Apple is able to restrict the phytoplasma infection via phytohormone signalling.
In apple and peach trees, SA and JA-Ile levels significantly increased in infected trees, indicating the involvement of defence pathways to phytoplasma colonization. The content of ABA in apple leaves also increased. Commonly, SA plays the central role for the interaction between biotrophic pathogens and host plants (Ma & Ma, 2016; Robert-Seilaniantz et al. , 2011) and an increase after Ca.P. mali infection was found earlier (Zimmermann et al., 2015). In contrast, the jasmonic acid pathway is induced in response to wounding, herbivore attack and necrotrophic pathogens (Heil & Ton, 2008). The development of different pathways in reaction to different threads enables plants to respond more specifically and is therefore resource-efficient. An antagonistic crosstalk between JA/ABA and SA was detected in several plant species (Flors et al. , 2005; Zimmermann et al., 2015). Not surprisingly, some bacteria species evolved the production of effector proteins that interfere with SA regulated defence responses by activating JA pathway (Chisholm et al. , 2006). This mechanism was also detected for phytoplasmas in Aster yellows-witches broom phytoplasma (AY-WB). AY-WB produces the SAP11 effector that down-regulates the plant defence response by lipoxygenase2 expression and JA production (Sugio et al. , 2011b). This down-regulation of defence mechanisms in AY-WB infected plants is advantageous to vector fitness (Sugio et al. , 2011b). Recently, an SAP11-like protein was detected in ‘Ca. P. mali’ that affected JA, SA and ABA pathways (Siewert et al. , 2014; Janik et al. , 2017).
Jasmonic acid plays a central role in induced plant defence e.g. by regulating the biosynthesis of herbivore-induced plant volatiles (Heil & Ton, 2008). Moreover, exogenous application of JA can be used to elicit plant defence responses similar to those induced by biting-chewing herbivores and mites that pierce cells and consume their contents. A low-dose JA application results in a synergistic effect on gene transcription and an increased emission of a volatile compound involved in indirect defence after herbivore infestation (Menzel et al., 2014). The induction of JA defence mechanisms in apple, pear and peach in response to psyllid feeding has not been proven yet. But infestations of Citrus plants with Asian Citrus Psyllid (ACP, Diaphorina citri ) lead to an upregulation of genes involved in the JA-pathway (Nehela et al. , 2018). Additionally, the infection of Citrus trees with the phloem dwelling proteobacterium CandidatusLiberibacter asiaticus induce the SA-pathway (Nehela et al. , 2018) and resulted in an increased emission of methyl salicylate from infected plants (Martini et al. , 2018).
Auxins (IAA and IBA) are shown to induce the recovery of periwinkle plants from ‘Ca. P. pruni’ and ‘Ca. P. asteris’ infections (Curković Perica, 2008), illustrating the importance of IAA in plant-pathogen interactions. Interestingly, the IAA concentration in infected P. persica plants was significantly lowered compared to healthy peach trees. A reduced auxin content was also detected in leaves of lime infected with ‘Ca. P. aurantifoliae’ (Zafari et al. , 2012). Imbalanced auxin concentration could be responsible for abnormal growth of infected peach trees (Figure 1c).
Overall, we have to consider that measurements are always just a snapshot of time. Different results from phytohormone analysis in AP-infected apple plants are reported in the literature (Janik et al., 2017; Zimmermann et al. , 2015), indicating that reactions to phytoplasma infections depend on season, cultivar and environmental conditions. Consequently, exact phytohormone and phloem sap analysis over a longer period (season) in correlation to respective phytoplasma titers are needed to draw reliable conclusions about symptom development.