The phytoplasma effector, phyllogen, structurally mimics host plant MADS transcription factors.

Phytoplasmas are obligate plant pathogens that include species causing severe developmental abnormalities in diverse plant families. Phytoplasma infections may result in dwarfism, witches' broom, virescence, and phyllody, the conversion of reproductive organs into leaf-like structures. Phyllody is due to the activity of phytoplasma effector proteins, called phyllogens, that interact with the host MADS transcription factor (MTF) network. This interaction was believed to be specific for members of the AP1/SEP/AGL6 superclade of tetrameric MTFs, key regulators of floral meristem identity and organogenesis. Here, we determined the molecular and atomic basis for the interaction between the phyllogen from onions yellow phytoplasma (PHYLOY) and the floral MTFs, APETALA1 (AP1), SEPALLATA3 (SEP3), and AGAMOUS (AG) using biochemical, structural, and genetic experiments. The DNA-binding patterns of homo and heteromeric MTFs with PHYLOY are shown using electrophoretic mobility shift assays, revealing broader interaction patterns between PHYLOY and MTFs that depend on highly conserved hydrophobic residues. Point mutations of these residues allowed gain or loss of interaction between MTFs and PHYLOY. Expressing PHYLOY under the control of the SEP3 and AP1 promoter in Arabidopsis further demonstrated whorl-specific phenotypes, recapitulating the in vitro interaction and DNA-binding patterns. These findings reveal how PHYLOY targets specific MTFs through structural mimicry, resulting in impaired MTF tetramer formation and homeotic conversion of floral organs due to loss of active MTF tetramers.