CsHB1, a HD-Zip II transcription factor, promotes somatic embryogenesis in citrus

In somatic embryogenesis (SE), somatic cells can develop into embryos in vitro. SE provides a very effective means in economically important plants for germplasm conservation, plant production and transformation. Here, we identified a citrus gene that encodes a homeodomain-leucine zipper (HD-ZIP) II transcription factor named CsHB1 that is expressed at elevated levels in embryogenic callus during SE. The overexpression of CsHB1 promoted the formation of embryos in Guoqing No. 1 Satsuma mandarin (Citrus unshiu) callus that has an attenuated capacity for embryogenesis. Thus, CsHB1 can promote the transformation of non-embryonic cells to an embryonic state and enhance SE. We also studied the genes expression affected by the overexpression of CsHB1 with RNA-seq and screened the genome-wide direct targets of CsHB1 with ChIP-seq to characterize its roles at the molecular level. The results demonstrated that CsHB1 could bind to many numbers of genes Gene ontology analysis revealed that target genes were enriched for DNA-binding transcription factor activity, cellular developmental process, response to hormone, and response to abiotic stimulus as well as response to light stimulus.