|Samac, Deborah - Debby|
Submitted to: Molecular Plant Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/15/1998
Publication Date: N/A
Citation: Interpretive Summary: Mineral nutrients are critical for plant growth and reproduction. One of the most important mineral nutrients is nitrogen, which is used to form amino acids, proteins, enzymes, and other plant components. Plants in the bean family, such as alfalfa, are able to form a symbiotic relationship with a soil bacterium that allows them to use nitrogen gas from the atmosphere, in a process called nitrogen fixation, to form amino acids. This process occurs in an organ called a root nodule. Plants lacking this ability rely on nitrogen in the form of nitrate or ammonia that is usually found in only limited amounts in soil and must be added as fertilizer for crop production. The enzyme aspartate aminotransferase plays a critical role in capturing symbiotically fixed nitrogen through synthesis of the amino acid aspartate. One form of the enzyme was found primarily in root nodule cells fixing nitrogen and is responsible for synthesis of aspartate from recently fixed nitrogen. Another form of the enzyme was located primarily in cells not fixing nitrogen and may play a role in maintaining the appropriate oxygen concentration in the root nodule. A short fragment of DNA from the gene promoter, the controlling portion of the gene, was found that is responsible for gene expression in the plant nodule. This promoter will be useful for engineering gene expression in alfalfa nodules for studies on regulation of nitrogen fixation and modifications of plants for enhanced nitrogen fixation. Success in the improvement of nitrogen uptake could have significant benefits for agriculture by reducing the need for and use of nitrogen fertilizer.
Technical Abstract: Aspartate aminotransferase (AAT) plays a critical role in the assimilation of symbiotically fixed nitrogen into aspartate and asparagine in legume root nodules. The enzyme occurs as a cytosolic form (AAT1) and a plastid form (AAT2)in alfalfa nodules. To further elucidate the functional role of each isozyme in root nodule metabolism, in situ hybridization was used to determine the pattern of transcript accumulation from the two genes. AAT2 transcripts were localized in infected cells throughout the symbiotic zone of effective alfalfa nodules, however, expression was reduced in ineffective nodules. The AAT1 gene was expressed in the uninfected cells of the invasion zone, symbiotic zone, the nodule parenchyma, and nodule vascular bundles of both effective and ineffective nodules. The AAT1 and AAT2 promoters were evaluated in transgenic alfalfa plants containing promoter B-glucuronidase (GUS) gene fusions. Histochemical staining patterns agreed with results from in situ localization. The distribution pattern of gene transcripts suggests that AAT1 has a role in maintenance of the O2 diffusion barrier in nodules and that AAT2 plays a major role in assimilation of recently fixed nitrogen. Promoter deletion analysis of the AAT2 promoter revealed that nodule-specific expression was retained in a promoter fragment of 300 bp.