In the present work, we compared hydrogen peroxide (H2O2) localisation and the activities/contents of antioxidant enzymes and metabolites in the leaf tissues of grapevine (Vitis vinifera L. cv. Prosecco) plants showing different sanitary status, namely diseased by Flavescence dorée, healthy or recovered. Polymerase chain reaction analysis revealed that the pathogen associated with Flavescence dorée (proposed as ‘Candidatus Phytoplasma vitis’) was detected in the leaf tissues of symptomatic plants, but was not observed in either the healthy or recovered plants. Hydrogen peroxide accumulated in the phloem plasmalemma of recovered grapevine leaves, but was not detected in either healthy or diseased material. When compared to diseased or healthy plants, recovered plants had distinctly lower extractable levels of catalase and ascorbate peroxidase, two enzymes primarily involved in the scavenging of excess H2O2 generated in different cell compartments. Among healthy, diseased and recovered leaves there was no significant difference in the amount of 2-thiobarbituric acid-reactive substances, which are assumed to reflect the extent of peroxidative breakdown of membrane lipids. Therefore, it is suggested that recovery from Flavescence dorée disease in grapevine might be associated with a long-term, sustained and tissue-specific accumulation of H2O2 in leaves, which reduces numbers or prevents further infection by Flavescence dorée phytoplasma. Recovered grapevine plants might be able to achieve such H2O2 accumulation through a selective and presumably stable downregulation of enzymatic H2O2 scavengers, without altering the levels of other antioxidant systems and without incurring an increased oxidative risk.

Recovery of apple trees from apple proliferation was studied by combining ultrastructural, cytochemical, and gene expression analyses to possibly reveal changes linked to recovery-associated resistance. When compared with either healthy or visibly diseased plants, recovered apple trees showed abnormal callose and phloem-protein accumulation in their leaf phloem. Although cytochemical localization detected Ca²⁺ ions in the phloem of all the three plant groups, Ca²⁺ concentration was remarkably higher in the phloem cytosol of recovered trees. The expression patterns of five genes encoding callose synthase and of four genes encoding phloem proteins were analyzed by quantitative real-time reverse transcription-polymerase chain reaction. In comparison to both healthy and diseased plants, four of the above nine genes were remarkably up-regulated in recovered trees. As in infected apple trees, phytoplasma disappear from the crown during winter, but persist in the roots, and it is suggested that callose synthesis/deposition and phloem-protein plugging of the sieve tubes would form physical barriers preventing the recolonization of the crown during the following spring. Since callose deposition and phloem-protein aggregation are both Ca²⁺-dependent processes, the present results suggest that an inward flux of Ca²⁺ across the phloem plasma membrane could act as a signal for activating defense reactions leading to recovery in phytoplasma-infected apple trees.