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Phloem Cytochemical Modification and Gene Expression Following the Recovery of Apple Plants from Apple Proliferation Disease

First, fifth, eighth, and tenth authors: Dipartimento di Biologia e Protezione delle Piante, Università di Udine, via delle Scienze, 208 I-33100 Udine Italy; second, third, and fourth authors: Dipartimento di Agrobiologia e Agrochimica, Università della Tuscia, Via S. Camillo de Lellis snc, I-01100 Viterbo Italy; sixth and seventh authors: Dipartimento di Scienze e Tecnologie Chimiche, Università di Udine, via Cotonificio 108, I-33100 Udine Italy; and ninth author: Dipartimento di Biotecnologie per il Monitoraggio Agro-Alimentare ed Ambientale, Università Meditterranea di Reggio Calabria, Contrada Melissari, I-89124 Reggio Calabria Italy.

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.

Additional keywords:calcium signaling, Malus domestica.