Two mutations in the truncated Rep gene RBR domain delayed the Wheat dwarf virus infection in transgenic barley plants

Pavel Cejnar, Ludmila Ohnoutková, Jan Ripl, Tomá? Vl?ko, Jiban Kumar Kundu

1 Division of Crop Protection and Plant Health, Crop Research Institute, Prague 16106, Czech Republic

2 Department of Chemical Biology and Genetics, Centre of the Region Hana for Biotechnological and Agricultural Research Palacky University, Olomouc 78371, Czech Republic

Abstract Wheat dwarf virus (WDV), an important cereal pathogen, is closely related to Maize streak virus (MSV), a model virus of the Mastrevirus genus. Based on its similarity to known MSV resistance strategies, a truncated part of the WDV replicationassociated (RepA) gene (WDVRepA215) and the WDV RepA gene with a mutated retinoblastoma-related protein (RBR)interaction domain (WDVRepA215RBRmut) were cloned into the pIPKb002 expression vector and transformed into immature embryos of spring barley cv. Golden Promise plants through Agrobacterium-mediated transformation. A detailed study of T1-generation plants infected by leafhoppers (Psammotettix alienus) fed on infection sources of variable strength was performed over a 5-week period encompassing the initial stages of virus infection. A DNA WDV TaqMan qPCR assay normalized using the DNA puroindoline-b SYBR Green qPCR assay for samples on a per week basis revealed an approximately 2-week delay in WDVRepA215RBRmut plants to WDVRepA215 plants before significant increases in the WDV viral levels occurred.Both WDVRepA215 and WDVRepA215RBRmut plants showed similar levels of transgenic transcripts over the screened period; however, the transgenic plants also showed increased numbers of infected plants compared to the control plants.

Keywords: Wheat dwarf virus (WDV), truncated Rep gene, RBR domain, qPCR screening

1. Introduction

Wheat dwarf virus (WDV), belonging to the genus Mastrevirus of the Geminiviridae family, is an important viral pathogen of cereal crops worldwide (Kundu et al. 2009;Nygren et al. 2015). It was first detected in the former Czechoslovakia (Vacke 1961). WDV is transmitted by a leafhopper, Psammotettix alienus, in a circulative, nonpropagative manner by both the larval instars and the adult leafhoppers (Lindsten and Vacke 1991). WDV is a singlestranded DNA virus with a monopartite circular genome of approximately 2.7 kb that is divided into several functional regions: a short intergenic region containing the origin of replication and sites of potential promoters for viral genes,which are grouped into two open reading frames for virus movement and coat protein; a long intergenic region; and a complementary open reading frame that produces two other virus proteins through differential splicing- replication protein (Rep) and replication-associated protein (RepA), see also in Fig. 1. The Rep protein is responsible for replicating the virus DNA, and RepA is responsible for enhancing the production of coat proteins and movement proteins and also for interactions with plant cell cycle mechanisms. Coat protein is the only virus protein that is present in the virus capsid and is responsible for proper circulation of the virus within the leafhopper insect vector and proper intracellular movement. The movement protein is required for proper viral cell-to-cell movement (Boulton 2002).

