Introduction
The fruit tree diseases apple proliferation (AP), pear decline (PD) and European stone fruit yellows (ESFY), are of high economic significance, causing annual crop losses of around half a billion Euro in Europe, alone (Eurostat 2009; Strauss, 2009). Intra- and interspecific differences in the response of fruit trees to these phytoplasma diseases have been observed over the last decades under both experimental and natural infection conditions (Fiore et al., 2019; Marcone & Rao, 2019). Intraspecific differences are explained by varying susceptibility of tree species and genotypes (rootstocks and cultivars) to phytoplasmas as well as virulence of phytoplasma strains (Kison and Seemüller, 2001; Koncz et al., 2017; Richter, 2002; Seemüller & Schneider, 2007; Seemüller et al., 1986). Fundamental differences between these plant-phytoplasma systems might be based on the degree of adaptation. However, only few studies provide firm data on host response, host–pathogen interaction and on anatomical, physiological and molecular basis of resistance (Seemüller & Harries, 2010), which, thus, is still poorly understood (Marcone & Rao, 2019). While the host plants of the 16SrX phytoplasmas belong to the Rosaceae, the causing agents of the diseases ’Candidatus Phytoplasma mali’, ’CandidatusPhytoplasma pyri’ and ’Candidatus Phytoplasma prunorum’ are also phylogenetically closely related and believed to be indigenous to Europe (Jarausch et al. , 2019a; Seemüller & Schneider, 2004). Phytoplasmas are very small bacteria without a cell wall. They have small linear chromosomes, lacking many genes that encode important metabolic functions such as amino and fatty acid synthesis (Kube et al., 2008; Oshima et al. , 2013). Therefore, they need to consume essential metabolites from their plant hosts.
Phytoplasmas are restricted to the phloem sieve elements in their host plants (Seemüller, 2002; Zimmermann et al., 2015). The phloem serves as main route for the long and short‐distance transport of mainly organic compounds (Hafke et al., 2005; van Bel, 1996). Sieve elements (SEs), companion cells (CCs) and phloem parenchyma cells (PPCs) are the three phloem cell types involved also in transport of defence- and stress related signalling molecules, such as RNA, proteins, and phytohormones (e.g. Dempsey & Klessig, 2012; Furch et al., 2014; Jung et al., 2009; Park et al., 2007). The sieve element sap is an energy-rich environment, sustaining phytoplasmas with nutrients and enabling them to distribute all over the plant. Therefore, upon phytoplasma infection the impairment of the phloem cells and the change in the phloem sap composition are most likely.
The distribution of secondary compounds plays a crucial role in plant communication and the induction of defence mechanisms against invading pathogens and attacking herbivores. It was previously shown that phytoplasmas produce and secrete effector proteins into phloem cells that induce physiological changes in infected host plants (Sugio et al. , 2011a). A number of non-specific symptoms, such as chlorosis, leaf yellowing, premature reddening, swollen leaf-veins, leaf curl and reduced vigor might be attributed to the impairment of the vascular system and the photosynthesis apparatus (Bertamini et al., 2002; Bertamini et al. , 2004; Maust et al., 2003). Additionally, abnormal growth, stunting, growth of witches’ brooms, reduced root size and dwarf fruits occur in phytoplasma infected plants indicating a disturbed hormone balance (Dermastia, 2019). Phytohormones are induced in reaction to abiotic and biotic stresses and lead to the induction of defence responses (Walling, 2000). The influence of phytoplasma infections on salicylic acid, jasmonates, auxins, abscisic acid, ethylene and cytokinine biosynthesis and pathways was recently reviewed by Dermastia (2019), illustrating the diverse and complex interactions between the specialized pathogens and their host plants.
In the case of phytoplasmas, the impact on vector insects that are crucial for the distribution of phytoplasmas, has to be taken into consideration. So far, all phytoplasmas of the group 16SrX causing important fruit crop diseases are vectored by jumping plant lice (Hemiptera: Psylloidea) or succinctly psyllids (Jarausch et al., 2019b). Psyllids nymphs and adults feed on plant phloem and occasionally on xylem sap (Gallinger & Gross, 2018, 2020; Weintraub & Beanland, 2006). Therefore, morphological changes of the plant vascular system may affect psyllid feeding behaviour and suitability of plants as hosts of vector insects. Additionally, phloem/xylem components may influence host choice and oviposition behaviour of psyllids (Gallinger & Gross, 2018, 2020; Mayer et al., 2011). In addition, to detect appropriate host plants for feeding and reproduction, volatile signals are used by many vectoring psyllid species during migration (Gallinger et al., 2019, 2020; Gross & Mekonen, 2005; Mayer et al. , 2008a,b, 2009; Soroker et al. , 2005; Weintraub & Gross, 2013). As plant volatile emission is frequently regulated by phytohormones, their changes in concentrations play an important role on the interplay of vector insects, plants and phytoplasmas (Gross, 2016).
In the present study, we explored how infections with specific fruit tree phytoplasmas (‘Ca .P. mali’, ‘Ca . P. pyri’ and ‘Ca . P. prunorum’) of the 16SrX group (Seemüller & Schneider, 2004), changed important morphological and physiological parameters of their respective host plants, all Rosaceae (Potter et al., 2007). We measured various parameters such as leaf morphology, plant vascular morphology, and callose deposition, determined physical phloem parameters (mass flow velocity and volumetric flow rate, relative density and dynamic viscosity), and analysed the content of several phytohormones in leaf tissues of healthy and phytoplasma-infected plants. The importance of measured parameters for symptom manifestation as well as the impact on vector insects and phytoplasma spread is discussed.