Plants can detect pathogens on the inside and outside of their cells. Two types of plasma membrane receptors are employed by plants to detect pathogens extracellularly: receptor-like kinases (RLKs) and receptor-like proteins (RLPs). These receptors recognize specific, pathogen-derived molecules called patterns. Subsequently, pattern recognition leads to the activation of plant immunity and, in many cases, to effective pathogen suppression and resistance. In this thesis, we describe the discovery of a novel pattern, the family of necrosis and ethylene-inducing peptide 1 (Nep1)-like proteins (NLPs). NLPs are secreted by several, mainly plant-associated, bacteria, fungi and oomycetes. Oomycete pathogens, such as the downy mildews and Phytophthora species, cause major crop losses and the cultivation of oomycete-resistant crops therefore, is a major focal point of plant breeders.
In a search for the function of NLPs, we generated transgenic Arabidopsis plants that express NLPs. Quite surprisingly, expression led to a reduced growth phenotype. Growth inhibition has often been associated with the activation of immunity: energy normally invested in plant development is used by the immune system. This proved to be the case here too: NLP-expressing plants were not only smaller but also highly resistant to downy mildew. The full protein is not always required to be functional as a proteinaceous pattern. For NLPs, the central domain of these proteins proved to be sufficient for the activation of immunity. This 24-amino acid peptide, called nlp24 is highly conserved. Treatment of plants with nlp24 peptides derived from NLPs of fungal, bacterial, and oomycete origin, led to the production of the defense-related plant hormone ethylene and high resistance to downy mildew. NLPs derived from fungi, bacteria and oomycetes thus act as molecular patterns in Arabidopsis.
Next, the question arose how Arabidopsis recognizes this NLP-derived pattern. We identified RLP23 as the NLP receptor. Furthermore, we showed that the RLK SOBIR1 is required for NLP recognition. It interacts with RLP23 to form a functional bipartite receptor complex. However, the function of non-cytotoxic NLPs remains elusive.
Arabidopsis is not the only plant species that recognizes NLPs. We determined that NLP treatment also triggered immunity in lettuce. Based on our research, lettuce lacks RLP23 and thus recognizes NLPs through a different receptor protein. To try to identify the NLP receptor in lettuce, wild lettuce species were tested for nlp24-sensitivity. Surprisingly, most wild lettuce species were NLP-unresponsive. Next, we tried to map nlp24-responsiveness using genetic crosses between cultivated lettuce and a wild lettuce variety. Our findings point to a genetic model in which 2 unlinked dominant genes encode redundant NLP receptors. Hopefully, future research will help decipher the location of the NLP receptor.
Finally, the author gives an overview of how the newly acquired knowledge in this thesis can be applied for breeding sustainable disease-resistant crops.