For geminiviruses, several different molecular engineering resistance strategies exist, but only a few are known for the Mastrevirus genus (Shepherd et al. 2009; Kis et al.2016; Zaidi et al. 2016). For Maize streak virus (MSV), a successful molecular engineering resistance strategy has already been performed (Shepherd et al. 2007a, b) based on the expression of the truncated MSV RepA protein in Digitaria sanguinalis and Zea mays plants. In MSV Rep and MSV RepA proteins, several structural and functional domains have been identified (Gutierrez 1999; Boulton 2002), and experiments with truncated MSV RepA proteins of different lengths expressed in transgenic D. sanguinalis plants produced different results. These results included the inhibition of plant growth, increased susceptibility to MSV or significantly decreased susceptibility to MSV (Shepherd et al. 2007b). An MSV RepA protein that was truncated to maintain only the first 219 amino acids but still contained functional DNA-binding domains (Koonin and Ilyina 1992), the RBR interaction domain (Xie et al. 1995) or the oligomerization domain (Horvath et al. 1998), was produced in the transgenic plants and reduced significantly the virus titres. Subsequently, a three-nucleotide mutation in the truncated MSV RepA protein gene sequence resulted in the complete inhibition of MSV in the tested plants (Shepherd et al. 2007b). This inhibition activity of MSV occurred even in response to infections caused by more divergent MSV isolates, but only to a reduced extent (Shepherd et al. 2014).For Wheat dwarf virus, the domains identified in the MSV Rep and RepA proteins were also identified in WDV either directly (Koonin and Ilyina 1992; Xie et al. 1995) or based on amino acid sequence homology between the MSV and WDV proteins (Missich et al. 2000). In this paper, we performed a functional analysis of the truncated RepA gene of WDV. Our result has shown that a two-point mutation in the RBR domain is able to delay the development of disease symptoms in the barley plants. The delayed disease symptoms also corresponded to the WDV titres detected by TaqMan qPCR.

2. Materials and methods

2.1. Construction of expression plasmids

Based on the amino acid sequence homology in the group of 123 WDV isolates obtained from the GenBank database, the MSV-Kom isolate (GenBank accession number AF003952;Shepherd et al. 2007b), an exact counterpart of the MSV RepA truncated 219-amino acid sequence, was identified as the first 215 amino acids in the WDV RepA protein and nucleotide sequences (ClustalX 2.1 Software; Larkin et al.2007). Two expression vectors, pIPKb002+WDVRepA215 and pIPKb 002+WDV RepA 215RBRmut, for the expression of the identified truncated WDV RepA sequence and the truncated WDV RepA sequence carrying a mutation in the RBR interaction domain, responsible for decreased susceptibility of maize plants to MSV, were manufactured.The identified 645-bp nucleotide sequence was amplified from the isolated DNA (GenElute? Plant Genomic DNA Miniprep Kit, Sigma-Aldrich, USA) from greenhouse plants inoculated with leafhoppers transmitting the wild-type Czech WDV isolate (GenBank accession number FJ546189). The primer pair used (WDVRepA215 CAfw+WDVRepA215-rv, see Table 1) also contained the CACC sequence for subsequent cloning and 6 nucleotides of the 3′ untranslated region (3′UTR) of the RepA gene sequence to maintain an exact analogue of the WDV RepA Kozak consensus sequence. An amplified and purified PCR product was cloned into the pENTR D-TOPO vector (pENTR?/D-TOPO?Cloning Kit, ThermoFisher Scientific, USA) according to the manufacturer’s instructions. Using the LR clonase reaction(Gateway?LR Clonase?II Enzyme mix, ThermoFisher Scientific, USA), the donor sequence was transferred into the pIPKb002 vector (Himmelbach et al. 2007) under the Z. mays ubiquitin-1 (ZmUbi-1) promoter, along with the hygromycin phosphotransferase gene (hpt) controlled by another ZmUbi-1 promoter for selection in plants.

For the WDVRepA215 product carrying the mutation in the RBR interaction domain (Shepherd et al. 2007b),a WDVRepA215CAfw+WDVRepA215rv PCR product was purified and cloned into the pGEM-T Easy vector(Promega, USA), and a site-directed mutagenesis using the WDVRepRBRsdm-fw+WDVRepRBRsdm-rv primer pair was performed. The mutation was confirmed by digesting with AflII. The extracted DNA of the pGEM-T Easy+WDVRepA215RBRmutvector was used as a template for the creation of the pIPKb002+WDV RepA 215RBRmutplasmid as described previously. Both final vectors,pIPKb002+WDVRepA215 and pIPKb002+WDV RepA -215RBRmut(see Fig. 1), were sequenced (GATC Biotech,Germany) to confirm the target sequence.

