Defence-triggering molecules of the parasitic plant Cuscuta and the recognition by cell surface receptors

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Cuscuta species are holoparasitic plants which infect both dicotyledonous and monocotyledonous hosts. With haustoria as specific infection organs, those parasites penetrate host plants to establish a connection to the vasculature and exhaust water, solutes and sugars. One notable exception is the cultivated tomato (Solanum lycopersicum) which shows defence responses and is resistant to Cuscuta reflexa. We discovered that tomato reacts to extracts of C. reflexa with immune responses normally activated after the recognition of microbe-associated molecular patterns (MAMPs). We identified the leucine-rich repeat receptor-like protein (LRR-RLP) Cuscuta Receptor 1 (CuRe1) which detects the parasite-associated molecular pattern, named Cuscuta factor, and confers responsiveness to this yet unknown molecule. CuRe1 also increased resistance to parasitic C. reflexa when heterologously expressed in otherwise susceptible plants. By comparison with other LRR-RLPs, CuRe1 has a few outstanding peculiarities; it has a second predicted transmembrane domain (TM) which seems essential for functionality, and CuRe1 does not require a ligand-dependent association with a co-receptor of the somatic embryogenesis receptor kinases (SERKs) to activate defence signalling. Concerning the Cuscuta factor, we know that it is present in many Cuscuta species; it is a cell-wall-derived, heterogeneous mix of small peptides (mw of 2-3 kDa) which all have a secondary modification (glycosylation) that is essential for the bio-activity. We successfully scaled up the Cuscuta factor purification protocol and made first steps in sequencing part of the peptide backbone. However, the full aa-sequence together with the secondary modification of the Cuscuta factor remains to be solved.In the proposed project we plan to work on both, CuRe1 and the Cuscuta factor in two mayor parts; in part A) "the Cuscuta factor", we aim to identify the sequence, structure and secondary modification of the Cuscuta factor. Furthermore we want to decipher the mechanism of receptor activation and its natural function for the parasite. In part B) "CuRe1 activation and related downstream signalling", we aim to work on the LRR-RLP CuRe1, its peculiarities as well as its extraordinary interaction partners and related initiated downstream signalling.