The Arabidopsis immune receptor EFR increases resistance to Xanthomonas and Xylella in transgenic sweet orange

Previous work showed that heterologous expression of the EFR pattern recognition receptor from Arabidopsis thaliana, a leucine-rich repeat receptor kinase recognizing bacterial EF-Tu (or the derived peptides elf18/elf26), provides broad-spectrum resistance against bacterial pathogens in different species. Transgenic sweet orange constitutively expressing AtEFR were generated to access whether this receptor would confer increased anti-bacterial resistance in citrus. After genetic confirmation of transgenic events, we confirmed AtEFR functionality by measuring reactive oxygen species (ROS) production, expression of immune marker genes, activation of MAP kinases and bacterial infection assays with Xylella fastidiosa subsp. pauca and Xanthomonas citri subsp. citri in transgenic events. Notably, detached leaves from transgenic plants infiltrated with X. citri (the causal agent of citrus canker) showed less symptoms and reduced bacterial population. Similarly, when inoculated with X. fastidiosa (the causal agent of CVC - Citrus Variegated Chlorosis) the transgenes perceived the bacteria and somehow restrained its growth through the upper parts of the plants. After 18 months, no or mild symptoms were observed. Altogether, our results in sweet orange indicate that EFR has great potential to improve broad-spectrum anti-bacterial disease resistance in perennial crops such as Citrus.