Research group Albert
Plant-Plant Interaction of Cuscuta spp. with Host Plants
The plant parasite Cuscuta spp. and its hosts
Living as obligate holoparasites, plants of the genus Cuscuta possess neither roots nor expanded leaves. To survive, Cuscuta develops haustoria, establishing connections directly to the vascular bundles of host plants and enwinding their stems. The parasite succeeds in representing itself as an attractive sink, and the penetration and accompanying wounding seems to go undetected by most host plants. Consequently, the attacked hosts do not try to fend off Cuscuta, which subsequently withdraws carbohydrates, nutrients and water from the host.
This situation differs from plant-microbe interactions. Given their significant evolutionary distance from microbes, over time all higher plants have established an innate immune system to defend against microbial pathogens. By means of pattern recognition receptors (PRRs), plants can sense so-called microbe-associated molecular patterns (MAMPs) and initiate defensive signaling programs that help the plant to restrict pathogen growth.
So how can plants detect plants? We are interested in identifying signals and bioactive molecules on the parasite’s side that trigger visible or measurable responses in the host plant, for plant development or for defense. On the host side, we study signaling and responses in the infested plant, focusing on perceptual systems that initiate downstream signaling and control cellular responses.
A chance exception: Cultivated tomato is resistant to Cuscuta reflexa
The relationship between cultivated tomato (Solanum lycopersicum) and Cuscuta reflexa offers a convenient tool for gaining some initial insights into how secreted parasitic signaling molecules might trigger a host plant reaction. Tomato is resistant to C. reflexa and actively responds with a clearly visible hypersensitive-like reaction precisely at the penetration site of C. reflexa haustoria (Fig. 2). This strong response leads to tomato cell-wall suberinisation and prevents the host from haustoria penetration, and consequently, C. reflexa dies off (Fig. 2). Such defensive reactions are unique to cultivated tomato and do not occur in other plants of the Solanaceae family.
Studying the invader: The parasite’s defense-triggering molecules
Extracts of C. reflexa stem and haustoria were tested for their capability to trigger typical well-known MAMP responses specific to tomato, such as ethylene production, MAP-kinase phosphorylation, oxidative burst, or induction of typical marker genes. The ethylene response seemed to work best in tomato but was absent in other related (non-resistant) solanaceaous plants (e.g., S. tuberosum, Nicotiana tabacum, or Nicotiana benthamiana), and so it was further used to screen Cuscuta-extracts for defense triggering molecules. Using stepwise purification of crude parasitic extracts by chromatography and monitoring of each purification step for its ability to trigger ethylene production, we purified an O-glycosylated 2.5 kDa peptide, which is the essential trigger of the ethylene response in cultivated tomato. For final detailed identification, biological material was scaled up and purification steps were optimized. The sequence analysis is currently in progress.
Studying the defense: Signal perception by resistant tomato
How might such defense-triggering molecules be sensed by tomato? To address this question, we first screened a collection of 20 wild tomato species for their resistance to C. reflexa and their ethylene response after treatment with parasitic extracts. Interestingly, we found three species that did not display resistance or ethylene production after treatment with C. reflexa extracts. For one species (Solanum pennellii), we obtained ~50 introgression lines (S. lycopersicum x S. pennellii) (The Tomato Genetics Resource Center; see tgrc.ucdavis.edu/), which were screened for ethylene response and resistance against C. reflexa. We mapped two loci that seem essential for ethylene response and/or resistance, respectively. One important gene locates to chromosome eight and encodes a Leucine-rich repeat receptor-like protein (LRR-RLP), which was termed Cuscuta Receptor 1 (CuRe1). After heterologous expression of the CuRe1 gene in N. benthamiana (usually insensitive to C. reflexa extracts), this plant started to respond by means of ethylene production when treated with crude C. reflexa extracts or with pure fractions of the identified glycopeptide. Peptide detection occurred in a highly sensitive and dose-dependent manner. In addition, after treatment with the C. reflexa glycopeptide, N. benthamiana with CuRe1 also showed an oxidative burst with a lag-phase of ~5 min. This indicates that CuRe1 of tomato is the corresponding receptor for the recently purified C. reflexa glycopeptide and may represent the switch for further downstream signaling and defense reactions.
A second interesting region was mapped to another chromosome. When this region is substituted in S. lycopersicum by the corresponding part of S. pennellii, plants are susceptible to C. reflexa in the same way as S. pennellii. Previously, no single gene could be identified as responsible for this resistance trait. However, the mapped ~1 Mbp sequence seems to contain a number of interesting candidate genes (such as cytosolic kinases) and transcription factors as putative downstream elements of CuRe1, or alternative receptor candidates which might act as complementary to or independent of CuRe1. Further analysis of all genes of interest will be indispensable in deciphering their role in tomato responses to a C. reflexa infection.
Objectives and future questions…
…on the parasite Cuscuta reflexa
· Identifying peptides/signals from C. reflexa that trigger defense responses
· Studying effects of the new isolated glycopeptide in susceptible plants
· Isolating and studying other signals of C. reflexa that manipulate/trigger the development of susceptible host plants
· Screening an mRNA library of C. reflexa (mRNA seq) for potential peptide signals
…on the resistant host Solanum lycopersicum
· Characterizing resistance-relevant genes
· Studying downstream signaling initiated by CuRe1
· Isolating interacting proteins/co-receptors of CuRe1
· Studying the defense-related signaling of tomato against C. reflexa running in parallel/independently from CuRe1
…on susceptible host plants
· How is Cuscuta manipulating and hooking up with the development of its host plant?
· How can the parasite successfully connect to the host vascular bundles?
· Which developmental signaling pathways are influenced in susceptible host plants?
Which host plant receptors perceive which kinds of parasitic signal?