|Lohar, Dasharath Prasa|
|Samac, Deborah - Debby|
Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2005
Publication Date: 1/3/2006
Citation: Lohar, D., Sharopova, N., Endre, G., Penuela, S., Samac, D.A., Town, C., Silverstein, K.A., VandenBosch, K.A. 2006. Transcript analysis of early nodulation events in Medicago truncatula. Plant Physiology. 140:221-234. Interpretive Summary: Plants in the legume family, which includes alfalfa, soybean, pea, common bean, and the model plant known as the barrel medic, form an important symbiotic association with soil microbes, collectively called rhizobia. The symbiosis results in the formation of a unique organ, the root nodule, that houses the bacteria within plant cells. The bacteria convert nitrogen gas into ammonia, which is used by the plant to make amino acids. The genes active in the early events of this interaction in barrel medic were identified using microarrays. The microarrays consisted of 6,144 different genes isolated from barrel medic and spotted onto glass slides. Using the microarrays, we identified hundreds of genes that are active from different metabolic processes from 1 to 72 hours after inoculating roots with the bacteria. Four different stages in the interaction were found with specific gene activity associated with each stage. The first stage at 1 hour after inoculation is characterized by activity of genes associated with defense against pathogenic microbes. In the second stage, at 6-12 hours after inoculation, activity of these genes is suppressed and genes involved in cell division, cell growth, and chemical signaling are active. Genes active in the third stage at 24-48 hours after inoculation include those involved in chromosome organization, cell division, and RNA and protein synthesis. The fourth stage is characterized by activity of genes involved in cell structure, protein synthesis, chromosome organization, and chemical signaling. These stages correspond to visible changes in the root. The suppression of genes involved in defense in the later stages is likely required for the bacteria to successfully colonize root cells. This is the first analysis of global gene activity at early time points in the symbiotic interaction. This study illustrates the complexity of the legume-rhizobia interaction and identified genes for further study of the control of the symbiotic association. Improving the effectiveness of the symbiosis, the number of nodules formed, and the amount of nitrogen fixed is important for improving crop yields with fewer inputs of costly nitrogen fertilizers.
Technical Abstract: Within the first 72 h of the interaction between rhizobia and their host plants, nodule primordium induction and infection occurs. We predicted that transcription profiling of early stages of the symbiosis between Medicago truncatula roots and Sinorhizobium meliloti would identify regulated plant genes that likely condition key events in nodule initiation. Therefore, using a microarray with about 6,000 cDNAs, we compared transcripts from inoculated and uninoculated roots corresponding to defined stages between 1 and 72 h after inoculation. Hundreds of genes of both known and unknown function were significantly regulated at each time point. Four stages of the interaction were recognized based on gene expression profiles, and potential marker genes for these stages were identified. Some genes that were regulated differentially in the first hours of the interaction belong to families encoding proteins involved in calcium transport and binding, reactive oxygen species metabolism, cytoskeleton and cell wall functions. Genes involved in cell proliferation were found to be up-regulated at 24 h after inoculation and later time points. Many defense response genes were up-regulated within 1 h after inoculation but down-regulated by 48 h after inoculation, likely facilitating infection thread progression into the root cortex. Additionally, a large number of genes putatively involved in signal transduction and transcriptional regulation were found to be differentially regulated in the inoculated roots at each time point. The findings shed light on the complexity of coordinated gene regulation and will be useful for continued dissection of the early steps in symbiosis.