2.2. Agrobacterium-mediated transformation

Fig. 1 Wheat dwarf virus (WDV) genome organization (WDV FJ546189 isolate) with the selected part of the replication-associated protein used (A) and organization of binary plasmid pIPKb002+WDVRepA215 (B). Rep, replication protein; RepA, replicationassociated protein; MP, movement protein; CP, coat protein; SIR, short intergenic region; LIR, long intergenic region; RBR,retinoblastoma-related proteins interaction domain; ColE, ColE1 origin of replication; pVS1, pVS1 origin of replication; spec,streptomycin/spectinomycin adenyltransferase; LB and RB, left and right T-DNA border; ZmUbi-1, maize ubiquitin promoter followed by maize ubiquitin intron; hpt, hygromycin phosphotransferase; T, terminator; GOI, gene of interest; attB1 and attB2, LR clonase reaction sites.

Table 1 Primers and probes used in the study

The transformation of the expression vector sequences into spring barley cv. Golden Promise cereal plants was performed as described by Cejnar et al. (2017) based on a previously published transformation protocol (Ohnoutkova et al. 2012). From 240 transformed embryos carrying the pIPKb002+WDVRepA215 vector, 4 indepen dent transgenic plants were regenerated, and another 10 albino plants were obtained. Among 150 embryos transformed with the pIPKb002+WDVRepA 215RBRmutvector, six inde pen dent transgenic plants were regenerated, and no albino plants were obtained. DNA was isolated from the leaf tissue of the regenerated plants using the procedure reported by Edwards et al. (1991), and the presence of the genes of interest and the hpt selection gene were confirmed(WDVRepA 215fw+WDVRepA215rv primer pair, F-Hyg/p204+R-Hyg/p204 primer pair).

2.3. Nucleic acid extraction protocols

RNA extraction from the barley plants was performed using TRIzol Reagent (TRIzol Reagent, ThermoFisher Scientific, USA) applied on the leaf tissue ground in liquid nitrogen according to the manufacturer instructions. DNA extraction was performed using the procedure by Edwards et al. (1991) and applied on leaf tissue ground in liquid nitrogen. For screening experiment 2, a combined plant RNA+DNA extraction protocol was used, meaning that the TRIzol Reagent protocol for RNA extraction was followed by the DNA extraction after RNA extraction protocol (Triant and Whitehead 2009).

2.4. RT-qPCR RNA assays

For screening levels of transgenic WDVRepA215 and WDVRepA215RBRmuttranscripts in plants, regardless of the presence and levels of the corresponding viral transcripts,the pIPKbRepA215 AB SYBR RT-qPCR RNA assay with mRNApIPKbRepA-fw+mRNApIPKbRepA-rv primer pair was performed. The forward primer mRNApIPKbRepA-fw was designed to match the part of the 3′UTR originally included in the pIPKb002 vector, which differs in sequence from the corresponding 3′UTR in WDV Rep. The reverse primer, mRNApIPKbRepA-rv, was designed to recognize the initial part of the transcript to maintain a small 175-bp amplicon. For the normalization of the mRNA expression levels, the PINB AB SYBR RT-qPCR RNA assay with the PinB-fw+PinB-rv primer pair was performed. RT-qPCR RNA assays were performed on an Applied Biosystems 7300 Real-Time PCR System (ThermoFisher Scientific,USA) in a total volume of 12 μL containing 6 μL of reaction pre-mix (Power SYBR?Green RNA-to-CT? 1-Step Kit,ThermoFisher Scientific, USA), 0.096 μL of RT Enzyme Mix (containing ArrayScript? UP Reverse Transcriptase and RNase Inhibitor), 2 μL of sample, 0.4 μmol L-1each primer and ddH2O to the final volume. All the assays were performed in triplicate for the standard and tested samples.Thermal profiles, qPCR standards and detected efficiencies are shown in Table 2.

