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).