Morphological and physiological adaptations in the vascular system of
infected fruit trees provide tolerance against phytoplasma diseases
Abstract
The host-pathogen combinations - Malus domestica (apple)/ ‘Candidatus
Phytoplasma mali´, Prunus persica (peach)/‘Ca. P. prunorum´ and Pyrus
communis (pear)/‘Ca. P. pyri´ show different courses of diseases
although the phytoplasma strains belong to the same 16SrX group. While
infected apple trees can survive for decades, peach and pear trees die
within some weeks or years. To this date, neither morphological nor
physiological differences caused by phytoplasmas have been studied in
these host plants. In this study, phytoplasma-induced morphological
changes of the vascular system as well as physiological changes of the
phloem sap and leaf phytohormones were analysed and compared with
non-infected plants. Unlike peach and pear, infected apple trees showed
substantial reductions in leaf and vascular morphology, affecting phloem
mass flow. In contrast, in pear mass flow and physicochemical
characteristics of phloem sap increased. No changes in phytohormone
levels were detected in pear but in apple and peach trees, where
defence- and stress-related phytohormones increased. Compared with peach
and pear trees, data from apple suggest that the long-lasting
morphological adaptations in the vascular system, which likely cause
reduced sap flow, triggers the ability of apple trees to survive
phytoplasma infection. Some phytohormone-mediated defences might support
the tolerance.