Polyamines (PAs) are small aliphatic amines present in all living cells. For more than 100 years in biology, they were misunderstood as “ptomaine or food poisoning” substances by toxicologists (Cohen et al., 1981). The largely known PAs in plants are putrescine (Put), spermidine (Spd), and spermine (Spm). In addition, cadaverine (Cad) and thermospermine (t-Spm), a Spm isomer, are also reported to exist in higher plants.

Polyamine homeostasis is determined by PA metabolism, conjugation, interconversion, chemical alteration and transport (Moschou and Roubelakis-Angelakis, 2014; Handa et al., 2018; Nguyen et al., 2018; Tiburcio and Alcazar, 2018; Podlesakova et al., 2019). Biochemical effects of PAs have been unraveled in many physiological processes, primarily in stability and function of proteins and nucleic acids (Handa et al., 2018), partly due to their positive charge that enables them to electrostatically interact with polyanionic molecules inside the cell. Polyamines correlate with numerous vital biochemical functions, including protein regulation (Takahashi and Kakehi, 2010; Sayas et al., 2019), regulation of chemiosmosis and photoprotection in chloroplasts (Ioannidis et al., 2016), ATP synthesis (Ioannidis et al., 2006), ion channeling (Pottosin et al., 2014a; Shabala et al., 2016), membrane fluidity (Paschalidis et al., 2010; Bleackley et al., 2014; Shabala et al., 2016; Dorighetto Cogo et al., 2018), and control of N/C balance (Moschou et al., 2012; Gupta et al., 2013; Majumdar et al., 2016). Exogenous PA application enhanced plant tolerance/resistance to several abiotic stress conditions, such as salinity, drought, water logging/flooding, osmotic stress, heavy metals, and extreme temperatures (Liu et al., 2006, 2015; Moschou et al., 2008c, 2012; Paschalidis et al., 2009a; Toumi et al., 2010; Moschou and Roubelakis-Angelakis, 2014; Gupta et al., 2016; Handa et al., 2018; Nguyen et al., 2018; Tiburcio and Alcazar, 2018; Pal et al., 2019; Yin et al., 2019). Polyamine application also enhanced tolerance to a few phytopathogenic infections in planta, such as Alternaria alternata (Estiarte et al., 2017), Phytophthora capsici (Koç, 2015), and Pseudomonas viridiflava (Rossi et al., 2015, 2018), and in vitro, such as Fusarium strains (Wojtasik et al., 2015) and Sclerotinia sclerotiorum (Garriz et al., 2003). The increase of host PA levels, either by using transgenic method or treatment with exogenous PAs, strongly decreased in planta growth of biotrophic pathogen Pseudomonas viridiflava, which was relieved by a PA oxidase (PAO) inhibitor (Marina et al., 2008). However, increase of leaf PA levels, by the same experimental approaches, led to increased necrosis in planta due to infection by Sclerotinia sclerotiorum, and the PA-induced increase of leaf necrosis after fungal infection was attenuated by inhibiting the activity of DAO and PAO (Marina et al., 2008). There is evidence that exogenous PA application modifies pathogenic responses depending on the strategy of the specific pathogen (Marina et al., 2008; Stes et al., 2011; Valdes-Santiago et al., 2012; Vilas et al., 2018).

Polyamines have crucial roles in a plethora of developmental procedures, including floral initiation and development (Liu et al., 2006, 2015; Liu and Moriguchi, 2007; Tavladoraki et al., 2016; Ahmed et al., 2017), leaf development and senescence (Kusano et al., 2008; Paschalidis et al., 2009b; Sobieszczuk-Nowicka et al., 2015; Sobieszczuk-Nowicka, 2017), fruit development and ripening (Liu et al., 2006; Liu and Moriguchi, 2007; Tsaniklidis et al., 2016; Fortes and Agudelo-Romero, 2018), and abiotic/biotic stress response (Alcazar et al., 2006, 2010; Moschou et al., 2008a; Liu et al., 2015, 2018; Montilla-Bascon et al., 2017).

