Genome-Wide Identification of the HD-ZIP Genes in Sweet Potato and Functional Role of IbHD-ZIP61 in Anthocyanin Accumulation and Salt Stress Tolerance

Sweet potato (Ipomoea batatas L.) is a vital dual-use crop, with some varieties being used as leafy vegetables that are rich in anthocyanins. Nevertheless, salinity stress is a challenge to their production. Homeodomain-leucine zipper (HD-ZIP) gene family members encode proteins participating in the regulation of plant defense and secondary metabolism, while the functional study of HD-ZIP genes in sweet potato is still limited. Herein, a total of 66 IbHD-ZIP genes were identified, which were expanded by segmental duplication. Based upon promoter cis-element information and precedent evidence, IbHD-ZIP61, belonging to subfamily I, was selected for functional studies. Functional characterization was conducted via ectopic expression in transgenic Nicotiana benthamiana. The overexpression of IbHD-ZIP61 significantly increased anthocyanin production under normal growth conditions by promoting anthocyanin biosynthetic genes AN1a, AN2, and DFR. Furthermore, transgenic plants displayed better salinity tolerance, which exhibited reduced growth inhibition, increased water status, decreased oxidative injury, as well as elevated activity of antioxidant enzymes. This study validated the coordinated regulation of anthocyanin pathway genes as well as pivotal pathways (NHX2, NCED1, P5CS) during salinity adaptation. These findings demonstrate that IbHD-ZIP61 is a transcription factor linking anthocyanin synthesis and salinity adaptation, thus making it a potential candidate for improving breeding in nutritionally superior and salinity-adapted edible crops such as sweet potato.