Classical and recently found phytohormones play an important role in plant growth and development, but plants additionally control these processes through small signalling peptides. Over 1000 potential small signalling peptide sequences are present in the Arabidopsis genome. However, to date, a mere handful of small signalling peptides have been functionally characterized and few have been linked to a receptor. Here, we assess the potential small signalling peptide outputs, namely the molecular, biochemical, and morphological changes they trigger in Arabidopsis. However, we also include some notable studies in other plant species, in order to illustrate the varied effects that can be induced by small signalling peptides. In addition, we touch on some evolutionary aspects of small signalling peptides, as studying their signalling outputs in single-cell green algae and early land plants will assist in our understanding of more complex land plants. Our overview illustrates the growing interest in the small signalling peptide research area and its importance in deepening our understanding of plant growth and development.

HighlightCLE26 plays an important role in regulating A. thaliana and monocot root architecture, and interacts with auxin signalling.

In plants, the generation of new cell types and tissues depends on coordinated and oriented formative cell divisions. The plasma membrane-localized receptor kinase ARABIDOPSIS CRINKLY 4 (ACR4) is part of a mechanism controlling formative cell divisions in the Arabidopsis root. Despite its important role in plant development, very little is known about the molecular mechanism with which ACR4 is affiliated and its network of interactions. Here, we used various complementary proteomic approaches to identify ACR4-interacting protein candidates that are likely regulators of formative cell divisions and that could pave the way to unraveling the molecular basis behind ACR4-mediated signaling. We identified PROTEIN PHOSPHATASE 2A-3 (PP2A-3), a catalytic subunit of PP2A holoenzymes, as a previously unidentified regulator of formative cell divisions and as one of the first described substrates of ACR4. Our in vitro data argue for the existence of a tight posttranslational regulation in the associated biochemical network through reciprocal regulation between ACR4 and PP2A-3 at the phosphorylation level.stem cells | columella | phosphorylation | kinase | phosphatase

Background: DEF6 is recruited to the immunological synapse upon T cell receptor-mediated signaling regulating inflammatory responses including Experimental Autoimmune Encephalomyelitis. Results: DEF6 is a substrate for ITK and forms granules co-localizing with P-bodies. Conclusion: DEF6 granule formation is likely to be mediated by unmasking a Q-rich coiled-coil region in the C terminus. Significance: Discovery of a potential link between T cell receptor-mediated signaling and translation regulation in P-bodies.

Cell–cell communication plays a crucial role in plant growth and development and relies to a large extent on peptide ligand–receptor kinase signaling mechanisms. The CRINKLY4 (CR4) family of receptor-like kinases is involved in a wide range of developmental processes in plants, including mediating columella stem cell identity and differentiation in the Arabidopsis thaliana root tip. Members of the CR4 family contain a signal peptide, an extracellular part, a single-pass transmembrane helix and an intracellular cytoplasmic protein kinase domain. The main distinguishing features of the family are the presence of seven “crinkly” repeats and a TUMOR NECROSIS FACTOR RECEPTOR (TNFR)-like domain in the extracellular part. Here, we investigated the evolutionary origin of the CR4 family and explored to what extent members of this family are conserved throughout the green lineage. We identified members of the CR4 family in various dicots and monocots, and also in the lycophyte Selaginella moellendorffii and the bryophyte Physcomitrella patens. In addition, we attempted to gain insight in the evolutionary origin of different CR4-specific domains, and we could detect “crinkly” repeat containing proteins already in single celled algae. Finally, we related the presence of likely functional CR4 orthologs to its best described signaling module comprising CLAVATA3/EMBRYO SURROUNDING REGION-RELATED 40 (CLE40), WUSCHEL RELATED HOMEOBOX 5 (WOX5), CLAVATA 1 (CLV1), and ARABIDOPSIS CR4 (ACR4), and established that this module likely is already present in bryophytes and lycophytes.

HighlightInformation collected using antagonistic peptide approaches can be very useful, but these approaches do not work in all cases and require insight on ligand-receptor interactions and peptide ligand structure.

Receptor kinases play important roles in plant growth and development, but only few of them have been functionally characterized in depth. Over the past decade CRINKLY 4 (CR4)-related research has peaked as a result of a newly discovered role of ARABIDOPSIS CR4 (ACR4) in the root. Here, we comprehensively review the available (A)CR4 literature and describe its role in embryo, seed, shoot, and root development, but we also flag an unexpected role in plant defence. In addition, we discuss ACR4 domains and protein structure, describe known ACR4-interacting proteins and substrates, and elaborate on the transcriptional regulation of ACR4 Finally, we address the missing knowledge in our understanding of ACR4 signalling.

Optimal development of root architecture is vital to the structure and nutrient absorption capabilities of any plant. We recently demonstrated that AtCLE26 regulates A. thaliana root architecture development, possibly by altering auxin distribution to the root apical meristem via inhibition of protophloem development. In addition, we showed that AtCLE26 application is able to induce a root architectural change in the monocots Brachypodium distachyon and Triticum aestivum. Here, we showed that application of the synthetic AtCLE26 peptide similarly affects other important agricultural species, such as Brassica napus and Solanum lycopersicum.