Transcriptional regulation of BZR1/PIF4 is involved in high temperature-induced early flowering process in Oncidium Grower Ramsay

Prolonged exposure to elevated temperatures has induced early flowering in Oncidium hybrid orchids by activating the ascorbate–glutathione (AsA–GSH) cycle. However, the intricacies of the phase transitions in response to thermal treatment remain ambiguous. Through transcription profiling, we identified Oncidium BRASSINAZOLE-RESISTANT 1 (BZR1), OgBZR1, and Oncidium PHYTOCHROME INTERACTING FACTOR-4 (PIF4), OgPIF4, whose expression levels increased with high-temperature treatment. BZR1, a pivotal brassinosteroid (BR) signaling regulator, has been proposed to interact directly with PIF4 to mediate BR and high-temperature responses. This study aimed to elucidate the involvement of OgBZR1 in the thermal activation of the early flowering process in Oncidium. The exogenous application of ep-brassinolide (ep-BL) effectively induced the expression of the flowering integrator genes FLOWERING LOCUS T (OgFT) and LEAFY (OgLFY) and activated the AsA–GSH cycle, resulting in an early flowering process. Subsequently, overexpression of OgBZR1 triggered early flowering in wild-type Arabidopsis plants but failed to induce early flowering in PIF4 quadruple mutants (pifq), suggesting that OgBZR1 may interact with OgPIF4 to promote early flowering in Arabidopsis. Furthermore, molecular evidence from yeast-2-hybrid assay, bimolecular fluorescent complementation analysis, and co-immunoprecipitation revealed an interaction between OgBZR1 and OgPIF4 proteins, supporting their synergistic and cooperative regulation in the thermally induced Oncidium flowering pathway. Thus, we propose that OgBZR1, in conjunction with OgPIF4, synergizes with AsA–GSH redox alterations to facilitate early flowering in Oncidium plants.