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Plant Cell Physiol. 2018 Nov 23. doi: 10.1093/pcp/pcy228. [Epub ahead of print]

A Novel Positive Regulator of the Early Stages of the Root Nodule Symbiosis Identified by Phosphoproteomics.

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Laboratorio de Genómica Funcional de Leguminosas. Facultad de Estudios Superiores Iztacala, Universidad Nacional Autónoma de México. Tlalnepantla, Estado de México, México.
Departments of Bacteriology and Agronomy, University of Wisconsin-Madison, Madison, WI, USA.
Current Address: Laboratoire de Recherche en Sciences Végétales, Université de Toulouse, CNRS, UPS, 24 Chemin de Borde Rouge, Auzeville, BP42617. 3126 Castanet Tolosan, France.
Current Address: School of Agriculture, University of Wisconsin-Platteville, USA.
Posgrado en Ciencias Biológicas, Universidad Nacional Autónoma de México, Coyoacan, Ciudad de México, México.
Unidad de Biomedicina, Facultad de Estudios Superiores Iztacala, Universidad Nacional Autónoma de México. Tlalnepantla, Estado de México, México.
Department of Biochemistry, University of Wisconsin, Madison, WI, USA.
Noble Research Institute, 2510 Sam Noble Parkway, Ardmore Oklahoma, USA.


Signals and signaling pathways underlying the symbiosis between legumes and rhizobia have been studied extensively over the past decades. In a previous phosphoproteomic study on the Medicago truncatula - Sinorhizobium meliloti symbiosis, we identified plant proteins that are differentially phosphorylated upon the perception of rhizobial signals, called Nod factors. In this study, we provide experimental evidence that one of these proteins, Early Phosphorylated Protein 1 (EPP1), is required for the initiation of this symbiosis. Upon inoculation with rhizobia, MtEPP1 expression was induced in curled root hairs. Down-regulation of MtEPP1 in M. truncatula roots almost abolished calcium spiking, reduced the expression of essential symbiosis-related genes (MtNIN, MtNF-YB1, MtERN1, and MtENOD40), and strongly decreased nodule development. Phylogenetic analyses revealed that orthologs of MtEPP1 are present in legumes and specifically in plant species able to host arbuscular mycorrhizal fungi, suggesting a possible role in this association too. Short chitin oligomers induced the phosphorylation of MtEPP1 like Nod factors. However, the down-regulation of MtEPP1 affected the colonization of M. truncatula roots by arbuscular mycorrhizal fungi only moderately. Altogether, these findings indicate that MtEPP1 is essential for the establishment of the legume-rhizobia symbiosis but might plays a limited role in the arbuscular mycorrhizal symbiosis.


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