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- With an unprecedented increase in the human population, higher agricultural production, enhanced food safety and the protection against alarming rise of antibiotic resistant pathogenic bacteria are amongst the main challenges of this century. This proposal centered on Rhizobium-legume symbiosis aims at contributing to these tasks by i) understanding the development of symbiotic nitrogen fixing cells for improvement of the eco-friendly biological nitrogen fixation, ii) gaining a comprehensive knowledge on polyploidy having a great impact on crop yields and iii) exploiting the strategies of symbiotic plant cells for the development of novel antibiotics. Symbiotic nitrogen fixation in Rhizobium-legume interactions is a major contributor to the combined nitrogen pool in the biosphere. It takes place in root nodules where giant plant cells host the nitrogen fixing bacteria. In Medicago nodules both the plant cells and the bacteria are polyploids and incapable for cell division. These polyploid plant cells produce hundreds of symbiotic peptides (symPEPs) that provoke terminal differentiation of bacteria in symbiosis and exhibit broad range antimicrobial activities in vitro. Permanent generation of polyploid cells is essential for the nodule development. It will be studied whether the complete genome is duplicated in consecutive endocycles, how different ploidy levels affect DNA methylation and expression profile and whether polyploidy is required for the expression of symPEP genes. The activity and mode of actions of symPEPs are in the focus of the proposal; i) how symPEPs achieve bacteroid differentiation and affect nitrogen fixation and ii) whether symPEP antimicrobial activities provide novel modes of antimicrobial actions and iii) whether ¿Sym-Biotics¿ could become widely used novel antibiotics. Their applicability as plant protecting and meat decontaminating agents as well as their in vivo efficiency in mouse septicemia models will be tested.