Abstract:

Annexins are a conserved， multi-member protein family. They are present in eukaryotes and participate in diverse stress-related biological processes by binding membrane phospholipids in a Ca²⁺-dependent way. Most studies of annexins have focused on vertebrates although the first plant annexin was reported early in 1989 from tomato （Lycopersicon esculentum）. To date， information on annexins in legumes， particularly forage legumes， is scarce. In this study， the phylogeny of annexins in Arabidopsis thaliana， Medicago truncatula and Medicago sativa， was analyzed. We then focused on investigating the expression pattern and biological functions of MtANN2 in root development and salt response. Quantitative RT-PCR demonstrated that MtANN2 was abundantly expressed in roots and was NaCl-inducible. RNA in situ hybridization showed a predominant expression of MtANN2 in the lateral root primordia. The loss-of-function mutantof AtANN2， the closest homolog of MtANN2 in Arabidopsis， displayed pleiotropic morphological abnormalities including dwarfism with fewer lateral roots and lower fresh weight， and the homozygous atann2 plants were hypersensitive to the salt treatment （100 mmol·L^-1）. The transgenic atann2 plants ectopically expressed MtANN2 were reminiscent of the wild type with similar fresh weight and the lateral root density between atann2 and Col-0. The results indicated that MtANN2 partly rescued the defects of atann2. Under the NaCl treatment， the germination rate and growth of the transgenic lines were similar to those of the wild type. Our molecular results suggest that MtANN2 is involved in root growth and salt response， and that the increase level of MtANN2 improves salt tolerance. Therefore， MtANN2 may serve as a potential target for breeding alfalfa tolerant to high salinity using the molecular approaches.

Key words: Medicago truncatula, annexin, expression pattern, ectopic expression, salt stress