were involved inthe host-virus interaction (Clarke et al., 1999).

Three different groups have shown that themetabolism of cytokinins is significantly alteredin three different host-virus combinations in favorof conjugation and a reduction of active formsfollowing infection. Clarke et al. (1999) notedthat the cytokinins are potent antioxidants(Musgrave, 1994), and that their supply also in-hibited the virus-induced decrease in several freeradical scavenging enzymes (Clarke et al., 2002).They suggest that the maintenance of free radicalscavenging activity is positively correlated withthe prevention of virus replication.

D. Plant-Virus Interactions with Auxins

The earliest experiments investigating the re-sponses of auxins to virus infection were in plantsthat showed conspicuous stunting. The decline inauxin activity was first described in potato infectedwith potato leaf-roll virus (Jahnel, 1939; Lucas,1939 cited in Pennazio and Roggero, 1996), andnumerous investigations have shown that virusinfection reduces both auxin activity and concen-tration (e.g., Smith et al., 1968; Lockhart andSemancik, 1970; Rao and Narasimham, 1974b;Rajagopal, 1977). However, other studies haveindicated that auxin levels rise during infection(van Steveninck, 1959; van Loon and Berbee, 1978).Studies using more physical techniques also dem-onstrated that auxin levels increased following vi-rus infection (Bulgary and Isac, 1988; Poggi-Polliniet al., 1990), although these increases paralleledthe appearance of severe symptomatology and mayhave occurred too late for any putative involve-ment of auxin in virus establishment.

However, in their investigation of Mal de RioCuarto virus infection of maize, Abdala et al.(1999) reported higher levels of IAA in diseasedthan in healthy tissue. MRCV infection of maizeinduces gall formation (enations) in the abaxial

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epidermis of leaves by causing phloem and xylemhyperplasia. Abdala et al. (1999) reported thatIAA levels were high in enations developed 36days after infection compared with control leavesof the same age, but no differences were foundlater between healthy and infected leaves withwell-developed enations. Interestingly, high lev-els of IAA oxidase activity were detected in ena-tions at the time of high IAA activity.

The mechanisms leading to changes in auxinlevels in virus-infected plants are still unknown.The catabolism of IAA may occur following fourdifferent routes: oxidative decarboxylation byperoxidase; oxidation not involving peroxidase;and conjugation to give either inactive esters orinactive amides (Bandurski et al., 1995;Kleczkowski and Schell, 1995). A number of re-ports indicate that peroxidase activity increasesrapidly following virus infection (van Loon, 1979;Lagrimini and Rothstein, 1987; Wahig, 1991;Buonaurio and Montalbini, 1993; Montalbini etal., 1995; Clarke et al., 2002), but this has usuallybeen associated with metabolism of H2O2, lignifi-cation, and cell-wall protein cross-linking (Ye etal., 1990).

After inoculation with potato A potyvirus,Muletarova et al. (1995) reported the appearanceof new peroxidase isoenzymes that acted as in-hibitors and suppressed auxin synthesis from pre-cursors. Interestingly, Rajagopal (1977) noted anincrease in tryptophan and phenylalanine (puta-tive precursors of IAA and PAA, respectively) inTMV-infected leaves compared with controls.

Changes in peroxidase activity immediatelyfollowing infection may account for the declinein auxin activity and concentration recorded inearlier studies. Reports on changes in the activityof IAA oxidase differ, an increase following virusinfection being noted by Lockhart and Semancik(1970) and Abdala et al. (1999) prior to a re-corded decline in auxin concentration, whereasNarayanasamy et al. (1972) attributed the decreasein IAA oxidase activity to an increased concentra-tion of phenolics.

Although it is apparent that virus infectioninduces changes in peroxidase enzymes, whetherthe virus acts directly on these enzymes or whetherthe alterations in enzyme activity are a conse-quence of virus infection remains to be deter-

mined. Moreover, little is known of the metabolicevents leading to the increase in auxin found insome virus-infected plants. However, an insightinto auxin regulation during virus infection isbeing gained by the introduction of Arabidopsismutants to the field.

