| Summary: | <p>Soybean and related legumes export symbiotically-fixed nitrogen from the nodules to the leaves as ureides. The ureide allantoin is hydrolyzed by allantoinase to allantoate then further degraded by other enzymes, releasing ammonia and carbon dioxide. This study aimed to identify allantoinase genes in soybean and their gene expression as well as enzyme activity patterns. The effects of water limitation and allantoin treatment on the expression and activity of allantoinase in N<sub>2</sub>-fixing plants were also evaluated. Enzyme activity and ureide content were evaluated using a spectrophotometric assay. Real time RT-PCR was used to quantify the amount of gene products. Four allantoinase genes were identified and were expressed, with <i>GmALN1</i> and <i>2</i> constantly expressed at higher levels. In seedlings, allantoinase was found to be actively synthesized more in cotyledons than in the embryonic axes, as seen by early enzyme activity and higher <i>GmALN 1</i> and <i>2</i> transcript levels. Allantoate produced in these tissues appeared to be mobilized to the developing axes. <i>GmALN1</i> and <i>2</i> were implicated in post-germination nitrogen assimilation during early seedling growth, while <i>GmALN3</i> and <i>4</i> were consistently expressed at very low levels, with an exception in nodules. Transcript abundance in the nodules of N<sub>2</sub>-fixing plants, supported by the high enzyme activity and ureide content observed, suggested an important role in the synthesis and transport of allantoate in these tissues. Allantoinase was also detected in non-fixing tissues but may play a different role in these tissues, most probably functioning in the turnover and salvage of purine nucleotides. The effect of exogenous allantoin during water limitation was investigated. The addition of allantoin prior to water limitation seemed to change the sensitivity of soybean to such stress, prolonging its ureide catabolic activity at least up to 5 days without water. Results of this study will aid in our understanding of how ureide catabolism is regulated during soybean development. This information may help address problems in legume crop improvement specifically in enhancing N<sub>2</sub>-fixation and yield capacity and in coping with water limitation stress.</p>
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