Cellular PA levels are largely dependent on the dynamic regulation/balance among their biosynthesis, transport, and catabolism interchange. Polyamine biosynthesis has been thoroughly studied in the abovementioned physiological processes and a number of excellent literature reviews refer to their role (Kusano et al., 2008; Paschalidis et al., 2009a; Rangan et al., 2014; Liu et al., 2015; Majumdar et al., 2016; Fortes and Agudelo-Romero, 2018; Handa et al., 2018; Tiburcio and Alcazar, 2018; Wuddineh et al., 2018; Podlesakova et al., 2019). Nevertheless, there is a substantial lack of information on PA catabolism; so far, the enzymes involved in this process and the potential functions of their genes remain poorly characterized. As far as substrate specificity is concerned, it is well known that PAs are catalyzed by two major categories of amine oxidases, copper-containing amine oxidases (CuAOs) and flavin-containing PA oxidases (PAOs) (Cona et al., 2006), with cell type-specific functions in plant tissue/organ differentiation and development (Tavladoraki et al., 2016).

Emerging evidence suggests that PA catabolism plays a critical signaling role in a variety of cellular and developmental processes in all organisms, mediated via regulation of their homeostasis in reaction to intercellular and/or intracellular signs, as developmentally generated by abiotic and/or biotic alarms. In an effort to elucidate the underlined biological mechanisms, the latest advances are updated here on the function of CuAOs and PAOs, as sources of bio-reactive products, such as H₂O₂, during developmental processes with emphasis in fruit ripening and senescence, and, moreover, in abiotic/biotic stress reactions. The present approach might help in unraveling the role/use of the PA catabolic pathway in plants as a focus area for innovative stress resistance/tolerance approaches.

Increasing studies report that PA catabolism is directly involved in plant development. Several evidence suggests that PA oxidation in the apoplast together with the generated reactive oxygen species (ROS) are involved in programmed cell death (PCD) and xylem differentiation (Corpas et al., 2019; Podlesakova et al., 2019). As early as 1998, Møller and McPherson found that AtCuAO localization in root xylem tissues is preceding and overlays with the synthesis of lignin in Arabidopsis (Møller and McPherson, 1998), and the PAO-generated apoplastic H₂O₂ levels considerably contribute to Zea mays leaf blade elongation (Rodriguez et al., 2009). In addition, the perturbation of PA catabolism by overexpressing the ZmPAO gene, as well as by down-regulating the S-adenosyl methionine decarboxylase (SAMDC) gene via RNA interference, in tobacco, promotes vascular cell differentiation and induces PCD in root cap cells (Moschou et al., 2008b; Tisi et al., 2011). Recently, the AtPAO5 has been reported to participate in the tightly controlled interplay between auxins and cytokinins, which are necessary for proper xylem differentiation (Alabdallah et al., 2017), and to regulate Arabidopsis growth through t-Spm oxidase activity (Kim et al., 2014).

Other studies suggest that PAs, along with ROS derived by their oxidation, control ion channeling in plant cells throughout normal and stress conditions, by affecting the plasma membrane ion transporting or acting as second messenger molecules (Pegg, 2014; Pottosin et al., 2014b). It has been reported that the Spd oxidase-produced H₂O₂ controls pollen plasma membrane hyperpolarization-activated Ca(²⁺)-penetrable canals and pollen tube growth (Wu et al., 2010). In Arabidopsis thaliana, differences in expression patterns are revealed for all of the AtPAO gene family members, as AtPAO1 was mainly found in the transition area among meristems and elongation root regions, as well as in anther tapetum, and AtPAO2 was most expressed in the pollen, quiescent center and columella initials, whereas AtPAO3 was predominantly identified in pollen, columella and guard cells. In addition, AtPAO5 was specifically expressed in the root vascular system and in hypocotyls (Fincato et al., 2012). Moreover, the gene structure of AtPAO5 was quite different from the other four AtPAO genes (Fincato et al., 2011). Its expression was detected during various growth stages, with the highest expression being observed in flowers, especially in sepals (Takahashi et al., 2010). AtPAO5 is classified as a cytosolic Spm oxidase/dehydrogenase protein undergoing proteasomal control (Ahou et al., 2014), that controls Arabidopsis growth via t-Spm oxidase activity (Kim et al., 2014; Liu et al., 2014d), while the rice OsPAO1 is a functional ortholog of AtPAO5 (Liu et al., 2014d) and the rice OsPAO7 is involved in lignin synthesis in anther cell walls (Liu et al., 2014c).

