Phytoplasmas are insect‐transmitted phytopathogenic bacteria, which secrete effector proteins that are often responsible for altering the plant morphology and behaviours of their vectors. Phytoplasma multifunctional effector proteins TENGU and SAP11 induce typical witches’ broom symptoms, but their mode of action remains unknown. Previously, we have identified a SAP11‐like effector from wheat blue dwarf phytoplasma, SWP1, which induces witches’ broom symptoms in Nicotiana benthamiana
. In this study, we observed that SWP1
‐expressing transgenic Arabidopsis thaliana
plants showed typical witches’ broom symptoms. On overexpression of SWP1 truncation mutants in N. benthamiana
, we identified that the coiled‐coil domain and nuclear localization were responsible for the induction of witches’ broom symptoms. In addition, using yeast two‐hybrid and bimolecular fluorescence complementation assays, we demonstrated that SWP1 interacts with A. thaliana
transcription factor TCP18 (BRC1), the key negative regulator of branching signals in various plant species. Moreover, in planta
co‐expression analysis showed that SWP1 promotes the degradation of BRC1 via a proteasome system. These findings suggest that the phytoplasma effector SWP1 induces witches’ broom symptoms through targeting of BRC1 and promoting its degradation.