Large-scale biological nitrogen fixation would mitigate the effects of the indiscriminate use of fertilisers. In addition, the ability to manipulate bacterial cell differentiation could be used to develop novel antibiotics without running the risk drug-resistant bacterial strains emerging. The EU-funded SYM-BIOTICS project studied the biological nitrogen fixation process to find eco-friendly solutions for increasing agricultural production and developing novel antibiotics. Biological nitrogen fixation in the Inverted Repeat-Lacking Clade (IRLC) legumes involves nodule-specific cysteine-rich (NCR) peptides that modify the properties of endosymbiont rhizobium bacteria. This host-directed differentiation of bacteria is a multistep process that culminates in the development of large polyploidy non-cultivable nitrogen-fixing bacteroids. NCR peptides – a win-win for us Researchers developed cutting-edge protocols and studied the role of polyploidy and irreversible differentiation in nitrogen-fixing bacteroids to determine the role of NCRs in this process. In ten selected IRLC legumes, they identified the NCR gene repertoire varying from a few to hundreds of genes and found a correlation between bacteroid morphology and the complexity of the NCR peptide families. Out of over 700 NCR peptides in Medicago truncatula, scientists identified the presence of around 150 peptides in these bacteroids, indicating their high stability. Several of these peptides, such as NCR247 and NCR169 proved to be critical for successful symbiosis, indicating their unique, irreplaceable functions. ‘The 700 NCRs arise from gene duplication events with some peptides having redundant functions while others are essential. Among these, several are specific for Medicago species like NCR169, while others are major players in the bacterial differential process,’ Prof. Eva Kondorosi, the principal investigator of this project explains. An interesting development is that some of these NCR peptides, more specifically the cationic ones, have antimicrobial properties. Prof. Kondorosi points out: ‘For example, NCR247 inhibits bacterial cell division and its interaction with many bacterial proteins alters the bacterium’s physiology with multiple mechanisms. This peptide in vitro effectively kills many pathogenic bacteria and fungi without cytotoxicity to human cells. In addition, attacking the microorganisms using many targets and pathways reduces the risk of resistance developing. These properties are thus ideal for the development of new antibiotics.’ Scientists tested around 40 synthetic NCR peptides on Gram-negative and Gram-positive bacteria, as well as fungi, with good results. Besides being non-toxic to human and animal cell lines, their performance was comparable to the commercially available antifungal amphotericin B. Furthermore, their multi-targeted mechanism of action minimises the emergence of drug-resistance. Plant resources, a boon for mankind This unprecedented study of how host organisms such as plants utilise NCR peptides to manipulate and modify microbes has provided novel insights with wide-ranging applications. Besides improving nitrogen fixation, this information could be used to develop resources for wastewater treatment, hydrogen production and environmental remediation. ‘We have excellent candidates for NCR peptide based antibiotics. However, our work is not ended. Due to the cost of chemical peptide synthesis, we need to reduce the size of peptides. Testing the combined action of antimicrobial peptides could prove to be the key to decreasing their minimal inhibitory concentration level’ Prof. Kondorosi concludes. Finally, a patent has been filed regarding the antimicrobial effects of NCR peptides. Thanks to the efforts of SYM-BIOTICS affordable and effective peptide-based antibiotics could now be available sooner than before.
SYM-BIOTICS, symbiosis, antibiotics, food security, NCR peptides