Phytoplasma infections affect the vascular morphology of apple
trees more than peach and pear trees.
Although apple trees survive phytoplasma infection for decades
(Seemüller et al., 2018), various significant reductionsin size were
found in leaves (width, length, midrib), tissues (vascular bundle,
phloem and xylem) and cells (sieve elements) when compared to
non-infected plants (Figures 1 and 2). In contrast, pear and peach trees
showed less significant differences between healthy and
phytoplasma-infected leaves; if any, we found significant increases for
leaf size and midrib ratio for peach and leaf width for pear (Table 3).
That seems surprising as it could be expected that plants with a higher
tolerance and survival rate would show a lower rate of symptoms than
plants demonstrating a higher mortality (Marcone & Rao 2019).
Additionally, the recovery phenomenon, describing the remission of
symptoms and the disappearance of phytoplasmas in the crown, was
observed for both, apple and apricot trees (Prunus armeniaca ),
but not for pear trees (Carraro et al., 2004; Musetti et al., 2013).
Hence, the morphological and physiological changes can be considered for
representing the ability to handle a phytoplasma infection and might be
the result of a selective adaptation. Intraspecific comparisons of these
parameters between plant genotypes of different sensitiveness to the
same phytoplasma isolate could be used in future studies to confirm or
refuse this hypothesis. In particular, the P.
communis /‘Ca. P. pyri´–system is of high scientific interest,
as disease severity shows huge variance from mild symptoms, such as
premature foliar reddening, to severe growth depression and the quick
decline of infected trees (Seemüller et al., 1986).
The pear and peach phloem reactions are very sensitive to
phytoplasma infections whereas apple is coping 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: However, depending
on the individual host-pathogen system, they are heterogeneous between
the systems and specific within. One reason for the heterogeneity might
be found in the formation of plant defence. A general defence response
to several (a)biotic stresses is an elevated
Ca2+-dependent deposition of callose that was already
reported for phytoplasma infections (Chen & Kim, 2009; Musetti et al.,
2013). We were able to show that P. communis and P.
persica trees responded to phytoplasma infections by blocking sieve
plates with callose. Phytoplasma effectors may cause regulating of
Ca2+ channels, which leads to sieve-tube occlusion
with dramatic effects on photoassimilate distribution as indicated by
the reduced volumetric flow rate in P. persica trees.
Surprisingly, the mass flow of infected P. communis trees was
increased though a simultaneous increase of phloem sap viscosity, which
reflects an increased sugar content. The reason may be an increased
pressure gradient (we calculated ~6.5 bar using the
van-‘t-Hoff equation) in infected trees, which drives the mass flow
against the resistance of the SEs. Thus, P. communis trees have
to take major effort with increased energy supply, which eventually
causes negative feedback. As the callose deposition in response to
phytoplasma infections never results in a restriction of the bacteria it
suggests that callose deposition only is a costly non-functional
leftover of general defence mechanisms. Strikingly, the infection with
’Ca . P. mali’ in apple trees did not lead to an increased callose
deposition. This might be due to the particular apple cultivar,
phytoplasma strain, specific defence mechanisms or an evolutionary
adaptation to the phytoplasma infection. The apple-phytoplasma
interaction might be older and better adapted compared to peach and
pear. Whether the callose deposition is directly or indirectly induced
by phytoplasmas, is an issue for future studies.
Callose deposition also is 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 carrying phytoplasmas. The occlusion
of sieve tubes inhibits the phloem flow and affects the feeding of
piercing-sucking insects on the phloem tissue of host plants (Will et
al. , 2009). Nevertheless, the brown plant hopperNilaparvata 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
still unknown. Yet, 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, therefore, increase the acquisition and spread of phytoplasmas in
pear orchards. However, a recent detailed phloem composition analysis ofPrunus trees revealed no major differences in the phloem
metabolite composition between ESFY infected and healthy trees
(Gallinger & Gross, 2020).