Genome-Wide Identification of Polyamine Oxidase (PAO) Family Genes: Roles of and in the Cold Tolerance of Pepper ( L.).

Polyamine oxidases (PAOs), which are flavin adenine dinucleotide-dependent enzymes, catalyze polyamine (PA) catabolism, producing hydrogen peroxide (HO). Several PAO family members have been identified in plants, but their expression in pepper plants remains unclear. Here, six PAO genes were identified in the 'Zunla-1' pepper genome (named - according to their chromosomal positions). The PAO proteins were divided into four subfamilies according to phylogenetics: CaPAO1 belongs to subfamily I; CaPAO3 and CaPAO5 belong to subfamily III; and CaPAO2, CaPAO4, and CaPAO6 belong to subfamily IV (none belong to subfamily II). , , and were ubiquitously and highly expressed in all tissues, was mainly expressed in flowers, whereas and were expressed at very low levels in all tissues. RNA-seq analysis revealed that and were notably upregulated by cold stress. CaPAO2 and CaPAO4 were localized in the peroxisome, and spermine was the preferred substrate for PA catabolism. and overexpression in significantly enhanced freezing-stress tolerance by increasing antioxidant enzyme activity and decreasing malondialdehyde, HO, and superoxide accumulation, accompanied by the upregulation of cold-responsive genes (, , , and ). Thus, we identified candidate PAO genes for breeding cold-stress-tolerant transgenic pepper cultivars.