Abstract: To investigate the function of calcium signal of apple trees during defense process in response to Alternaria alternata apple pathotype（A. alternata AP），this study analyzed the leaf cellultra-structure，distribution change of calcium and relative expression of MdCPKs during the plant- pathogen interaction period. The susceptible and resistant cultivars to A. alternata AP，Red Delicious and Jonathan apple were used as materials. The variation of Ca2+ subcellular localization and relative expression of MdCPKs in leaves were detected by transmission electron microscope and qRT-PCR analysis when leaves were attacked by A. alternata AP. The results were as follows：The cell structure of Red Delicious and Jonathan remained intact and calcium antimonate deposits localized mainly in the vacuoles and intercellular spaces of the mesophyll cells under the resting conditions，while the density of calcium precipitation in intercellular spaces is higher in Jonathan than that in Red Delicious. Calcium gathered in the cytoplasms and remarkable reduced in the vacuoles in Red Delicious，while it not only gathered in the cytoplasms but also in the sieve tubes in Jonathan，declined in the vacuoles and intercellular spaces，and tended to locate in the outside of the cell wall and vacuole membrane when leaves were inoculated with A. alternata AP at 8 hpi（hours post inoculation）. Calcium in the leaf cells of Red Delicious tended to move from the cells to the intercellular spaces at 18 hpi，while a large number of calcium localized in the cytoplasms，vacuoles and sieve tubes in Jonathan at this time. When inoculated A. alternata AP at 24 hpi，the cell structure of Red Delicious had been distorted，plasmalemma were broken and sieve tube wall was lignified，and calcium mainly localized in the cytoplasms，while the cell structure of Jonathan still kept integrity and calcium localized in the vacuoles and cytoplasms. Some cells in Red Delicious were invaded by hyphae at 36 hpi. Calcium deposits mainly localized in the vacuoles in the undamaged cells，and distributed in intercellular spaces disorderly around the damaged cells，however，the calcium level kept dynamic homeostasis in the leaf cells of Jonathan at this time. The relative expression of MdCPKs in Red Delicious and Jonathan showed different characters during the different stages of A. alternata AP infection. Most MdCPKs reached to the highest expression level at 24 hpi in Jonathan while that delayed at 36 hpi in Red Delicious，and the relative expression of MdCPKs in Jonathan were higher than in Red Delicious. The results indicated that calcium signal involved in the response of apple to A. alternata AP infection. The increasing of cytoplasm calcium in Jonathan and in Red Delicious mainly resulted from the inflow of extracellular spaces and the release of organelle respectively when they attacked by A. alternata AP. Moreover，MdCPKs in Jonathan responded to the infection earlier and more intensely than they did in Red Delicious.

Key words: apple, Alternaria alternata apple pathotype, localization of calcium, Calcium-Dependent Protein Kinase（CDPK）