Trehalose metabolism in root nodules of the model legume Lotus japonicus in response to salt stress

Abstract

The effect of NaCl stress (50 mM) and validamycin A treatment (30 μM) on growth and nitrogen fixation of Lotus japonicus was investigated in plants cultured under symbiotic and hydroponics conditions for teen weeks (flowering stage). Validamycin A was used as a potent trehalase inhibitor, and was able to produce a five‐fold increase in the level of trehalose during salt treatment, concomitant with an enhance in biomass (20%) in salinized plants. Alterations of nodule metabolism related to some carbohydrates and some enzyme activities were also examined. The shoot and total plant dry weight were severely affected by saline conditions decreasing by 40% and only 15–20% in plant treated without or with validamycin A, respectively. Nitrogenase activity (E.C. 1.7.9.92) was inhibited almost 40% by salt stress and no effect of validamycin was observed. Based on these results, L. japonicus might be considered as a salt‐sensitive legume. In addition, the saline conditions also inhibited the enzyme activities of sucrose synthase (E.C. 2.4.1.13), alkaline invertase (E.C. 3.2.1.26) and trehalose‐phosphate synthetase (E.C. 2.4.1.15). The validamycin A treatment mainly decreased enzyme activities: sucrose synthase, trehalose‐phosphate phosphatase (E.C. 3.1.3.12) and trehalase (E.C. 3.2.1.28). On the other hand, a high concentration of the carbohydrates, starch, sucrose and glucose, seems not to be the mechanism induced in L. japonicus to protect nodules exposed to NaCl because all these sugars decreased in such conditions. Results of the present study support the possible role of trehalose as an osmoprotectant under salt stress.