Sheng et al. (1998) isolated an Arabidopsismutant that displayed a severely dwarfed pheno-type and loss of apical dominance following in-fection with turnip vein clearing virus (TVCV).The wild-type phenotype following such infec-tion is a mild reduction in height and on occasionscurled stems. The mutant (vid1 — virus-inducibledwarf) displays a normal phenotype in the ab-sence of virus. Most notably, auxin (as 2,4-D)spray repressed the virus-induced phenotype ofthe vid1 plants, whereas the application of GA(not specified) had no effect (Sheng et al., 1998).Because the plants were responsive to auxin, theauthors suggested that the auxin signal transduc-tion pathway was unaffected. They proposed thatbecause systemic viral infection is thought to in-terfere with the intercellular transport of the hostplant, the vid1 mutation affects this transport path-way. As a consequence of this, they suggestedthat the directional transport of auxin is disrupted,leading to the reduction in height and the releaseof apical dominance after virus infection. Inter-estingly, Smith et al. (1968) had earlier suggestedthat virus infection may reduce auxin transport intomato infected with curly top virus.

In contrast to this work, Mayda et al. (2000)suggested that changes in auxin perception mightbe critical to the activation of at least some de-fense-related genes. This conclusion is based onwork with the CEVI-1 gene that is induced insusceptible tomato after infection with viral(ToMV) and subviral (viroid) pathogens, but isnot induced by auxin, SA, ethylene, MeJA, orwounding. This gene encodes an anionic peroxi-dase. Mayda et al. (2000) searched for Arabidopsismutants constitutively expressing this gene andshowed that these mutants were also insensitiveto auxin. They suggested that activation of CEVI-1gene expression correlated with a defect in per-ception of auxin by the plant. Further, Mayda etal. (2000) showed that the 5' promoter region ofthe CEVI-1 gene from tomato contains the twoAuxREs that are characteristic of auxin-regulated

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genes. However, they proposed that auxin actu-ally represses the expression of this peroxidase,and that auxin sensitivity is altered in diseasedplants, and it is this that mediates the activation ofspecific defense-related genes.

From the above two reports it is clear thatdifferent plant-virus combinations may be affect-ing auxin homeostasis differently. Unfortunately,in-depth investigations of auxin turnover itself inthese systems is lacking.

E. Plant-Virus Interactions withGibberellins

Despite being an obvious area for research,the stunting of growth that results from infectionby a range of viruses has led to only a limitednumber of studies of GA content following virusinfection. Most groups reported decreased GA-like activity in virus-infected tissues. For example,Russell and Kimmins (1971) reported a decreasein GA3-like activity in barley infected with barleyyellow dwarf virus, although they suggested thatthe GAs were qualitatively similar in diseasedand healthy plants. Sridhar et al. (1987) reporteda marked decrease in both free and bound GA-like substances in plants infected with rice tungrovirus, while Bailiss (1968) suggested that theremight be a slight decrease in GAs during infec-tion of tomatoes by tomato aspermy virus. Subse-quently, Bailiss (1974) and Aharoni et al. (1977)reported that the concentration of GA-like sub-stances (GA1- and GA3-like) was reduced in CMV-infected cucumber seedlings, which was corre-lated with the appearance of stunting. However,increased levels of ABA and ethylene were alsorecorded (Aharoni et al., 1977; Marco et al., 1976).While Rao et al. (1977) also reported less GA-likeactivity in infected leaves, they also reported thatthere was no change in GA-inhibitory compoundsin their assay when investigating the GA-likeactivity of Citrus sinensis leaves infected withmosaic virus, which ultimately causes leaf yel-lowing and abscission.

Aharoni et al. (1977) noted that the applica-tion of GA3 enhanced hypocotyl growth in healthyseedlings, but was ineffective when applied toinfected seedlings. They suggested that this might

be due to either a change in GA metabolism ininfected tissues and/or enhanced tissue levels ofABA or ethylene.

Ben-Tal and Marco (1980) reported a signifi-cant reduction in GA concentration in CMV-in-fected cucumber, but only after stunting becameapparent. More significantly, they noted a quali-tative change in GA profile before the stuntingsymptom became visible. They confirmed thisqualitative difference by showing a difference inthe metabolism of 3H-GA3 between healthy andinfected plants, suggesting the possibility that GAturnover to less or nonactive forms might be in-volved in virus infection.

