Putrescine (Put) is the starting point of the polyamines (PAs) pathway and the most common PA in higher plants. It is synthesized by two main pathways (from ornithine and arginine), but recently a third pathway from citrulline was reported in sesame plants. There is strong evidence that Put may play a crucial role not only in plant growth and development but also in the tolerance responses to the major stresses affecting crop production. The main strategies to investigate the involvement of PA in plant systems are based on the application of competitive inhibitors, exogenous PAs treatments, and the most efficient approaches based on mutant and transgenic plants. Thus, in this article, the recent advances in understanding the role of this metabolite in plant growth promotion and protection against abiotic and biotic stresses will be discussed to provide an overview for future research.

The apoplast comprises the intercellular space, the cell walls, and the xylem. Important functions for the plant, such as nutrient and water transport, cellulose synthesis, and the synthesis of molecules involved in plant defense against both biotic and abiotic stresses, take place in it. The most important molecules are ROS, antioxidants, proteins, and hormones. Even though only a small quantity of ROS is localized within the apoplast, apoplastic ROS have an important role in plant development and plant responses to various stress conditions. In the apoplast, like in the intracellular cell compartments, a specific set of antioxidants can be found that can detoxify the different types of ROS produced in it. These scavenging ROS components confer stress tolerance and avoid cellular damage. Moreover, the production and accumulation of proteins and peptides in the apoplast take place in response to various stresses. Hormones are also present in the apoplast where they perform important functions. In addition, the apoplast is also the space where microbe-associated molecular Patterns (MAMPs) are secreted by pathogens. In summary, the diversity of molecules found in the apoplast highlights its importance in the survival of plant cells.

Summary: Cis‐(+)‐12‐oxo‐phytodienoic acid (OPDA) is likely to play signaling roles in plant defense that do not depend on its further conversion to the phytohormone jasmonic acid. To elucidate the role of OPDA in Solanum lycopersicum (tomato) plant defense, we have silenced the 12‐oxophytodienoate reductase 3 (OPR3) gene. Two independent transgenic tomato lines (SiOPR3‐1 and SiOPR3‐2) showed significantly reduced OPR3 expression upon infection with the necrotrophic pathogen Botrytis cinerea. Moreover, SiOPR3 plants are more susceptible to this pathogen, and this susceptibility is accompanied by a significant decrease in OPDA levels and by the production of JA‐Ile being almost abolished. OPR3 silencing also leads to a major reduction in the expression of other genes of the jasmonic acid (JA) synthesis and signaling pathways after infection. These results confirm that in tomato plants, as in Arabidopsis, OPR3 determines OPDA availability for JA biosynthesis. In addition, we show that an intact JA biosynthetic pathway is required for proper callose deposition, as its pathogen‐induced accumulation is reduced in SiOPR3 plants. Interestingly, OPDA, but not JA, treatment restored basal resistance to B. cinerea and induced callose deposition in SiOPR3‐1 and SiOPR3‐2 transgenic plants. These results provide clear evidence that OPDA by itself plays a major role in the basal defense of tomato plants against this necrotrophic pathogen.

Summary: Hexanoic acid‐induced resistance ( H x‐ IR ) is effective against several pathogens in tomato plants. Our study of the mechanisms implicated in H x‐ IR against P seudomonas syringae pv. tomato DC 3000 suggests that hexanoic acid ( H x) treatment counteracts the negative effect of coronatine ( COR ) and jasmonyl‐isoleucine ( JA ‐ I le) on the salicylic acid ( SA ) pathway. In H x‐treated plants, an increase in the expression of jasmonic acid carboxyl methyltransferase ( JMT ) and the SA marker genes PR 1 and PR 5 indicates a boost in this signalling pathway at the expense of a decrease in JA ‐ I le. Moreover, H x treatment potentiates 12‐oxo‐phytodienoic acid accumulation, which suggests that this molecule might play a role per se in H x‐ IR . These results support a positive relationship between the SA and JA pathways in H x‐primed plants. Furthermore, one of the mechanisms of virulence mediated by COR is stomatal re‐opening on infection with P . syringae . In this work, we observed that H x seems to inhibit stomatal opening in planta in the presence of COR , which suggests that, on infection in tomato, this treatment suppresses effector action to prevent bacterial entry into the mesophyll.

NH₄⁺ nutrition provokes mild toxicity by enhancing H₂O₂ accumulation, which acts as a signal activating systemic acquired acclimation (SAA). Until now, induced resistance mechanisms in response to an abiotic stimulus and related to SAA were only reported for exposure to a subsequent abiotic stress. Herein, the first evidence is provided that this acclimation to an abiotic stimulus induces resistance to later pathogen infection, since NH₄⁺ nutrition (N-NH₄⁺)-induced resistance (NH₄⁺-IR) against Pseudomonas syringae pv tomato DC3000 (Pst) in tomato plants was demonstrated. N-NH₄⁺ plants displayed basal H₂O₂, abscisic acid (ABA), and putrescine (Put) accumulation. H₂O₂ accumulation acted as a signal to induce ABA-dependent signalling pathways required to prevent NH₄⁺ toxicity. This acclimatory event provoked an increase in resistance against later pathogen infection. N-NH₄⁺ plants displayed basal stomatal closure produced by H₂O₂ derived from enhanced CuAO and rboh1 activity that may reduce the entry of bacteria into the mesophyll, diminishing the disease symptoms as well as strongly inducing the oxidative burst upon Pst infection, favouring NH₄⁺-IR. Experiments with inhibitors of Put accumulation and the ABA-deficient mutant flacca demonstrated that Put and ABA downstream signalling pathways are required to complete NH₄⁺-IR. The metabolic profile revealed that infected N-NH₄⁺ plants showed greater ferulic acid accumulation compared with control plants. Although classical salicylic acid (SA)-dependent responses against biotrophic pathogens were not found, the important role of Put in the resistance of tomato against Pst was demonstrated. Moreover, this work revealed the cross-talk between abiotic stress acclimation (NH₄⁺ nutrition) and resistance to subsequent Pst infection.