Rice is one of the most consumed crops in the world
Rice is one of the most consumed crops in the world(1). In many countries such as China, Japan and Philippines, rice plants have been affected with Rice dwarf virus (RDV), which causes a severe disease to these rice plants (2). RDV is a virus member of genus Phytoreovirus, which is a family of Reoviridae (2,3). RDV has an icosahedral double-shelled structure with a diameter of 693 Å (2,3,4). Its genome contains 12 segments of double-stranded RNA (dsRNA), which is designed in order from S1 to S12 for their mobility on polyacrylamide gel (2,3). Among rice plants, RDV is widespread and it has been considered as the first possible cause in the overall rice production reduction (2). Plans that are infected with this virus are stunted and do not have the ability to bear seeds(2). Transmission of RDV to the host plant is by an insect vector which is leafhopper, and this process is accumulated after the replication of the virus in the insect vector (2,3). RDV have the ability to replicates in insects and in cells of graminaceous plants, however, its transmission can be only by an insect (3). This virus does not stimulate neoplasia, however, it can affect rice plants by inhibiting the growth of these plants and the occurrence of white chlorotic areas on the leaves of the plants, which leads in reducing the yield of the rice plants (2,3,4). Approaches to genetic resistance have been introduced to protect rice plants from this virus (2). However, there are no information or reports states that naturally occurring genes in rice plants can have resistance to RDV (2). Until now, plants viruses are partially controlled by some traditional cultivation methods such as early detection, cross-protection, chemical control of insect vector, crops rotation and breeding for resistance (3). In the 1980s, Agrobacterium was successfully used as a vector to transfer a foreign DNA into a nuclear genome, this approach has induced the introduction of genetic engineering for the improvement of many kinds of plants with enhancing the virus resistance in them (3). Plant biotechnology has many goals and one of these goals is genetically modified cereals, such as rice and wheat that have the ability to enhance the resistance to diseases and insects (2). Today, many antiviral techniques are being used, either by enhancing the natural plant’s defense against diseases or designing new materials and strategies, which in most cases are in the end also based on the plant’s natural resistance mechanism (3). In this review paper, three genetic engineering methods will be discussed which are RNA silencing, ribozyme-mediated resistance and comparing these tools with each other and other strategies.
1.1 RNA silencing
Genetic modification of plants by using virus-derived genes to grant resistance is an effective method that results in pathogen-derived resistance (2,3). An important mechanism for induction of pathogen-derived resistance is the RNA silencing of gene expression that is known as post-transcriptional gene silencing (Vazquez Rovere et al. , 2002), which was first found in plants (2) Two types of RNA play major roles in RNA silencing: dsRNA, which acts as a trigger for the sequence-specific degradation of RNA; and short interfering RNA (siRNA), which provides the specificity for the RNA-induced silencing complex that degrades the target RNA (Hannon, 2002) (2). When mRNA molecule is totally homologous to the dsRNA, dsRNA can then make RNA silence or induce its degradation. Due to this mechanism, RNAi phenomena have been introduced for the suppression and inhibition of gene expression in plants (2). Moreover, there were efforts to engineer RNAi-mediated resistance to other plant viruses, which resulted in varying degrees of successful achievement, such as immunity in plants and the delay of the disease symptoms (2). The degree differences of resistance might result from the differences of target areas in the viral genome (2). For the RDV situation, the genetic resistance for rice plants has targeted two proteins which are protein P4 (capsid outer protein) and P5(minor core protein) (2). The results were that these transgenic rice plants have partial resistance to the virus disease and the symptoms appearance were delayed (2). The replication of RDV is in its viroplasm, which has the nonstructural protein Pns12 (2). What was discovered is that the protein Pns12 can activate the viroplasm-like inclusions formation even when other proteins are absent(2). Also, Pns12 is the first detectable protein in the infected insect-vector cells with playing a major role in viral infection at the initial phase of the proliferation of the virus in the infected rice plant (2). With these important observations, it gives a suggestion that Pns12 genes can be a target for suppression of the RDV proliferation in infected rice plants (2). In Inverted-repeated (IR) concept, this IR encodes the dsRNA, which matches a 500-bp piece of S12 genome of the virus, and S12 encodes the Pns12. With this construct, it will lead to plant protection (2). Other nonstructural protein that can be involved in the assembly of RDV is Pns4. Pns4 produces minitubulues with a diameter of 10 nm at the late phase of the viral infection, therefore, it was chosen as another target (2). Even though that RNAi is considered to be highly gene specific targeting , however, siRNA can estimate off- target gene silencing problem effects.