Abstract
As the legume-rhizobia symbiosis is established, the plant recognizes bacterial-signaling molecules, Nod factors (NFs), and initiates transcriptional and developmental changes within the root to allow bacterial invasion and the construction of a novel organ, the nodule. Plant mutants defective in nodule initiation (Nod(-)) are thought to have defects in NF-signal transduction. However, it is unknown whether WT plants respond to NF-independent bacterial-derived signals or whether Nod(-) plant mutants show defects in global symbiosis-associated gene expression. To characterize plant gene expression in the establishment of the symbiosis, we used an Affymetrix oligonucleotide microarray representing 9,935 Medicago truncatula expressed sequences. We identified 46 sequences that are differentially expressed in plants exposed for 24 h to WT Sinorhizobium meliloti or to the invasion defective S. meliloti mutant, exoA. Eight of these genes encode nucleolar proteins, which are implicated in ribosome biogenesis. We also identified differentially expressed transcription factors, signaling components, defense response proteins, stress response proteins, and several previously uncharacterized genes. NF appears both necessary and sufficient to induce most changes. Six of seven Nod(-) M. truncatula mutants (nfp, dmi1, dmi2, dmi3, nsp1, and nsp2) showed no transcriptional response to S. meliloti, suggesting that the encoded proteins are required for initiating new transcription. The Nod(-) mutant hcl, however, exhibits a reduced transcriptional response to S. meliloti, indicating that the machinery responsible for initiating new transcription is at least partially functional in this mutant.