Localization of calreticulin and calcium ions in mycorrhizal roots of Medicago truncatula in response to aluminum stress.

Aluminum (Al) toxicity limits growth and symbiotic interactions of plants. Calcium plays essential roles in abiotic stresses and legume-Rhizobium symbiosis, but the sites and mechanism of Ca2+ mobilization during mycorrhizae have not been analyzed. In this study, the changes of cytoplasmic Ca2+ and calreticulin (CRT) in Medicago truncatula mycorrhizal (MR) and non-mycorrizal (NM) roots under short Al stress [50 μM AlCl3 pH 4.3 for 3 h] were analyzed. Free Ca2+ ions were detected cytochemically by their reaction with potassium pyroantimonate and anti-CRT antibody was used to locate this protein in Medicago roots by immunocytochemical methods. In MR and NM roots, Al induced accumulation of CRT and free Ca2+. Similar calcium and CRT distribution in the MR were found at the surface of fungal structures (arbuscules and intercellular hyphae), cell wall and in plasmodesmata, and in plant and fungal intracellular compartments. Additionally, degenerated arbuscules were associated with intense Ca2+ and CRT accumulation. In NM roots, Ca2+ and CRT epitopes were observed in the stele, near wall of cortex and endodermis. The present study provides new insight into Ca2+ storage and mobilization in mycorrhizae symbiosis. The colocalization of CRT and Ca2+ suggests that CRT is essential for calcium mobilization for normal mycorrhiza development and response to Al stress.