Two CCAAT ‐box‐binding transcription factors redundantly regulate early steps of the legume‐rhizobia endosymbiosis

Summary During endosymbiotic interactions between legume plants and nitrogen‐fixing rhizobia, successful root infection by bacteria and nodule organogenesis requires the perception and transduction of bacterial lipo‐chitooligosaccharidic signal called N od factor ( NF ). NF perception in legume roots leads to the activation of an early signaling pathway and of a set of symbiotic genes which is controlled by specific early transcription factors ( TF s) including CYCLOPS / IPD 3, NSP 1, NSP 2, ERN 1 and NIN . In this study, we bring convincing evidence that the M edicago truncatula CCAAT ‐box‐binding NF ‐ YA 1 TF , previously associated with later stages of rhizobial infection and nodule meristem formation is, together with its closest homolog NF ‐ YA 2 , also an essential positive regulator of the NF ‐signaling pathway. Here we show that NF ‐ YA 1 and NF ‐ YA 2 are both expressed in epidermal cells responding to NF s and their knock‐down by reverse genetic approaches severely affects the NF ‐induced expression of symbiotic genes and rhizobial infection. Further over‐expression, transactivation and Ch IP ‐ PCR approaches indicate that NF ‐ YA 1 and NF ‐ YA 2 function, at least in part, via the direct activation of ERN 1 . We thus propose a model in which NF ‐ YA 1 and NF ‐ YA 2 appear as early symbiotic regulators acting downstream of DMI 3 and NIN and possibly within the same regulatory complexes as NSP 1/2 to directly activate the expression of ERN 1 .