Symbiotic phenotype of the S. meliloti hfq knock-out mutants. (a) Nodule formation kinetics of the S. meliloti 1021 wild-type strain and its mutant derivative 1021Δhfq determined as the number of nodules per plant (left plot) and % nodulated plants (right plot). Each point represents the mean ± standard error of determinations in two independent sets of 24 plants grown hydroponically in test tubes. Dpi, days post inoculation. (b) Competition assays between the S. meliloti wild-type strain 2011 and its hfq insertion mutant derivative 2011-3.4. Nodule occupancy (expressed as % of invaded nodules by each strain) was determined in plants grown in either Leonard assemblies or agar plates and co-inoculated with both strains at 1:1 ratio. (c) Symbiotic efficiency of the 1021 and 1021Δhfq strains. Left histogram, % nitrogen fixing nodules induced by each strain in plants grown either in test tubes (two sets of 24 plants) or agar plates (5 plates of 10 plants) 30 dpi. Right panels: growth of 1021- and 1021Δhfq-inoculated plants 30 dpi in Leonard jars and dry-weigh of the same plants expressed as the mean ± standard error from measurements in 24 individual plants. Ni, not inoculated.