B08

Functional diversity of the Arabidopsis thaliana SHRK family

ShRK signalling in plant-microbe interactions
Receptor-like kinases (RLKs) are modular proteins that enable plant cells to monitor their environment and e.g. detect the presence of microbes. Most RLKs consist of an extracellular sensor domain that perceives exogenous or endogenous stimuli and an intracellular domain that executes downstream signalling. The extracellular domain can be comprised of different domains and architectures; most members of the LRRI-RLK family, for example, contain an extracellular malectin-like domain (MLD), which is connected to leucine-rich repeats (LRRs) via a GDPC amino acid sequence motif (MLD-LRR-RKs). MLD-LRR-RLKs are implicated in the accommodation of beneficial (Symbiosis Receptor-like kinase, SymRK; Lotus japonicus) and detrimental (e.g. SymRK-homologous Receptor-like Kinases, AtSHRKs; Arabidopsis thaliana) microbes. Aggregated data on SymRK and AtSHRKs suggest a complex regulation of MLD-LRR-RLKs. We found that at least three distinct AtSHRK paralogs are involved in the modulation of the interaction of the obligate biotrophic oomycete and the causative agent of downy mildew disease Hyaloperonospora arabidopsidis (Hpa) with A. thaliana. We will explore the mechanistic details of SHRK signalling and the functional consequences of the ancestral sequence diversification among SHRK paralogs. We have identified a suite of putative SHRK interactors via yeast two-hybrid screens. These comprise diverse interaction specificities with either all three or only one or two SHRKs pointing towards SHRK-specific interactomes and thus potentially distinct signalling cascades. We will launch comparative biochemical, biophysical, genetic, phytopathological, and cell biological approaches to study the role of each SHRK and its interactors in the Arabidopsis-Hpa interaction. Overall, we expect to illuminate signalling mechanisms of MLD-LRR-RLKs and increase our understanding of the Arabidopsis-Hpa association. In the long run, our knowledge about MLD-LRR-RLKs might contribute to the targeted manipulation of diseases in agriculturally relevant crops.

The malectin-like domain leucine-rich repeat receptor kinase (MLD-LRR-RK) SymRK belongs to the LRR-I family of receptor kinases and is a key component in the successful establishment of plant root symbioses1-3. SymRK is subjected to proteolytic cleavage in planta giving rise to a free MLD and a truncated and highly-unstable receptor protein, which preferentially interacts with NFR54. The overabundance of SymRK results in the spontaneous activation of symbiosis signalling in the absence of rhizobia5. Release of the MLD as well as phosphorylation in the kinase domain are crucial for proper symbiotic development in the epidermis but not in the cortex6-7. Taken together, these data imply a complex regulation of MLD-LRR-RKs to fine-tune receptor activation and signal attenuation. Intriguingly, MLD-LRR-RKs are not only implicated in the accommodation of beneficial microbes (SymRK; Lotus japonicus). In Arabidopsis thaliana, SymRK homologous receptor kinase 1 (SHRK1) and SHRK2 support the reproductive success of the obligate biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa) and contribute to the proper development of the oomycete within plant cells8. However, it is unknown how SHRKs impact the Arabidopsis-Hpa interaction and the underlying signalling events remain elusive. It is our long-term goal to illuminate the mechanistic details of SHRK signalling in plant-microbe interactions.

References
1. Stracke, S. et al. A plant receptor-like kinase required for both bacterial and fungal symbiosis. Nature (2002).
2. Markmann, K., Giczey, G. & Parniske, M. Functional adaptation of a plant receptor-kinase paved the way for the evolution of intracellular root symbioses with bacteria. PLoS Biol. (2008).
3. Gherbi, H. et al. SymRK defines a common genetic basis for plant root endosymbioses with arbuscular mycorrhiza fungi, rhizobia, and Frankiabacteria. Proc. Natl. Acad. Sci. (2008).
4. Antolín-Llovera, M., Ried, M. K. & Parniske, M. Cleavage of the symbiosis receptor-like kinase ectodomain promotes complex formation with nod factor receptor 5. Curr. Biol. (2014).
5. Ried, M. K., Antolín-Llovera, M. & Parniske, M. Spontaneous symbiotic reprogramming of plant roots triggered by receptor-like kinases. Elife. (2014).
6. Kosuta, S. et al. Lotus japonicus symRK-14 uncouples the cortical and epidermal symbiotic program. Plant J. (2011).
7. Saha, S. et al. Gatekeeper Tyrosine Phosphorylation of SYMRK Is Essential for Synchronizing the Epidermal and Cortical Responses in Root Nodule Symbiosis. Plant Physiol. (2016).
8. Ried, M. K. et al. A set of Arabidopsis genes involved in the accommodation of the downy mildew pathogen Hyaloperonospora arabidopsidis. PLoS Pathog. (2019).

Principal Investigator: Dr. Martina Ried, Leibniz Institute of Plant Biochemistry