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 âLandwirtschaftliche
Rentenbankâ number 28RF4IP008.