Viruses as triggers of DNA rearrangements in host plants

Viruses as triggers of DNA rearrangements in host plants

Larisa Andronic

Institute of Genetics and Plant Physiology of the Academy of Sciences of Moldova, MD 2002, 20 St. Padurii,Chisinau, Republic of Moldova (e-mail: [email protected]). Received 14 September 2011,

accepted 30 March 2012.

Andronic, L. 2012. Viruses as triggers of DNA rearrangements in host plants. Can. J. Plant Sci. 92: 1083�1091. Assessmentof microsporogenesis in tomato (Lycopersicon esculentum, cultivars Fachel, Nistru and Prizior) infected with tomatoaspermy virus or potato virus X and barley (Hordeum vulgare L., cultivars Galactic, Sonor, Unirea) infected with barleystripe mosaic virus showed deviations in the conjugation of homologous chromosomes and segregation of genetic material,expressed in the disruption of chromatin cohesion between homologous chromosomes. The evidence of meiotic division intargeted genotypes indicates the effect of viral infection on chiasmata number and position, promoting the redistributionof chiasmata. On the basis of cytological study, significant changes and the induction of additional exchanges offset byasynapsis were established in early diakinesis. Different parameters, determined at particular stages of meiosis, such aschromosome aberration and the mean percentage of abnormal pollen mother cells, served as cytogenetic evaluation ofmicrosporogenesis in virus-infected tomato or barley cultivars. The study of meiotic stability in anaphase and telophaseI and II revealed a significant increase in different types of abnormalities: elimination or/and lagging chromosomes,formation of chromosome and chromatid bridges with or without fragments. Reviewed examples provide data regardinggenetic rearrangements in host plants as a response to viral infection.

Key words: Viral pathogenesis, chiasma, chromosome aberration, meiotic division, mitotic recombination,DNA rearrangements

Andronic, L. 2012. Les virus comme declencheur de rearrangements de l’ADN dans les plantes hotes. Can. J. Plant Sci. 92:1083�1091. L’evaluation du morfosporogenese de tomates (Solanum lycopersicum L., les varietes Fachel, Nistru et Prizior)infeces par le virus aspermy de tomatrs ou virus X de pomme de terre, de l’orge (Hordeum vulgare L., varietes Galactic,Sonor, Unirea) infectes par le virus de la mosaıque striee de l’orge a montre des ecarts dans la conjugaition deschromosomes homologues, la segregation du materiel genetique, exprimee dans la perturbation de la cohesion entre lachromatine des chromosomes homologues. La preuve de la perturbance en division meiotique chez les genotypes indiquel’effect de l’infection virale sur le nombre et la position de chiasmas, la promotion de la redistribution des chiasmas. Sur lesbases de l’etude cytologique de diacinese ont ete etablis au debut des changements significats et dans l’induction d’echangessupplementaires compensees par asynapsis. Differents parametres, determines a des moments particuliers de la meiosis, telsque aberrations chromosomiques et le pourcentage moyen de cellules meres de pollen anormaux ont servi evaluationcytogenetique des microsporogenese dans les varietes de tomatoes ou l’orge infectees du virus. L’etude de la stabilite dansla meiose anaphase et telophase I et II ont revele une augmentation significative des differents types d’aberrations :elimination et/ou a fragments, la formation du chromosome et de ponts chromatides avec ou sans fragments. Exemples surles donnees concernant les rearrangements de materiel genetique dans les plants comme une response a l’infection virale.

Mots cles: Pathogenese virale, chiasma, aberration chromosomique, meiose, recombinaison meiotique,rearrangements de l’ADN

Viruses are the most damaging of plant pathogens,and induce a variety of responses from their hosts (Khanand Dijkstra 2002; Burch-Smith et al. 2004). It is knownthat virus infections dramatically affect plant physiol-ogy, decreasing photosynthesis, increasing respirationand altering carbohydrate levels (Zaitlin and Hull 1987;Rowland et al. 2005). The modification of these physio-logical processes by viral diseases is one of the primarycauses of decreasing crop productivity.

Previous work has established that viral infectioncan contribute to chromosome breaks (Nagar et al.1995, 2002; Bass et al. 2000), activation of transpos-ing elements (Dellaporta et al. 1984; Johns et al. 1985;Kalendar et al. 2000; Ikeda et al. 2001), chromatin con-densation and cell proliferation (Nagar et al. 2002), and

modification of gene expression (Stratford and Covey1988).

