…. African cassava mosaic virus (ACMV) has been linked to famine and indirectly to the deaths of thousands of people. Accordingly, several groups have intensively investigated approaches for breeding or engineering resistance to ACMV ….
Now, a study in Genome Biology by Mehta et al. has applied the CRISPR–Cas approach in cassava. Unexpectedly, they found that Cas9-mediated cleavage of the virus was insufficient to confer resistance to infection in the face of rapid genome replication. Further, the authors say that a CRISPR–Cas approach to protection against geminivirus infection may not be viable, and may in fact drive accelerated evolution of geminiviruses.
The failure to demonstrate a resistant phenotype in cassava could have been due to insufficient expression of Cas9 and/or the sgRNA, such that the genome cleavage frequency was simply outstripped by genome replication, as the authors in fact speculate.
It is also possible that the initial infecting dose of ACMV was too high, given that it was delivered into cassava plants via recombinant A. tumefaciens by dipping de-leaved stems into bacterial suspension. This does not mimic natural infection by viruliferous whiteflies …. and could mean that the cleavage machinery was simply overwhelmed right from the initiation of infection.
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