Genome-wide characterization of the homeodomain leucine zipper gene family in Limonium bicolor and functional analysis in salt gland development

Salinity severely limits plant development and growth; studying halophytes can inform efforts to increase the salt tolerance of salt-sensitive crop plants. Limonium bicolor (also known as Forget Me Not) is a classic halophyte with strong salt tolerance, which it achieves by excreting excess salt through epidermal salt glands. Homeodomain leucine zipper (HD-ZIP) transcription factors have important roles in plant development and adaptation to abiotic stresses. In this study, we performed a genome-wide characterization of the L. bicolor HD-ZIP (LbHDZ) gene family and explored its role in salt gland development and plant tolerance to salt stress. We identified 36 LbHDZ genes and systematically analyzed them at the whole genome level. To determine the possible roles of these LbHDZ genes in salt gland development, we performed RNA-seq at different stages of salt gland development and salt treatment. Subsequently, we conducted an in-depth study of LbHDZ14 and LbHDZ23, the expression levels of which changed significantly during early salt gland development and salt stress. LbHDZ14 and LbHDZ23 encode nucleus-localized transcription factors. Knockout mutants revealed that LbHDZ14 and LbHDZ23 are involved in salt gland formation and salinity tolerance. Overall, our results provide a foundation for further elucidating the functions of HD-ZIP genes in L. bicolor, screening key salt-tolerance genes and improving the salt tolerance of crops.