Pathogen-derived resistance in potato to Potato virus Y—aspects of stability and biosafety under field conditions

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.

Populational survey of arthropods on transgenic common bean expressing the rep gene from Bean golden mosaic virus

Genetically modified (GM) crops is considered the fastest adopted crop technology in the history of modern agriculture. However, possible undesirable and unintended effects must be considered during the research steps toward development of a commercial product. In this report we evaluated effects of a common bean virus resistant line on arthropod populations, considered as non-target organisms. This GM bean line (named M1/4) was modified for resistance against Bean golden mosaic virus (BGMV) by expressing a mutated REP protein, which is essential for virus replication. Biosafety studies were performed for a period of three years under field conditions. The abundance of some species was significantly higher in specific treatments in a particular year, but not consistently different in other years. A regular pattern was not observed in the distribution of insects between genetically modified and conventional treatments. Data analyses showed that minor differences observed can be attributed to random variation and were not consistent enough to conclude that the treatments were different. Therefore the present study indicates that the relative abundance of species are similar in transgenic and non-transgenic fields.

Engineering broad-spectrum resistance against RNA viruses in potato

RNA silencing technology has become the tool of choice for inducing resistance against viruses in plants. A significant discovery of this technology is that double-stranded RNA (dsRNA), which is diced into small interfering RNAs (siRNAs), is a potent trigger for RNA silencing. By exploiting this phenomenon in transgenic plants, it is possible to confer high level of virus resistance by specific targeting of cognate viral RNA. In order to maximize the efficiency and versatility of the vector-based siRNA approach, we have constructed a chimeric expression vector containing three partial gene sequences derived from the ORF2 gene of Potato virus X, Helper Component Protease gene of Potato virus Y and Coat protein gene of Potato leaf roll virus. Solanum tuberosum cv. Desiree and Kuroda were transformed with this chimeric gene cassette via Agrobacterium tumefaciens-mediated transformation and transgenic status was confirmed by PCR, Southern and double antibody sandwich ELISA detection. Due to simultaneous RNA silencing, as demonstrated by accumulation of specific siRNAs, the expression of partial triple-gene sequence cassette depicted 20% of the transgenic plants are immune against all three viruses. Thus, expression of a single transgene construct can effectively confer resistance to multiple viruses in transgenic plants.

Role of siRNAs and miRNAs in the processes of RNA-mediated gene silencing during viral infections

Phenomenon of RNA-induced gene silencing is a highly conservative mechanism among eukaryotic organisms. Several classes of small RNAs (siRNAs and miRNAs) 21-25 nt in length, which play a significant role in the processes of development of an organism, occurred important components of antiviral defence in animals and plants. This review shortly describes the main stages of gene silencing mechanism, features of antiviral RNA silencing in plants, invertebrates, mammals, ways of suppression of RNA-interference by viruses, as well as possible approaches of utilization of abovementioned phenomenon for struggling against viral infections.

Differentiation of Potato virus Y strains using improved sets of diagnostic PCR-primers

Potato virus Y (PVY) is one of the most important viruses of potato world-wide, several strain groups are recognized. In the past two decades, novel PVY variants have appeared causing necrotic symptoms on potato tubers. Implicated are two groups of recombinant strains: PVY(N)W and PVY(NTN), and NA-PVY(NTN). While the first two are recombinants between PVY-N- and O-strains the latter is a recombinant between an N-strain and an unknown PVY strain or other Potyvirus. Available biological and molecular data on PVY suggest that classification of PVY strains has to be revised. Some drawbacks have been found with recently published primers used in RT-PCR based differentiation of PVY strains as some defined isolates could not be identified correctly. Consequently we developed new primers using both recently available sequences and newly generated complete sequences of PVY strains. The reliability of these newly developed primers and procedures was successfully demonstrated on nearly 100 biologically and serologically characterised PVY isolates.

Development and assessment of a potato virus X-based expression system with improved biosafety

Over the last decade, plant virus-based vectors have been developed and successfully exploited for high-yield production of heterologous proteins in plants. However, widespread application of recombinant viruses raises concerns about possible risks to the environment. One of the primary safety issues that must be considered is the uncontrolled spread of the genetically engineered virus from experimental plants to susceptible weeds or crops. Using a movement-deficient Potato virus X (PVX)-based transient gene expression vector which harbors the beta-glucuronidase (gus) gene, we established a plant viral expression system that provides containment of the recombinant virus and allows for safe and efficient protein production. By deletion of the viral 25k movement protein gene, systemic spread of the modified virus in non-transgenic Nicotiana benthamiana plants was successfully inhibited. In transgenic N. benthamiana plants expressing the 25K viral movement protein, this deficiency was complemented, thus resulting in systemic infection with the movement-deficient virus. While no differences in virus spread and accumulation were observed compared to infection caused by wild-type PVX in non-transgenic plants, the movement protein transgenic plants exhibited none of the normal symptoms of viral infection. Several biosafety aspects were investigated including the potential for recombination between the defective virus and the movement protein transgene, as well as complementation effects in non-transgenic plants doubly infected with the defective and the wild-type virus. Furthermore, the applicability of the safety system for the production of heterologous proteins was evaluated with gus as a model gene. With respect to the stability of the gus insert and the expression level of the GUS protein, there were no differences between the novel system developed and the conventional PVX-based expression system.