New Abscisic Acid Derivatives Revealed Adequate Regulation of Stomatal, Transcriptional, and Developmental Responses...

The phytohormone abscisic acid (ABA) plays an important role in plant stress response, mainly against desiccation. Hence, ABA receptor agonists may function as agents to enhance drought tolerance in crops. ABA exhibits diverse functions that impact plant development and are regulated by various ABA receptor subfamilies. Indeed, we previously reported that 3'-alkyl ABAs exhibit diverse receptor specificities and that 3'-butyl ABA induced a drought stress response without eliciting growth inhibitory effects in Arabidopsis seedlings. Thus, to further investigate plant responses induced by 3'-butyl ABA, as well as the receptors that control the opposing stress and growth responses, we designed new 3'-alkyl ABA derivatives. In addition to the 3'-alkyl chain, a cyclopropyl group was attached to position 3 of ABA to occupy the C6 cleft in the ABA-binding pocket of the receptors, which served to increase the binding affinity and specificity to a certain receptor set. Additionally, the inhibitory activity of pyrabactin resistance 1 (PYR1) and PYR1-like (PYL1) proteins against type 2C protein phosphatase increased following incorporation of the 3-cyclopropyl group in all tested 3'-alkyl ABAs. Interestingly, 3'-butyl ABA induced the highest tolerance against drought stress, compared with 3-cyclopropyl derivatives. To investigate the molecular mechanism underlying the effects elicited by different chemical treatments, those of ABA derivatives on stomatal closure, growth, and gene expression were studied. Evaluation of the receptors activated by ABA derivatives and the plant responses revealed the induction of PYR1, PYL1, PYL2, and PYL5, mediated stomatal closure, and regulated transcription, consequently leading to drought tolerance in plants.