Conclusion
In the three investigated fruit crops the infection with specific phytoplasmas induced different morphological and physiological responses in the particular host plants. As apple trees generally survive a phytoplasma infection more often and much longer than peach and pear, some unique apple-specific responses are the most interesting and indicative features that could explain how a plant might become tolerant against a phytoplasma. Based on the results obtained, the long-lasting changes in the structure of the vascular system with all physiological consequences on the sap flow found in apple trees provides a promising step towards a deeper understanding of host plant defence against phytoplasma. In spite of the growing understanding of this pathosystem, it seems clear that the complexity of these interactions is not fully elucidated yet, and many open questions remain: Does the plant perceive a phytoplasma infection at all? If so, what does the plant recognize? Is there a MAMP/DAMP/effector present that induces an increased defence response in the SEs? What are the specific events during infections in the host in terms of time, place and extent? How do the antagonists interact on the molecular level? All these questions implicate more investigation on the molecular level and strongly suggest approaches such as RNAseq and transgenic approaches.
Acknowledgements: We thank Sebastian Faus and Katharina Piwowarczyk (JKI, Dossenheim, Germany) for excellent assistance in the laboratory. We thank Felix Hergenhahn (JKI, Dossenheim, Germany) for grafting and cultivation of the plants. We thank Andrea Lehr (MPI for chemical Ecology, Jena) for technical support. This work was supported by the Deutsche Forschungsgemeinschaft (Grant FU 969/2-1 to AF and MZ). Jannicke Gallinger was supported by a fund of the “Landwirtschaftliche Rentenbank” number 28RF4IP008.