Melatonin is among one of the promising agents able to protect agricultural plants from the adverse action of different stressors, including salinity. We aimed to investigate the effects of melatonin priming (0.1, 1.0 and 10 µM) on salt-stressed potato plants (125 mM NaCl), by studying the growth parameters, photochemical activity of photosystem II, water status, ion content and antioxidant system activity. Melatonin as a pleiotropic signaling molecule was found to decrease the negative effect of salt stress on stolon formation, tissue water content and ion status without a significant effect on the expression of Na⁺/H⁺-antiporter genes localized on the vacuolar (NHX1 to NHX3) and plasma membrane (SOS1). Melatonin effectively decreases the accumulation of lipid peroxidation products in potato leaves in the whole range of concentrations studied. A melatonin-induced dose-dependent increase in Fv/Fm together with a decrease in uncontrolled non-photochemical dissipation Y(NO) also indicates decreased oxidative damage. The observed protective ability of melatonin was unlikely due to its influence on antioxidant enzymes, since neither SOD nor peroxidase were activated by melatonin. Melatonin exerted positive effects on the accumulation of water-soluble low-molecular-weight antioxidants, proline and flavonoids, which could aid in decreasing oxidative stress. The most consistent positive effect was observed on the accumulation of carotenoids, which are well-known lipophilic antioxidants playing an important role in the protection of photosynthesis from oxidative damage. Finally, it is possible that melatonin accumulated during pretreatment could exert direct antioxidative effects due to the ROS scavenging activity of melatonin molecules.

The considerable decrease in crop productivity associated with the expansion of saline soils is an acute problem in agriculture. Endophytic fungi positively affect plant fitness under salinity conditions. The effects of potato inoculation with the conidia of the Beauveria bassiana strain Sar-31 on growth (the weight of fresh and dry biomass, shoot and root length, numbers of stolons and leaves, and the leaf surface) and physiological indices (the concentration of pigments, free proline and malondialdehyde, and antioxidant enzymes’ activity) were evaluated under moderate chloride salinity (100 mM). The results indicated that the plant’s association with the fungus mitigated the negative impact of salinity probably because of the activation of antioxidant enzymes and accumulation of free proline in potato tissues. Moreover, under the influence of B. bassiana Sar-31, the number of stolons significantly increased, which is one of the main characteristics of potato as an agricultural crop. Thus, Sar-31 may be a promising candidate for further investigation of its ability to stimulate growth and increase the stress tolerance of potato plants.

Endophytic entomopathogenic fungi are promising agents for the promotion of plant growth, the activation of immunity, and protection against phytopathogens. However, physiological changes in plants after treatment with fungi are insufficiently studied. We investigated the effect of potato inoculation with conidia from Metarhizium robertsii and Beauveria bassiana on the growth (fresh and dry weight, length of shoots and roots, counts of stolons and leaves, and total surface area of leaves) and physiological parameters (pigment contents, free proline and malondialdehyde content, and activity of antioxidant enzymes) at the initial stage of the plant–fungus interaction (seven days) under hydroponic conditions. The results showed that the fungi could act as an immune-modulating factor for plants based on the increase in malondialdehyde and proline contents. At the same time, we observed growth retardation and a decrease in the content of photosynthetic pigments, which may be caused by a tradeoff between plant growth and the immune response.