2.5. qPCR DNA assays

WDV DNA titre analysis was performed using a WDV Roche TaqMan qPCR DNA assay and was run on a Roche LightCycler?480 Instrument II (Roche Applied Science, Germany) in a total volume of 12 μL containing 6 μL of reaction pre-mix (LightCycler?480 Probes Master,Roche Applied Science, Germany), 2 μL of DNA sample,0.4 μmol L-1of each primer, 0.2 μmol L-1of TaqMan probe and ddH2O up to the final volume. For the normalization of the extracted DNA from the samples of different growth stages,PINB Roche SYBR qPCR DNA assay was performed. The assay was based on the detection of puroindoline-b copies(2 copies are present in wheat genome (Gautier et al. 2000),and 4 copies are hordoindoline-b1 and hordoindoline-b2 in barley genome (Galassi et al. 2012)) in extracted DNA. The assay was run on a Roche LightCycler?480 Instrument II in a total volume of 12 μL containing 6 μL of reaction pre-mix(LightCycler?480 SYBR Green I Master, Roche Applied Science, Germany), 2 μL of DNA sample, 0.4 μmol L-1of each primer and ddH2O up to the final volume. Thermal profiles for both qPCR DNA assays, qPCR standards and detected efficiencies are shown in Table 2. For the derivatives of the experimental and theoretical meltingcurves of PINB Roche SYBR qPCR DNA assay, see Appendix A.

Table 2 RT-qPCR RNA assays and qPCR DNA assays used in the study

2.6. Plant growth conditions and virus inoculation

All the plants were grown under controlled conditions(19/15°C, 16/8 h daylight/night, spectrum adjusted with high-pressure sodium bulbs) and sown in soil mixed with the substrate (2:1 autoclaved soil: autoclaved substrate ratio). In screening experiments 1-3, approximately twice as many seeds as required for testing were sown and allowed to grow under controlled conditions. At the 2-leaf stage(DC 12 by Zadoks, Zadoks et al. (1974)), approximately 5 days before inoculation, the first leaves were removed from all plants, and RNA was isolated and analysed using the pIPKbRepA215 and PINB AB SYBR RT-qPCR RNA assays.Among the plants that were positive for WDVRepA215 or WDVRepA215RBRmuttranscription, the plants with middle and high levels of transcription were selected. After prescreening of the RNA of the tested plants, leafhoppers were allowed to feed on the selected WDV source of infection for 5 days prior to inoculation and then for 6 days on the tested plants (approximately DC12-13 by Zadoks). Each plant was exposed to 3 infected leafhoppers per plant.

2.7. Molecular per week screening of WDV-inoculated plants

For detailed screening of the changes in the virus titres on a molecular basis, the plants were sampled once per week,starting at the 2nd week after the start of inoculation. To reduce the damage to the plants during screening, each tested plant was sampled by cutting one leaf each week —the oldest leaf on the main shoot. This procedure allowed us to obtain samples of tissue associated with the plant phloem network, i.e., the tissue targeted by WDV (Dinant 2004),without interrupting the remaining plant phloem network of the stem and the other leaves. The screening of one leaf per plant per week allowed us to eliminate the effects of slight differences between the individual plants and differences in the progression of their infection. The controlled growing conditions for the barley plants were sufficient to produce at least one new leaf per week after approximately the 2-leafstage in both infected and uninfected plants.

2.8. Phenotype screening of WDV-inoculated plants

For WDV, a significant dwarfing of the WDV-infected plants is one of the main symptoms. However, when a leaf from the main shoot was cut each week for the molecular screening, the reported height of the whole plant could then be seriously affected. Here, two other parameters were measured: the height from the soil level (visible height) up to the axil of the first leaf of the main shoot and the visible height up to the highest axil of the leaf present on the whole plant. In detailed studies, the distance between these two parameters is reported.

