Abstract

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 Medicago truncatula is the nodule cysteine-rich (NCR) group of defensin-like (DEFL) genes. We used a custom Affymetrix microarray to catalog the expression changes of 566 NCRs at different stages of nodule development. Additionally, bacterial mutants were used to understand the importance of the rhizobial partners in induction of NCRs. Expression of early NCRs was detected during the initial infection of rhizobia in nodules and expression continued as nodules became mature. Late NCRs were induced concomitantly with bacteroid development in the nodules. The induction of early and late NCRs 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 NCRs were required for promoter activity. These cis-element motifs were found to be unique to the NCR family among all annotated genes in the M. truncatula genome, although they contain sub-regions with clear similarity to known regulatory motifs involved in nodule-specific expression and temporal gene regulation.

Highlights

  • In legumes, biological nitrogen fixation results from the mutualistic interaction of root cells with rhizobia in specialized organs called nodules [1]

  • No nodule cysteine-rich (NCR) were identified in expressed sequence tag (EST) derived from nodules of Glycine max and Lotus japonicus, which led to the hypothesis that NCRs are specific to the inverted-repeat loss clade (IRLC) of legumes [10], [11]

  • NCR Expression Patterns are Dependent on Nodule Maturation and Rhizobial Development

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Summary

Introduction

Biological nitrogen fixation results from the mutualistic interaction of root cells with rhizobia in specialized organs called nodules [1]. This interaction leads to modification of gene expression in both host and bacteria [2]. Three independent studies found that these NCRs appear to be legume-specific and are part of a large (.300 members) gene family [9], [10], [11]. No NCRs were identified in ESTs derived from nodules of Glycine max and Lotus japonicus, which led to the hypothesis that NCRs are specific to the inverted-repeat loss clade (IRLC) of legumes [10], [11]

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