Perception of the parasitic plant Cuscuta reflexa, as an invader, by the tomato Solanum lycopersicum.

Abstract

Cuscuta spp. are assigned to the family of the Morning glories (Convolvulaceae) and live as obligate holoparasitic plants, which infect the shoot of the host plants. They have a broad host spectrum and infect nearly all dicot plants with a few notable exceptions. For example, tomato (Solanum lycopersicum) is able to fend off the parasite’s attack with an active defense. It responds to extracts of C. reflexa in a similar manner as known for the detection of microbe-associated molecular patterns (MAMPs) (e.g. increased ethylene biosynthesis and the production of reactive oxygen species (ROS)). Using recombinant inbred lines between the resistant S. lycopersicum and the susceptible S. pennellii allowed for mapping of the corresponding locus within the tomato genome, responsible for the response against Cuscuta. Further mapping led to the identification of the leucine-rich repeat receptor like protein (LRR-RLP) CuRe1 (Cuscuta Receptor 1). CuRe1 perceives a molecular pattern from C. reflexa and subsequently induces the defense responses described above. Initial characterization indicated for a proteinaceous defense trigger with potential secondary modifications. To identify this parasite associated molecular pattern (ParAMP) we used chromatographic purification techniques (SPE, FPLC, HPLC) and mass spectrometry (MS). After purification and subsequent analyses, we found multiple 2-3 kDa peptides in different fractions of the C. reflexa extract that could be correlated with the CuRe1 activation. After in-depth MS analyses and partial peptide sequencing, we found that these peptides are degradation products of the same precursor protein, a class II C. reflexa glycine-rich protein (CrGRP). We could specify the epitope within the protein to a 21 aa long peptide with six cysteine residues. Synthesized peptides as well as heterologously expressed CrGRP were able to induce CuRe1-dependent defense responses. These findings demonstrate CrGRP as the ParAMP which is specifically recognized by CuRe1 and enables tomato to detect C. reflexa as a pathogenic invader. Notably, GRPs are widespread among plants, microbes and animals. Tomato itself possesses this type of GRP with a comparable six Cysteine motif. Nevertheless, these tomato homologs do not activate CuRe1. Future work will address the question why this motif serves as a specific target for Cuscuta perception.