Resistance to multiple viruses in transgenic tobacco expressing fused, tandem repeat, virus-derived double-stranded RNAs

Critical points for the design and application of RNA silencing constructs for plant virus resistance

Control of plant virus diseases is a big challenge in agriculture as is resistance in plant lines to infection by viruses. Recent progress using advanced technologies has provided fast and durable alternatives. One of the most promising techniques against plant viruses that is cost-effective and environmentally safe is RNA silencing or RNA interference (RNAi), a technology that could be used alone or along with other control methods. To achieve the goals of fast and durable resistance, the expressed and target RNAs have been examined in many studies, with regard to the variability in silencing efficiency, which is regulated by various factors such as target sequences, target accessibility, RNA secondary structures, sequence variation in matching positions, and other intrinsic characteristics of various small RNAs. Developing a comprehensive and applicable toolbox for the prediction and construction of RNAi helps researchers to achieve the acceptable performance level of silencing elements. Although the attainment of complete prediction of RNAi robustness is not possible, as it also depends on the cellular genetic background and the nature of the target sequences, some important critical points have been discerned. Thus, the efficiency and robustness of RNA silencing against viruses can be improved by considering the various parameters of the target sequence and the construct design. In this review, we provide a comprehensive treatise regarding past, present and future prospective developments toward designing and applying RNAi constructs for resistance to plant viruses.

Catch Me If You Can! RNA Silencing-Based Improvement of Antiviral Plant Immunity

Viruses are obligate parasites which cause a range of severe plant diseases that affect farm productivity around the world, resulting in immense annual losses of yield. Therefore, control of viral pathogens continues to be an agronomic and scientific challenge requiring innovative and ground-breaking strategies to meet the demands of a growing world population. Over the last decade, RNA silencing has been employed to develop plants with an improved resistance to biotic stresses based on their function to provide protection from invasion by foreign nucleic acids, such as viruses. This natural phenomenon can be exploited to control agronomically relevant plant diseases. Recent evidence argues that this biotechnological method, called host-induced gene silencing, is effective against sucking insects, nematodes, and pathogenic fungi, as well as bacteria and viruses on their plant hosts. Here, we review recent studies which reveal the enormous potential that RNA-silencing strategies hold for providing an environmentally friendly mechanism to protect crop plants from viral diseases.

The Tug-of-War between Plants and Viruses: Great Progress and Many Remaining Questions

Plants are persistently challenged by various phytopathogens. To protect themselves, plants have evolved multilayered surveillance against all pathogens. For intracellular parasitic viruses, plants have developed innate immunity, RNA silencing, translation repression, ubiquitination-mediated and autophagy-mediated protein degradation, and other dominant resistance gene-mediated defenses. Plant viruses have also acquired diverse strategies to suppress and even exploit host defense machinery to ensure their survival. A better understanding of the defense and counter-defense between plants and viruses will obviously benefit from the development of efficient and broad-spectrum virus resistance for sustainable agriculture. In this review, we summarize the cutting edge of knowledge concerning the defense and counter-defense between plants and viruses, and highlight the unexploited areas that are especially worth investigating in the near future.

Temperature and CO2 Level Influence Potato leafroll virus Infection in Solanum tuberosum

We determined the effects of atmospheric temperature (10-30 ± 2°C in 5°C increments) and carbon dioxide (CO2) levels (400 ± 50 ppm, 540 ± 50 ppm, and 940 ± 50 ppm) on the infection of Solanum tuberosum cv. Chubaek by Potato leafroll virus (PLRV). Below CO2 levels of 400 ± 50 ppm, the PLRV infection rate and RNA content in plant tissues increased as the temperature increased to 20 ± 2°C, but declined at higher temperatures. At high CO2 levels (940 ± 50 ppm), more plants were infected by PLRV at 30 ± 2°C than at 20 or 25 ± 2°C, whereas PLRV RNA content was unchanged in the 20-30 ± 2°C temperature range. The effects of atmospheric CO2 concentration on the acquisition of PLRV by Myzus persicae and accumulation of PLRV RNA in plant tissues were investigated using a growth chamber at 20 ± 2°C. The M. persicae PLRV RNA content slightly increased at elevated CO2 levels (940 ± 50 ppm), but this increase was not statistically significant. Transmission rates of PLRV by Physalis floridana increased as CO2 concentration increased. More PLRV RNA accumulated in potato plants maintained at 540 or 940 ± 50 ppm CO2, than in plants maintained at 400 ± 50 ppm. This is the first evidence of greater PLRV RNA accumulation and larger numbers of S. tuberosum plants infected by PLRV under conditions of combined high CO2 levels (940 ± 50 ppm) and high temperature (30 ± 2°C).