Fruits usually keep higher PA levels at early developmental stages and are followed by a continuing decrease thereafter, especially at ripening stage (Fortes and Agudelo-Romero, 2018). This phenomenon has been reported in both climacteric and non-climacteric fruits, such as apple (Biasi et al., 1988), avocado (Kushad et al., 1988), peach (Liu and Moriguchi, 2007; Ziosi et al., 2009), mango (Malik and Singh, 2004), olive (Gomez-Jimenez et al., 2010), tobacco (Paschalidis and Roubelakis-Angelakis, 2005a), strawberry (Guo et al., 2018), raspberry (Simpson et al., 2017), oil palm (Teh et al., 2014), tomato (Rastogi and Davies, 1991; Tassoni et al., 2006; Mattoo et al., 2007; Van de Poel et al., 2013; Tsaniklidis et al., 2016; Liu et al., 2018), and grapevine (Paschalidis et al., 2009b; Agudelo-Romero et al., 2013; Fortes et al., 2015). As PA contents largely depend on the balance between anabolism and catabolism, it is necessary to unravel this balance during fruit ripening.

Although the precise roles of PA catabolism in fruit ripening are poorly understood, current studies reveal their tight interplay. High expression levels of CuAOs and PAOs during fruit ripening denote the involvement of PAs in associated physiological processes. For example, the free and conjugated PAs dramatically decrease during grape ripening, together with an up-regulation of the CuAO and PAO genes/enzymes and an increase of the H₂O₂ content (Agudelo-Romero et al., 2013), as well as an increase in γ-aminobutyric acid (GABA), a major product of PA catabolism (Fortes et al., 2015). These data suggest that increased PA oxidation might lead to decrease in PA titers. As PAO-derived ROS usually act as secondary messengers, the up-regulation of CuAOs/PAOs during ripening might establish an adequate ROS source for signaling actions driving to ripening hastening. In peach fruit, a jasmonate-induced ripening delay was closely related to increased PA levels (Ziosi et al., 2009). A few studies have unraveled strong indications for the interactions among PAs, PAO-derived products, and hormones, such as abscisic acid (ABA), cytokinins, auxins, and ethylene, aiming on their coordinated action in signaling pathways of several physiological processes, like fruit ripening and stress response (Podlesakova et al., 2019) (Agudelo-Romero et al., 2013). Inhibition of PA catabolism in grape with guazatine, a potent inhibitor of PAO activity, led to profound changes in amino acids, carbohydrates, and hormonal metabolism (Agudelo-Romero et al., 2013).

