Regulatory Patterns of a Large Family of Defensin-Like Genes Expressed in Nodules of <i>Medicago truncatula</i>

<div><p>Root nodules are the symbiotic organ of legumes that house nitrogen-fixing bacteria. Many genes are specifically induced in nodules during the interactions between the host plant and symbiotic rhizobia. Information regarding the regulation of expression for most of these genes is lacking. One of the largest gene families expressed in the nodules of the model legume <i>Medicago truncatula</i> is the nodule cysteine-rich (<i>NCR</i>) group of defensin-like (<i>DEFL</i>) genes. We used a custom Affymetrix microarray to catalog the expression changes of 566 <i>NCR</i>s at different stages of nodule development. Additionally, bacterial mutants were used to understand the importance of the rhizobial partners in induction of <i>NCR</i>s. Expression of early <i>NCR</i>s was detected during the initial infection of rhizobia in nodules and expression continued as nodules became mature. Late <i>NCR</i>s were induced concomitantly with bacteroid development in the nodules. The induction of early and late <i>NCR</i>s was correlated with the number and morphology of rhizobia in the nodule. Conserved 41 to 50 bp motifs identified in the upstream 1,000 bp promoter regions of <i>NCR</i>s were required for promoter activity. These <i>cis</i>-element motifs were found to be unique to the <i>NCR</i> family among all annotated genes in the <i>M. truncatula</i> genome, although they contain sub-regions with clear similarity to known regulatory motifs involved in nodule-specific expression and temporal gene regulation.</p>