Isolation of a molecular marker of soybean nodule senescence

cDNA markers differentially expressed in mature and senescent soybean (Glycine max [L.] Merr. cv. Labrador) nodules were isolated by mRNA differential display. Three clones were confirmed to have a differential expression by dot-blot analysis. Two of them were down-regulated during senescence and showed a total identity with the nodulin 22 precursor cDNA sequence. Nodulin 22 has been suggested to be involved in metal exchanges between the symbiotic partners; the down-regulation of this gene suggests that these exchanges are reduced during the senescence process. The third clone (DD15) was up-regulated: no transcripts were detected in mature functioning nodules but senescent ones exhibited a high level of expression. This clone exhibited high homology with several soybean cDNA ESTs from the USDA-ARS soybean genetics and genome database. This gene family appears to encode proteins involved in the remobilisation occurring during natural or stress induced senescence or the utilisation of reserves during the germination process. On the other hand, DD15 also exhibits significant homology with an EST from the model plant Medicago truncatula, which corresponds to a gene expressed in cell cultures upon elicitor induction. Our results suggest that DD15 could constitute a good marker of soybean nodule senescence.