24-epibrassinolide effects on in vitro callus tissue formation, growth, and regeneration in wheat varieties with contrasting drought resistance

Effects were investigated of kinetin replacement on 24-epibrassinolide (24-EB) in the in vitro cultured embryonic explants that were obtained from two varieties of spring soft wheat (Triticum aestivum L.)— Bashkirskaya 26 (drought resistant) and Salavat Yulaev (weakly resistant), differing in drought resistance, on the calli formation, its growth indices, contents of ABA and cytokinins, morphological and histological parameters, as well as their regenerative capacity. The resistant Bashkirskaya 26 variety, in contrast to the Salavat Yulaev variety, was characterized by a significantly higher frequency of calli formation in the culture of immature embryos on the 24-EB induction medium, higher increase in fresh and dry weights, and a large number of morphogenetic centers. On the medium containing kinetin, the Salavat Yulaev calli were characterized by an increased level of ABA throughout the experiment with a maximum of 15–25 days, whereas, in the Bashkirskaya 26 calli, the maximum ABA accumulation occurred on the seventh to 11th day of cultivation, after which a decrease in the hormone content was observed. It was found that calli of both varieties cultivated on the 24-EB medium against the background of absence in the ABA content changes were characterized by an increased content of endogenous cytokinins, especially significant in Bashkirskaya 26 calli. Calli of both varieties were characterized by a high regenerative capacity in all the studied variants of the regeneration medium. At the same time, the maximum capacity for regeneration and formation of regenerants with a single callus were revealed by replacing kinetin with 24-EB, especially pronounced in a resistant variety. The combination of the results obtained demonstrates the efficacy of 24-EB introducing instead of kinetin into the in vitro culture medium for explants of two varieties of spring soft wheat that differ in drought resistance, as evidenced by an increase in the frequency of callus formation from immature embryos, as well as the number of morphogenetic centers in the resulting calli.