Three tandemly aligned LEA genes from Medicago truncatula confer differential protection to Escherichia coli against abiotic stresses

Late embryogenesis abundant (LEA) proteins are important for abiotic stress tolerance in diverse organisms. Within the LEA protein superfamily, group 4 members are characterized by a conserved N-terminal region and a structurally disordered C-terminal region that varies regarding length and amino acid content. Previous in vitro assays have suggested that the conserved N-terminal region shared by group 4 LEA proteins is critical for forming an amphipathic α-helix and protecting enzymatic activities from the adverse effects of desiccation or freezing. However, the cellular roles of the varying C-terminal region remain largely to be characterized. Medicago truncatula contains five subgroup LEA4B proteins encoding loci of which three are tandemly arranged on chromosome 7 due to local gene duplication events. In this study, abiotic stresses and addition of abscisic acid (ABA) induced the transcription of the four LEA4B genes. Escherichia coli cells overexpressing the three tandemly aligned LEA genes indicated significantly increased tolerance to salt, osmotic, heat, and freezing stresses. However, the extent of the protective effects on the survival and growth of bacterial cells differed among the LEA proteins, potentially because of variations in the C-terminal region. This possibility was further supported by the observation that the protective effects of the native truncated MtLEA3140, which only contains a conserved N-terminal region, were inferior to the effects of the full-length mutant version. The results suggest that the structurally flexible C-terminal region of group 4 LEA proteins plays roles in protecting cells from damages caused by various abiotic stresses, and provide clues for elucidating the mechanisms underlying the intracellular functions of these proteins.