Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 3, 2005
Publication Date: March 1, 2006
Citation: Tesfaye, M., Samac, D.A., Vance, C.P. 2006. Insights into symbiotic nitrogen fixation in Medicago truncatula. Molecular Plant-Microbe Interactions. 19:330-341. Interpretive Summary: Nitrogen is one of the most important nutrients needed for plant growth. Plants in the legume family obtain much of the needed nitrogen through a symbiotic relationship with soil bacteria collectively called rhizobia. Presence of particular rhizobia causes formation of a unique structure on legume roots called nodules in which the bacteria reside and convert or "fix" nitrogen gas into compounds that can be transferred to the plant. The barrel medic, a legume plant closely related to the crop plant alfalfa, is a useful model for identifying the genes important in nitrogen uptake and use. A computer comparison of thousands of genes expressed in nodules with genes expressed in roots identified 530 genes that potentially are expressed during the symbiotic interaction. Expression of these genes and others potentially involved in carbon and nitrogen metabolism was measured in roots, nodules producing nitrogen compounds, and nodules that were unable to fix nitrogen. One group of genes that were found to have enhanced expression in nitrogen-fixing nodules were similar to plant disease resistance genes. Also, genes for enzymes in the pathways for production of organic acids and for nitrogen uptake compounds were simultaneously expressed at similar levels in nitrogen-fixing nodules. The computer comparison of genes accurately identified 78% of the genes to be expressed in nodules. These techniques are useful tools for identifying genes involved in the symbiotic interaction and nitrogen uptake and for measuring expression of many genes simultaneously. Insights into the molecular mechanisms involved in rhizobial symbiosis have the potential to improve the efficiency of biological nitrogen fixation by plants and reduce costly fertilizer inputs.
Technical Abstract: In silico analysis of the Medicago truncatula gene index release 8.0 at The Institute for Genomic Research identified about 530 putatively symbiosis-specific tentative consensus sequences (TCs) clustered from 2,700 expressed sequence tags (ESTs) derived solely from Sinorhizobium meliloti-inoculated root and nodule tissues. A majority (53%) of the putatively symbiosis-specific TCs were derived exclusively from nitrogen-fixing and senescent nodules. A cDNA filter array was constructed using putative symbiosis-specific ESTs as well as ESTs representing selected carbon and nitrogen metabolic pathways. The array was used to analyze transcript abundance of M. truncatula roots and nodules following inoculation by a wild-type S. meliloti strain, a mutant strain that forms ineffective nodules, an uninoculated root control as well as roots following nitrate or ammonium treatments. A total of 81 ESTs were upregulated in both effective and ineffective nodules, and 78% of these ESTs were identified by in silico analysis as symbiosis-specific. One group of symbiosis-specific transcripts identified by the in silico analysis encodes eight genes with similarity to putative plant disease resistance (R) genes. The expression of symbiosis-specific R genes was enhanced in effective nodules, but transcripts were also detected in ineffective nodules at 14 days post-inoculation (dpi). Genes for enzymes involved in organic acid synthesis (carbonic anhydrase, phosphoenolpyruvate carboxylase, malate dehydrogenase, and citrate synthase) along with genes involved in nitrogen metabolism (asparagine synthetase and glutamine synthetase) were co-expressed in effective nodules of M. truncatula. The coordinated expression of genes for organic acid and nitrogen metabolism is expected to play a vital role in the regulation of nitrogen assimilation during Medicago symbiosis.