Polyamines are plant metabolites involved in many processes under physiologically normal and stressful conditions. Cadaverine is one of the least studied plant polyamines. The relationship between its physiological effects and the formation of signaling mediators, in particular, reactive oxygen species (ROS), has hardly been specially studied. The aim of this work was to study the possible protective effect of cadaverine on wheat (Triticum aestivum L.) seedlings under heat stress and its relationship with the formation and detoxification of ROS by antioxidant enzymes. Etiolated seedlings of soft winter wheat variety Doskonala were used in the work. We treated three-day-old seedlings with cadaverine at concentrations ranging from 0.05 to 2.5 mM by adding it to the root incubation medium. In some variants of the experiment, we treated seedlings with a hydrogen peroxide scavenger dimethylthiourea (DMTU - 150 μM), a diamine oxidase inhibitor aminogunidine (1 mM) or an inhibitor NADPH oxidase imidazole (10 μM), as well as the indicated inhibitors in combination with cadaverine. The hydrogen peroxide content and the activity of antioxidant enzymes were determined in the roots of seedlings a certain time after treatment with the studied compounds. One day after the treatment of seedlings with cadaverine, ROS antagonists, and a combination of effectors, the seedlings were subjected to damaging heating in a water thermostat (10 min at 45 °C). 24 h after heating, we assessed the content of the products of lipid peroxidation (LPO) in the roots and, after 3 days, the survival of seedlings. Incubation in the presence of cadaverine increased the resistance of seedlings to damaging heat (See Fig. 1). The highest relative number of surviving seedlings was observed in the variant with 1 mM cadaverine treatment. Under the effect of cadaverine, the content of hydrogen peroxide in the roots increased (See Fig. 2). We observed a noticeable effect 1-4 h after the start of treatment, with a maximum after 2 h. Treatment of seedlings with a scavenger of hydrogen peroxide DMTU removed the manifestation of the effect of an increase in the content of H2 O2 in the roots caused by the action of cadaverine (See Fig. 3). This effect was also completely eliminated by the diamine oxidase inhibitor aminoguanidine and was almost unchanged in the presence of the NADPH oxidase inhibitor imidazole. The effect of heat stress on seedlings caused an increase in the content of the LPO products in them. Treatment with cadaverine markedly reduced this manifestation of oxidative stress. The antioxidant DMTU and the diamine oxidase inhibitor aminoguanidine largely neutralized the protective effect of cadaverine (See Fig. 4a). At the same time, the NADPH oxidase inhibitor imidazole had almost no effect on the manifestation of the effect of cadaverine on the LPO products content in roots. Under the influence of DMTU and aminoguanidine, but not imidazole, the positive effect of cadaverine on the survival of seedlings after damaging heating was also leveled out (See Fig. 4b). The treatment of seedlings with cadaverine caused a change in the activity of antioxidant enzymes in the roots (superoxide dismutase - SOD, catalase, and guaiacol peroxidase) (See Fig. 5). DMTU and aminoguanidine neutralized the effect of cadaverine-induced increase in the activity of catalase and guaiacol peroxidase, but had almost no effect on the increase in SOD activity in roots induced by this diamine (See Fig. 6). The NADPH oxidase inhibitor imidazole did not significantly affect the manifestation of the effect of increasing the activity of antioxidant enzymes when seedlings are treated with cadaverine. We can conclude that one of the signaling mediators involved in the regulation activity of catalase and guaiacol peroxidase and in the induction of heat resistance of wheat seedlings by exogenous cadaverine is hydrogen peroxide, which is formed during the oxidation of cadaverine by diamine oxidase. At the same time, the modification of SOD activity in the roots of wheat seedlings with cadaverine, apparently, can occur without the participation of ROS.
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