Increasing evidences have showed that the plant PA catabolism is involved in various abiotic stresses responses, especially in salinity. Previously, it has been reported that GABA generated by CuAO-mediated PA oxidation exerts a substantial role in salinity stress response (Su et al., 2007). The PAOs exerting multifaceted roles on plant growth and salt stress response have been identified in, among others, tobacco, grapevine, sweet orange, tomato, and Arabidopsis (Moschou et al., 2008b; Paschalidis et al., 2010; Fincato et al., 2011; Wang and Liu, 2016; Gemes et al., 2017; Hao et al., 2018). Salinity induces tobacco cells to secrete exodus of Spd to the apoplast, where it is oxidized by PAO, thus generating abundant H₂O₂ and leading to enhanced PCD (Moschou et al., 2008a,b). A PAO gene of sweet orange (CsPAO4) has been further characterized functioning in PA terminal catabolism and playing an important role against salinity (Wang and Liu, 2016). This CsPAO4 was overexpressed in tobacco, which significantly promoted the germination of transgenic seeds, while prominently inhibited the vegetative growth and root elongation of transgenic plants under salinity (Wang and Liu, 2016). The PAO activity provided a significant apoplastic production of ROS, which partly contributed to the maize leaf blade elongation under salt stress (Rodriguez et al., 2009). On the other hand, the peroxisomal AtPAO5 loss-of-function mutation in Arabidopsis thaliana exhibits constitutively higher t-Spm levels and activates metabolic and transcriptional reprogramming promoting salinity stress protection (Zarza et al., 2017). Moreover, PAO inhibitor treatment significantly decreased the H₂O₂ and NO production in tomato under salinity (Takacs et al., 2016), which indicates that PAO may contribute to H₂O₂ and NO production in order to cope with salinity and that the terminal activities of CuAO and PAO might play a role in cell death induction under lethal salt stress. The PA catabolism is also involved in many other abiotic stress responses, among others, in improved thermotolerance in Nicotiana tabacum by underexpressing the apoplastic PA oxidase (Mellidou et al., 2017), in aluminum-induced oxidative stress of wheat (Yu et al., 2018), in selenium-induced H₂O₂ production in Brassica rapa (Wang et al., 2019), and in wound-healing by producing the necessary H₂O₂ for suberin polyphenolic domain and lignin synthesis catalyzed by peroxidase (Angelini et al., 2008).

Plants have developed a series of strategies to thwart pathogen attack (Vilas et al., 2018). The production of ROS is one of the defense responses against pathogen attack. Hydrogen peroxide may act either as an antimicrobial means preventing pathogen from growing or contributing as a signaling molecule, which induces the activation of protecting genes (Corpas et al., 2019).

As the terminal catabolism of PAs is followed by the generation of H₂O₂, PA catabolism is, thus, involved in pathogen defense response. The ornithine decarboxylase (ODC) activity increased 20-fold during the hypersensitive response (HR) to tobacco mosaic virus (TMV) infection; however, the levels of Put, Spd, and Spm were not greatly altered, as expected (Negrel et al., 1984), while the activities of arginine decarboxylase (ADC), ornithine decarboxylase (ODC), and CuAO were all obviously increased (Marini et al., 2001). In addition, the PAO expression level and PA titers were also increased in tobacco plants resistant to TMV (Yoda et al., 2003, 2006), suggesting that both PA biosynthesis and catabolism are activated in the host during pathogen infection, where appropriate.

The H₂O_2, resulted from increased activities of CuAO and PAO, might be the cause for the HR observed in barley after powdery mildew infection (Cowley and Walters, 2002). The increase of host PA levels limited bacterial growth, while inhibition of the PAO host enzymes increased the infection (Marina et al., 2008). It has been reported that DAO and PAO activities might play role in promoted defense against biotrophic or hemibiotrophic pathogens. However, these activities enhanced the infection of necrotrophic pathogens (Marina et al., 2008; Yoda et al., 2009; Moschou et al., 2009a). Similarly, the accumulation and further oxidation of free PAs was detected in the apoplast of tobacco leaves during tobacco defense against infection by microorganisms with contrasting pathogenesis strategies (Marina et al., 2008). This response affected the pathogen’s ability to colonize host tissues and was detrimental for plant defense against necrotrophic pathogens, but it might be beneficial for plant defense against biotrophic pathogens because the former fed on necrotic tissue while the latter depended on living tissue for successful host colonization (Marina et al., 2008). Therefore, apoplastic PAs were suggested to play significant roles in plant-pathogen interactions and lead to significant changes in host susceptibility to different kinds of pathogens through regulation of host PA levels, particularly in the leaf apoplast (Marina et al., 2008). Similarly, tobacco plants overexpressing a ZmPAO unraveled a preinduced disease tolerance against the biotrophic bacterium Pseudomonas syringae pv tabaci and the hemibiotrophic oomycete Phytophthora parasitica var nicotianae (Moschou et al., 2009a), showing a critical role for a PAO-generated H₂O₂ apoplastic barrier for these fungi and bacteria. The PA catabolism also contributed to a resistance state through modulation of the immune response in grapevine following osmotic stress and/or after Botrytis cinerea infection (Hatmi et al., 2018). The pretreatment of stressed berries with appropriate inhibitors of DAO and PAO further increased PA level and greatly lowered defense responses, leading to higher susceptibility to B. cinerea (Hatmi et al., 2018). It is evident that the host PA apoplastic catabolism and the mediated H₂O₂ accumulation play an important signaling role in plant-pathogen interactions. However, the specific mechanisms of PA catabolism against plant resistance to pathogens are often more complicated. Further research is needed to clarify the exact role of PA catabolism in biotic stress resistance, in an effort to help plants cope with adverse environmental conditions and survive.