The contribution of viral infection to the enhance-ment of somatic (Kovalchuk et al. 2003; Dong 2004;Filkowski et al. 2004) and meiotic recombination(Chiriac et al. 2006), resulting in rearrangements thatcould, potentially, be transmitted to the next generation(Kovalchuk et al. 2003; Boyko et al. 2007; Marii andChiriac 2009) has been recognized. Exposure of maize

Abbreviations: A-T I, anaphase�telophase I; A-T II, anaphase�telophase II; BSMV, barley stripe mosaic virus; PMC, pollenmother cells; PVX, potato virus X; S, overall number of chiasmata;SI, sum of interstitial chiasmata; ST, sum of terminal chiasmata;TAV, tomato aspermy virus

Can. J. Plant Sci. (2012) 92: 1083�1091 doi:10.4141/CJPS2011-197 1083

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plants to barley stripe mosaic virus (BSMV) seems toactivate transposable elements and cause mutations inthe non-infected progeny of infected plants (Kovalchucet al. 2003). The induction by the BSMV of an inher-ited effect may mean that the virus has a non-cell-autonomous influence on genome stability. The authorsreport a threefold increase in homologous recombina-tion frequency in both infected and non-infected tissueof tobacco plants with either tobacco mosaic virus oroilseed rape mosaic virus. A similar increase in DNArecombination was also observed in the progeny ofthe infected plants, indicating that pathogen-inducedrecombination can lead to heritable adaptations to envi-ronmental stresses (Dong 2004).

Much research regarding plant viruses reveals thatthe character of the systemic symptoms of virus infec-tion in plants is determined by the expression of bothhost and virus genes. The consequences of viral infectionare highly variable, leading to a continued lack of under-standing of these effects.

The objectives of this study were: (i) to evaluatemeiotic division in tomato (dicot) and barley (monocot)under viral infection; (ii) to identify the meiotic recom-bination in virus-infected plants; and (iii) to elucidatethe cytogenetic effect of viral infection in different host�pathogen systems.

MATERIALS AND METHODSThe biological materials used in this study were the cul-tivated tomato, Lycopersicon esculentum (syn. Solanumlycopersicum, 2n�24), cultivars Fachel, Nistru andPrizior, and three cultivars of spring barley (Hordeumvulgare L., 2n�14), Galactic, Sonor and Unirea.

Tomato plants were grown in pots or in the fieldfollowing a standard technique. The experimental plantswere infected at the three- to four-leaf stage with tomatoaspermy virus (TAV, isometric virions, RNA genomicnucleic acid component) or potato virus X (PVX,filamentous virions, RNA genomic nucleic acid compo-nent). The source of the TAV was Nicotiana glutinosasystemically infected with chrysanthemum isolate, andthe source of the PVX was N. tabacum ‘Samsun’ infectedwith PVX.

Barley plants from experimental lots were mechani-cally inoculated with a BSMV (tripartite, ARN segmen-ted genome) extract. Plants at the two-leaf stage fromexperimental lots were infected twice, at 2-d intervals,with inoculum of BSMV using carborundum powder.Inoculum was prepared by grinding leaf tissue in dis-tilled water (1:2). Initially, the extract was preparedfrom field-grown barley plants that expressed symptomsspecific to those induced by BSMV and presented apositive response to the immunosorbent electron micro-scopy test (Milne and Lesemann 1984), using polyclonalanti-BSMV serum (kindly provided by J. G. Atabekov).The immunosorbent electron microscopy was adaptedto detect BSMV particles in homogenates of barleyleaves. The grids were treated for 5 min with antiserum

to BSMV diluted 1:1000, incubated with extracts ofhomogenate tissues for 12 h, washed, and negativelystained with 1% uranyl acetate.

All three cultivars investigated, Galactic, Sonor andUnirea, were susceptible to BSMV and showed externalsymptoms 10�14 d after mechanical inoculation.

Mock-inoculated plants that responded negatively tothe virological test served as controls.

