Heterologous expression of the immune receptorEFRinMedicago truncatulareduces pathogenic infection, but not rhizobial symbiosis
AbstractInterfamily transfer of plant pattern recognition receptors (PRRs) represents a promising biotechnological approach to engineer broad-spectrum, and potentially durable, disease resistance in crops. It is however unclear whether new recognition specificities to given pathogen-associated molecular patterns (PAMPs) affect the interaction of the recipient plant with beneficial microbes. To test this in a direct reductionist approach, we transferred theBrassicaceae-specific PRR ELONGATION FACTOR-THERMO UNSTABLE RECEPTOR (EFR) fromArabidopsis thalianato the legumeMedicago truncatula, conferring recognition of the bacterial EF-Tu protein. ConstitutiveEFRexpression led to EFR accumulation and activation of immune responses upon treatment with the EF-Tu-derived elf18 peptide in leaves and roots. The interaction ofM. truncatulawith the bacterial symbiontSinorhizobium melilotiis characterized by the formation of root nodules that fix atmospheric nitrogen. Although nodule numbers were slightly reduced at an early stage of the infection inEFR-Medicagowhen compared to control lines, nodulation was similar in all lines at later stages. Furthermore, nodule colonization by rhizobia, and nitrogen fixation were not compromised byEFRexpression. Importantly, theM. truncatulalines expressingEFRwere substantially more resistant to the root bacterial pathogenRalstonia solanacearum. Our data suggest that the transfer of EFR toM. truncatuladoes not impede root nodule symbiosis, but has a positive impact on disease resistance against a bacterial pathogen. In addition, our results indicate thatRhizobiumcan either avoid PAMP recognition during the infection process, or is able to actively suppress immune signaling.Significance StatementCrop engineering helps reducing the economic and environmental costs of plant disease. The genetic transfer of immune receptors across plant species is a promising biotechnological approach to increase disease resistance. Surface-localized pattern-recognition receptors (PRRs), which detect conserved characteristic microbial features, are functional in heterologous taxonomically-diverse plant species, and confer broad-spectrum disease resistance. It was unclear whether PRR transfer negatively impacts the association of the recipient plants with symbiotic microbes. Here, we show that a legume engineered with a novel PRR recognizing a conserved bacterial protein becomes more resistant to an important bacterial pathogen without significant impact on nitrogen-fixing symbiosis with rhizobia. This finding is of particular relevance as attempts to transfer this important symbiosis into non-legume plants are ongoing.
