Brassinosteroids Suppress Ethylene Biosynthesis via Transcription Factor BZR1 in Pear and Apple Fruit

The plant hormone ethylene is important for the ripening of climacteric fruit, such as pear (), and the brassinosteroid (BR) class of phytohormones affects ethylene biosynthesis during ripening, although via an unknown molecular mechanism. Here, we observed that exogenous BR treatment suppressed ethylene production during pear fruit ripening, and that the expression of the transcription factor was enhanced by epibrassinolide (EBR) treatment during pear fruit ripening. PuBZR1 was shown to interact with PuACO1, which converts 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene, and suppress its activity. We also observed that BR-activated PuBZR1 bound to the promoters of and of , which encodes ACC synthase, and directly suppressed their transcription. Moreover, PuBZR1 suppressed the expression of transcription factor by binding its promoter, and PuERF2 bound to the promoters of and . We concluded that PuBZR1 indirectly suppresses the transcription of and through its regulation of PuERF2. Ethylene production and the expression profiles of the corresponding apple () homologs showed similar changes following EBR treatment. Together, these results suggest that BR-activated BZR1 suppresses ACO1 activity and the expression of and , thereby reducing ethylene production during pear and apple fruit ripening. This likely represents a conserved mechanism by which exogenous BR suppresses ethylene biosynthesis during climacteric fruit ripening.