Abstract
Tracheary Element Differentiation Inhibitory Factor (TDIF) belongs to the family of post-translationally modified CLE (CLAVATA3/embryo surrounding region (ESR)-related) peptide hormones that control root growth and define the delicate balance between stem cell proliferation and differentiation in SAM (shoot apical meristem) or RAM (root apical meristem). In Arabidopsis, Tracheary Element Differentiation Inhibitory Factor Receptor (TDR) and its ligand TDIF signaling pathway is involved in the regulation of procambial cell proliferation and inhibiting its differentiation into xylem cells. Here we present the crystal structures of the extracellular domains (ECD) of TDR alone and in complex with its ligand TDIF resolved at 2.65 Ǻ and 2.75 Ǻ respectively. These structures provide insights about the ligand perception and specific interactions between the CLE peptides and their cognate receptors. Our in vitro biochemical studies indicate that the interactions between the ligands and the receptors at the C-terminal anchoring site provide conserved binding. While the binding interactions occurring at the N-terminal anchoring site dictate differential binding specificities between different ligands and receptors. Our studies will open different unknown avenues of TDR-TDIF signaling pathways that will enhance our knowledge in this field highlighting the receptor ligand interaction, receptor activation, signaling network, modes of action and will serve as a structure function relationship model between the ligand and the receptor for various similar leucine-rich repeat receptor-like kinases (LRR-RLKs).
Highlights
Since the discovery of the first plant peptide hormone systemin in 1991 [1], secreted small peptides have been recognized as essential mediators in intercellular communication that governs plant growth, development, interaction with environment, and other biological responses [2]
We have crystallized the extracellular domain of A. thaliana Tracheary Element Differentiation Inhibitory Factor Receptor (TDR) alone and ecdTDR in complex with a synthetic treachery element differentiation inhibitory factor (TDIF) peptide
We have determined the apo-TDR structure by molecular replacement using the FLS2-extracellular domains (ECD) structure (PDB ID 4MN8) as an initial search model, and the structure of the ecdTDR/TDIF complex structure was solved by molecular replacement using the apo-TDR structure as the search model
Summary
Since the discovery of the first plant peptide hormone systemin in 1991 [1], secreted small peptides have been recognized as essential mediators in intercellular communication that governs plant growth, development, interaction with environment, and other biological responses [2]. Two CLE genes, CLE41 and CLE44, encode a CLE peptide which is hydroxylated and proteolytically processed into a 12 amino acid residue peptide, known as tracheary element differentiation inhibitory factor (TDIF) [12]. Structure-guided mutational studies in both reports have indicated a conserved binding mode between CLE peptides and their cognate LRR-RLK receptors centered on residue G6 of CLEs [27, 28]. Further studies are entailed in order to understand the binding specificities of CLE ligands to different LRR-RLKs. In this study, we use TDIF-TDR interaction as an example to understand the structural basis of differential CLE peptide recognition by LRR-RLKs in plant growth and development. Our results corroborate the previous structural studies of TDIF-TDR(PXY) interactions, and suggests a differential binding mode of CLEs/LRR-RLKs interactions. The results will help to reveal how short peptide ligands activate different LRR-RLKs in general
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