Brassinosteroids (BRs) are plant steroid hormones known mainly for promoting organ growth through their combined effect on cell expansion and division. In addition, BRs regulate a broad spectrum of plant developmental and physiological responses, including plant architecture, vascular differentiation, male fertility, flowering, senescence, photomorphogenesis and tolerance to biotic and abiotic stresses. Recently, a complete core BR signaling pathway was defined in which BR signals are conveyed from the cell surface to the nucleus through sequential signaling modules. A major challenge now is to understand precisely how this signaling pathway controls the different BR-regulated actions. The current identification of direct targets of BRASSINAZOLE-RESISTANT1 (BRZ1) and BR-INSENSITIVE-EMS-SUPPRESSOR1 (BES1)/BZR2 transcription factors suggests that BR signaling pathway controls growth and interacts with other signaling pathways mainly at the transcriptional level.
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