Host-secreted antimicrobial peptide enforces symbiotic selectivity in Medicago truncatula

Wang Q, Q. and Yang S, S. and Liu, J. and Terecskei, Kata and Ábrahám, Edit and Domonkos, Ágota and Szűcs, Attila and Körmöczi, Péter and Wang, Ting and Kondorosi, Éva and Kaló, Péter and Kereszt, Attila (2017) Host-secreted antimicrobial peptide enforces symbiotic selectivity in Medicago truncatula. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 114 (26). pp. 6854-6859. ISSN 0027-8424

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Abstract

Legumes engage in root nodule symbioses with nitrogen-fixing soil bacteria known as rhizobia. In nodule cells, bacteria are enclosed in membrane-bound vesicles called symbiosomes and differentiate into bacteroids that are capable of converting atmospheric nitrogen into ammonia. Bacteroid differentiation and prolonged intracellular survival are essential for development of functional nodules. However, in the Medicago truncatula-Sinorhizobium meliloti symbiosis, incompatibility between symbiotic partners frequently occurs, leading to the formation of infected nodules defective in nitrogen fixation (Fix-). Here, we report the identification and cloning of the M. truncatula NFS2 gene that regulates this type of specificity pertaining to S. meliloti strain Rm41. We demonstrate that NFS2 encodes a nodule-specific cysteine-rich (NCR) peptide that acts to promote bacterial lysis after differentiation. The negative role of NFS2 in symbiosis is contingent on host genetic background and can be counteracted by other genes encoded by the host. This work extends the paradigm of NCR function to include the negative regulation of symbiotic persistence in host-strain interactions. Our data suggest that NCR peptides are host determinants of symbiotic specificity in M. truncatula and possibly in closely related legumes that form indeterminate nodules in which bacterial symbionts undergo terminal differentiation.

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