We have identified the three genes pssCDE in Rhizobium leguminosarum bv. trifolii TA1. Even though they were almost identical to earlier identified pssCDE genes of R. leguminosarum, they differed in gene lengths and gene overlaps. The predicted gene products of pssCDE genes shared significant homology to prokaryotic glycosyl transferases involved in exopolysaccharide synthesis. The Tn5 insertion in pssD created the nonmucoid mutant that induced non-nitrogen-fixing nodules. The microscopic analysis of the nodules, induced on Trifolium pratense by the pssD133 mutant, showed abnormally enlarged infection threads densely packed with bacteria, which were released from the infection threads in an unusual way. The symbiosomes were observed very rarely and the nodule remained almost empty. Symbiotic phenotype of the pssD133 suggested a correlation between this mutation and defective endocytosis of bacteria into nodule cells.

An ability to form symbiotic associations with rhizobia and to utilize atmospheric nitrogen makes legumes ecologically successful. High iron content in legume grains, partially relocated from root nodules, is another-nutritional-advantage of this group of plants. The ferritin complex is the major cell iron storage and detoxification unit and has been recognized as a marker of many stress-induced responses. The possible participation of ferritin in nodule formation and functioning was investigated here. Correlation of increased accumulation of both ferritin polypeptide and mRNA with actual in situ localization of ferritin allowed ferritin synthesis in the developing, indeterminate-type root nodules to be related to differentiating bacteroid tissue. This kind of tissue, in contrast to the determinate-type nodules, is present in lupin nodules at almost all stages of their development. Interestingly, it was found that, in this type of nodule, senescence starting in the decaying zones induces ferritin accumulation in younger, still active, tissues. Based on the presented data, and in correlation with previous results, some aspects of the regulation of expression of lupin ferritin genes are also discussed.

In this review, the anatomy of indeterminate legume root nodule is briefly summarized. Next, the indeterminate nodule meristem activity, organization and cell ultrastructure are described in species with a distinct nodule meristem zonation. Finally, the putative primary endogenous factors controlling nodule meristem maintenance are discussed in the context of the well-studied root apical meristem (RAM) of Arabidopsis thaliana.