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A tale of how phytoplasma effectors alter plant-pathogen-insect interactions: It's a mad MADS world.
A. M. MACLEAN (1), Z. Orlovskis (1), A. Sugio (1), H. Kingdom Gibbard (1), S. A. Hogenhout (1). (1) John Innes Centre, Norwich, United Kingdom

Pathogens that rely upon multiple hosts to complete their life cycles often modify behavior and development of these hosts to coerce them into improving pathogen fitness. Host coercion is particularly important for obligate biotrophs that are reliant upon their hosts for survival. Phytoplasma are obligate phytopathogens that are transmitted by phloem-feeding insects such as leafhoppers. We wish to elucidate the mechanisms by which Aster Yellows phytoplasma strain Witches' Broom (AY-WB) modulates the development of plants to make these more attractive to the AY-WB insect vector <i>Macrosteles quadrilineatus</i>. We identify two novel AY-WB effector proteins - SAP11 and SAP54 - that target highly conserved host processes to transform phytoplasma-infected plants into preferred sites for leafhopper ovipositioning. We show that SAP11 acts to destabilize members of the TCP family of transcription factors, thereby suppressing jasmonate-mediated defence against insects. In contrast, SAP54 initiates the degradation of MADS-domain transcription factors, transforming fertile flowers into sterile leaf-like structures. <i>M. quadrilineatus </i>produce an increased number of nymphs on SAP11- and SAP54-expressing Arabidopsis, indicating that these effectors have the ability to modulate plant-insect interactions. Thus, an obligate bacterial pathogen coerces the development of plants to influence the behaviour of its insect vector, maximizing the transmission of AY-WB to new hosts.

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