Two coupled components of the mitogen-activated protein kinase cascade MdMPK1 and MdMKK1 from apple function in ABA signal transduction.

Plant mitogen-activated protein kinase (MAPK) cascades are involved in a range of biotic and abiotic stress responses, but many members of the MAPK family involved in signal transduction of the stress-related hormone ABA remain to be identified and how they regulate ABA signaling is still unclear. Here we characterized biochemically an apple MAPK signaling cascade MdMKK1-MdMPK1, which is transiently activated by ABA. Expression of MdMKK1 or MdMPK1 in the reference plant Arabidopsis (Arabidopsis thaliana) confers ABA hypersensitivity in both seed germination and seedling growth, showing that MdMKK1 and MdMPK1 are positively involved in ABA signaling. Expression of MdMKK1 or MdMPK1 up-regulates expression of several ABA-responsive transcription factor-encoding genes including ABI5. Furthermore, MdMPK1 phosphorylates the Arabidopsis ABI5 protein through the unique residue Ser314, showing that ABI5 is a potential direct downstream component of MAPK in ABA signaling. These findings indicate that the apple MdMKK1-MdMPK1-coupled signaling cascade may function in ABA signaling by regulating both expression and the phosphorylation status of the important ABA signaling component ABI5 or ABI5-like transcription factors.