The presence of virions in infected tomato plantsand their absence in healthy plants was establishedthrough enzyme-linked immunosorbent assay (ELISA)(Clark and Adams 1977) and immunosorbent electronmicroscopy. Commercial antisera (Sediag, France) wereused in the ELISA according to the manufacturer’sinstructions. For cytological analysis, tomato or barleyanthers undergoing meiosis were subjected to directfixation in an acetic acid:alcohol (3:1) solution. Afterfixation, buds were transferred to 70% ethanol andstored at 48C. The frequency and distribution ofchiasmata were measured based on observations of tem-porary preparations obtained by acetocarmine stain-ing investigated under light microscope (BIOLAR).Bivalents were grouped by configuration according tochiasma number and position during early diakinesis:1 interstitial (I), 2 interstitial (II), 1 terminal (T), 1terminal�1 interstitial (T�I), 1 terminal�2 interstitial(T�II), 1 terminal�3 interstitial (T�III), 2 terminal(TT), 2 terminal�1 interstitial (TT�I), and univalents(0) (Imai and Moriwaki 1982). The total number ofinterstitial chiasmata (SI), terminal chiasmata (ST) andthe overall number of chiasmata (S) per pollen mothercells (PMC) were also assessed. Meiotic characteristics,such as the number and distribution of chiasmata,were studied in at least 60 diakinetic cells. Chromosomebehaviour in anaphase and telophase I and II wasevaluated in more than 300 meiocytes per phase.

Statistical analysis of data was carried out usingStatgraphics Plus for Windows (version 2.1; MicrosoftCorp., Redmond, WA) and Microsoft Excel. The con-tribution of variation sources was computed followingthe ANOVA test results (Clewer and Scarisbrick 2001).

RESULTSCytological examination of tomato and barley microspo-rocytes revealed morphological succession dependingon the degree of compaction and chromosome-specificconjugation according to the stage of meiosis (Fig. 1).In the first sub-stage, instead of the leptotene andzygotene stages, a synezetic knot occurred, consistingof thin chromatin strands surrounding the nucleolus andeventually totally covering it (Fig. 1A, B, C, F, G, H).Later, paired chromosomes unravelled from the knotand appeared as thick strands. In diakinesis, typicalmeiotic bivalents were established in healthy and virus-infected tomatoes (Fig. 1K, L, M).

The structural changes of the nucleoli and thetendency to undergo chromosome condensation withthe transition from one phase to another are illustrated

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in Fig. 1. As synaptic cohesion is achieved, the forma-tion of short, thick bivalent chromosomes is seen(Fig. 1D, E, N, O). The result of homologous chromo-some association is the formation of 12 bivalents inthe normal tomato PMC and 7 in the barley PMC,which can be more easily examined in diplotene anddiakinesis. Differences between experimental variantscould be observed only in the diakinesis stage, whenit was possible to examine bivalent profiles and thepresence or absence of univalents. The bivalent config-uration allows assessment of the location of homologouschromosome association.

In cytological investigations, the frequency and posi-tion of chiasmata are good estimators for the evaluationof meiotic recombination (Korol et al. 1994). Accordingto their location, the chiasmata are considered the cyto-logical expression of crossing-over, reflecting the point

of physical exchange between regions of homology ofnon-sister chromatids. Proceeding from this, the tomatobivalents were examined corresponding to chiasmatadistribution along the chromosome arms (Fig. 2).

In control plants, the value of interstitial chiasmataranged from 3.69 per bivalent in diakinetic PMC of cv.Nistru to 4.35 in those of cv. Fachel and 3.93 in cv.Prizior, while the terminals had 13.97, 12.66 and 12.28,respectively. Average chiasmata frequency values were16.69, 17.66 and 17.17 in cultivars Fachel, Nistru andPrizior, respectively. According to the results obtainedfrom the treatments, modifications in the number ofchiasmata per bivalent were found in the variantsinduced by TAV and PVX (Table 1).