The ability of brassinosteroids (BS) of different chemical structures (for example, 24-epibrassinolide (EBL) and 24-epicastasterone (ECS)) to induce priming in rapeseed (Brassica napus L.) plants, increasing resistance to “delayed” salt stress (150 mM NaCl), was studied. After 2 weeks of growth of rapeseed seedlings in a hydroponic setup in Hoagland-Snyder (HS) medium, they were transferred to the same medium supplemented with EBL or ECS (10 nM) for 4 h. The plants were then kept on the HS medium without BS for 20 h. Then they were grown on HS nutrient medium in the absence or in the presence of 150 mM NaCl for 6 days. Salinization inhibited growth processes (stem length, number of leaves, assimilating surface area, raw mass of plants (by 30–35%)); decreased levels of chlorophylls a and b and carotenoids (by 38–44%), tissue water content (by 3.54%), and osmotic potential of cells (1.7 times); and increased lipid peroxidation (LPO) (by 62%) in leaves. At the same time, salt stress stimulated the level of proline in leaves by six times and in stems and roots by 11 times, the activity of superoxide dismutase (SOD) by 2.3 times, and the activity of peroxidase by 26%. The concentration (at %) of Na and Cl under salinization increased in all parts of the plants, but the concentration of K, Ca, Mg, S, P, Al, and Fe ions decreased. Treatment of plants with BS (10 nM, 4 h) stimulated growth processes under optimal conditions, which was more typical for ECS, which also increased the content of chlorophyll a and carotenoids (by 11 and 18% in leaves, respectively). BS stimulated the activity of antioxidant enzymes (EBL increased the activity of SOD by 4.6 times, ECS increased the activity of SOD and peroxidase (by 48%)), which could affect the development of the priming state in plants. Pretreatment of rape seedlings with BS increased their resistance to subsequent salt stress. The protective effect of BS manifested at the levels of growth responses (primarily in ECS), water status, and osmotic potential of leaves. The LPO level decreased due to the activation of SOD and peroxidase in the case of EBL and ECS, respectively. The accumulation of ions in plants subjected to hormonal priming depended on the chemical structure of BS. It has been suggested that the protective effect of BS priming is explained by the ability of hormones to activate antioxidant enzymes before stress and to influence the inorganic ion transport system in such a way that plants accumulate ions more actively with subsequent salinization, resulting in a decrease in the osmotic potential of cells and maintenance of the water status. The obtained results can be useful in the development of technology for increasing the salt tolerance of rapeseed.

The aim of this work was to study the ability of 28-homobrassinolide (HBL) and 28-homocastasterone (HCS) to increase the resistance of barley (Hordeum vulgare L.) plants to drought and to alter their endogenous brassinosteroid status. Germinated barley seeds were treated with 0.1 nM HBL or HCS solutions for two hours. A water deficit was created by stopping the watering of 7-day-old plants for the next two weeks. Plants responded to drought through growth inhibition, impaired water status, increased lipid peroxidation, differential effects on antioxidant enzymes, intense proline accumulation, altered expression of genes involved in metabolism, and decreased endogenous contents of hormones (28-homobrassinolide, B-ketones, and B-lactones). Pretreatment of plants with HBL reduced the inhibitory effect of drought on fresh and dry biomass accumulation and relative water content, whereas HCS partially reversed the negative effect of drought on fresh biomass accumulation, reduced the intensity of lipid peroxidation, and increased the osmotic potential. Compared with drought stress alone, pretreatment of plants with HCS or HBL followed by drought increased superoxide dismutase activity sevenfold or threefold and catalase activity (by 36%). The short-term action of HBL and HCS in subsequent drought conditions partially restored the endogenous B-ketone and B-lactone contents. Thus, the steroidal phytohormones HBL and HCS increased barley plant resistance to subsequent drought, showing some specificity of action.

Heavy metals and aluminum are among the most significant abiotic factors that reduce the productivity and quality of crops in acidic and contaminated soils. The protective effects of brassinosteroids containing lactone are relatively well-studied under heavy metal stress, but the effects of brassinosteroids containing ketone are almost unstudied. Moreover, there are almost no data in the literature on the protective role of these hormones under polymetallic stress. The aim of our study was to compare the stress-protective effects of lactone-containing (homobrassinolide) and ketone-containing (homocastasterone) brassinosteroids on the barley plant’s resistance to polymetallic stress. Barley plants were grown under hydroponic conditions; brassinosteroids, increased concentrations of heavy metals (Mn, Ni, Cu, Zn, Cd, and Pb), and Al were added to the nutrient medium. It was found that homocastasterone was more effective than homobrassinolide in mitigating the negative effects of stress on plant growth. Both brassinosteroids had no significant effect on the antioxidant system of plants. Both homobrassinolide and homocastron equally reduced the accumulation of toxic metals (except for Cd) in plant biomass. Both hormones improved Mg nutrition of plants treated with metal stress, but the positive effect on the content of photosynthetic pigments was observed only for homocastasterone and not for homobrassinolide. In conclusion, the protective effect of homocastasterone was more prominent compared to homobrassinolide, but the biological mechanisms of this difference remain to be elucidated.