‘Recovery’ from apple proliferation disease: an integrated approach.

Recovery is the spontaneous remission, sometimes permanent, from disease symptoms. Phytoplasmas surviving in the roots are not able to recolonise the plant crown. The causes that induce recovery remain still unknown and its physiological bases are poorly understood. In this research the modifications in the phloem tissue related to recovery-induced resistance in apple have been investigated through ultrastructural, chemical, cytochemical and gene expression analyses of leaf tissues from recovered, healthy and apple proliferation-diseased plants. Ultrastructural observations detected abnormal callose and P-protein accumulations in the phloem of recovered apple plants. Callose synthesis and Pprotein plugging, which are Ca2+-dependent, would form physical barriers preventing the in planta movement. The cytochemical localization by potassium pyroantimonate detected the presence of Ca2+ ions in the phloem in all the three groups of plants; however the Ca2+ concentration was remarkably higher in the cytosol of the recovered apple plants. This observation would support the hypothesis that resistance mechanisms would be related to an increased Ca2+- dependent signaling activities. Apple genes coding for callose synthases and phloem proteins were identified by an in silico approach. The expression patterns of five genes encoding callose synthases (MDCALS1/5) and of four genes encoding phloem proteins (MDPP2-1/3 and MDERG1) were analysed by quantitative real time RT-PCR. Four of the nine analysed genes were up-regulated in recovered plants in comparison to healthy and diseased ones, supporting the hypothesis that recovered apple plants were able to develop resistance mechanisms dependent from Ca2+ signal activities