The role of hormonal system and lectin in the protective effect of 24-epibrassinolide on wheat (Triticum aestivum L.) seedlings differing in drought adaptation strategies under water deficit

A major challenge of modern plant science is unravelling the mechanisms of plant tolerance to drought which leads to disruption of all metabolic events and loss of productivity of plants. Two ecologo-geographical groups (ecotypes) of wheat have formed in the Russian Federation, which differ significantly in their adaptation strategies to drought - the forest-steppe West Siberian and steppe Volga ecotypes. The differences between ecotypes are especially characteristic in the initial stage of ontogenesis. The cultivars of the forest-steppe West Siberian ecotype are characterized by delayed germination and seedling growth. In contrast, seedlings of the steppe Volga ecotype develop rapidly and are more tolerant in comparison with plants of the forest-steppe West Siberian ecotype in the early stage of development. Growth regulators play an important role in regulation of drought tolerance in plants. There has been increasing amount of data demonstrating the involvement of steroidal phytohormones brassinosteroids, characterized by a combination of pronounced growth-stimulating and protective effects, in the regulation of wheat tolerance to water deficit. In this work, for the first time it was revealed the protective effect of 24-epibrassinolide (EBR) on wheat plants of the steppe Volga ecotype to drought, which is based on the mitigation of rapid stress-induced shifts in the content of endogenous phytohormones and the accumulation of protective protein wheat germ agglutinin (WGA). At the same time, the protective effect of EBR on plants of the forest-steppe West Siberian ecotype was not detected, which was due to preservation of stress-induced imbalance of the endogenous hormonal system and insignificant wheat germ agglutinin (WGA) accumulation. The aim of this work is to establish the role of changes in the hormonal system and the content of wheat lectin in the protective effect of 24-epibrassinolide on wheat plants of Zauralskaya Zhemchuzhina (forest-steppe West Siberian ecotype) and Ekada 70 (steppe Volga ecotype) cultivars in the initial stage of their ontogenesis under conditions of simulated soil drought. The work was conducted in 2024. Wheat seeds were pre-soaked for 3 hours in a solution of 0.4 µm of 24-epibrassinolide or distilled water (control) and sown on a soil in 15 liter pots (30 seeds per pot) and grown under controlled conditions. Then, some of the 3-day-old seedlings were exposed to early soil drought, which was modeled by limited watering until the soil moisture dropped to 30 % of the total water retention capacity of soil. In the control pots, the soil moisture was maintained at 70 % of the total water retention capacity of soil. Physiological and biochemical parameters were analyzed at the initial stage of wheat development (7-9 days). Growth was estimated by changes in the fresh and dry weights of untreated and EBR-pretreated seedlings under normal and drought conditions. The contents of abscisic acid (ABA), indolylacetic acid (IAA), cytokinins (CK) and WGA in the same 10 seedlings (0.9-1.0 g of fresh weight) on 7, 8 and 9 days were determined by enzyme immunoassay (ELISA) using specific to the studied phytohomones or WGA polyclonal rabbit antibodies and peroxidase-labeled anti-rabbit antibodies. Damage to the cell membranes of seedlings was assessed by the content of malondialdehyde (MDA), the end product of lipid peroxidation, on 9th day. It was revealed that the content of endogenous phytohormones in seedlings of Ekada 70 and Zauralskaya Zhemchuzhina cultivars underwent significant changes under drought. At the same time, more significant changes in the hormonal balance were found in the seedlings of the Zauralskaya Zhemchuzhina cultivar, which was reflected in a stronger inhibition of their growth. In conditions of drought, plants of the Ekada 70 variety were distinguished by an earlier and significantly higher accumulation of WGA (by 86% more than Zauralskaya Zhemchuzhina seedlings), which may indicate the involvement of wheat lectin in the formation of their drought tolerance. Pre-sowing EBR treatment mitigated the negative effect of drought on the growth and hormonal status of Ekada 70 seedlings during their germination under stress, which, however, was not typical for plants of the Zauralskaya Zhemchuzhina cultivar. At the same time, EBR pretreatment reduced by 30 % the stress-induced accumulation of WGA in Ekada 70 plants, which may indicate a decrease in the stress pressure on hormone-treated seedlings of this cultivar. This is supported by data on a decrease in the amount of malondialdehyde (MDA) by 26 % in the tissues of EBR-pretreated plants of the steppe Volga ecotype under drought. However, in the Zauralskaya Zhemchuzhina cultivar, the protective effect of EBR pretreatment on the content of WGA and membrane structures under stressful conditions was almost not expressed.