Genome-wide identification of the AP2 subfamily in broomcorn millet and functional characterization of <italic>PmAP2-1</italic> and <italic>PmAP2-9</italic> in salt tolerance

Broomcorn millet is an ancient cereal crop that originated in China and plays a foundational role in the development of Chinese agricultural civilization. The AP2 subfamily has been shown to regulate plant growth, development, and responses to abiotic stresses; however, the number and its specific functions in broomcorn millet remain unclear. In this study, we performed a genome-wide identification of members of the AP2 subfamily in broomcorn millet using bioinformatics approaches based on its genome sequence. Analyses included gene structures and expression patterns under salt stress. A total of 20 PmAP2 subfamily members (PmAP2-1 to PmAP2-20) were identified at the genome-wide level, distributed across 15 chromosomes. Phylogenetic analys is clustered all PmAP2 encoded proteins into three subgroups: euANT, basalANT, and euAP2. Promoter cis-acting element analysis revealed that genes of the PmAP2 subfamily are involved in multiple biological processes, including plant hormone and abiotic stress, growth and development, and light responsive. Interspecies synteny analysis identified 3 and 27 syntenic gene pairs between broomcorn millet and Arabidopsis thaliana and Oryza sativa, respectively. Expression profiling indicated that PmAP2 genes exhibit both cultivar and tissue specificity, and the expression of all 20 members was induced by salt stress. PmAP2-1 and PmAP2-9, two salt stress-induced upregulated genes from broomcorn millet, were introduced into Arabidopsis thaliana via genetic transformation, and stably heritable transgenic lines were obtained. Root growth in transgenic Arabidopsis thaliana was affected by salt stress, with root length decreasing as the NaCl concentration increased. Under severe salt stress of 75 mmol L-1 and 100 mmol L-1 NaCl, the root lengths of transgenic plants showed significant differences compared with the wild type. These findings suggest that PmAP2-1 and PmAP2-9 may serve as potential target genes for the genetic improvement of salt tolerance in other staple crops.