The contribution of SERK family receptors to immunity against Xylella fastidiosa

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Plant diseases represent a significant threat to global food security. One of the pathogens ranked in the “list of priority pests” for Europe is Xylella fastidiosa. This insect-transmitted bacterium is the cause of the OLIVE QUICK DECLINE SYNDROME (OQDS) and important for outbreaks over the past eight years. A sustainable strategy to manage OQDS is to breed disease resistance into susceptible hosts. However, only few resistant olive cultivars have been identified and classical breeding approaches will be long-term, taking little advantage of the molecular knowledge on plant immune mechanisms. This project will evaluate the processes that control the infection of X. fastidiosa in susceptible and resistant olive. Having established X. fastidiosa infection in the genetic model A. thaliana, we identified that BRASSINOSTEROID INSENSITIVE 1-ASSOCIATED KINASE (BAK1) controls X. fastidiosa infection success. BAK1 belongs to the family of leucine-rich repeat-receptor-like kinases called SOMATIC EMBRYOGENESIS RECEPTOR KINASEs (SERK). BAK1/SERK3 forms complexes with pattern recognition receptors (PRR) such as the EF-Tu RECEPTOR (EFR) to induce PRR-triggered immunity (PTI). Transcriptome profiling in olive revealed that most OeSERK3 homologues were downregulated in the susceptible “Ogliarola salentina” cultivar. In resistant “Leccino”, more OeSERKs were upregulated in response to X. fastidiosa infection, and we speculate that some could function as co-receptors with PRRs. We also established callus and cell suspension cultures of both resistant and susceptible olive cultivars, which we explore for characterizing prototypic PTI responses as well as the generation of transgenic olives, i.e. transformed with EFR. Our goal is to answer two key questions: 1. How do OeSERKs regulate PTI and the response to X. fastidiosa infection? Profiling OeSERKs from olive and comparing cultivated varieties will reveal candidate genes subjected to functional analysis. OeSERK interacting proteins will be explored across the two cultivars. 2. What are the PTI signatures associated with olive immunity and Xylella’s infection success? Comparative transcriptome profiling of a resistant and susceptible olive cultivar together with assessing prototypic PTI responses to a panel of immune triggers in the olive family are likely to provide insights into this question. Being a woody perennial and non-model plant, our overall aim is to better understand PTI in olive and explore the cultivar and family genetic diversity for PTI profiling and the identification of molecular components associated with immunity. At its completion, this project will lead to new insights into the olive immune system and identify OeSERKs and interactors involved in the response to X. fastidiosa. Understanding immune co-receptor diversity is essential to engineer resistance into susceptible crop genotypes.