Different strategies for salt tolerance in determined and indeterminate nodules of Lotus japonicus and Medicago truncatula

In this study, we examine how indeterminate and determined nitrogen-fixing root nodules of model legumes Lotus japonicus and Medicago truncatula adapt their non-structural carbohydrate pool during salt stress, with particular emphasis on trehalose, a compatible solute abundant in nodules of some legumes. M. truncatula and L. japonicus plants were inoculated with Sinorhizobium meliloti and Mesorhizobium loti, respectively, and the effect of 50 mM sodium chloride (NaCl) added to the nutrient solution was studied in a time-course experiment. Sucrose and pinitol were the predominant carbohydrates in nodules of both legumes, contributing to osmoprotection in nodules of L. japonicus under salt stress. Trehalose concentration increased under salt stress in L. japonicus nodules; however, compared with sucrose and pinitol, its concentration was too low to contribute efficiently to osmoregulation. By contrast, proline showed a dramatic increase in nodules and leaves of M. truncatula under salt stress, contributing to osmotic adjustment in this species. Results found in this study showed different mechanisms for salt tolerance in determined and indeterminate nodules of model legumes L. japonicus and M. truncatula that might be a general feature in the mentioned different types of nodules.