Genome-Wide Identification and Functional Analysis of Polyamine Oxidase Genes in Maize Reveal Essential Roles in Abiotic Stress Tolerance

Abstract

Polyamines (PAs) play a critical role in growth and developmental processes and stress responses in plants. Polyamine oxidase (PAO) is a flavin adenine dinucleotide (FAD)-dependent enzyme that plays a major role in PA catabolism. Here, for the first time, <i>PAO</i> genes in maize were screened for the whole genome-wide and nine <i>ZmPAO</i> genes were identified in this study, named as <i>ZmPAO1</i>-<i>9</i>. Based on structural characteristics and a comparison of phylogenetic relationships of <i>PAO</i> gene families from seven representative species, all nine PAO proteins in maize were categorized into three distinct subfamilies. Further, chromosome location and schematic structure revealed an unevenly distribution on chromosomes and evolutionarily conserved structure features of <i>ZmPAO</i> genes in maize, respectively. Furthermore, transcriptome analysis demonstrated that <i>ZmPAO</i> genes showed differential expression patterns at diverse developmental stages of maize, suggesting that these genes may play functional developmental roles in multiple tissues. Further, through qRT-PCR validation, these genes were confirmed to be responsive to heat, drought and salinity stress treatments in three various tissues, indicating their potential roles in abiotic stress responses. Eventually, to verify the biological function of <i>ZmPAO</i> genes, the transgenic <i>Arabidopsis</i> plants overexpressing <i>ZmPAO6</i> gene were constructed as a typical representative to explore functional roles in plants. The results demonstrated that overexpression of <i>ZmPAO6</i> can confer enhanced heat tolerance through mediating polyamine catabolism in transgenic <i>Arabidopsis</i>, which might result in reduced H₂O₂ and MDA accumulation and alleviated chlorophyll degradation under heat stress treatment, indicating that <i>ZmPAO6</i> may play a crucial role in enhancing heat tolerance of transgenic <i>Arabidopsis</i> through the involvement in various physiological processes. Further, the expression analysis of related genes of antioxidant enzymes including glutathione peroxidase (GPX) and ascorbate peroxidase (APX) demonstrated that <i>ZmPAO6</i> can enhance heat resistance in transgenic <i>Arabidopsis</i> through modulating heat-induced H₂O₂ accumulation in polyamine catabolism. Taken together, our results are the first to report the <i>ZmPAO6</i> gene response to heat stress in plants and will serve to present an important theoretical basis for further unraveling the function and regulatory mechanism of <i>ZmPAO</i> genes in growth, development and adaptation to abiotic stresses in maize.