A glutathione–brassinosteroid–miR156 positive feedback loop becomes gradually attenuated during the vegetative phase change in apple

The vegetative phase change (VPC) is precisely controlled by microRNA156 (miR156) in flowering plants. We previously showed that mature miR156 is mainly produced from MdMIR156a5 in apple (Malus domestica Borkh.) and that MdMIR156a5 expression and miR156 abundance were positively correlated with glutathione (GSH) contents during the VPC. In this study, we identified three transcription factor genes co-expressed with MdMIR156a5: the REVEILLE member MdRVE1-1, PHYTOCHROME-INTERACTING FACTOR 4 (MdPIF4), and BES1/BZR1 HOMOLOG 4 (MdBEH4). MdRVE1-1 activated MdMIR156a5 transcription by directly binding to its promoter, whereas MdBEH4 and MdPIF4 bound to the MdRVE1-1 promoter to indirectly promote MdMIR156a5 expression. MdBEH4 also directly induced MdPIF4 expression. Application of the brassinosteroid (BR) epibrassinolide or the GSH precursor 2-oxothiazolidine-4-carboxylate elevated BR and GSH contents, increased expression of MdBEH4, MdPIF4, MdRVE1-1, MdMIR156a5, and BR and GSH biosynthesis genes, and increased miR156 abundance, indicative of reciprocal promotion of GSH and BR biosynthesis and MdMIR156a5 expression. Overexpression of MdMIR156a6 in apple plants resulted in higher BR and GSH contents, higher expression of MdBEH4, MdPIF4, MdRVE1-1, MdMIR156a5, and BR and GSH biosynthesis genes, and higher miR156 abundance, defining a GSH-BR-miR156 positive feedback loop that becomes gradually attenuated during the VPC. These results establish that the MdBEH4-MdPIF4-MdRVE1-1-MdMIR156a5 module regulates miR156 abundance by integrating GSH and BR signaling pathways during the VPC in apple.