Polyamines (PAs) are positively charged amines that are present in all organisms. In addition to their functions specific to growth and development, they are involved in responding to various biotic and abiotic stress tolerance functions. The appropriate concentration of PA in the cell is maintained by a delicate balance between the catabolism and anabolism of PAs, which is primarily driven by two enzymes, namely diamine oxidase and polyamine oxidase (PAO). PAOs have been found to be localized in multiple subcellular locations, including peroxisomes. This paper presents a holistic account of peroxisomal PAOs. PAOs are flavin adenine dinucleotide-dependent enzymes with varying degrees of substrate specificity. They are expressed differentially upon various abiotic stress conditions, namely heat, cold, salinity, and dehydration. It has also been observed that in a particular species, the various PAO isoforms are expressed differentially with a spatial and temporal distinction. PAOs are targeted to peroxisome via a peroxisomal targeting signal (PTS) type 1. We conducted an extensive bioinformatics analysis of PTS1s present in various peroxisomal PAOs and present a consensus peroxisome targeting signal present in PAOs. Furthermore, we also propose an evolutionary perspective of peroxisomal PAOs. PAOs localized in plant peroxisomes are of potential importance in abiotic stress tolerance since peroxisomes are one of the nodal centers of reactive oxygen species (ROS) homeostasis and an increase in ROS is a major indicator of the plant being in stress conditions; hence, in the future, PAO enzymes could be used as a key candidate for generating abiotic stress tolerant crops.

Abiotic stress is a major bottleneck for crop productivity. To counter abiotic stresses, plants have developed several strategies, and the accumulation of polyamine (PA) serving as an osmolyte is one of them. The cellular pool of PAs is primarily regulated by polyamine oxidases (PAOs) either by terminal catabolism or by back conversion (BC) of polyamines. The role of PAO in abiotic stress tolerance has also been reported. Polyamine oxidases are primarily localized in the cytosol, cell wall, and apoplastic regions; however, lately, their peroxisomal localization has also been demonstrated. In this study, we reported the detection of polyamine oxidase isoform 3 from Oryza sativa (OsPAO3) in peroxisome as early as 12 h post-transformation under in vitro conditions using fluorescent microscopy. The gene was also found to be significantly upregulated by salinity, dehydration, cold, and heat stress. We have also demonstrated that the expression of OsPAO3 gene was mediated by a second messenger, calcium. The upregulation of OsPAO3 by salinity, dehydration, cold, and heat stresses suggests that it could be a suitable candidate for providing tolerance to plants against abiotic stress combination or stress matrix, which is a common feature in agricultural field conditions. Furthermore, the data provided here would be valuable in understanding the abiotic stress-mediated signal transduction network of PAOs.