Stress conditions are accompanied by ROS accumulation and induce a composite signaling system recognized by endogenous plant cell sensors and transferred via secondary messengers to kinases, which lead to differentiations in gene expressions and related metabolites by means of the corresponding transcription factors in a plethora of processes identified as stress responses (Skopelitis et al., 2006; Waszczak et al., 2018).

In addition to several pathways, as, for example, photorespiration and electron transferring in chloroplasts and mitochondria, ROS are produced by apoplastic enzymes or enzymes that have different subcellular localization (Moschou and Roubelakis-Angelakis, 2014; Waszczak et al., 2018; Bordenave et al., 2019). The NADPH oxidase (Papadakis et al., 2005; Papadakis and Roubelakis-Angelakis, 2005; Andronis et al., 2014; Gemes et al., 2016), peroxidases (Papadakis et al., 2005; Paschalidis and Roubelakis-Angelakis, 2005b), oxalate oxidase (Angelini et al., 2008), xanthine dehydrogenase (Zarepour et al., 2010), and PAOs (Paschalidis and Roubelakis-Angelakis, 2005b; Moschou et al., 2008a,b,c; Paschalidis et al., 2009a, 2010; Takahashi et al., 2010; Gupta et al., 2016; Tavladoraki et al., 2016; Hao et al., 2018; Wu et al., 2018; Corpas et al., 2019) are included in these pathways, depending on each specific occasion.

Polyamines, as key compounds in plant physiology, are involved in this stress-signaling scheme, playing essential roles in the control of plant stress tolerance (Moschou and Roubelakis-Angelakis, 2014). Furthermore, numerous protein kinases are transcriptionally or posttranscriptionally regulated by PAs (Moschou and Roubelakis-Angelakis, 2014). Almost 3.5 centuries since their discovery – 1,678 in human semen – PAs still remain fundamental research interests, as they are widely implicated in a plethora of developmental and stress signaling responses. Proteomic and transcriptomic analyses on the PA-stress interplay and identification of over- or underexpressed key related genes, among others, ADC, ODC, SAMDC, Spd synthase (SPDS), Spm synthase (SPMS), CuAOs, and PAOs (Liu et al., 2006, 2015; Liu and Moriguchi, 2007; Tanou et al., 2014; Corpas et al., 2019) may offer a new insight into the molecular mechanisms controlling stress responses. Polyamines partially reversed the NaCl-induced phenotypic and physiological disturbances and systematically up-regulated the expression of PA biosynthesis (ADC, SAMDC, SPDS, and SPMS) and catabolism (DAO and PAO) genes, reprograming the oxidative and nitrosative status and the proteome of citrus plants exposed to salinity stress (Tanou et al., 2014). Recent transcriptomic analyses of the effect of Spd or norspermidine on Arabidopsis indicate up-regulation of the response to heat stress and denatured proteins, inhibiting protein ubiquitylation, both in vivo and in vitro, and this interferes with protein degradation by the proteasome, a situation known to deplete cells of amino acids (Sayas et al., 2019). Furthermore, by in situ RNA–RNA hybridization approaches, the spatial contribution of ODC1, 2; ADC2; and CuAO gene transcripts has been largely elucidated in developing tomato fruits in order to decode the potential connection of PA anabolism/catabolism to developmental processes, like fruit ripening (Tsaniklidis et al., 2016).