In most of the above cases only low-resolu-tion chromatographic techniques and bioassayshave been used to quantify GA-like substances inthe healthy and virus-infected plants. The morerecent work of Zhang et al. (1997) using a GA-based ELISA confirmed the earlier studies report-ing a decrease of GAs in diseased compared withhealthy tissue. As they also noted increased ABAlevels in diseased tissue, they advocated an im-balance of plant hormones leading to symptomdevelopment. However, the differential metabo-lism noted by Ben-Tal and Marco (1980) war-rants further investigation.

GAs are biosynthesized from the isoprenoidmetabolic pathway and share common intermedi-ates with both cytokinin and ABA. The possibil-ity of an integrated response involving all threehormones should be investigated, especially asZhang et al. (1997) reported a decrease in bothcytokinin and GA and an increase in ABA invirus-infected banana leaves.

F. Plant-Virus Interactions with Ethylene

Generally, ethylene production appears to beincreased by virus infection and has been associ-ated with the development of necrotic or chloroticlesions (e.g., Ross and Williamson, 1951; Lockhartand Semancik, 1970; Marco and Levy, 1979;Fraser and Whenham, 1982; van Loon andAntoniw, 1982; Pennazio and Roggero, 1992a;Ohtsubo et al., 1999), correlated with the suppres-sion of hypocotyl elongation (Marco et al., 1976;Levy and Marco, 1977) or associated with other

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growth abnormalities (e.g., Gold and Faccioli,1972). The majority of research has focused onsystems in which a hypersensitive response (HR)occurs, but Chaudhry et al. (1998) specificallyshowed enhanced ethylene production in leavesof tobacco systemically infected with cucumbermosaic virus.

It has been established by several groups thatthe ethylene production is of plant origin via theACC-dependent pathway (e.g., de Laat et al.,1981a,b; de Laat and van Loon, 1982, 1983;Roggero and Pennazio, 1988; Pennazio andRoggero, 1990, 1992a; Chaudhry et al., 1998).Recently, Knoester et al. (1995) demonstratedthat virus infection increased the transcription ofgenes coding for both ACC synthase and ACCoxidase.

The role of ethylene in the development ofnecrotic lesions remains a contentious issue. deLaat and van Loon (1982, 1983) demonstrated anincrease in ethylene following infection of to-bacco by TMV, just prior to the appearance ofnecrotic lesions. Levels of methionine andS-adenosylmethionine remained constant in in-fected tissue, but there was an increased produc-tion of ACC. The increase in ACC preceded theincrease in ethylene production and was restrictedto the cells surrounding the necrotic area. Utiliz-ing a temperature-based strategy, which synchro-nizes the induction of the HR, de Laat and vanLoon (1983) showed that ethylene production com-menced 2 h after the shift from 30 to 20°C, 6 hbefore necrosis developed. They concluded thatthe increase in ethylene was not determined bythe onset of necrosis, but by a much earlier event.

This was supported by Ohtsubo et al. (1999),also using a temperature-induced synchronouslesion formation system, who showed that accu-mulation of ACC oxidase mRNA and increasedethylene production preceded and promoted le-sion formation. Siefert et al. (1995) reported anincrease in ACC and cyanide in tobacco leavesinfected with TMV. They concluded that the de-velopment of necrotic lesions during the HR wasfrom cyanide produced during the conversion ofACC to ethylene.

The use of biosynthetic inhibitors, ethyleneaction inhibitors, or hypobaric conditions have allbeen shown to relieve virus-induced symptoms,

including reducing lesion size (Ohtsubo et al.,1999; c.f. Pennazio and Roggero, 1990, 1992b),delaying chlorotic lesion development (Marco andLevy, 1979), and increasing hypocotyl elongationand preventing virus-induced epinasty (Levy andMarco, 1976), all of which support a causativerole of ethylene in the development of these symp-toms.

