Plant and animals live in a microbial world which results in complex host-microbe interactions from beneficial to detrimental ones. The most intimate interactions have co-evolved in mutualistic symbiotic interactions where the partners complement each other usually with missing nutrients.

Our model system is the Sinorhizobium - Medicago symbiosis resulting in the formation of root nodules where bacteria inside the plant cells are irreversibly converted to non-cultivable large polyploid nitrogen fixing bacteroids (Fig. 1). The symbiosis is of mutual benefit; the bacteria by reducing the atmospheric nitrogen to ammonia satisfy the nitrogen need of the host, while the plant provides carbon sources and energy for the bacteria.

Bacteroid differentiation is under host control and mediated by at least 600 symbiotic peptides belonging to the Nodule-specific Cysteine Rich peptide (NCR) or glycine rich peptide (GRP) families. These peptides are delivered to the endosymbionts via the secretory pathway and localize to the bacterial membranes and/or cytosol indicating multiple interactions with the bacteria. NCRs and GRPs are unique for Medicago and related legumes and exclusively expressed in the polyploid infected plant cells, however, the distinct members are produced at different stages of the bacteroid and symbiotic cell differentiation. Our objectives are to characterize the activities and modes of action of symbiotic plant peptides and identify master regulators involved in the differentiation of nitrogen fixing bacteria. Since the nodule is a remarkable source of novel bioactive peptides we test the peptide activities also beyond symbiosis in vitro on other microbes and organisms for potential biotech applications. Interestingly, both the host cells and the endosymbionts are polyploid therefore we investigate the significance of endoploidy in the plant and bacterial symbionts. In addition, we employ genomics tools (induced mutants and natural variants, sequencing, etc.) to identify further genomic determinants and fine-tuners required for the intracellular accommodation of the microsymbionts and to study other symbiotic and pathogenic interactions of different microbes.

Figure 1. Differentiation of bacteroids and the infected plant cells in M. truncatula nodules. A. Root nodules. B. Longitunal section of a nodule. I. nodule meristem, II. infection zone, III. nitrogen fixation zone. C. Gradual elongation of the bacteroids in zone II and zone III.

Selected publications

Van de Velde, W. & al. (2010). Plant peptides govern terminal differentiation of bacteria in symbiosis. Science 327: 1122-1126.

Kondorosi E & al. (2013) A Paradigm for Endosymbiotic Life: Cell Differentiation of Rhizobium bacteria by Host Plant Factors. Annu. Rev. Microbiol. 67: 611–628.

Tiricz H. & al. (2013) Antimicrobial nodule-specific cysteine-rich peptides induce membrane depolarization associated changes in Sinorhizobium meliloti. Appl. Env. Microbiol. 79: 6737-6746.

Maróti, G. & al. (2011). Natural roles of antimicrobial peptides as antibiotics in microbes, plants and animals. Res. Microbiol. 162, 363-374.

Kereszt A. & al. (2011) Innate immunity effectors and virulence factors in symbiosis. Curr Opin Microbiol. 14:76-81