Polyamines may further alleviate the unfavorable stress effects by activating the antioxidant machinery (Podlesakova et al., 2019). Spd and Spm, and to a lesser extent, Put, inhibit NADPH-oxidase, whereas Put prevents the induction of PCD (Papadakis and Roubelakis-Angelakis, 2005; Andronis and Roubelakis-Angelakis, 2010; Andronis et al., 2014). Abiotic and biotic stress may cause radical alterations in PA metabolism. Several model systems, like Arabidopsis thaliana, have helped in deciphering the role of PAs and elucidating their metabolic paths (Gupta et al., 2016). The preservation of an appropriate balance of the PA catabolic pathways with the H₂O₂ dual role under normal and stress conditions has helped in illuminating the plant adaptation mechanisms (Paschalidis and Roubelakis-Angelakis, 2005b; Paschalidis et al., 2010; Gupta et al., 2016). ABA is an upstream signal for the induction of the polyamine catabolic pathway in the apoplast of grapevine, thus, amine oxidases are producing H₂O₂ which signals stomatal closure (Paschalidis et al., 2010). When the titers of H₂O₂ are below a threshold, expression of tolerance effector genes is induced, while when it exceeds this threshold, the PCD syndrome is induced (Paschalidis et al., 2010). Polyamines also increase nitric oxide and ROS in guard cells of Arabidopsis thaliana during stomatal closure (Agurla et al., 2018) and during growth inhibition in Triticum aestivum L seedlings (Recalde et al., 2018). In addition, the redox gradient across plasma membranes may play an essential role in climate changes, as a redox signaling regulator (Gupta et al., 2016).

Plant life and stress go hand-to-hand. During growth, in order to overcome abiotic stress conditions, plants develop a remarkable organ/tissue/age-specific PA-related phenotypic plasticity (Paschalidis and Roubelakis-Angelakis, 2005a). Under favorable conditions, a balanced hypogeous and hypergeous PA homeostasis is critical to allow constant water/nutrient uptake and photosynthetic flux, respectively. For example, PA genes/metabolites may contribute to an accurate adaptation of the shift between advancement in cell cycle/cell division, that pushes the growth of very young root/shoot primordia toward cell expansion, differentiation, and lignification (Paschalidis and Roubelakis-Angelakis, 2005b; Paschalidis et al., 2009a). On the contrary, plant growth under abiotic stress might, among other effects, wound/wilt the leaf surface or increase evaporation, rendering plant susceptibility. In this case, plants constantly examine whether or not the environmental signals are favorable for their development/growth, and might redirect a PA-associated phenotypic plasticity, involving Η₂Ο₂, the product of PA catabolism, either for growth or for stress adaptation, e.g., via spermidine-mediated stomatal closure (Paschalidis et al., 2009b, 2010). It is also specified that the seriousness/type of reaction (s) to (a)biotic stress is a cell/tissue/organ/age-specific route, related to PA catabolism (Paschalidis et al., 2009b, 2010). The assessment of the antioxidant genes/machinery, along with the photosynthetic factors, the intracellular cation titers, and the PA interplay in over/underexpressing ZmPAO plants under prolonged/varying salinity (Gemes et al., 2016, 2017) and heat (Mellidou et al., 2017) stress have highlighted a plant ontogenetic stage-specific role for PA oxidase and Η₂Ο₂ during plant developmental reactions to (a)biotic stress conditions.