Confirmed statistical differences in the total numberof chiasmata were recorded for all three cultivarsevaluated, Fachel, Nistru and Prizior, affected by

Fig. 1. Male meiocytes at various stages: (A) Leptotene in tomato cv. Fachel, control. (B) Leptotene in tomato cv. Fachel infectedwith tomato aspermy virus (TAV). (C) Leptotene in tomato cv. Fachel infected with potato virus X (PVX). (D) Leptotene in barleycv. Galactic, control. (E) Leptotene in barley cv. Galactic, infected with barley stripe mosaic virus (BSMV). (F) Zygotene in tomatocv. Fachel, control. (G) Zygotene in tomato cv. Fachel infected with TAV. (H) Zygotene in tomato cv. Fachel infected with PVX. (I)Pachitene in barley cv. Galactic, control. (J) Pachitene in barley cv. Galactic, infected with BSMV. (K) Early diakinesis in cv. Fachel,control. (L) Early diakinesis in cv. Fachel, infected with TAV. (M) Early diakinesis in cv. Fachel, infected with PVX. (N) Earlydiakinesis in barley cv. Galactic, control. (O) Early diakinesis in barley cv. Galactic, infected with BSMV.

Fig. 2. Tomato bivalent types according to chiasma position at early diakinesis (for various cultivated tomato, SolanumlycopersicumL.). (A) Bivalent with one interstitial chiasma. (B) Bivalent with two interstitial chiasmata. (C) Bivalent with one terminalchiasma. (D) Bivalent with one terminal and one interstitial chiasma. (E) Bivalent with one terminal and two interstitial chiasmata.(F) Bivalent with two terminal chiasmata. (G) Bivalent with two terminal and one interstitial chiasma. (H) Univalents.

ANDRONIC * VIRUSES REARRANGE DNA IN HOST PLANTS 1085

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PVX or TAV. For cv. Nistru, significant changes werenoted only in plants infected with tomato aspermy.In this genotype, the increase in the overall interstitialchiasmata was accompanied by a decrease in terminalchiasmata. This fluctuation contributed to the lack ofstatistical confirmation of the total amount of chiasmatavariation. Similar tendencies were observed for cv.Prizior infected with TAV. Proximal trends of increas-ing interstitial chiasmata followed by reductions of theterminal chiasmata have also been described for cv.Prizior, while in cv. Fachel increases in both chiasmatatypes have been found.

Chromosome pairing in barley at diakinesis wastypical of diploidy (2n�4x�14), being associatedpredominantly as bivalents, but univalents were alsorecorded (Fig. 1f). Such chromosome pairing is ex-pected, based on random associations of seven bivalentstypical for diploid genotypes. In Fig. 3, barley bivalenttypes according to the number and position of chias-mata and some meiotic abnormalities are illustrated.

In control plants, the number of total chiasmataranged between 13.52 and 14.21 in cultivars Unireaand Sonor, respectively (Table 2). The experimentshave demonstrated that barley plants infected withBSMV responded with changes in the number andposition of chiasmata. Significant differences were ob-served for all three host-virus combinations. The great-est changes recorded were those in the number ofinterstitial chiasmata. The tendency in the variation innumber of chiasmata was found to be similar to that forvirus-infected tomatoes.

Based on the examination of barley microsporogen-esis, it was established that the fluctuation in the number

of chiasma is caused by changes in bivalent types.Within each treatment variant, modifications in theratios of divalent types were found. In all cases, thebivalents were more frequently conjugated in terminaland interstitial positions or in both distal arms. It issignificant that the bivalents with terminal chiasmatawere frequently found in healthy and virus-infectedtreatments. Modifications induced by BSMV causedan increase in the rate of ‘‘TT�I’’ and ‘‘T�I’’ bivalenttypes, and the overall number of chiasmata per PMCdepends primarily on the bivalents with interstitialchiasmata (Fig. 4). Meanwhile, in all the experimentalvariations an increase in univalents was noted. Theprevalence of bivalent types ‘‘T’’ and ‘‘T�I’’ wasreported in our previous publications and for othergenotype-pathogen combinations (Chiriac et al. 2006;Marii and Chiriac 2009).

