Thesis presented March 04, 2022
Abstract: Phytoplasmas are obligate plant pathogenic bacteria that spread by insects and infect plant phloem cells. Phytoplasma infections can result in witch’s broom, dwarfism and phyllody, the homeotic conversion of floral organs to leaf-like structures. The economic loss due to phytoplasma infections of crop plants is already very important and there are no effective treatments. Thus, understanding how phytoplasmas interact with the host plant and rewire developmental pathways is key to both understanding the phytoplasma-induced phenotypes and developing strategies and successful treatments for phytoplasma-infected plants.
Recent studies have demonstrated that phytoplasmas produce virulence factors responsible for phyllody, called phyllogens. Phyllogens are small alpha-helical proteins that interact with host plant MADS transcription factors (TFs). MADS TFs form different heteromeric tetramers and each tetramer regulates a distinct set of target genes important for flowering and floral organ development. Phyllogen binding to MADS TFs results in the proteasomal degradation of the host MADS TFs and prevents the formation of flowers.
How phyllogens interact with only a subset of floral MADS TFs is unclear. To understand this, we studied the homocomplexes and heterocomplex of the MADS TFs, SEPALLATA3 and AGAMOUS, and their interactions with the phyllogen, PHYL1OY of “
Candidatus Phytoplasma asteris” Onion Yellow’s strain. Using a combination of
in vitro techniques, in vivo assays and structural studies, we have determined that PHYL1OY targets the tetramerization interface of the MADS TFs. The monomer PHYL1OY binds to a dimer of MADS proteins
via the hydrophobic surface needed for tetramerization and requires the presence of highly conserved leucine residues present in a subset MADS TFs. These experiments define the protein-protein interactions necessary for PHYL1OY interactions with MADS TFs and demonstrate the elegant use of structural mimicry by a bacterial protein.
Keywords:
MADS transcription factors, Phyllogen, Protein interaction, SEPALLATA3, PHYL1OY, Floral organogenesis
On-line thesis.