RNA interference (RNAi) involves small naturally occurring molecules of double stranded RNA (dsRNA) targeting genes for expression knockdown by recognition of the target gene messenger RNA (mRNA) and interfering with the translation into protein. In this way, plant genes can be targeted to remove unwanted metabolites and in pests, pathogens and viruses, essential genes can be silenced leading to an effective control. Since these small RNAs recognize target gene mRNAs based on sequence complementarity, systems can be designed which target only genes with homologous sequences in a very narrow range of species. The exponential increase in available genomic and transcriptomic sequence data allows the specific design of these dsRNAs, minimizing the risk for off-target effects or silencing effects in non-target organisms.
RNAi has several unique features compared to other forms of crop biotechnology which offer unique opportunities to breeders for varietal improvements. One of these characteristics is the fact that RNAi leads to a knockdown effect, rather than a complete knockout. This could be important when lower levels of gene expression are required. dsRNA small molecules have high mobility through the plant vascular system and can move inside the plant from one point of application to the entire plant. Therefore, dsRNA produced in part of the plant (eg. rootstock) can spread in all parts (eg. canopy) so to confer resistance to disease to the full plant, including fruit that will result not genetically modified but protected by the presence of target specific highly degradable dsRNA small molecule. The dsRNA small molecule can be applied topically via a spray-based approach and/or via the application of modified micro-organisms and virus. This approach excludes the use of DNA recombinant technologies in plant so to not be classified as a GMO plant, as expected in many directives and in particular in the EU directive 2001/18.
Worldwide, several virus resistant plants have been approved (e.g. plum, squash and papaya) and many more applications are being developed. As with other technologies, pest and pathogen resistance management is important and new crop protection applications need to be accompanied by effective stewardship and resistance management plans. In addition, plant resistance to a wide range pests and diseases is being studied, particularly to insect vectors of pathogens and a range of fungal pathogens such as cereal rusts or fruit grey mould.
A more recent innovation is the use of topical applications of dsRNA to induce gene silencing as a new strategy for plant protection or growth regulation. Technical advances in the production of dsRNA and formulations to improve the efficacy, stability and persistence of extracellular dsRNA mean that it is now realistic to consider using dsRNA as a biopesticide. It can be applied as foliar sprays (spray induced gene silencing – SIGS) or as soil additives and there is considerable commercial interest in this because the cost of production, the specificity and improved biosafety compared with chemical pesticides and some alternative biocontrol strategies. SIGS are also being considered for weed control by targeting specific genes in a weed that do not occur in crops or other weed genera. Such a strategy would be very useful for controlling grass weeds in a range of graminaceous crops such as wheat and rice. Topical applications would typically contain dsRNAs which are produced in bacteria or synthesised in vitro. Thus they are not like other agrochemicals and are different from other biocontrol agents which exploit proteins such as peptides or Cry toxins. The sRNA may be produced using GM bacteria but the product can be formulated to not contain any GM material except the RNA molecule designed for a very specific gene target in a pest, pathogen or weed in order to avoid non-target effects and with limited persistence in the environment. Thus, it represents a novel type of (bio)pesticide and it is important that regulations governing plant protection products (PPPs) are adapted to allow introduction of this technology.
RNAi is a technology which is already being exploited and with great future potential in a range of areas of crop production and protection. It can make a major contribution towards integrated pest management and the sustainable agricultural strategies needed worldwide to secure current and future food production. GM RNAi plants are being assessed and regulated using existing regulatory frameworks. However SIGS needs to be classified in the regulatory framework as novel PPPs.