Cytological study of meiosis revealed aberrationssuch as laggards, precocious chromosome migration,and bridges with/without fragments (Fig. 5). Accordingto cytogenetic research in A-T I and II, laggards wereobserved more frequently. Chromosomal or chromatidbridges with/without fragments were often noticed intomatoes infected with TAV. It is important to under-line that in infected plants, microspores with two ormore aberrations increased. Cell fusion and the absenceof cytokinesis were also recorded in treatments, leadingto unbalanced microspores and abnormal tetrads (dyad,triad, poliad) found among the tetrad. Cytogeneticanalysis of meiosis in tomato cultivars showed a rela-tively small percentage of aberrant cells in healthyplants, the level of which ranged between 6.89 and9.63% in Nistru and Fachel, respectively. At the same

Table 1. Mean value of the chiasma number per pollen mother cell (n�12) in healthy and virus-infected tomato cultivars

Chiasma position

Genotypes Treatment Interstitial chiasmata Terminal chiasmata Total chiasmata number

Fachel Control 4.3590.153 12.65590.273 16.69090.249TAV 6.41490.182*** 13.86290.297** 20.27690.371***PVX 3.58690.176* 13.82890.179*** 17.37990.175**

Nistru Control 3.69090.233 13.96690.246 17.65590.278TAV 5.17290.165*** 12.58990.331*** 17.79390.379PVX 3.72490.178 14.03490.219 17.75990.288

Prizior Control 3.93190.297 13.27990.289 17.17290.289TAV 6.48390.346*** 11.89790.323*** 18.34590.337**PVX 4.75990.177* 13.65590.281 18.34590.229***

*, **, *** Significant at P50.05, P50.01, and P50.001, respectively.

Fig. 3. Barley bivalent types according to chiasma position at early diakinesis (for various cultivated barley, Hordeum vulgare L.).(A) Bivalent with one interstitial chiasma. (B) Bivalent with two interstitial chiasmata. (C) Bivalent with one terminal and oneinterstitial chiasma. (D) Bivalent with one terminal and three interstitial chiasma. (E) Bivalent with two terminal and one interstitialchiasmata. (F) Bivalent with two terminal chiasmata. (G) Univalents.


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time, the meiocytes formed had an increasing percentageof the aberrant PMC. Thus, the rate of PMC withaberrations increased 3.02�4.44 times in A-T I in Facheland 3.93�2.7 times in Prizior infected with TAV or PVX(Table 3).

According to the data obtained, independent of thehost genotype, the infection induced by the TAV maygenerate a significant increase in the aberrant PMC inthe second vs. first division. For example, in cv. Fachelinfected with TAV, the abnormal PMC are 53% higherthan in the second division. In the analyzed tomatocultivars infected with PVX, the frequency of aberrantPMC was similar in A-T II and A-T I. It was establishedthat the percentage of aberrant cells in the healthytomato cultivars declined in the second division. Ragaband Abbel-Rahem (1989) explained the reduction ofaberrations in the second division by eliminatingaberrant PMC and/or as a result of the DNA repairprocesses.

Precocious chromosome migration to the poles inmetaphase (Fig. 6B and C), laggards in anaphase(Fig. 6E, F and K), telophase with bridges (Fig. 6Hand I), and telophases with micronuclei and unequallydistributed genetic materials (Fig. 6L) were the mainabnormalities established in barley cultivars infectedwith BSMV.

The percentage of abnormal PMC in healthy barleyranged from 11.1% in A-T I cv. Galactic to 13.67% incv. Sonor, having a tendency to decrease in A-T II(Table 4).

In A-T I, virus infection caused a threefold increaseover the control of aberrant PMC in cv. Galactic, a 2.7-fold increase in cv. Sonor and 2.6-fold increase in cv.Unirea. In cv. Unirea, BSMV tended to maintain thesame depressive effect in A-T II. At the same time, therate of aberrant PMC in the second meiotic division wasslightly lower than in the first division in virus-infectedcv. Galactic and cv. Sonor.

Cytogenetic studies performed by us suggest thatBSMV, like TAV and PVX in tomato, causes significantchanges in the number and position of chiasmata andthe rate of chromosome aberration in meiotic division,contributing to the essential abnormality in the forma-tion of microspores.

DISCUSSIONOn the basis of the data obtained, we assume that theobserved deviations in meiotic recombination couldresult from an effect of viral infection on genetic ma-terials and peculiar chromosome behaviour. In ourexperiments, relevant changes in bivalent pattern cor-relation were established in all analyzed tomato and

Table 2. Mean value of the chiasma number per pollen mother cell (n�7) in healthy and virus-infected barley cultivars

Chiasma position

Genotypes Treatments Interstitial chiasmata Terminal chiasmata Total chiasmata number

Galactic Control 6.17290.263 7.93190.302 14.06990.216Virus 7.75890.288*** 7.65590.212*** 15.41490.304***

Sonor Control 6.27690.257 7.96690.308 14.20790.213Virus 7.82790.253*** 7.68990.233*** 15.55290.300***

Unirea Control 6.10390.239 7.44890.219 13.51790.183Virus 7.34590.206*** 7.37990.201*** 14.75890.214***

*** Significant at P50.001.

Fig. 4. Distribution of bivalent types in healthy (A) and virus-infected (B) barley cultivars.


