Viruses Offer Efficient Delivery for Precise Plant Breeding

Despite the technology's great potential, the application of genome editing in plant molecular breeding is limited by the lack of straightforward and efficient methods for introducing genome editing reagents into plants.

To deliver genome editing reagents into plants, conventional plant transformation-based methods frequently require extended tissue culture, which is a labor-intensive and technically difficult procedure for many elite crop cultivars. This bottleneck makes it impossible to economically edit the genomes of many elite crop cultivars.

Furthermore, homology-directed repair (HDR)-based genome editing techniques are very flexible, but they frequently require the delivery of a significant amount of donor DNA, which can be challenging to accomplish with many traditional techniques. Utilizing both DNA and RNA viruses, these technological obstacles have been addressed.

The essential elements required for plant genome editing have been delivered via RNA viruses. Positive-strand RNA viruses' (PSVs') ability to infect germline cells in plants and cause heritable edits makes up for their limited capacity.

Consequently, PSV-based delivery strategies have primarily been used to introduce guide RNA molecules—which frequently exceed the size limit that PSVs can accommodate—into plant hosts that express the corresponding Cas nuclease gene.

Negative-strand RNA viruses (NSVs) can accommodate longer DNA fragments—like one encoding the complete sequence-specific nuclease machinery-though they hardly ever infiltrate germline cells. Therefore, if heritable edits are desired, NSVs-based delivery strategies frequently involve a subsequent plant regeneration process.

To achieve precise edits through HDR, donor DNA that has been artificially supplied is typically used as the repair template. The donor DNA must be supplied at a high copy number for this to work.

A family of plant DNA viruses known as the “geminiviruses” is capable of highly efficient genome replication in plant cells. They are the perfect vectors for delivering repair donors because of this characteristic. It has been successful in using genome-specific replicons to produce desired editing effects in tobacco, tomatoes, wheat, and other plants.

For viral vectors currently on the market, cargo capacity and vector mobility generally trade off. PSVs are promising tools for tissue culture-free gene editing; however, because of the viral vector's limited capacity, they are dependent on an already-existing Cas-expressing line.

NSV-based vectors can accommodate the full CRISPR/Cas machinery, allowing genome editing without the need for transgenes.

However, the recovery of plants with heritable edits frequently depends on a subsequence tissue culture procedure. Similarly, GVRs are altered to become minimally mobile, non-infectious replicon vectors to accommodate additional nucleotide sequences. Creating viral vector systems that can deliver cargo into germline cells in plants while also having enough capacity to support the entire CRISPR/Cas components is preferable.

Condensed sequence-specific nucleases, on the other hand, are excellent candidates for delivery via virus-based systems. Additionally, it is important to investigate novel elements that could be fused with the delivered cargos to improve the genome editing reagents' systemic movement and accomplish germline edits.

Moreover, broad-spectrum viral delivery systems are necessary to apply to a wider variety of crop species. To lessen the unintentional burden on people and the ecosystem, further research into the biosafety and risk assessment of the application of viral vectors is also worthwhile.

In conclusion, employing viral vectors to transfer genome editing components may overcome many of the current technical obstacles to plant genome editing. By exploiting new viral species and modifying existing viruses to perform better, more effective delivery systems that can easily produce heritable edits may be developed in the future.