During abiotic stress conditions, PAs (mainly Spd) are secreted in the apoplast and oxidized by PAOs (they refer to both CuAOs and PAOs, but, for simplicity, they are depicted only as PAOs, throughout the model presentation) (Figure 1), resulting in PA catabolism intermediates. The level of PAO-mediated Spd oxidation results in: (①) moderate apoplastic PAO oxidizing Spd at a small percentage producing modest (beneficial) H₂O₂ (and 1,3-diaminopropane) contents, that act as signaling molecules, inducing a ROS-dependent protective pathway, thus triggering abiotic stress tolerance reactions; (②) high apoplastic PAO, over a specific threshold, oxidizing Spd considerably faster, producing high (harmful) H₂O₂ levels, and resulting in down-regulation of pro-survival genes and execution of a specific PCD pathway in plants under abiotic stress conditions (Figure 1; Moschou et al., 2008b; Moschou and Roubelakis-Angelakis, 2014; Gupta et al., 2016; Corpas et al., 2019).

A possible scenario below may be postulated in order to explain the stress signaling/defense. Abiotic stress induces the production of intracellular and extracellular H₂O₂, higher PAs, and second messengers like Ca²⁺ (Wu et al., 2010). The higher PA levels, when oxidized, generate additional H₂O₂ that activates the plant antioxidant machinery. Indeed, under salt stress conditions, with increased levels of endogenous PAs induced by exogenously applied Spd, PAO activity is further enhanced, thus contributing to H₂O₂ accumulation, subsequently inducing enhanced antioxidant defense, which is helpful for growth (Wu et al., 2018). A cold-responsive ethylene-responsive factor from Medicago falcata was demonstrated to confer cold tolerance by upregulating polyamine turnover, antioxidant protection, and proline accumulation (Zhuo et al., 2018). ABA endogenous contents are also activated by stress, which may trigger ROS-related routes involving PAs (Gupta et al., 2016). A well-organized protection mechanism comprising of PAs, Ca²⁺, ABA, and H₂O₂ coordinates an adaptation response of plants to stress (Figure 1; Skopelitis et al., 2006; Moschou et al., 2008b; Paschalidis et al., 2010; Toumi et al., 2010; Moschou and Roubelakis-Angelakis, 2014; Gupta et al., 2016; Majumdar et al., 2016; Gemes et al., 2017; Handa et al., 2018).

Under abiotic stress conditions, ABA triggers the PA machinery in tolerant/sensitive grapevine genotypes (Toumi et al., 2010). The abiotic tolerance stress signal (③) is received by plants generating essential signal molecules like ABA that are involved in an augmentation of PA synthesis (Figure 1). Tolerant plants showed higher PA synthesis, as compared with the sensitive, giving rise to higher PA levels (Toumi et al., 2010). Regardless the genotype competence in withstanding stress, PAs follow the secretion way and are oxidized in the apoplast by PAOs (Paschalidis et al., 2010; Toumi et al., 2010). In this way, higher intracellular PA titers and higher PA synthesis, together with the apoplastic PAO-derived H₂O₂, are participating in a “positive feedback loop” helping to maintain homeostasis and enhance tolerance through activation of further defense mechanisms. On the contrary, lower PA titers/anabolism enhance PCD syndrome (Paschalidis et al., 2010; Toumi et al., 2010; Gemes et al., 2017).

This model/hypothesis elucidates the role of mostly the intercellular PAs. In abiotic-induced PCD of down-regulated SAMDC tobacco plants, the cellular Spd and Spm levels were reduced, but, unexpectedly, these plants showed similar, to the wild type, PA levels and oxidation in the apoplast (Moschou et al., 2008b). The plants with silenced SAMDC unravel a PA-dependent trade-off between growth and tolerance reactions (Mellidou et al., 2016) and the stimulation of the ADC pathway acts as a positive feedback loop to maintain the PA homeostasis (Toumi et al., 2010).

