Abstract
Legumes have the unique ability to host nitrogen-fixing Rhizobium bacteria as symbiosomes inside root nodule cells. To get insight into this key process, which forms the heart of the endosymbiosis, we isolated specific cells/tissues at different stages of symbiosome formation from nodules of the model legume Medicago truncatula using laser-capture microdissection. Next, we determined their associated expression profiles using Affymetrix Medicago GeneChips. Cells were collected from the nodule infection zone divided into a distal (where symbiosome formation and division occur) and proximal region (where symbiosomes are mainly differentiating), as well as infected cells from the fixation zone containing mature nitrogen fixing symbiosomes. As non-infected cells/tissue we included nodule meristem cells and uninfected cells from the fixation zone. Here, we present a comprehensive gene expression map of an indeterminate Medicago nodule and selected genes that show specific enriched expression in the different cells or tissues. Validation of the obtained expression profiles, by comparison to published gene expression profiles and experimental verification, indicates that the data can be used as digital “in situ”. This digital “in situ” offers a genome-wide insight into genes specifically associated with subsequent stages of symbiosome and nodule cell development, and can serve to guide future functional studies.
Highlights
Legume plants have the unique ability to host nitrogen-fixing bacteria, collectively called rhizobia, in a newly formed organ, the so-called root nodule
Laser Capture Microdissection of Medicago Nodules To isolate distinct nodule cells at different stages of symbiosome development we used laser-capture microdissection (LCM), which allows the rapid and specific isolation of cells/tissues based on conventional histological identification [28]
Three week old Medicago nodules typically contain an active meristem at the apex, a well-defined infection zone and an active fixation zone
Summary
Legume plants have the unique ability to host nitrogen-fixing bacteria, collectively called rhizobia, in a newly formed organ, the so-called root nodule. Symbiosomes fix atmospheric nitrogen into ammonium which is transferred to the plant in return for carbohydrates [2]. This symbiosis is one of the most important sources of biologically fixed nitrogen and allows legumes to grow in nitrogen poor soil conditions, without the need of chemical fertilizer. To better understand this ecologically and agriculturally important interaction a key goal is the identification of the transcriptome changes that are associated with the different stages of the interaction and to link gene expression to the corresponding developmental processes. The developmentally structured organization of Medicago nodules makes them an ideal system to study the different stages of nodule and symbiosome development
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