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barley genotypes, while the essential modifications ofthe mean number of interstitial and terminal chiasmatawere not confirmed for all treatments. Nevertheless, in

the present investigation, trial differences in the distribu-tion of chiasma per bivalent were observed. Essentialchanges in the position of terminal and interstitial chias-mata were found in all treatments with viral infection.

Analysis of variance of the means of the relativedistance between various bivalent types according tochiasmata positions showed that the interstitial associa-tions are stable and do not move to the chromosometermini. Also, in some treatments, especially those inwhich virus infection was applied, the induction of newsynaptic cohesions was observed, confirmed by theincreases in the bivalent types with one or two inter-stitial chiasmata (‘‘TT�I’’; ‘‘T�II’’).

Similar effects were observed in plants carryingsupernumerary B chromosomes, which induce changesin pairing of A chromosomes (Bell and Burt 1990). Theredistribution of chiasmata is very significant, because itleads to increasing recombination between genes locatedin the same linkage groups (Korol et al. 1994).

Fig. 5. Results of microsporogenesis in tomatoes cultivars. (A) Normal anaphase. (B) Anaphase I with bridges. (C) Anaphase I withprecocious chromosome migration. (D) Telophase II with micronucleus (arrowhead). (E) Telophase II with unbalancedchromosome distribution. (F) Telophase II with six nuclei. (G) Normal tetrad. (H) Abnormal sporads. (I) Polyad.

Table 3. Frequency of aberrant pollen mother cells (PMC) in healthy and

virus-infected tomato cultivars

Percent of the aberrant PMC

Genotypes Treatment A-T I A-T II

Fachel Control 9.6391.23 9.5491.06TAV 29.1391.22*** 44.7290.98***PVX 42.7693.40*** 42.2990.73***

Nistru Control 6.8991.28 4.9090.22TAV 27.8593.29*** 26.1192.77***PVX 18.6292.35*** 22.7192.48***

Prizior Control 7.8291.22 7.1391.03TAV 30.6590.99*** 42.6491.03***PVX 28.5890.48*** 30.6590.99***



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According to Wright et al. (2003), bivalents withthree chromatin associations, especially those such as‘‘TT�I’’, are attributed the main role in inducinggenetic diversity. It is important to note that in ourinvestigations it was found that the generation of newexchanges within chromosomes was accompanied byan increase in univalents. Such compensation is con-sidered an interchromosomal effect in the regulation of

meiotic recombination. The cause of univalent induc-tion is considered to be asynapsis or desynapsis. In thecase of asynapsis, synaptic arrest was established,resulting from the failure of conjugation between homo-logous segments. In desynapsis, the primary cohesionis achieved, but in diplotene�diakinesis the synaptone-mal complex is disassembled, leaving unsynapsedchromosomes.

Fig. 6. Results of microsporogenesis in barley cultivars. (A) Normal metaphase. (B) Metaphase I with precocious bivalent migration(arrowhead). (C) Metaphase I with bivalents elimination (arrowhead). (D) Normal anaphase I. (E) Anaphase I with precociouschromosome and laggard. (F) Telophase I with laggard (arrowhead). (G) Normal telophase I. (H) and (I) Telophase I with bridge(arrowhead). (J) Normal telophase II (arrowhead). (K) Telophase II with laggard (arrowhead). (L) Unequal chromosomesegregation (arrowhead).


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Chromosomal figures observed in diakinesis revealedsome synaptic cohesion and chiasma formation orfailure/losses of conjugation, generating univalents.These data lead to the conclusion that viral infectionimpacts the achievement of meiotic conjugation. It isconsidered that the abnormality in meiotic recombina-tion causes the deviation in the following meiotic stagesand leads to a reduction in the number of normalmicrosporocytes.

