The application of a heat-inducible CRISPR/Cas12b (C2c1) genome editing system in tetraploid cotton (G. hirsutum) plants.

The CRISPR/Cas9 and Cas12a (Cpf1) tools have been used on a large scale for genome editing. A new effector with a single nuclease domain, a relatively small size, low-frequency off-target effects and cleavage capability under high temperature has been recently established and designated CRISPR/Cas12b (C2c1). Cas12b has also shown temperature inducibility in mammalian systems. Therefore, this system is potentially valuable for editing the genomes of plant species, such as cotton, that are resistant to high temperatures. Using this new system, mutants of upland cotton were successfully generated following Agrobacterium-mediated genetic transformation under a range of temperatures. Transformants (explants infected by Agrobacterium) exposed to 45 °C for 4 days showed the highest editing efficiency. No off-target mutation was detected by whole-genome sequencing. Genome edits by AacCas12b in T0 generation were faithfully passed to the T1 generation. Taken together, CRISPR/Cas12b is therefore an efficient and precise tool for genome editing in cotton plants.

High-efficient and precise base editing of C•G to T•A in the allotetraploid cotton (Gossypium hirsutum) genome using a modified CRISPR/Cas9 system.

The base-editing technique using CRISPR/nCas9 (Cas9 nickase) or dCas9 (deactivated Cas9) fused with cytidine deaminase is a powerful tool to create point mutations. In this study, a novel G. hirsutum-Base Editor 3 (GhBE3) base-editing system has been developed to create single-base mutations in the allotetraploid genome of cotton (Gossypium hirsutum). A cytidine deaminase sequence (APOBEC) fused with nCas9 and uracil glycosylase inhibitor (UGI) was inserted into our CRISPR/Cas9 plasmid (pRGEB32-GhU6.7). Three target sites were chosen for two target genes, GhCLA and GhPEBP, to test the efficiency and accuracy of GhBE3. The editing efficiency ranged from 26.67 to 57.78% at the three target sites. Targeted deep sequencing revealed that the C→T substitution efficiency within an 'editing window', approximately six-nucleotide windows of -17 to -12 bp from the PAM sequence, was up to 18.63% of the total sequences. The 27 most likely off-target sites predicted by CRISPR-P and Cas-OFFinder tools were analysed by targeted deep sequencing, and it was found that rare C→T substitutions (average < 0.1%) were detected in the editing windows of these sites. Furthermore, whole-genome sequencing analyses on two GhCLA-edited and one wild-type plants with about 100× depth showed that no bona fide off-target mutations were detectable from 1500 predicted potential off-target sites across the genome. In addition, the edited bases were inherited to T1 progeny. These results demonstrate that GhBE3 has high specificity and accuracy for the generation of targeted point mutations in allotetraploid cotton.

Whole genome sequencing reveals rare off-target mutations and considerable inherent genetic or/and somaclonal variations in CRISPR/Cas9-edited cotton plants.

The CRISPR/Cas9 system has been extensively applied for crop improvement. However, our understanding of Cas9 specificity is very limited in Cas9-edited plants. To identify on- and off-target mutation in an edited crop, we described whole genome sequencing (WGS) of 14 Cas9-edited cotton plants targeted to three genes, and three negative (Ne) control and three wild-type (WT) plants. In total, 4188-6404 unique single-nucleotide polymorphisms (SNPs) and 312-745 insertions/deletions (indels) were detected in 14 Cas9-edited plants compared to WT, negative and cotton reference genome sequences. Since the majority of these variations lack a protospacer-adjacent motif (PAM), we demonstrated that the most variations following Cas9-edited are due either to somaclonal variation or/and pre-existing/inherent variation from maternal plants, but not off-target effects. Of a total of 4413 potential off-target sites (allowing ≤5 mismatches within the 20-bp sgRNA and 3-bp PAM sequences), the WGS data revealed that only four are bona fide off-target indel mutations, validated by Sanger sequencing. Moreover, inherent genetic variation of WT can generate novel off-target sites and destroy PAMs, which suggested great care should be taken to design sgRNA for the minimizing of off-target effect. These findings suggested that CRISPR/Cas9 system is highly specific for cotton plants.