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AP2-ERF Transcription Factors Mediate Nod Factor–Dependent Mt ENOD11 Activation in Root Hairs via a Novel cis-Regulatory Motif^[W]

^a Laboratory of Plant Microbe Interactions, Centre National de la Recherche Scientifique–Institut National de la Recherche Agronomique, BP 52627, Castanet-Tolosan Cedex, France
^b Institut Fédératif de Recherche 40 Pôle de Biotechnologie Végétale, BP17 Auzeville, 31326 Castanet-Tolosan, France

⁴ Address correspondence to fniebel{at}toulouse.inra.fr.

Rhizobium Nod factors (NFs) are specific lipochitooligosaccharides that activate host legume signaling pathways essential for initiating the nitrogen-fixing symbiotic association. This study describes the characterization of cis-regulatory elements and trans-interacting factors that regulate NF-dependent and epidermis-specific gene transcription in Medicago truncatula. Detailed analysis of the Mt ENOD11 promoter using deletion, mutation, and gain-of-function constructs has led to the identification of an NF-responsive regulatory unit (the NF box) sufficient to direct NF-elicited expression in root hairs. NF box–mediated expression requires a major GCC-like motif, which is also essential for the binding of root hair–specific nuclear factors. Yeast one-hybrid screening has identified three closely related AP2/ERF transcription factors (ERN1 to ERN3) that are able to bind specifically to the NF box. ERN1 is identical to an ERF-like factor identified recently. Expression analysis has revealed that ERN1 and ERN2 genes are upregulated in root hairs following NF treatment and that this activation requires a functional NFP gene. Transient expression assays in Nicotiana benthamiana have further shown that nucleus-targeted ERN1 and ERN2 factors activate NF box–containing reporters, whereas ERN3 represses ERN1/ERN2-dependent transcription activation. A model is proposed for the fine-tuning of NF-elicited gene transcription in root hairs involving the interplay between repressor and activator ERN factors.

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