SydR plays a crucial role in S. meliloti/M. truncatula symbiosis. (A) Nodulation kinetics of M. truncatula plants inoculated with WT, ΔsydR, and ΔsydR-ptrp:sydR strains. (B) Representative images of M. truncatula root nodules after inoculation with WT (1), ΔsydR (2), and ΔsydR-ptrp:sydR (3). (C) Nitrogen fixation activity, determined by ARA at 21 dpi. (D) Number of white and pink nodules at 21 dpi. (E) Nodule number (left graph) and ARA (right graph) from WT (black bars) and ΔsydR-pnodA:sydR (white bars) inoculated roots at 21 dpi. The values shown are the means ± SEM of three independent experiments. Non-parametric Kruskal-Wallis and post hoc Conover-Iman tests with Benjamini-Hochberg correction (A and C), and Mann-Whitney test (E) were used to assess the statistical significance of differences compared to the WT-inoculated roots (*P < 0.05).