Apple trees are able to restrict the phytoplasma infection via
phytohormone signalling.
Besides the direct and local defence mechanisms, an activation and
systemic distribution of signalling compounds such as phytohormones via
the SEs, may be induced by phytoplasmas and may have an impact on the
plant’s defence. Moreover, typical symptoms, such as development of
witches’ broom, smaller fruits, reduced leaf and vascular morphology of
diseased apple trees, can be explained with an infection-induced
imbalance of phytohormones. In apple and peach trees, SA and JA-Ile
levels significantly increased in infected trees, indicating the
involvement of defence pathways to phytoplasma colonization.
Furthermore, the content of ABA in apple leaves increased as well.
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 of SA in apple treesafterCa. 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 more 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 reducing 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 compounds 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 yet been proven. However, infestations of Citrusplants with Asian citrus psyllid (ACP, Diaphorina citri ) led 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 Candidatus Liberibacter asiaticus
induced 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) were 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
concentrations might be responsible for abnormal growth of infected
peach trees (Figure 1c).
Overall, it has to be considered that such measurements merely represent
a snapshot of time. Indeed, various 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
titres are needed to draw reliable conclusions about symptom
development.