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 weeks to few 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 area, affecting phloem mass flow. In contrast, in infected pear mass flow and physicochemical characteristics of phloem sap increased. Additionally, an increased callose deposition was detected in pear and peach leaves but not in apple trees in response to phytoplasma infection. The phytohormone levels in pear were not affected by an infection, while in apple and peach trees concentrations of defence- and stress-related phytohormones were 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.
Highlights
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 [1,2]
This study reveals anatomical and physiological changes in apple trees, which may result from an enhanced ability to sense phytoplasma infection and provoke adequate and successful cellular and physiological defence responses
The successful infections resulted in known visible disease symptoms: witches’ broom and enlarged stipules in apple trees, premature foliar reddening in pear trees and chlorosis and suberization in peach trees (Fig 1 and Table 1)
Summary
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 [1,2]. 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 [3,4]. Only few studies provide firm data on host response, host–pathogen interaction and on anatomical, physiological and molecular basis of plant resistance [10], which is still poorly understood [4]. While the host plants of the 16SrX phytoplasmas belong to the Rosaceae, the causing agents of the diseases ’Candidatus Phytoplasma mali’, ’Candidatus Phytoplasma pyri’ and ’Candidatus Phytoplasma prunorum’ are phylogenetically closely related [11]. Phytoplasmas are very small bacteria without a cell wall. As all bacteria including phytoplasmas have generally circular chromosomes, the 16SrX phytoplasmas used in this study, Ca. P. mali, Ca. P. pyri and Ca. P. prunorum, have small linear chromosomes. Phytoplasmas lack many genes that encode important metabolic functions such as amino and fatty acid synthesis [12,13] and they need to consume essential metabolites from their plant hosts
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