Cloning and functional verification of endogenous U6 promoters for developing an efficient CRISPR/Cas9-mediated genome editing system in kenaf (Hibiscus cannabinus L.)

The U6 promoter plays a pivotal role in the CRISPR/Cas9 system by driving the transcription of single guide RNA (sgRNA), which directs Cas9 to achieve precise genome editing. Endogenous U6 promoters typically exhibit superior transcriptional activation efficiency compared to exogenous counterparts, thereby enhancing the efficacy of genome editing. However, the endogenous U6 promoter in kenaf (Hibiscus cannabinus L.) remains uncharacterized. In this study, we conducted a homologous search of the kenaf genome using the Arabidopsis U6 (AtU6-26) RNA sequence as a reference, identifying two candidate promoters, HcU6-1 and HcU6-14. Promoter fragments were amplified from the genomic DNA of kenaf cultivar 'Fuhong 952' and subsequently cloned into a GUS fusion expression vector. Histochemical staining revealed transcriptional activity for both promoters, with HcU6-14 demonstrating significantly stronger activity. To evaluate editing efficiency, we constructed a CRISPR/Cas9 vector containing HcALS sgRNA, driven by either the kenaf U6-14P promoter or the cotton U6-9P (GbU6-9P) promoter. Kenaf hairy roots were regenerated via Agrobacterium rhizogenes K599-mediated transformation. Sequencing analysis of ALS gene fragments from these hairy roots confirmed successful targeted editing when using the kenaf U6-14P promoter, whereas no base mutations were detected with the cotton U6 promoter. These findings highlight the superior editing efficiency of the kenaf U6 promoter and provide a critical foundation for advancing functional genomics research in kenaf.