However, Knoester et al. (1997, 1998), usingsense and antisense constructs to ACC oxidase, aswell as tobacco transformed with the etr1-1 mu-tant gene, considered that transgenic tobacco plantswith altered ethylene levels or reduced sensitivityto ethylene were not impaired in their ability toform local lesions after TMV infection, and thatethylene was not essential for the HR. In address-ing this claim, Ohtsubo et al. (1999) inferred thatKnoester’s system lacked the elegance of theirown, and concluded that ethylene is an absoluterequirement for lesion development and the HR.However, Kachroo et al. (2000) also suggestedthat the HR response of Arabidopsis to turnipcrinkle virus was not dependent on ethylene.

Tobias et al. (1989), using a range of TMVstrains and Capsicum annuum host-combinations,examined the relationship between ethylene syn-thesis, necrosis, and pathogenesis-related (PR)protein accumulation. Necrotic infections stimu-lated ethylene synthesis, but symptomless leaveswith enhanced ethylene production were alsofound. The nonnecrotic strain of TMV did notsignificantly enhance ethylene production. Caseswere found where ethylene was increased withoutPR protein accumulation and, conversely, wherePR proteins increased without major ethylene in-creases.

However, there are numerous reports in theliterature linking the induction of PR proteinswith ethylene production following virus infec-tion (see references cited in Ohtsubo et al., 1999).Knoester et al. (1998), for example, suggestedthat increased ethylene production following vi-rus infection was essential for the induction of PRproteins. They showed that ethylene-insensitivetransgenic tobacco reacted to TMV with reducedexpression of basic PR genes. Interestingly, whilethese plants still exhibited the HR to TMV, theyhad become susceptible to normally nonpatho-genic soil fungi. Tonero et al. (1997) also re-

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ported that pathogenesis-related proteins wereinduced in TMV-infected tomato plants follow-ing accumulation of ACC.

Most recently, Ohtsubo et al. (1999) showedthat inhibitors of ethylene biosynthesis and actionreduced both basic PR-1 protein gene expressionand also that of PI-II, but not that of the acidicPR-1 gene. Further, induction of these genes wasincreased in ACC-treated leaves, especially intransgenic plants overexpressing ACC oxidase.Ohtsubo et al. (1999) concluded that the expres-sion of basic PR genes in tobacco is controlled bythe endogenous ethylene produced during locallesion formation.

While Ohtsubo et al. (1999) concluded thatethylene is not a byproduct but an indispensablefactor for HR, the conflicting conclusions drawnby both Knoester et al. (1997, 1998) and Kachrooet al. (2000) need to be resolved. However, sev-eral groups suggest that lesion formation doesdepend on the activity of the ethylene biosynthe-sis pathway. The presence or absence of ethyleneis also strongly correlated with a number of othervirus-induced symptoms. Further, recent workimplicates ethylene along with JA in signal trans-duction pathways leading to up-regulation of de-fense-related genes (Section III) in addition to thebasic PR proteins and PI-II.

G. Plant–Virus Interactions withJasmonic Acid

Jasmonic acid is regarded as a key compoundin plant defense reactions to both biotic and abi-otic stimuli. Until recently, most reports concernedthe involvement of JA in responses to wounding,bacterial, or fungal attack (see references cited inSeo et al., 2001). However, there are several veryrecent reports on interactions between JA andplant viruses (Preston et al., 1999; Clarke et al.,2000b; Dhondt et al., 2000; Seo et al., 2001).

The earliest investigations were by Ravnikaret al. (1990), who reported that reduced potatovirus M content in potato meristems was corre-lated with an increased JA concentration. In amore recent report, Petrovic et al. (1997) showedincreased JA in roots of diseased plants and de-creased JA in shoots of PVY-infected potato plant-

lets relative to healthy plantlets and concludedthat JA may play a role in plant response to viralinfection.

Recently, Clarke et al. (2000b) reported thatboth JA and dihydrojasmonic acid increased fol-lowing infection of Phaseolus vulgaris withWClMV. These increases occurred after a rapidbut transient increase in JA alone, which wasascribed to a wound response to inoculation. Theysuggested that the second rise in JA activity, whichwas coincident with the increase in virus titer andthat reached a plateau within 5 days postinocula-tion, was due to membrane damage as the virusreplicated. Jasmonic acid is synthesised from li-nolenic acid, which is released from damagedcellular membranes (Vick and Zimmerman, 1984;Conconi et al., 1996).

