Ability of Lactone- and Ketone-Containing Brassinosteroids to Induce Priming in Rapeseed Plants to Salt Stress

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.