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.