The pear and peach phloem reactions are very sensitive to
phytoplasma infections whereas apple has arranged with the infection
All observed results regarding the particular morphological (Figures 1
to 4) and functional measurements (Figure 5) illustrate well the
consequences of a phytoplasma infection for a plant: They are
heterogeneous and specific and depend on the individual host-pathogen
interaction. One reason for the heterogeneity might be found in plant
defence responses. A fundamental defence response to several (a)biotic
stresses is an elevated Ca2+ dependent deposition of
callose and was already reported for phytoplasma infections (Chen &
Kim, 2009; Musetti et al., 2013). We could show that P. communisand P. persica trees reacted to phytoplasma infections with
blocking of sieve plates with callose. Phytoplasma effectors may cause
gating of Ca2+ channels leading to sieve-tube
occlusion with dramatic effects on photoassimilate distribution as
indicated by the reduced volumetric flow rate in P. persicatrees. Surprisingly, the mass flow of infected P. communis trees
was increased, by a simultaneous increase of phloem sap viscosity, which
reflects an increased sugar content. The reason has to be an increased
pressure gradient (~6.5 bar) of infected trees, which
drives the mass flow against the resistance. P. communis trees
have to bring a major effort with increased energy supply that result at
the end in die back. In contrast, the infection with ’Ca . P.
mali’ did not lead to an increased callose deposition in apple trees.
This might be due to apple cultivar, phytoplasma strain specific
mechanisms or an evolutionary adaptation to the phytoplasma infection.
The callose deposition in response to phytoplasma infections never
results in a restriction of the bacteria and therefore is only a costly
non-functional leftover of general defence mechanisms. The
apple-phytoplasma interaction is maybe the oldest of our three
investigated interactions where both partners coexist without destroying
each other. If the callose deposition is directly or indirectly induced
by phytoplasmas is an issue for prospective surveys.
Callose deposition is also a defence mechanism against phloem-feeding
and is induced by phloem feeding insects (Hao et al. , 2008; Will
et al., 2013). Therefore, callose concentrations are of great importance
for phloem-feeding vector insects of phytoplasmas. The occlusion of
sieve tubes inhibits the phloem flow and the feeding of piercing-sucking
insects on the phloem tissue of host plants (Will et al. , 2009).
Furthermore, the brown plant hopper Nilaparvata lugens is able to
overcome this plant defence by activating and secreting a hydrolysing
enzyme, which induces the degradation of callose in SEs (Hao et
al. , 2008). Whether psyllid species transmitting AP, PD and ESFY
(AP: C. picta ; PD: C. pyri, C. pyrisuga and C.
pyricola ; ESFY: C. pruni ) have evolved such mechanisms to
overcome this particular plant defence is unknown. Nevertheless, it was
shown that phloem ingestion of C. pruni was not influenced by
phytoplasma infection of its host plants (P. persica and P.
insititia ), indicating that callose deposition in infected peach plants
does not affect vector feeding (Gallinger & Gross, 2020).
In general, sugars (e.g. sucrose) are known to stimulate feeding of
phloem-feeding insects, such as aphids (Arn & Cleere, 1971; Mittler &
Dadd, 1963). Thus, the detected higher sugar concentration in infected
pear phloem could increase probing and feeding behaviour of psyllids and
increase the acquisition and spread of phytoplasmas in pear orchards.
However, recently a detailed phloem composition analysis ofPrunus trees revealed no major differences in the phloem
metabolite composition between ESFY infected and healthy trees
(Gallinger & Gross, 2020). Therefore, to study whether or not the
concentration of micronutrients in the phloem of diseased apple and pear
trees is affected by phytoplasma colonization is a goal of our future
work.