Evidence that RNA silencing-mediated resistance to beet necrotic yellow vein virus is less effective in roots than in leaves

AtCDKA;1 silencing in Arabidopsis thaliana reduces reproduction of sedentary plant-parasitic nematodes

The activity of the Arabidopsis thaliana cyclin-dependent kinase AtCDKA;1 is important throughout G(1)/S and G(2)/M transitions and guarantees the progression of the cell cycle. Inhibitor studies have shown that activation of the cell cycle is important for the development of nematode feeding sites. The aim of this study was to silence the expression of the AtCDKA;1 gene in nematode feeding sites to interfere with their development. Therefore, sense and antisense constructs were made for the AtCDKA;1 gene and fused to a nematode-inducible promoter which was activated in nematode feeding sites at an earlier time point than AtCDKA;1. Two transgenic A. thaliana lines (S266 and S306) containing inverted repeats of the AtCDKA;1 gene and with reduced AtCDKA;1 expression in seedlings and galls were analysed in more detail. When the lines were infected with the root-knot nematode Meloidogyne incognita, significantly fewer galls and egg masses developed on the roots of the transgenic than wild-type plants. Infection of the AtCDKA;1-silenced lines with Heterodera schachtii resulted in significantly fewer cysts compared with controls. The S266 and S306 lines showed no phenotypic aberrations in root morphology, and analysis at different time points after infection demonstrated that the number of penetrating nematodes was the same, but fewer nematodes developed to maturity in the silenced lines. In conclusion, our results demonstrate that silencing of CDKA;1 can be used as a strategy to produce transgenic plants less susceptible to plant-parasitic nematodes.

Resistance of transgenic tobacco plants incorporating the putative 57-kDa polymerase read-through gene of Tobacco rattle virus against rub-inoculated and nematode-transmitted virus

Nicotiana tabacum plants were transformed with the 57-kDa read-through domain of the replicase gene of Tobacco rattle virus (TRV) isolate SYM. From a total of six lines containing the viral transgene, four displayed various levels of resistance to TRV infection. Transgenic plants from line 81G were highly resistant to foliar rub-inoculation with the homologous isolate, or with isolates TRV-PpK20 and TRV-PLB, which are almost identical to TRV-SYM in RNA1 sequence. Moreover, 81G plants were moderately resistant to the serologically and genetically distinct, highly pathogenic isolate TRV-GR. Resistance characteristics of line 81G remained stable over six generations. No unambiguous correlation was established between number of transgene insertion loci and level of resistance. Transgene-specific mRNA was clearly detected in plants from susceptible lines but only at an early developmental stage in resistant plants, indicating the operation of a RNA silencing resistance mechanism. Following challenge using viruliferous vector nematodes carrying TRV-PpK20 or by rub inoculation of roots, 81G plants did not show any symptoms and virus was not detected in leaves. However, virus was detected in roots but without apparent effects on plant growth and often at low concentration. When challenged with nematodes carrying TRV-GR, symptoms in aerial parts of 81G plants were less severe and much delayed compared to non-transgenic plants, although younger plants showed less resistance than older ones. No difference was detected in transgene transcript accumulation between leaves and roots of 81G plants. This is the first work reporting a broad level of pathogen derived resistance against two geographically and genetically distinct TRV isolates transmitted directly by their nematode vectors and provides further insight into the expression of transgenic resistance against naturally transmitted soil-borne viruses.

Identification of amino acids of the beet necrotic yellow vein virus p25 protein required for induction of the resistance response in leaves of Beta vulgaris plants

The RNA3-encoded p25 protein of beet necrotic yellow vein virus (BNYVV) is responsible for the production of rhizomania symptoms of sugar beet roots (Beta vulgaris subsp. vulgaris). Here, it was found that the presence of the p25 protein is also associated with the resistance response in rub-inoculated leaves of sugar beet and wild beet (Beta vulgaris subsp. maritima) plants. The resistance phenotype displayed a range of symptoms from no visible lesions to necrotic or greyish lesions at the inoculation site, and only very low levels of virus and viral RNA accumulated. The susceptible phenotype showed large, bright yellow lesions and developed high levels of virus accumulation. In roots after Polymyxa betae vector inoculation, however, no drastic differences in virus and viral RNA accumulation levels were found between plants with susceptible and resistant phenotypes, except at an early stage of infection. There was a genotype-specific interaction between BNYVV strains and two selected wild beet lines (MR1 and MR2) and sugar beet cultivars. Sequence analysis of natural BNYVV isolates and site-directed mutagenesis of the p25 protein revealed that 3 aa residues at positions 68, 70 and 179 are important in determining the resistance phenotype, and that host-genotype specificity is controlled by single amino acid changes at position 68. The mechanism of the occurrence of resistance-breaking BNYVV strains is discussed.