Dhondt et al. (2000) analyzed tobacco leavesexhibiting a hypersensitive response to TMV. Theyshowed that an increase of soluble phospholipaseA2 activity occurred at the onset of necrotic lesionappearance. Subsequently, 12-oxophytodienoic acidaccumulated followed by lesser amounts of JA.Clarke et al. (2000b) and Dondt et al. (2000) bothsuggested that JA levels increased rather late invirus infection and probably were not involved inthe onset of any defense response.

However, in a very recent note, Seo et al.(2001) monitored changes in JA in a temperature-dependent synchronized hypersensitive cell deathsystem using TMV-infected tobacco leaves. Un-der these circumstances they showed a transientincrease in JA that was preceded by activation ofWIPK and followed by accumulation of tran-scripts of the basic PR-1 gene. Peak JA accumu-lation coincided with the onset of electrolyte leak-age. It should be noted that inoculation occurred48 h before plants were shifted to the cooler HR-inductive temperatures. By this stage the transientwound-induced peak of JA observed by Clarke etal. (2000) would no longer have been detected. Incontrast to Clarke et al. (2000b) and Dhondt et al.(2000), Seo et al. (2001) implicate JA in signaltransduction during virus infection.

However, Preston et al. (1999) suggested thatTMV infection inhibited wound-induced JA-me-diated responses. They were working with to-bacco plants that were inoculated with TMV andthen wounded some 4 days later. The JA level

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was determined at just one time point, 90 minafter wounding. Local lesions had developed by48 h, so any immediate JA responses to woundingor to lesion development would not have beenassessed. Over the time frame of their experi-ment, no JA response to wounding was observed.They suggested that TMV inoculation inhibitedthe production of the wound signal due to thelikely inhibition of JA biosynthesis by the el-evated SA levels induced by virus inoculation(Preston et al., 1999). In contrast, in thenonhypersensitively responding system used byClarke et al. (2000b), JA and DJA were seen toaccumulate simultaneously with virus replication.

The results of Preston et al. (1999) also ap-pear in contrast to those of Sano et al. (1994,1996) and Sano and Ohashi (1995), who reportedthat tobacco plants expressing a gene coding fora small GTP-binding protein had enhanced en-dogenous cytokinins and enhanced resistance toTMV. Further, these plants were shown to havean enhanced response to wounding that includedan accumulation of both JA and SA. Sano et al.(1996) proposed that cytokinins regulated the ac-cumulation of SA and JA in wounded and/orvirus-infected transgenic plants. They suggestedthat following wounding of wild-type plants thatan increase in JA occurred that inhibited SA bio-synthesis. However, in virus-infected transgenicplants, they suggested that this JA inhibition ofSA did not occur, and SA was synthesized withthe subsequent induction of the hypersensitiveresponse. Petrovic et al. (1997, 1999) also sug-gested an interaction between JA and cytokininsmight be occurring during virus infection.

It would appear worthwhile for some com-parative work to be carried out on endogenousJA in hypersensitively and nonhypersensitivelyresponding systems. This would need to becarried out over a time course encompassingthe initial wound response during inoculation,and during and after the HR itself and the timeperiod during which either systemic acquiredresistance or systemic infection becomes es-tablished. However, both the application of JAand the use of JA mutants have certainly im-plicated the jasmonates in virus-inducedupregulation of defense-related genes (Sec-tion III).

H. Plant-Virus Interactions with SalicylicAcid

Salicylic acid, its glucose conjugate, and me-thyl ester have been shown to increase markedlyfollowing virus infection (e.g., Enyedi et al., 1992;Seskar et al., 1998). The biosynthesis and metabo-lism of SA is well characterized. Salicylic acid issynthesized from trans-cinnamic acid via benzoicacid, a reaction catalyzed by benzoic acid 2-hy-droxylase (BA2H). This enzyme is expressed con-stitutively in tobacco but is highly induced by in-oculation with TMV or application of benzoic acid(e.g., Yalpani et al., 1993; Leon et al., 1993, 1995),indicating de novo biosynthesis of SA in responseto virus infection. Phytotoxic accumulation of SAis probably controlled by conjugation with glucoseto form β-O-D-glucosylsalicylic acid (SAG). Thebiosynthesis of SAG is via the extracellular actionof UDPglucose:SA glucosyltransferase, the activ-ity of which increases in the immediate vicinity oflocal lesions following TMV infection of tobacco(Enyedi and Raskin, 1993; Lee et al., 1995), pos-sibly to prevent localized damage from accumulat-ing SA.

