The 1021Δ hfq mutant is impaired in the survival within the nodule cells. Representative enlarged images of nodules induced in alfalfa plants by the 1021 (a) and 1021Δhfq (e) strains. Bright-field microscopy of longitudinal sections of the same nodules (b and f); the zones characterizing the histology of nitrogen-fixing indeterminate nodules are indicated in (b). Merged images of the same nodule sections observed with green and blue filters (520 nm and 470 nm, respectively) (c and g). Magnification of the images of central nodule tissues (d and h); 1021Δhfq-induced nodules are scarcely invaded by bacteria and show signs of premature senescence: degradation of leghemoglobin (arrows) and cell debris (double arrowheads). Scale bars, 250 μm.