The Effects of High Temperature on Infection by Potato virus Y, Potato virus A, and Potato leafroll virus

We examined the effects of temperature on acquisition of Potato virus Y-O (PVY-O), Potato virus A (PVA), and Potato leafroll virus (PLRV) by Myzus persicae by performing transmission tests with aphids that acquired each virus at different temperatures. Infection by PVY-O/PVA and PLRV increased with increasing plant temperature in Nicotiana benthamiana and Physalis floridana, respectively, after being transmitted by aphids that acquired them within a temperature range of 10-20°C. However, infection rates subsequently decreased. Direct qRT-PCR of RNA extracted from a single aphid showed that PLRV infection increased in the 10-20°C range, but this trend also declined shortly thereafter. We examined the effect of temperature on establishment of virus infection. The greatest number of plants became infected when N. benthamiana was held at 20°C after inoculation with PVY-O or PVA. The largest number of P. floridana plants became infected with PLRV when the plants were maintained at 25°C. PLRV levels were highest in P. floridana kept at 20-25°C. These results indicate that the optimum temperatures for proliferation of PVY-O/PVA and PLRV differed. Western blot analysis showed that accumulations of PVY-O and PVA coat proteins (CPs) were lower at 10°C or 15°C than at 20°C during early infection. However, accumulation increased over time. At 25°C or 30°C, the CPs of both viruses accumulated during early infection but disappeared as time passed. Our results suggest that symptom attenuation and reduction of PVY-O and PVA CP accumulation at higher temperatures appear to be attributable to increased RNA silencing.

Transgenic resistance

Transgenic resistance to plant viruses is an important technology for control of plant virus infection, which has been demonstrated for many model systems, as well as for the most important plant viruses, in terms of the costs of crop losses to disease, and also for many other plant viruses infecting various fruits and vegetables. Different approaches have been used over the last 28 years to confer resistance, to ascertain whether particular genes or RNAs are more efficient at generating resistance, and to take advantage of advances in the biology of RNA interference to generate more efficient and environmentally safer, novel "resistance genes." The approaches used have been based on expression of various viral proteins (mostly capsid protein but also replicase proteins, movement proteins, and to a much lesser extent, other viral proteins), RNAs [sense RNAs (translatable or not), antisense RNAs, satellite RNAs, defective-interfering RNAs, hairpin RNAs, and artificial microRNAs], nonviral genes (nucleases, antiviral inhibitors, and plantibodies), and host-derived resistance genes (dominant resistance genes and recessive resistance genes), and various factors involved in host defense responses. This review examines the above range of approaches used, the viruses that were tested, and the host species that have been examined for resistance, in many cases describing differences in results that were obtained for various systems developed in the last 20 years. We hope this compilation of experiences will aid those who are seeking to use this technology to provide resistance in yet other crops, where nature has not provided such.

Engineered resistance in potato against potato leafroll virus, potato virus A and potato virus Y

Transgenic potato plants of Solanum tuberosum cultivar Vales Sovereign were generated that expressed fused, tandem, 200 bp segments derived from the capsid protein coding sequences of potato virus Y (PVY strain O) and potato leafroll virus (PLRV), as well as the cylindrical inclusion body coding sequences of potato virus A (PVA), as inverted repeat double-stranded RNAs, separated by an intron. The orientation of the expressed double-stranded RNAs was either sense-intron-antisense or antisense-intron-sense RNAs, and the double-stranded RNAs were processed into small RNAs. Four lines of such transgenic potato plants were assessed for resistance to infection by PVY-O, PLRV, or PVA, all transmitted by a natural vector, the green-peach aphid, Myzus persicae. Resistance was assessed by the absence of detectable virus accumulation in the foliage. All four transgenic potato lines tested showed 100% resistance to infection by either PVY-O or PVA, but variable resistance to infection by PLRV, ranging from 72 to 96% in different lines. This was regardless of the orientation of the viral inserts in the construct used to generate the transgenic plants and the gene copy number of the transgene. This demonstrates the potential for using tandem, fused viral segments and the inverted-repeat expression system to achieve multiple virus resistance to viruses transmitted by aphids in potato.

Comparative analysis of RNA silencing suppression activities between viral suppressors and an endogenous plant RNA-dependent RNA polymerase

RNA silencing is an evolutionarily conserved system that functions as an antiviral mechanism in eukaryotes, including higher plants. To counteract this, several plant viruses express silencing suppressors that inhibit RNA silencing in host plants. Here, we show that both 2b protein from peanut stunt virus (PSV) and a hairpin construct (designated hp-RDR6) that silences endogenous RNA-dependent RNA polymerase 6 (RDR6) strongly suppress RNA silencing. The Agrobacterium infiltration system was used to demonstrate that both PSV 2b and hp-RDR6 suppressed local RNA silencing as strongly as helper component (HC-Pro) from potato virus Y (PVY) and P19 from tomato bush stunt virus (TBSV). The 2b protein from PSV eliminated the small-interfering RNAs (siRNAs) associated with RNA silencing and prevented systemic silencing, similar to 2b protein from cucumber mosaic virus (CMV). On the other hand, hp-RDR6 suppressed RNA silencing by inhibiting the generation of secondary siRNAs. The small coat protein (SCP) of squash mosaic virus (SqMV) also displayed weak suppression activity of RNA silencing. Agrobacterium-mediated gene transfer was used to investigate whether viral silencing suppressors or hp-RDR6 enhanced accumulations of green fluorescence protein (GFP) and β-glucuronidase (GUS) as markers of expression in leaf tissues of Nicotina benthamiana. Expression of both GFP and GUS was significantly enhanced in the presence of PSV 2b or CMV 2b, compared to no suppression or the weak SqMV SCP suppressor. Co-expression with hp-RDR6 also significantly increased the expression of GFP and GUS to levels similar to those induced by PVY HC-Pro and TBSV P19.