3. Results

3.1. Evaluation of T1 transgenic plants with WDVRepA215 and WDVRepA215RBRmut transcripts shows a 2-week delay in significant increase of virus titre

Half of the tillers of the T0successfully regenerated transgenic WDVRepA215 and WDVRepA215RBRmutbarley plants from 4 different WDVRepA 215 transgenic events(J1-1 to 4) and from 6 WDVRepA 215RBRmuttransgenic events (K5-1 to 6) was moved to separate pots at the end of the tillering stage (DC 28-30 by Zadoks). To limit the potential acquired mature resistance (Lindblad and Sigvald 2004), the group was pre-screened for resistance to WDV.The leafhoppers were allowed to feed on a strong source of infection (FJ546189 and FJ546193 isolate mixture) for 5 days prior to inoculation and then for the subsequent 4 days on the inoculated plants, with approximately 15 leafhoppers per tiller group. The plants presented only mild symptoms of infection (decreased growth, more stunted and wilted leaves than in the non-inoculated control group), and their leaf tissue was sampled at 40 days following the start-of-inoculation (dpi). One tested plant died at approximately 30 dpi. The DNA was extracted and quantified by UV spectrophotometry, and the number of puroindoline-b reference gene DNA was also determined by the PINB Roche SYBR qPCR DNA assay. The WDV virus titres were detected by the WDV Roche TaqMan qPCR DNA assay and showed highly variable results, including plants with extremely high or extremely low virus titres. For the T1screening, only the transgenic events with extremely low virus titres were selected.

For the detailed study of WDV infection progress,an infection source of 98342 WDV/PINB (WDV isolate FJ546193) was selected. The screening experiment 1 included 3 inoculated plants from each promising transgenic event (J1-1, K5-1, K5-3), 3 inoculated wild-type plants(barley cv. Golden Promise) and 4 non-inoculated controls(one for each tested group, i.e., 16 tested plants in total).At the 3-leaf stage (DC13), the infected leafhoppers were allowed to feed on each plant for another 6 days. For the next 5 weeks, the plant phenotypes were screened, each week one leaf was cut to characterize the progression of WDV infection at the molecular level, and the DNA was extracted. Once every 2 weeks, the mRNA levels of WDVRepA215 or WDVRepA215RBRmutwere monitored in the extracted RNA. Both the WDV titre levels and the phenotypic observations (see Fig. 2-IB and IC) confirmed that all transgenic plants were infected. However, in the inoculated control plants, only 1 of 3 control plants was infected. Significant increases in the virus titres during the screening period were visible for K5-1, K5-3 and the control plant, starting at approximately 4 weeks post-inoculation,and for J1-1 plants starting at approximately 2 weeks earlier at 2 weeks post-inoculation. The plants did not show any other significant differences after the end of screening when maintained in the controlled greenhouse (see Appendix B).

Fig. 2 Detailed screening of J1 (WDVRepA215), K5 (WDVRepA215RBRmut), GP (wild-type control) and GP-hpt (control hpt+) plants in the first 6 weeks after Wheat dwarf virus (WDV) inoculation in screening experiments 1 (I), 2 (II) and 3 (III, extra-low strength of infection source experiment): inoculated plants (white filled shapes) and uninoculated control plants (green filled shapes); plants in which WDV was detected (red lines) or not detected (green lines) at 6 weeks after inoculation. A, mRNA levels of WDVRepA215 and WDVRepA215RBRmut transgenic transcripts as determined by the pIPKbRepA215 AB SYBR RT-qPCR RNA and PINB AB SYBR RT-qPCR RNA assays. B, WDV DNA titres detected by the WDV Roche TaqMan DNA qPCR assay normalized to the amount of DNA determined by the PINB Roche SYBR DNA qPCR assay. C, height of the plants measured from the axil of the first leaf of the main shoot up to the highest axil of the leaf present on the whole plant.