Recently, Guo et al. (2000) monitored SAlevels during the temperature-mediated synchro-nous induction of HR in tobacco. Large increasesin the levels of SA and SAG were not detecteduntil 6 and 8 h after HR induction, respectively,enabling the separation in time of SA-inducedand early SA-independent gene expression (Sec-tion III).

By monitoring endogenous SA levels in vi-rus-inoculated and uninoculated leaves, usingmutants over- or underexpressing SA, and thenahG gene to hydrolyze and inactivate SA, thereis strong evidence for a role for SA in the activa-tion of cell death, in pathogen localization, indefense gene activation, and in the induction ofSAR. For a recent comprehensive review seeDempsey et al. (1999).

While it is accepted that SA is a necessarycomponent of SAR, whether it is the “long-dis-tance” signal between the site of infection and thesystemically responding leaf is still debated(Vernooij et al., 1994 vs. Shulaev et al., 1995 andreferences in Dempsey et al., 1999). More recently,the volatile methyl-ester of SA (methylsalicylate)

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has been shown to increase in parallel with SAafter infection and has been implicated in plant-to-plant and within plant communication followinginfection (Shulaev et al., 1997; Seskar et al., 1998).

Recently, it has been demonstrated that SAactivates mitogen-activated protein (MAP) kinases(Zhang and Klessig, 1997). The SA-induced pro-tein kinases identified by Zhang and Klessig (1997)are thought to regulate PR protein expression andthe expression of several genes (R genes) involvedin bacterial, fungal, nematode, and virus resis-tance (Romeis et al., 1999). However, althoughvirus infection and the application of SA leads tothe production of PR proteins, these proteins donot appear to have antiviral activity (e.g., Dempseyet al., 1999).

Seeking a mechanism by which SA mightinduce resistance to viruses, and noting that SAinduces the plant mitochondrial alternative oxi-dase (AOX), Chivasa et al. (1997), Chivasa andCarr (1998) and Naylor et al. (1998), showed thatsalicylhydroxamic acid (SHAM), an inhibitor ofAOX, antagonized not only the SA-induced ac-quired resistance in HR tobacco, but also the re-duction in virus replication in the non-HR plants.Interestingly, SHAM did not inhibit the produc-tion of PR-1 protein or resistance to fungal orbacterial pathogens. They proposed that the de-fense signal transduction pathway separates down-stream of SA into two branches. One is SHAMsensitive, and by deduction probably requiresactivity of the AOX pathway. The other is SHAMinsensitive, and leads to the production of anti-fungal and antibacterial mechanisms (reviewedby Murphy et al., 1999).

Chivasa et al. (1997) showed that SA inducedinterference with replication in the directly inocu-lated tissue. However, Naylor et al. (1998), notingthat viral particles were not necessarily restrictedwithin local lesions, also proposed that SA couldinduce an inhibition of long-distance movementand interfere with the exit of the virus from theleaf. Significantly, both resistances required op-eration of the SHAM-sensitive branch of the de-fense signal transduction pathway (Chivasa andCarr, 1998; Naylor et al., 1998).

Interestingly, Clarke et al. (2000a) recentlyshowed that cytokinin, JA, and SA could all inter-fere with replication of WClMV at the subgenomic

dsRNA level in the inoculated tissue. On the otherhand, virus infection has been shown to cause areduction in active cytokinins and in ROS scav-enging enzymes (Clarke et al., 1999; Clarke et al.,2002), leading these authors to suggest that aminimal level of ROS species is required for viralreplication. The alternative oxidase also lowersROS (Maxwell et al., 1999) and appears as part ofa plant defense pathway against virus infection(Murphy et al., 1999). The possibility of a director indirect link between the cytokinin-inducedresistance and the AOX pathway and ROS iscurrently under investigation (Jameson and Galis,unpublished data).

