Nodulin gene expression was analyzed in effective and ineffective root nodules of alfalfa (Medicago sativa L. cv Iroquois) elicited by three different Rhizobium meliloti mutants: an exoB mutant having defective acidic exopolysaccharide that does not fluoresce on plates containing the fluorescent brightener Calcofluor; fix21, a spontaneous mutant that has defective
lipopolysaccharide and is Calcofluor bright; and a Rhizobium mutant resulting from a Tn5 insertion in the nifH gene of the nif operon. The ineffective nodules elicited by these various mutant rhizobia are blocked at different stages of nodule development and have unique phenotypes. A distinctive pattern of
nodulin gene expression as determined by in vitro translations of total nodule
RNA characterizes each nodule phenotype. Seventeen nodulins are found in effective nodules including five leghemoglobins. Only one
nodulin gene is expressed in the bacteria-free nodules elicited by the exoB mutant. Other
nodulin genes (
leghemoglobin and nine others) are expressed in fix21-induced nodules. The genes for nodule-enhanced
glutamine synthetase as well as for all the other nodulins are expressed in nodules induced by the nifH mutant. The expression of genes for the nodulins, including
leghemoglobin, is independent of the
nitrogen-fixing ability of the nodule and appears to correlate with the differentiation of densely cytoplasmic host cells in the nodule and, to some extent, with bacterial release from
infection threads.