The biological replicate (screening experiment 2) from the previous study was run using a final (preselected)group of 3 inoculated plants of the T1J1-1 transgenic event, 3 inoculated plants of K5-1, 3 inoculated wildtype barley cv. Golden Promise controls and 3 barley cv.Golden Promise controls with only the hpt selection gene and without the WDVRepA215 or WDVRepA215RBRmuttransgene (hpt+control plants, artefacts of screening for positive transformants from previous Agrobacterium transformations). In each group, one plant was included as an a non-inoculated control. The selected source of infection was determined to be approximately the same strength (78 315 WDV/PINB, WDV isolate FJ546180) as applied previously, and the inoculation conditions were also identical. However, during the screening in this experiment,a combined RNA+DNA extraction protocol was applied. The results are shown in Fig. 2-B and C. All the J1-1- and K5-1-inoculated plants, none of the inoculated hpt+control plants and one of the wild-type barley cv. Golden Promise plants were infected. At the molecular level, all the J1-inoculated plants exhibited earlier significant increases in WDV titres(2 weeks post-inoculation) compared to the K5-1 plants and an infected wild-type plant (approximately 4 weeks post-inoculation). Based on the levels of mRNA transcripts in screening experiment 1 (Fig. 2-IA), the levels of mRNA transcripts in the weekly screening experiment 2 (Fig. 2-IIA)confirmed the stable levels of transgenic transcripts in both the WDV-infected and -uninfected plants.

3.2. WDVRepA215 and WDVRepA215RBRmut plants are susceptible to infection even from extra-lowstrength infection sources

To confirm whether the transgenic insert affected the susceptibility of the plant to WDV through susceptibility to the reduced virus inoculation dose, screening experiment 3 was performed with an extremely weak source of infection of 24 855 WDV/PINB (a wild-type plant 2 weeks after inoculation was used as a new source of infection, WDV isolate FJ546193). The final tested group of plants included 3 inoculated plants of J1-1, 4 inoculated plants of K5-1, 4 inoculated plants of barley cv. Golden Promise wild-type and 4 inoculated hpt+control plants. There were 15 inoculated plants in total. In this experiment, no non-inoculated plants were added. The DNA and RNA from the leaf tissue were extracted, and the results of the DNA screening for the WDV,phenotype screening and the levels of RNA transcripts from the sampled period up to the 6th week post-inoculation are shown in Fig. 2 (IIIA-IIIC). With an extremely weak source of infection, one J1-1 and one K5-1 plants were infected,and no control plants (either wild-type or with antibiotic selection only) were infected. Even in this experiment,an approximately 2-week shift in the start of the detectable increase in the WDV titre was observed between J1-1- and K5-1-infected plants.

3.3. Stability of WDVRepA215, WDVRepA 215RBRmut and barley puroindoline-b mRNA levels during the first 6 weeks of barley plant growth

As a reference gene for testing the mRNA levels in each of the first 6 weeks of barley plant growth, the puroindoline-b gene was selected. The small number of copies present in the wheat and barley genomes decreases the possibility of many different promoters. For validation of the stability of the transcription of puroindoline-b genes in barley at different growth stages and with or without WDV infection, the data from detailed studies (screening experiments 1-3) were collected, and a two-way ANOVA was performed (number of puroindoline-b copies per ng of extracted RNA in samples) in R Software (v 3.4). The results did not show any significant dependence on the week of sampling (Pr(＞F)=0.55),the WDV infection present (Pr(＞F)=0.37) or both factors(Pr(＞F)=0.15). This result allowed the use of the levels of the puroindoline-b gene transcripts as reference values for normalization of the mRNA transcript levels. We must note that, for example, the β-tubulin 6 gene (Jarosova and Kundu 2010) did not pass this two-way ANOVA test and showed a significant dependence on the week of sampling (Cejnar P,unpublished data). All the transgenic plants in screening experiments 1-3 confirmed the stable levels of transgenic transcripts in both the WDV-infected and uninfected plants(Fig. 2-IA, IIA, and IIIA).

