Phoma medicaginis colonizes Medicago truncatula root nodules and affects nitrogen fixation capacity

The ability of Phoma medicaginis to infect legume nodules and the subsequent effect on their functioning was investigated in Medicago truncatula lines, TN.8.20 and Jemalong6, inoculated with an efficient Sinorhizobium meliloti strain. Colonization of nodules by P. medicaginis strain Pm8 was confirmed by morphological and molecular analyses. P. medicaginis effect was more relevant on the most susceptible line, TN.8.20, where it decreased the nitrogen fixation capacity (NFC) significantly in nodules from 62 days after sowing (das) till the end of nodule life cycle; however in Jemalong6 nodules, the NFC inhibition was significant only at the flowering stage. Analyses of stress indices as structure, lipid peroxidation and leghemoglobin content did not show significant structural or metabolic alterations in infected nodules. Superoxide dismutase (SOD) activity was increased in nodules of both M. truncatula lines up to 62 das. After 78 das, increase of SOD activity continued only in Jemalong6 nodules. Changes in peroxidase (POX) activity were correlated to those revealed on NFC. Indeed, POX was decreased in nodules of TN.8.20 line but remained almost unchanged in Jemalong6. These results, suggest that POX represents a strong component of the defence mechanisms in nodules of resistant M. truncatula lines, where it could be involved in reinforcing the cell wall barrier. The stimulation of SOD activity is also a key mechanism in maintaining nodule integrity via controlling ROS overproduction when counteracting fungal attacks.