Abstract
Lipo-chitooligosaccharides (LCOs), signal compounds produced by N2-fixing rhizobacteria after isoflavone induction, initiate nodule formation in host legumes. Given LCOs' structural similarity to pathogen-response-eliciting chitin oligomers, foliar application of LCOs was tested for ability to induce stress-related genes under optimal growth conditions. In order to study the effects of LCO foliar spray under stressed conditions, soybean (Glycine max) seedlings grown at optimal temperature were transferred to sub-optimal temperature. After a 5-day acclimation period, the first trifoliate leaves were sprayed with 10−7 M LCO (NodBj-V (C18∶1, MeFuc)) purified from genistein-induced Bradyrhizobium japonicum culture, and harvested at 0 and 48 h following treatment. Microarray analysis was performed using Affymetrix GeneChip® Soybean Genome Arrays. Compared to the control at 48 h after LCO treatment, a total of 147 genes were differentially expressed as a result of LCO treatment, including a number of stress-related genes and transcription factors. In addition, during the 48 h time period following foliar spray application, over a thousand genes exhibited differential expression, including hundreds of those specific to the LCO-treated plants. Our results indicated that the dynamic soybean foliar transcriptome was highly responsive to LCO treatment. Quantitative real-time PCR (qPCR) validated the microarray data.
Highlights
The legume-rhizobia N2-fixation symbiosis is one of the most intensively studied and best characterized plant-microbe interactions because it is the most inexpensive and environmental-friendly source of nitrogen for crop production [1]
We have studied the effects of LCO from B. japonicum strain 532C on the gene expression profile of soybean leaves following spray application at a sub-optimal growth temperature (15uC), corresponding to typical spring field conditions of eastern Canada
The four gene lists of interest in the microarray experiment were generated from four pair-wise comparisons of gene expression profiles (Fig. 1); i.e., gene list 1 was the collection of genes differentially expressed between the dH2Otreated control and 1027 M LCO-treated plants, for leaves harvested immediately (0 h) after the spray treatment; whereas gene list 2 was the list of genes differentially expressed between the dH2O-treated control and 1027 M LCO-treated plants, for leaves harvested after 48 h of foliar spray
Summary
The legume-rhizobia N2-fixation symbiosis is one of the most intensively studied and best characterized plant-microbe interactions because it is the most inexpensive and environmental-friendly source of nitrogen for crop production [1]. Lipo-chitooligosaccharides (LCOs), known as Nod factors, are synthesized by rhizobia and excreted as the host-specific rhizobia-to-plant signals [4,5]. They can be perceived by multiple receptors in host roots and trigger a cascade of signaling events, which are essential for bacterial invasion of the host roots, leading to the formation of N2fixing root nodules [6]. The host plant responses upon exposure to LCOs can be categorized as nodulation-related and nonnodulation-related. The effects of LCOs outside the context of nodulation are of particular interest
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