Moser, Mirko

Abstract Background: 'Candidatus Phytoplasma mali', the causal agent of apple proliferation disease, exerts influence on its host plant through various effector proteins, including SAP11CaPm which interacts with different TCP transcription factors. This study examines the transcriptional response of the plant upon early expression of SAP11CaPm. For that purpose, leaves of Nicotiana occidentalis H.-M. Wheeler were Agrobacterium-infiltrated to induce transient expression of SAP11CaPm and changes in the transcriptome were recorded until 5 days post infection. Results: The analysis revealed that presence of SAP11CaPm in leaves leads to downregulation of genes involved in defense response and related to photosynthetic processes, while expression of genes involved in metabolic pathways was enhanced. Conclusions: The results indicate that early SAP11CaPm expression might be important for the colonization of the host plant since phytoplasmas lack many metabolic genes and are thus dependent on metabolites from their host plant.

Phytoplasmas are the causative agent of numerous diseases of plant species all over the world, including important food crops. The mode by which phytoplasmas multiply and behave in their host is poorly understood and often based on genomic data. We used yeast two-hybrid screening to find new protein–protein interactions between the causal agent of apple proliferation ‘Candidatus Phytoplasma mali’ and its host plant. Here, we report that the ‘Ca. P. mali’ strain PM19 genome encodes a protein PM19_00185 that interacts with at least six different ubiquitin-conjugating enzymes (UBC; E2) of Arabidopsis thaliana. An in vitro ubiquitination assay showed that PM19_00185 is enzymatically active as E3 ligase with A. thaliana E2 UBC09 and Malus domestica E2 UBC10. We show that a nonhost bacteria (Pseudomonas syringae pv. tabaci) can grow in transgenic A. thaliana plant lines expressing PM19_00185. A connection of phytoplasma effector proteins with the proteasome proteolytic pathway has been reported before. However, this is, to our knowledge, the first time that a phytoplasma effector protein with E3 ligase activity has been reported.