Brassinosteroid and brassinosteroid-mimic differentially modulate Arabidopsis thaliana fitness under drought

Brassinosteroids (BRs) are widely used to promote plant growth/development and alleviate environmental stresses' adverse effects. However, its low stability in the field precludes large-scale application, challenging research, and more stable and cost-effective analogues. The most commonly used is 24-Epibrassinolide (EBL), yet, due to its high production cost, the study of cheaper molecules with similar/higher activity constitutes a priority. In this study, we analyzed, under drought, the effects of EBL and DI-31, a synthetic functional analogue, through a physiological and biochemical approach in Arabidopsis thaliana wild-type plants. Additionally, differential BRs/ABA interactions were detected and further analyzed in abscisic acid (ABA) mutants, assays in stomata with ABA-closure inhibitors, and analysis of ABA-stress-responsive genes expression via qRT-PCR. Similar to EBL, DI-31 induced dose-responsive growth and stomatal closure curve. Compared to EBL, DI-31 induced oxidative burst in a stronger but delayed manner; and increased biomass and foliar area under drought, preventing more effectively the relative water content fall under stress. Although both, EBL and DI-31, enhanced drought-response, the DI-31 action was more effective, durable, and differed in regulating several ABA/stress-response indicators. DI-31/ABA interactions under drought were confirmed in ABA-mutants, where the analogue compromised the activation of ABA-regulated proteins. Moreover, the analogue mediates stomatal closure through paths partially alternative to the ABA-controlled and specifically repressed stress-responsive genes regulated by AREB/ABF transcriptional factors. These findings confirm the DI-31 practical value as growth-promoter and defence-enhancer, with stronger and longer-term activity than EBL, constituting an environmentally-friendly and cost-effective alternative to increase plant fitness under drought, precluding large biomass penalty.