Recognition between leguminous plants and the specific rhizobial strains that nodulate them is mediated via a regulon of nodulation (nod) genes present in the bacteria. These nod genes are induced by flavonoids secreted from legume roots. Many of the nod gene products are involved in the synthesis of host-specific signals that are recognised by appropriate legume hosts. Recently (Lerouge et al., 1990), the signal molecule made by one strain of Rhizobium meliloti was identified as an acylated and sulphated, tetraglucosamine glycolipid and there is strong evidence that Rhizobium leguminosarum makes related but structurally distinct signals.
On the basis of these observations it is now possible to make sense of several similarities that have been recognised between nod gene products and enzymes of known function. Thus, for example, it appears that the nodM gene product is involved in the formation of glucosamine precursors of the signal molecule, whilst other gene products are likely to be involved in specific substitutions that confer host specificity to the signal molecule.
In addition to those nod gene products that are involved in the synthesis of the glycolipid, it is evident that there are other genes which may carry out a different role. Of particular interest is the nodO gene which encodes a secreted Ca2+-binding protein that has the potential to interact directly with plant cells. In the absence of the nodFEL genes, nodO is necessary for nodulation, indicating that the NodO protein can compensate for the loss of nodFEL function during infection.
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