The soybean (Glycine max L. Merrill) root nodule is an organ specialized for nitrogen fixation resulting from a symbiosis between the bacterium Bradyrhizobium japonicum and the roots. Root nodule senescence, its last stage of development, was the focus of this study. The progression of soybean root nodule senescence was examined using nitrogenase activity. During nodule development, nitrogenase activity was first observed with the appearance of nodules and peaked just prior to flowering, which marks the onset of nodule senescence. Accordingly, nitrogenase activity declined after flowering. Although nitrate and dark treatment both caused significant declines in nitrogenase activity, darkness caused a greater decline in nitrogenase activity than nitrate.
The SAN1A gene, originally isolated from senescent root nodules (Chan, 1995), and two other members of the SAN1 gene family, were characterized. These genes represent a novel family of highly conserved, tandemly repeated genes in soybean. SAN1A and SAN1B encode predicted proteins of 352 and 353 amino acids, respectively, whereas SAN1C encodes a truncated protein of 126 amino acids and is likely a pseudogene. Despite their occurrence in tandem, gene conversion does not appear to have occurred within the evolutionary history of these genes. During natural senescence, SAN1A and SAN1B expression patterns suggested that SAN1A may play a role throughout nodule development, whereas SAN1B is likely to be more important during senescence. Both nitrate and dark treatment caused a decrease in SAN1A expression and an increase in SAN1B expression. Although both SAN1A and SAN1B were expressed in all soybean tissues examined, their patterns of expression differed. These results suggest that SAN1A and SAN1B have divergent functions or that SAN1B may replace SAN1A under conditions such as senescence. Preliminary results also suggest that SAN1A may be light regulated. Finally, a test of SAN1C expression in soybean tissues showed that it is expressed, despite the fact that it encodes a truncated protein.
Database searches suggested that SAN1 genes are related to plant dioxygenases. A gene tree and BLASTP searches revealed that the SAN1 gene products are most closely related to uncharacterized dioxygenases identified in Arabidopsis, rice, and maritime pine, with the closest, identifiable dioxygenases being gibberellin 20-oxidases. Thus, the SAN1 genes may encode paralogues of known dioxygenases or novel dioxygenases. The evidence collected to date suggests that the SAN1 genes encode dioxygenases with roles in various aspects of plant metabolism. Further characterization of this family may yield more clues about their role(s).
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/29378 |
| Date | January 2006 |
| Creators | Webb, Candace June |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 332 p. |
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