A biotic stress-induced increase in PAO gene/enzyme occurred in overexpressing PAO tobacco plants infected by Pseudomonas syringae pv tabaci (Moschou et al., 2009a). Under biotic stress conditions, mostly Spm is secreted in the apoplast and oxidized by the respective enriched PAO, causing a H₂O₂ buildup (④, biotic defense) that protects plants from phytopathogenic bacteria (Figure 1). In this context, overexpressing PAO plants reveal a preinduced tolerance against diseases, such as the biotrophic bacterium Pseudomonas syringae pv tabaci and the hemibiotrophic oomycete Phytophthora parasitica var nicotianae (Moschou et al., 2009a). PAO and DAO activities promote defense against biotrophic or hemibiotrophic pathogens and, by contrast, these activities favor the spread of the lesions provoked by necrotrophic pathogens (Marina et al., 2008; Yoda et al., 2009; Moschou et al., 2009a). Oxidation of others polyamines, such as t-Spm, is also involved in response to pathogenic bacteria, increasing Arabidopsis resistance to Pseudomonas viridiflava (Marina et al., 2013). This is probably related to the ability of plant PAOs to oxidize t-Spm in a wide range of tissues and organs, as occurs when other PAs such as Spm are accumulated throughout the plant (Marina et al., 2013). In addition to that, pathogens activate their own and the plant PA metabolism during the compatible interaction between tomato and Pseudomonas syringae (Vilas et al., 2018). This activation results in the accumulation of Put in whole leaf tissues, as well as in the apoplastic fluids, which is explained by the induction of its synthesis in plant cells and also on the basis of its excretion by bacteria (Vilas et al., 2018). Ralstonia solanacearum also produces abundant Put, acting as a virulence metabolite and accelerating wilt disease, possibly reducing ROS in the host (Lowe-Power et al., 2018). The present dual abiotic and biotic stress protection scheme may represent an innovative route for generating tolerant transgenic plants to a variety of environmental and phytopathogenic stress factors.

During development, several molecules exist inside common plant tissues in normal environmental and phytopathogenic-free states, concerting an orthodox plant behavior. However, as soon as normal conditions are substituted by stressful ones, these molecules begin to work as stress-relievers. In this work, PAs are suggested to work as such molecules, i.e., as “orthodox-concerters” under normal conditions and as “stress-relievers” under stressful ones. Polyamines have established duties inside plants; however, when they are found in adverse conditions, they may reveal novel functions, not expected until that time. Polyamines, PA oxidases, and the generated H₂O₂ all have specific roles in sustaining plant developmental procedures, such as fruit ripening and senescence. Furthermore, in this work, the role for the concerted action of PA catabolism and its products, in reaction to both abiotic and biotic stress are discussed. The PA oxidation will surely remain a fascinating area for scientific examination, as its concerted action with the generated H₂O₂ is shown to classify specific stressful parameters and build an effective defense device.

To date, many attempts have been made to investigate the roles of PA catabolism in plant growth, development, fruit ripening, and responses to biotic and abiotic stresses. Therefore, the understanding of the roles played by CuAOs and PAOs in these processes has progressed significantly during the recent decades, especially in rice and Arabidopsis. However, many key questions remain unanswered. Firstly, current studies show that the homeostasis regulation of PAs in plants is rather complex. So far, the information about specific regulatory mechanisms in PA biosynthesis and catabolism is very limited. Although it has been revealed, among others, that the transcription of PA biosynthetic genes is regulated by several transcription factors under stress (Paschalidis and Roubelakis-Angelakis, 2005a; Paschalidis et al., 2009a; Sun et al., 2014; Wu et al., 2016; Liu et al., 2018), relatively less information is available on the transcriptional regulation of PA catabolism. Secondly, although many members of CuAOs and PAOs involved in PA back-conversion pathway have been identified, the explicit role of the PA back-conversion reactions in PA homeostasis and associated physiological processes remains obscure. Last, but not least, although a dual signaling role for PA catabolism and the generated H₂O₂ under abiotic and/or biotic plant stress conditions has been revealed, further study will enable researchers to better elucidate this role by using new era technology.

WW, KP, J-CF, and JS wrote the paper. J-HL conceived the work and finalized the MS.