Parasitic molecular cues during plant-plant interaction - recognition and influence on host plants

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Plants of the genus Cuscuta live as holoparasites and infect nearly all dicot plants by penetrating stems with haustoria. Haustoria connect to the vascular bundles of their hosts and withdraw water, carbohydrates and nutrients. In a recent work, we identified a receptor (Cuscuta receptor 1, CuRe1) of resistant tomato as a critical component to recognize a Cuscuta glycopeptide, defined as Cuscuta factor 1 (CuF1). Upon CuF1-perception, CuRe1 initiates defense responses and also leads to increased resistance when expressed in otherwise susceptible host plants. The recognition of C. reflexa as alien by CuRe1 is an exception of tomato. Usually, the parasite stays unrecognized by its host and actively manipulates it towards susceptibility: Parenchymal cells of the vascular bundles start to divide, differentiate and finally connect to the parasitic cells of both Xylem and Phloem. What type of molecular cues initiate and regulate these mechanisms of susceptibility? In own preliminary work, we cloned promoters of host plant genes that are up-regulated during a Cuscuta infection. Using them as promoter::reporter-gene (luciferase) constructs in plant systems, these constructs serve as tools to detect luciferase activity upon treatment with stepwise purified Cuscuta extracts and will finally help to identify new Cuscuta factors (CuFs) that manipulate host plant signaling. First results indicate the presence of different new CuFs which will be further characterized and identified. Surprisingly, CuF1 was also able to trigger the luciferase expression in case of one promoter::luciferase construct indicating a relevant role of CuF1 besides its exclusive role of triggering defense in tomato. Such alternative CuF1-induced signaling also hints to a CuRe1-independent perception mechanism existing in susceptible hosts.In a complementary project we aim to identify the detection mechanisms for parasitic CuFs in host plants and want to gain knowledge about the susceptibility related signaling. We will use Arabidopsis receptor mutants which collections are available in our laboratory. Mutant plants will be grown, infected with Cuscuta and further analyzed according altered susceptibility. Subsidiary, we have diverse Aarabidopsis accessions at hand (1001 Genomes project) that we will check in addition. Relevant genes of obtained phenotypes with disturbed susceptibility will be identified either by classical map-based cloning or by using Genome-wide association studies (GWAS). Novel components of CuF-perception or related signaling components will be further analyzed with well-established methods such as studies on receptor-ligand interaction, localization, protein-protein interaction, etc. Furthermore, we will prove the biological relevance by determining the Cuscuta biomass after growth on mutants or transgenic plants. This will help to decipher the molecular and biological role of novel components required for a plant-plant interaction.