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.