III. DEFENSE-RELATED GENES

One might expect that more reports on theeffects of GAs, cytokinins, auxin, and ABA inrelation to the up- or down-regulation of genesrelated to defense against viruses should soonappear. The products of defense-related genes thatare induced by pathogens (as opposed to the resis-tance genes) are referred to as pathogenesis-re-lated proteins. It appears accepted that ethyleneinduces the expression of basic PR proteins,whereas SA is strongly implicated in the induc-tion of the PR genes associated with SAR. How-ever, as PR genes have not been shown to exhibitantiviral activity (Dempsey et al., 1999), this in-duction may have little to do with the control ofvirus replication and spread. However, it is in-creasingly evident that defense-related genes showcomplex expression patterns in response to SA,JA, and ethylene (see recent reviews by Dong etal., 1998; Pieterse and van Loon, 1999 and refer-ences cited in Guo et al., 2000).

While in some systems it appears that there isa direct antagonism operating between JA and SAin the development of systemic resistance to mi-crobes and insect herbivores (Niki et al., 1998;Felton et al., 1999; Thaler et al., 1999; Preston etal., 1999), it now appears, based on work withArabidopsis, that defense genes can be classifiedinto three groups: the first group is induced by JAbut not by SA, the second group by SA but notJA, and the third group is induced by both JA andSA (Song et al., 2000). Ethylene is also frequently

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linked with JA as a necessary component (seereview by Pieterse and van Loon, 1999).

Here we consider a few recent examples toemphasize the increasing complexity of the signaltransduction chains operating following infectionwith virus. These support the contention that thereare both SA-dependent and SA-independent path-ways for the induction of defense against virus(see also reviews by Pieterse and van Loon, 1999;Dempsey et al., 1999; Murphy et al., 1999).

Kachroo et al. (2000) utilized the ethylene-insensitive (etr1-1) and JA-insensitive (coi1-1)Arabidopsis mutants, as well as NahG transgenics,to show that both the HR and TCV resistancewere dependent on SA but not on ethylene or JAduring the hypersensitive response of Arabidopsisto turnip crinkle virus. However, TCV resistancewas unaffected by mutations in NPR1, a key com-ponent of SA-regulated resistance pathways tobacterial and fungal pathogens (see references inKachroo et al., 2000). Kachroo et al. (2000) con-cluded that TCV resistance requires a SA-depen-dent, but NPR1-independent signaling pathway.They also noted that the pathway proposed byChivasa et al. (1997) and Chivasa and Carr (1998)would also be SA dependent but NPR1 indepen-dent.

Recently, Guo et al. (2000), utilizing a syn-chronous HR-activation system, and focusing ona relatively early time frame in the induction ofresistance, isolated a group of genes that wasinduced both locally and systemically in tobaccoresistant to infection by TMV. By utilizing bothNahG transgenic tobacco and ethylene action and/orsynthesis inhibitors, they deduced that inductionof most of the genes in this family were SAindependent, but ethylene dependent, although asmall subset were also ethylene independent.

Mitter et al. (1998) also suggested that, inboth Arabidopsis and tobacco, a pathogen- (in-cluding TMV-) induced, salicylate-independentsystemic signaling pathway exists. They suggestthat this pathway, which leads to the up-regula-tion of a plant defensin, involves ethylene and JAsignaling components. They showed that JA andTMV infection, but not SA, led to the systemicinduction of the PDF1.2 promoter in tobacco.

Song et al. (2000) specifically focused on theactivation of the antiviral protein gene (PIP2)

induced locally and systemically in Phytolaccainsularis by wounding. This gene codes for aribosome-inactivating protein. Song et al. (2000)showed that it could be induced by ABA and JAbut not by SA. They suggest that the PIP2 genebelongs to the group of defense-related genes thatare regulated via the JA-dependent but SA-inde-pendent pathway, and function to prevent virusesfrom entering through wounded sites and spread-ing to the rest of the plant.