Viral infections induced by TAV and PVX in tomatoand BSMV in barley have also been found to beinvolved in the induction of abnormalities of micro-sporogenesis revealed at A-T I and II and at the sporadstage. It is known that various types of aberrationsreflect the spectrum of structural changes and determinethe intensity of different mutagenic factors (Raicu et al.1983). Thus, the presence of laggard chromosomes inA-T is the cytological expression of duplications ordeletions, and chromatic bridges the results of theinversions (Handel 1998).

Plant responses, including those associated with viralinfection, are based on the ability to recognize the stressand to produce mobile signals that can activate specificresponses in distant tissues. If a resistance gene is absent(compatible interaction), then the interaction between aplant and a pathogen is more ambiguous (Kathiria et al.2010). The authors previously reported that the compa-tible interaction between TMV and tobacco (Nicotianatabacum ‘SR1’) plants lacking the TMV resistance Ngene results in the production of a systemic signal. Thissignal leads to an increase in the frequency of somatichomologous recombination, and is locally generated atthe site of infection and is capable of spreading fasterthan the virus, altering genome stability in non-infectedtissue (Kovalchuk et al. 2003).

According to our experiments, prophase I occursduring a stage of penetration and multiplication of thevirus particles in the sporophytic tissues (tapetum,endotecium). Based on ultrastructural analysis, theabsence of the TAV and PVX particles in tomatoesmicrospores was noted. For the BSMV, the presence ofvirions in pollen grains of the evaluated barley cultivarswas reinforced (unpublished personal data). It is knownthat BSMV particles can be observed attached to

microtubules of the spindle apparatus undergoing meio-sis and mitosis (Mayhew and Carroll 1974). Recently, theformation of a ribonucleoprotein complex containingviral genomic material of the BSMV and messengerRNAs was established (Lim et al. 2009). In spite ofestablished differences in the localization of virus parti-cles, similarities were found in the repercussion ofmeiotic events in tomato infected with TAV or PVXand barley attacked by BSMV. Our results confirm thepostulate that the viral pathogen triggers a systemicsignal that is transmitted to non-treated tissue. Filkowskiet al. (2004) hypothesized a potential involvementof free radicals in the induction of the systemic effect,and reported a significant induction of homologousrecombination in tobacco plants treated with radical-generating agents, short wave (UVC radiation) or roseBengal. Importantly, the increase in recombination wasobserved in local (treated), as well as systemic (non-treated) tissue. Our previous results showed that BSMV,like different doses of gamma radiation (100 Gy, 150 Gy,250 Gy), contributes to the significant increase inexchange of the sister chromatids (Andronic et al. 2010).

The systematic recombination signal, generated atthe site of infection and capable of spreading faster thanthe virus, should alter genome stability in non-infectedtissue (Boyko et al. 2007). The authors show thatmethylation is a well-explored process of genome main-tenance, whereby methyl groups tend to make chroma-tin less accessible to various remodelling processes, andhypomethylation can be suggested as a pathway thatfacilitates the rearrangement of certain loci.

We have previously reported data regarding theinduction of host DNA rearrangements confirmedthrough assessment of chlorophyll mutations in barley(Andronic et al. 2010a) and tomato exemplars with newnonspecific characters association for parental forms(Andronic et al. 2010b).

The changes in microsporogenesis presented in thisarticle strengthen our previous postulate, that plantviruses can be used to induce genetic variability insusceptible hosts.

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T. B. 2010a. Genotoxicity of barley stripe mosaic virus ininfected host plants. Centr. Eur. J. Biol. 5: 633�640.Andronic, L., Jacota, A. G. and Bujoreanu, V. V. 2010b.

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Kovalchuk, I. 2007. Transgenerational changes in the genomestability and methylation in pathogen-infected plants (virus-induced plant genome instability). Nucleic Acids Res. 35:1714�1725.

Table 4. Frequency of aberrant pollen mother cells (PMC) in healthy and

virus-infected barley cultivars

Percent of the aberrant PMC

Genotypes Treatment A-T I A-T II

Galactic Control 11.1091.51 10.2491.23BSMV 32.8991.72*** 24.3791.12***

Sonor Control 13.6791.14 13.5891.51BSMV 36.5691.34*** 31.4690.89***

Unirea Control 12.1390.65 10.3690.45BSMV 31.2291.07*** 26.8191.31***



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Viruses as triggers of DNA rearrangements in host plants