Genome-wide identification and expression profiling of the <i>bZIP</i> gene family in <i>Chrysanthemum indicum</i> and the functional analysis of <i>CibZIP29</i> under cadmium stress

Recent industrial activities have increased cadmium (Cd) contamination in soils, negatively affecting crop growth and agricultural development. Chrysanthemum indicum represents a significant wild species within the Chrysanthemum genus. Its distinctive resistance and adaptability serve as valuable references for the enhancement of ornamental chrysanthemum breeding programs. However, the role of bZIP family of Cd stress in C. indicum remains unclear. This study conducted a genome-wide identification and expression analysis of bZIP genes in C. indicum, identifying 65 CibZIP genes distributed across nine chromosomes. Phylogenetic analysis grouped them into 12 subgroups, supported by conserved motifs, gene, and protein structure analyses. Tandem and segmental duplications were found to be key drivers of CibZIP gene family expansion, with Ka/Ks analysis indicating purifying selection of duplicated genes. Promoter analysis revealed cis-acting elements involved in plant development and stress response. Transcriptomic data showed that CibZIP genes are highly expressed in different tissues and play significant roles in Cd stress tolerance. RT-qPCR analysis further confirmed that specific CibZIP genes, especially CibZIP29, are upregulated in response to Cd stress. Overexpression of CibZIP29 in transgenic Arabidopsis enhanced Cd tolerance, providing new insights into the molecular mechanisms underlying Cd stress response in C. indicum. These findings contribute to understanding the role of CibZIP genes in Cd tolerance and will support future research on Cd stress mechanisms in Chrysanthemum.