Metabolism and regulation of cellular polyamine levels are crucial for living cells to maintain their homeostasis and function. Polyamine oxidases (PAOs) terminally catabolize polyamines or catalyse the back-conversion reactions when spermine is converted to spermidine and Spd to putrescine. Hydrogen peroxide (H₂O₂) is a by-product of both the catabolic and back-conversion processes. Pharmacological and genetic approaches have started to uncover the roles of PAO-generated H₂O₂ in various plant developmental and adaptation processes such as cell differentiation, senescence, programmed cell death, and abiotic and biotic stress responses. Many of these studies have revealed that the superoxide-generating Respiratory Burst Oxidase Homolog (RBOH) NADPH oxidases control the same processes either upstream or downstream of PAO action. Therefore, it is reasonable to suppose that the two enzymes co-ordinately control the cellular homeostasis of reactive oxygen species. The intricate relationship between PAOs and RBOHs is also discussed, posing the hypothesis that these enzymes indirectly control each other’s abundance/function via H₂O₂.

Plants can be regenerated from various explants/tissues via de novo shoot meristem formation. Most of these regeneration pathways are indirect and involve callus formation. Besides plant hormones, the role of polyamines (PAs) has been implicated in these processes. Interestingly, the lateral root primordia (LRPs) of Arabidopsis can be directly converted to shoot meristems by exogenous cytokinin application. In this system, no callus formation takes place. We report that the level of PAs, especially that of spermidine (Spd), increased during meristem conversion and the application of exogenous Spd improved its efficiency. The high endogenous Spd level could be due to enhanced synthesis as indicated by the augmented relative expression of PA synthesis genes (AtADC1,2, AtSAMDC2,4, AtSPDS1,2) during the process. However, the effect of PAs on shoot meristem formation might also be dependent on their catabolism. The expression of Arabidopsis POLYAMINE OXIDASE 5 (AtPAO5) was shown to be specifically high during the process and its ectopic overexpression increased the LRP-to-shoot conversion efficiency. This was correlated with Spd accumulation in the roots and ROS accumulation in the converting LRPs. The potential ways how PAO5 may influence direct shoot organogenesis from Arabidopsis LRPs are discussed.

Lateral root primordia (LRPs) of Arabidopsis can be directly converted to shoot meristems (SMs) by the application of exogenous cytokinin. Here, we report that Arabidopsis POLYAMINE OXIDASE 5 (AtPAO5) contributes to this process, since the rate of SM formation from LRPs was significantly lower in the pao5-2 knockout mutant. Furthermore, the presented experiments showed that AtPAO5 influences SM formation via controlling the thermospermine (T-Spm) level. Gene expression analyses supported the view that the pao5-2 mutation as well as exogenous T-Spm downregulate the expression of the class 3 haemoglobin coding genes AtGLB1 and AtGLB2. AtGLB1 and 2 have been reported to augment cytokinin sensitivity, indirectly inhibiting the expression of type-A ARABIDOPSIS RESPONSE REGULATORs (ARRs). In agreement, the same ARR-coding genes were found to be upregulated in the pao5-2 mutant. Although GLB proteins might also control cytokinin-induced nitric oxide (NO) accumulation, we could not find experimental evidence for it. Rather, the negative effect of NO-donor treatment on AtPAO5 gene expression and SM formation was seen. Nevertheless, a hypothetical pathway is set up explaining how AtPAO5 may affect direct shoot meristem formation, controlling cytokinin sensitivity through T-Spm and GLBs.