The ‘Candidatus Phytoplasma mali’ effector protein SAP11CaPm interacts with MdTCP16, a class II CYC/TB1 transcription factor that is highly expressed during phytoplasma infection

‘ Candidatus Phytoplasma mali’, is a bacterial pathogen associated with the so-called apple proliferation disease in Malus × domestica . The pathogen manipulates its host with a set of effector proteins, among them SAP11 CaPm , which shares similarity to SAP11 AYWB from ‘ Candidatus Phytoplasma asteris’. SAP11 AYWB interacts and destabilizes the class II CIN transcription factors of Arabidopsis thaliana , namely AtTCP4 and AtTCP13 as well as the class II CYC/TB1 transcription factor AtTCP18, also known as BRANCHED1 being an important factor for shoot branching. It has been shown that SAP11 CaPm interacts with the Malus × domestica orthologues of AtTCP4 (MdTCP25) and AtTCP13 (MdTCP24), but an interaction with MdTCP16, the orthologue of AtTCP18, has never been proven. The aim of this study was to investigate this potential interaction and close a knowledge gap regarding the function of SAP11 CaPm . A Yeast two-hybrid test and Bimolecular Fluorescence Complementation in planta revealed that SAP11 CaPm interacts with MdTCP16. MdTCP16 is known to play a role in the control of the seasonal growth of perennial plants and an increase of MdTCP16 gene expression has been detected in apple leaves in autumn. In addition to this, MdTCP16 is highly expressed during phytoplasma infection. Binding of MdTCP16 by SAP11 CaPm might lead to the induction of shoot proliferation and early bud break, both of which are characteristic symptoms of apple proliferation disease.