The HD-ZIP Gene Family in Watermelon: Genome-Wide Identification and Expression Analysis Under Abiotic Stresses

Homeodomain-leucine zipper (HD-ZIP) transcription factors are one of the plant-specific gene families involved in plant growth and response to adverse environmental conditions. However, little information is available on the HD-ZIP gene family in watermelon. In this study, forty were systemically identified in the watermelon genome, which were subsequently divided into four distinctive subfamilies (I-IV) based on the phylogenetic topology. HD-ZIP members in the same subfamily generally shared similar gene structures and conserved motifs. Syntenic analyses revealed that segmental duplications mainly contributed to the expansion of the watermelon HD-ZIP family, especially in subfamilies I and IV. HD-ZIP III was considered the most conserved subfamily during the evolutionary history. Moreover, expression profiling together with stress-related -elements in the promoter region unfolded the divergent transcriptional accumulation patterns under abiotic stresses. The majority (13/23) of in subfamilies I and II were downregulated under the drought condition, e.g., , , , , , and . On the contrary, most HD-ZIP genes were induced by cold and salt stimuli with few exceptions, such as and under cold stress and and under the salt condition. Notably, the gene was predominantly downregulated by three stresses whereas was upregulated, suggesting their possible core roles in response to these abiotic stimuli. Collectively, our findings provide promising candidates for the further genetic improvement of abiotic stress tolerance in watermelon.