Cell-to-cell communication is essential for plant growth, defence and development, with peptide hormones being key players in the regulation ofthese signalling networks. Peptide hormones are produced as pre-pro-peptides which undergo proteolytic cleavage resulting in a short peptide sequence. In addition, many of these peptides are post-transcriptionally modified by tyrosine sulfation, proline hydroxylation or hydroxyproline arabinosylation in order to obtain the functional peptide hormone. In Arabidopsis many peptide families have been described, among those are the well characterized INFLUORESCENCE DEFICIENT ABSCISSION (IDA)/IDA-LIKE (IDL) and the CLAVATA 3 / EMBRYO SURROUNDING REGION (CLE) family. Peptide hormones from both families actas ligands for the Leucine-Rich Repeat Receptor Kinases (LRR-RKs), the largest subfamily of membrane receptor kinases. It has been shown, that IDA/IDL peptides act as ligand for HAESA(HAE)/HAE-LIKE(HSL) receptors in order to control the lateral root emergence, floral organ_and leaf abscission, while CLE peptides are recruited by CLAVATA1 (CLVl)/BARELY ANY MERISTEMl (BAMl) LRR-RLK receptors, controlling meristem maintenance and vasculature development. A recent study provided biochemical and functional data on the possibility that the HAESA-LIKEl (HSLl) receptor can sense CLE9/10 and other CLE family members, using SERK 1 as a co-receptor to modulate stomatal development.
In the following thesis we could confirm that HSL1 can sense CLE peptides, but our biochemical and in planta work shows that HSL1 recognizes the IDL peptide family with higher affinity and is able to substitute the HAE receptor in the regulation of floral abscission. Comparative structural analysis of the HSLl/IDL and BAMl/CLE complexes reveal key structural features as well as different peptide modifications that drive the distinct recognition affinities. Moreover, expression pattern analysis reveale"d, that HSLl, IDL4, BAMl, and CLE9/10 are co-expressed during stomatal development indicating a new potential role for IDL4 in the stomatal development process. Moreover, the expression pattern analysis showed a. broad distribution of different IDLs in various tissues in Arabidopsis suggesting their involvement in different developmental processes. As a continuation of this work, we found a potential role of the HAE/IDL6 complex during lateral root development. Biochemical assays and quantitative analysis oflateral root emergence indicate a possible novel fonction of the HAE/IDL6 complex during this developmental process. The identification of key structural elements that drive the specific recognition of IDL/CLE peptides opens new avenues in the understanding on how receptors can differentiate between various signais presented at the cell surface in physiological conditions. Further characterization of IDL2, IDL4 and IDL6 could shed light on the impact of different HSL/IDL complexes during various developmental processes in Arabidopsis.