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 upregulated
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.L'articolo é disponibile sul sito dell'editore: http://www.apsjournals.apsnet.or
