Four allantoinase genes are expressed in nitrogen-fixing soybean

Soybean (Glycine max L. [Merr]) plants export nitrogen from the nodules as ureides during symbiotic biological nitrogen fixation. Ureides also play a role as nitrogen storage compounds in the seeds and are broken down in germinating seedlings. In this work we identified four soybean genes encoding allantoinase (E.C. 3.5.2.5), an enzyme involved in both ureide production in nodules and ureide catabolism in leaves and other sink tissues. We examined ureide content, allantoinase enzyme activity and expression of these genes, which we term GmALN1 through GmALN4, in germinating seedlings and in vegetative tissues from 45 day old soybean plants. GmALN1 and GmALN2 transcripts were measured in all tissues, but similar levels of expression of GmALN3 and GmALN4 was only observed in nodules. The soybean allantoinase genes seem to have arisen through tandem gene duplication followed by a whole genome duplication. We looked for evidence of the tandem duplication in common bean (Phaseolus vulgaris L.) and present evidence that it occured sometime in the bean lineage before these two species diverged, but before soybean became a tetraploid.

► Four allantoinase genes identified in soybean.
► Ureide content, allantoinase activity and gene expression determined and discussed.
► Two genes are expressed only in the nodule.
► A second, new, allantoinase gene isolated from common bean.
► Discuss evolution of legume allantoinase.