4. Discussion

The different timing in the progression of virus infection between the transgenic plants- the plants with WDVRepA215 transcripts compared with the plants with WDVRepA215RBRmuttranscripts, i.e., with two-point mutations in the RBR interaction domain - was confirmed in three detailed screening experiments. The tested twopoint mutation in the WDV RBR interaction domain was not lethal for WDV infection in barley, whereas the closely related MSV virus in maize, even for WDV, the conserved sequence of the RBR interaction domain, possibly played a supporting role in the mechanism of infection through its potential interaction with host RBR proteins. Even if it does not halt the infection, disruption of this mechanism is still likely detectable as a source of the delay observed in the WDV K5 plants compared with the WDV J1 plants, in which the regressed timing of infection in the K5 plants was very similar to the timing observed in the wild-type plants.

All three screening experiments also support the hypothesis that the transgenic insert of the first 215 amino acid codons of the WDV DNA sequence (FJ546189 isolate)enhance the susceptibility of the barley plant to the wild-type WDV infection, in contrast to the closely related MSV (see Shepherd et al. 2007a, 2014). However, even in maize plants with their increased resistance to MSV, enhanced susceptibility was detected with shorter transgenic inserts of the MSV Rep gene. In maize, the increased susceptibility of plants translating shortened MSV Rep proteins was likely due to the DNA binding domains that are potentially responsible for enhanced replication, which could also be the cause of WDV in barley, in which these domains were also detected in the WDV sequence (Koonin and Ilyina 1992). For MSV, the corresponding resistance strategy significantly reduced the levels of tested virus isolates,with as little as 60.6% amino acid identity (Shepherd et al.2014), thus validating the robustness of the amino acid substitutions. Here, all the experiments were performed with wild-type WDV isolates, which together showed at least 88% amino acid identity, and none of them showed any different symptoms or altered disease progression in the experiments. The detailed screening studies showed that the virus titres in inoculated transgenic and non-transgenic plants of the barley cv. Golden Promise at the 5th or 6th week post-inoculation reached very similar virus titres;thus, accumulation of the virus in the tested leaf tissues was likely not affected by either the WDVRepA215 or WDVRepA215RBRmutproducts. The detailed studies with the lower strength infection sources showed the enhanced susceptibility of the transgenic plants to infection through the reduced levels of initial virus doses required for infection compared with the wild-type plants, thus affecting the general probability of infection of the plant. The potential mechanism of such a resistance breakdown behaviour could be either the effect of different virus-host interactions that evolved through the specialization of mastreviruses to their preferred hosts (WDV for barley, MSV for maize) or due to other mutations that occurred as a side effect of any other reaction to some other virus-host interaction evolutionary event (see also Shepherd et al. 2005; Guiu-Aragones et al.2015; Pinel-Galzi et al. 2016).

5. Conclusion

The detailed screening experiments showed the progression of WDV during the first 6 weeks of infection in wild-type barley and WDVRepA215 or WDVRepA215RBRmutbarley plants. The significant increase in the WDV titres in response to tested two-point mutations in the tested transgene RBR domain region were delayed by approximately 2-week intervals as a probable result of the potentially disrupted infection mechanism. However, neither the transgene nor the included mutation were lethal for virus infection. The enhanced susceptibility of the transgenic plants to WDV compared with the wild-type plants was identified as having increased susceptibility to sources of low infection strength.

Acknowledgements

This work was equally supported by the Czech Ministry of Education, Youth and Sports funding programme LH12161,and the Czech Ministry of Agriculture funding programme MZE RO0417. All wheat dwarf virus isolates were obtained from the Virus Collection of the Crop Research Institute,Prague (Virus Collection 2016).

Appendicesassociated with this paper can be available on http://www.ChinaAgriSci.com/V2/En/appendix.htm