Synthesis and bioactivity of brassinosteroid borates

A regioselective synthesis of boric acid esters derived from 24-epibrassinolide and 24-epicastasterone has been developed. The esterification proceeds under mild conditions with high efficiency, yielding the corresponding 22,23-borates of steroidal phytohormones as the sole products. Their plant growth-stimulating activity was evaluated using spring wheat (Triticum aestivum L.) as a model system. The compounds were applied via seed soaking in solutions at concentrations of 10^–7 and 10^–9 M for 24 hours. Germination rates were assessed after 7 days, while seedling height, root length, and fresh weight of both seedlings and roots were measured 10 days after the start of the treatment. The synthesized conjugates were found to enhance seed germination and stimulate the growth of above-ground parts (by 10–15 % relative to the control), as well as increase biomass accumulation (by 13–23 %) and root weight (by 8–22 %). These novel brassinosteroid derivatives (borates) exhibited higher plant growth-stimulating activity than the native hormones themselves.

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