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.