Molecular functions of class II Glycine-rich proteins (GRPs) in plant parasitism and plant immunity

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Cuscuta spp. (dodder) are root- and leaf-less holoparasitic plants which infect dicotyledonous plants by winding around the shoots and penetrating host tissues. With multicellular infection organs, called haustoria, Cuscuta spp. connect to the host’s vasculature and withdraw water, solutes, nutrients and carbohydrates. In this manner, dodders harm host plants and pose an extra threat to many crops in addition to other phytopathogens and herbivores. Cultivated tomato as one notable exception displays an active defense against Cuscuta reflexa including immune responses like the induction of ethylene, reactive-oxygen species as well as a type of hypersensitive response (HR), in common finally leading to a full resistance of tomato against C. reflexa. In previous works we identified the leucine-rich repeat receptor protein Cuscuta Receptor 1 (CuRe1) as a critical component of tomato’s immune system to recognize C. reflexa and to induce the defense responses described. As a pathogen-associated molecular pattern (PAMP) of the parasitic plant C. reflexa, we recently identified a C. reflexa glycine-rich protein (CrGRP) that localizes to the parasite’s cell walls and that seems present in all parts and tissues of Cuscuta plants. CrGRP physically interacts with CuRe1 and we identified a 21 amino acid long peptide epitope, crip21 (cysteine-rich peptide 21), that is sufficient to bind to CuRe1 and to induce tomato immune responses in nanomolar ranges. After BLAST-analyses, homologs of CrGRPs can be found in all other organisms (microbes, fungi, arthropods and vertebrates) and other plants, even in tomato. The Solanum lycopersicum GRP (SlGRP) or Slcrip21 peptide, however, was not able to be recognized by tomato CuRe1 and we identified a single aa residue (Tyr11) that abolishes its function as defense-trigger and also demonstrates that CrGRP is not acting as potential damage-associated molecular pattern (DAMP). The original function of CrGRP and SlGRP are unknown and after literature research, it turned out that in general not much is known about the molecular function or physiological role of such GRPs and derived peptides in plants. In the proposed work, we therefore would like to address the following questions; i) What functions has CrGRP for Cuscuta spp. and how are those related to susceptibility or resistance of hosts? ii) what physiological roles and molecular functions do have GRPs for host plants in general and during an ongoing infection process by dodder? iii) How do GRPs associate with other proteins or (cell wall) components and does it matter for Cuscuta – host interaction? iv) Peptide motifs with high similarity to crip21 can be found in other parasitic plants, in insect or in microbial phytopathogens – do they have any function as trigger or suppressor of plant defense? and v) How are potential receptors for crip21 are distributed among the Solanaceae and how efficient do they recognize crip21-like peptides from other phytopathogens?