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Innate immunity in the model plant Arabidopsis thaliana: perception of Microbe-Associated Molecular Patterns (MAMPs)

Supervision: Dr. Stefanie Ranf (TUM Junior Fellowand DFG-Emmy Noether Junior Research Group Leader)

Plants are permanently exposed to diverse classes of potentially pathogenic microbes. Despite, plants are mostly healthy as they have evolved an efficient immune system. Plants, however, do not possess specialized roaming immune cells like vertebrates, but rely solely on a genetically determined innate immune response that can be executed by all plant cells. During attempted infection of plants, pathogens can be detected through “Microbe-Associated Molecular Patterns” (MAMPs). Conserved and indispensable microbial components such as fungal chitin, bacterial peptidoglycans, lipopolysaccharides or flagellin are typical MAMPs. Perception of such patterns by specific cell-surface “pattern recognition receptors” (PRRs) of the host initiates immune reactions that, if successful, lead to MAMP-triggered immunity (MTI). Pathogens, in turn, can evade recognition or suppress defence responses, while plants evolve new recognition strategies. Hence, a dynamic evolutionary arms-race exists between pathogenic microbes and their potential host plants. Understanding the underlying molecular mechanisms of plant innate immunity may support plant breeding and biotechnology in keeping pace with the adaptation of pathogens and thus in strengthening plant health.

Extracellular MAMP perception by host PRRs at the cell-surface induces intracellular signalling cascades and subsequent defence reactions (Ranf et al. 2015). One of the earliest signalling events activated after MAMP perception are rapid changes in the cytosolic Ca2+ concentration ([Ca2+]cyt), which are a prerequisite for establishment of downstream responses, such as reactive oxygen species accumulation, activation of protein kinases, and induction of defence gene expression (Ranf et al., 2011). Despite the pivotal role of Ca2+ as second messenger in MAMP signalling, our knowledge about the participating components and their upstream regulation and connection to the receptor complex is still fragmentary. Seedlings of the model plant Arabidopsis thaliana expressing the Ca2+-reporter aequorin allow direct monitoring of Ca2+ signalling events and thus constitute a sensitive and quantitative measure of MAMP perception and early signal transduction. To unravel the missing links of early MAMP signalling, we apply reverse and forward genetic screening for Arabidopsis mutants with altered MAMP-induced [Ca2+]cyt elevations in a high-throughput manner (Ranf et al., 2012). Currently, we identify several of such mutants and characterise respective proteins for their mechanistic function in innate immunity. Ongoing work further involves the identification of new specificities in perception mechanisms of MAMPs.

 

Ranf, S., Eschen-Lippold, L., Pecher, P., Lee, J. and Scheel, D. (2011) Interplay between calcium signalling and early signalling elements during defence responses to microbe- or damage-associated molecular patterns. Plant J, 68, 100-113.

Ranf, S., Grimmer, J., Pöschl, Y., Pecher, P., Chinchilla, D., Scheel, D. and Lee, J. (2012) Defense-related calcium signaling mutants uncovered via a quantitative high-throughput screen in Arabidopsis thaliana. Mol Plant, 5, 115-130.

Ranf S, Gisch N, Schäffer M, Illig T, Westphal L, Knirel YA, Sánchez-Carballo PM, Zähringer U, Hückelhoven R, Lee J, Scheel D (2015) A lectn S-domain receptor kinase mediates lipopolysaccharide sensing in Arabisopsis thaliana. Nature Immunology 16: 426–433