In addition to defense pathways activated asa consequence of viral recognition (i.e., the HRand systemic responses), it appears logical thatantiviral defenses would be up-regulated viawound-induced signal transduction pathways.Clearly, with viruses there are multiple defensepathways operating.

IV. GALL-FORMING VIRUSES

It is surprising that few researchers have tar-geted those viruses that induce hyperplasia andsubsequent tumorigenic growths in which onemight expect cytokinin and/or auxin metabolismto be perturbed. As noted in Section II.D, Abdalaet al. (1999) conducted such a study on MRCV-infected maize, reporting elevated levels of bothIAA and IAA oxidase in during gall formation.

Latham et al. (1997) also recorded the induc-tion of cell division by beet curly top virus (BCTV)and isolated the C4 gene as the gene that alone issufficient to initiate cell division. The gene acts ina cell nonautonomous fashion similar to the regu-latory gene knotted1 (kn1). Ectopic expression ofboth C4 and kn1 resemble that of cytokininoverexpression (Latham et al., 1997; Hewelt etal., 2000, respectively). How C4 induces cell di-vision is unknown. Latham et al. (1997) suggestit may be due to perturbation of the hormonebalance or a more direct effect on cyclins, cyclin-dependent kinases, or other cell-signaling mecha-nisms. Neither endogenous cytokinin nor auxinlevels were measured during the formation oftumorigenic growths induced by BCTV. Interest-ingly, Manes et al. (2001) have shown up-regula-tion of the CylinD3 transcript during induction ofgall formation by the bacterium Rhodococcusfascians, an effect mimicked by application of Z.

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It appears that there has been little work onother gall-forming viruses with respect to the planthormones.

V. CONCLUSIONS

The effects of virus infection on the biosyn-thesis and metabolism of plant hormones are ob-viously extremely complex. The nature of theclassic plant hormones and the jasmonates do notlend themselves to facile analysis. Obviously lack-ing are in-depth hormone analyses in numerousplant-virus combinations, with the analyses cov-ering extended yet detailed time frames and tis-sues. The logistics of such exercises would bebeyond any single laboratory. There are examplesof changes in biosynthesis and/or metabolism ofABA, GAs, auxin, and cytokinins in one or moresystems. Changes in sensitivity to a particularsignal compound must also be kept in mind. How-ever, whether these changes are directed by theinvading virus, are indirectly a consequence ofthe redirection of assimilates, or directly of virus-induced cellular damage remain to be determined.Further, whether these changes in metabolism arelinked to SA, JA, or ethylene remain to be deter-mined, although the work of Sano et al. (1996)indicates that there may be a direct link with thecytokinins.

Elevated endogenous levels of ethylene, SA,and JA appear to be a direct consequence of virusinfection and have been shown to be tightly tem-porally and spatially coordinated with the induc-tion of the HR. These compounds are all impli-cated in signal transduction chains leading to theinduction of defense-related genes in bothhypersensitively responding and systemically in-fected systems. However, care must be taken toensure that conclusions drawn from the responsesto applications of compounds are not because ofthe use of pharmacological concentrations. Theuse of mutants and transgenic plants is extensivein this area, but data obtained from such plantsshould still be underpinned by endogenous analy-ses.

With the increasingly rapid discovery of planthormone biosynthetic, metabolic, perception, andsignal transduction genes, such as witnessed for

the cytokinins during 2001 (Takei et al., 2001;Inoue et al., 2001), the use of microarray technol-ogy and the application of gene silencing tech-niques (eg Smith et al., 2000) aimed at eithervirus genes or plant hormone genes, rapid ad-vances could be made in addressing this tantaliz-ing, but underresearched, area of the plant hor-mone biology.

If Murphy et al. (1999) and Kachoo et al.(2000) are correct and resistance to viruses inplants “is activated via a mechanism distinct fromthose used for other microbial pathogens”, re-viewers will need to be more discriminating andnot refer generically to “necrotising pathogens”and “disease resistance mechanisms”.

ACKNOWLEDGMENTS

This review was completed while PEJ was onsabbatical leave at CSIRO, Plant Industry,Canberra.

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