Studies On Recognition Of Bacterial Elongation Factor EF-Tu By Arabidopsis AtEFR And Endogenous Receptor In Oryza Sativa

In nature,plants are immune to the majority of pathogenic microbes,largely due to perception of microbe-associated molecular patterns(MAMPs)by cell surface-localized pattern recognition receptors(PRRs),referred to as MAMP-triggered immunity(PTI).The second branch of the plant immune system is termed effector-triggered immunity,which occurs after recognition of pathogen effectors by host intracellular resistance proteins.Rice,one of the staple food crops in the world,is subject to infection of many virulent bacteria,which often causes massive yield losses in agriculture.Breeding and deployment of rice resistant cultivars is one of the most effective strategies.Effector-triggered immunity has been very effective in controlling some important plant diseases,but it only protects plants from certain pathogen strains carrying specific Avr proteins and is easily overcome by evolving pathogens.It is difficult for pathogens to evolve to evade PRR recognition given the conserved and essential nature of MAMPs.Therefore,PRR-mediated immunity is speculated to be more stable and persistent than R gene-mediated resistance.However,relatively fewer PRR genes have been reported to enhance disease resistance in crop plants through breeding and transgenic approaches so far.Here,it was explored how to utilize the elongation factor Tu(EF-Tu)receptor(EFR)in Arabidopsis to enhance bacterial disease resistant in rice through transgenic technology.EFR in Arabidopsis specifically recognizes the N-terminal acetylated elf18 region of bacterial EF-Tu and thereby activates plant immunity.The receptor was reported to be species-specific in cruciferous plants.In this study,the transgenic rice plants and cell cultures with constitutive expression of AtEFR were developed to investigate whether AtEFR senses EF-Tu and thus enhances bacterial resistance in the monocot plants.It was demonstrated that the Xanthomonas oryzae-derived elf18 peptide was not able to induce defense responses in the wild-type rice plants and cell suspensions.In contrast,elfl8 induced oxidative burst and mitogen-activated protein kinase activation in the AtEFR transgenic rice cells and plants,respectively.Furthermore,pathogenesis-related genes,such as OsPBZl,were upregulated dramatically in transgenic rice plants and cell lines in response to elf18 stimulation.More importantly,pretreatment with elfl8 triggered strong resistance to Xanthomonas oryzae pv.oryzae PX099A in the transgenic plants,which was largely dependent on the AtEFR expression level.These plants also exhibited enhanced resistance to rice bacterial brown stripe,but not to rice fungal blast,suggesting the specificity of EFR to recognize bacterial EF-Tu.A recent study demonstrated that rice was responsive to EF-Tu derived from Acidovorax avenae subsp.avenae N1141 by recognizing the EFa50 region.In this study,glutathione S-transferase(GST)-tagged EF-Tu proteins derived from several different phytopathogenic bacteria were expressed and purified in Escherichia coli.Oxidative burst assays showed that the ability of these different EF-Tu proteins to trigger reactive oxygen species(ROS)generation in the rice suspension-cultured cells was distinct.Comparison of the inducing activities of EF-Tu from PX099A and A.avenae subsp.avenae RS-1 suggested a novel recognition site in PX099A EF-Tu sensed by rice.A series of truncated EF-Tu proteins of PX099A were subsequently constructed and purified.ROS burst assays demonstrated that the peptide of PX099A EF-Tu comprising Glu246 to Leu280 was active to induce plant immunity,indicating that this peptide is a novel region recognized by rice.Collectively,the results indicate that PRRs from dicot plants can be utilized in the moncot crop plants to improve plant resistance against different pathogens,which offers a novel approach to enhance plant immunity through biotechnology.Furthermore,the study uncovered a novel site in EF-Tu of PX099 recognized by rice.The finding lays a foundation for futher elucidation of molecular mechanisms underlying rice recognition of EF-Tu.