RNA4-encoded p31 of beet necrotic yellow vein virus is involved in efficient vector transmission, symptom severity and silencing suppression in roots

RNA3 and RNA4 of beet necrotic yellow vein virus (BNYVV) are not essential for virus multiplication, but are associated with vector-mediated infection and disease development in sugar beet roots. Here, a unique role for RNA4 in virus transmission, virulence and RNA silencing suppression was demonstrated. Mutagenic analysis revealed that the RNA4-encoded p31 open reading frame (ORF) was involved in efficient vector transmission and slight enhancement of symptom expression in some Beta species. No effects of RNA4 on virus accumulation in infected tissue were observed. Furthermore, the p31 ORF was involved in the induction of severe symptoms by BNYVV in Nicotiana benthamiana plants without affecting viral RNA accumulation. In contrast, RNA3-encoded p25, previously identified as a major contributor to symptom induction in sugar beet, had no such effect on N. benthamiana. In two different silencing suppression assays, neither p31 nor p25 was able to suppress RNA silencing in leaves, but the presence of p31 enhanced a silencing suppressor activity in roots without alteration in viral RNA accumulation. Thus, BNYVV p31 plays a multifunctional role in efficient vector transmission, enhanced symptom expression and root-specific silencing suppression.

Efficient dsRNA-mediated transgenic resistance to Beet necrotic yellow vein virus in sugar beets is not affected by other soilborne and aphid-transmitted viruses

Rhizomania caused by Beet necrotic yellow vein virus (BNYVV) is one of the most devastating sugar beet diseases. Sugar beet plants engineered to express a 0.4 kb inverted repeat construct based on the BNYVV replicase gene accumulated the transgene mRNA to similar levels in leaves and roots, whereas accumulation of the transgene-homologous siRNA was more pronounced in roots. The roots expressed high levels of resistance to BNYVV transmitted by the vector, Polymyxa betae. Resistance to BNYVV was not decreased following co-infection of the plants with Beet soil borne virus and Beet virus Q that share the same vector with BNYVV. Similarly, co-infection with the aphid-transmitted Beet mild yellowing virus, Beet yellows virus (BYV), or with all of the aforementioned viruses did not affect the resistance to BNYVV, while they accumulated in roots. These viruses are common in most of the sugar beet growing areas in Europe and world wide. However, there was a competitive interaction between BYV and BMYV in sugar beet leaves, as infection with BYV decreased the titres of BMYV. Other interactions between the viruses studied were not observed. The results suggest that the engineered resistance to BNYVV expressed in the sugar beets of this study is efficient in roots and not readily compromised following infection of the plants with heterologous viruses.

Synergism of a DNA and an RNA virus: enhanced tissue infiltration of the begomovirus Abutilon mosaic virus (AbMV) mediated by Cucumber mosaic virus (CMV)

Replication of the begomovirus Abutilon mosaic virus (AbMV) is restricted to phloem nuclei, generating moderate levels of virus DNA. Co-infection with Cucumber mosaic virus (CMV) evidently increased AbMV titers in Nicotiana benthamiana, tobacco, and tomato, resulting in synergistic symptom enhancement. In situ hybridization revealed that in double-infected leaves an increased number of nuclei contained elevated amounts of AbMV. Additionally, the begomoviral phloem-limitation was broken. Whereas CMV 3a movement protein-expressing tobacco plants did not exert any similar influence, the presence of CMV 2b silencing suppressor protein lead to enhanced AbMV titers and numbers of infected vascular cells. The findings prove that AbMV can replicate in nonvascular cells and represent the first report on a true synergism of an RNA/ssDNA virus combination in plants, in which CMV 2b protein plays a role. They indicate considerable consequences of mixed infections between begomo- and cucumoviruses on virus epidemiology and agriculture.

Lower levels of transgene silencing in roots is associated with reduced DNA methylation levels at non-symmetrical sites but not at symmetrical sites

Transgene transcripts were recently shown to accumulate at higher levels in roots, relative to leaves, of silenced-transgenic Nicotiana benthamiana plants and to be inversely related with the accumulation of small interfering RNAs (siRNAs), suggesting that RNA silencing is less active in roots than in leaves (Andika et al., 2005. Mol. Plant-Microbe Interact. 18: 194). Here we show that the lower transgene RNA silencing activity in roots was associated with lower transgene methylation levels at non-symmetrical CpNpN context but not at symmetrical CpG or CpNpG context in three sets of transformant plants with different exogenous genes. In contrast, such a difference between roots and leaves was not observed for the Tnt1 retrotransposon: no Tnt1 transcript was detected in roots or in leaves of N. benthamiana, while equal levels of Tnt1-derived siRNA accumulation and Tnt1 methylation were found. From our data and previously reported information, we suggest that roots have less of an activity that acts at the step of generation of siRNAs.