Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: June 5, 2005
Publication Date: June 5, 2005
Citation: Tesfaye, M.T., Samac, D.A., Vance, C.P. 2005. Bacterial symbiosis triggers the expression of plant resistance genes and induces the transcription of organic acid metabolism and ammonium assimilation genes in Medicago truncatula. Model Legume Congress, June 5-9, 2005, Pacific Grove, California. Paper No. S48, p. 126. Technical Abstract: Nitrogen (N) is the mineral nutrient needed in greatest abundance by plants and most plant species can utilize a wide range of N compounds including applied nitrate and ammonium fertilizers. An important aspect of legumes including Medicago truncatula (Mt) is their unique ability to obtain N via bacterial symbiosis with soil bacteria, collectively called rhizobia. The acquisition of nitrogen in symbiosis with soil bacteria is expected to trigger a wide variety of molecular and physiological changes in the host. We used in silico analysis to identify bacterial symbiosis-specific transcripts and employed a cDNA array analysis to study the transcriptional events elicited during bacterial fixation of nitrogen following inoculation with a wild-type effective Sinorhizobium meliloti strain and a mutant strain that forms ineffective nodules. The cDNA filter array contained selected nodule-specific transcripts as well as probes representing selected carbon and nitrogen metabolic pathways. We found that 62% of the ESTs that showed up-regulated expression in effective and bacterial-conditioned ineffective nodules at 14 days post-inoculation (dpi) were identified by in silico analysis as being symbiosis-specific transcripts. One group of functionally important symbiosis-specific TCs identified by our in silico analysis encodes genes with similarity to plant resistance (R) genes. This group comprises 8 TCs clustered from 19 ESTs: 6 TCs were clustered from 14 ESTs derived exclusively from cDNA libraries at 2 – 4 dpi, whereas 2 TCs were clustered from 5 ESTs derived from inoculated root and N-fixing nodule cDNA libraries. Despite the low number of ESTs clustered in each TC, the expression of those ESTs encoding plant R genes was enhanced in effective nodules, and transcripts were also detected in bacterial-controlled ineffective nodules at 14 dpi. The functional importance of symbiosis-specific R genes in nitrogen-fixing nodules is not clear. We also show that genes involved in the early steps of the tricarboxylic acid (TCA) cycle, specifically cytoplasmic aconitase, NADP-isocitrate dehydrogenase, and glyoxysomal MDH, were up-regulated in effective nodules of Mt at 14 dpi. The expression of these genes is expected to play a vital role in maintaining the alpha-ketoglutarate level and therefore in the regulation of nitrogen assimilation. Our observation that these TCA cycle genes along with those genes responsible for primary ammonium assimilation (GS, GOGAT, AS, GDH, and AAT) are expressed in N-fixing nodules of Mt at 14 dpi confirms that genes for organic acid synthesis and N-assimilating genes are co-expressed during biological nitrogen fixation.