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 course 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 to few years period. So far, in these phytoplasma-plant
interactions neither morphological nor physiological differences have
been studied in the hosts. 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, causing negative effects on
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. If compared with
peach and pear trees, the 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.