Spatial range, temporal span, and promiscuity of CLE-RLK signaling

An updated model of shoot apical meristem regulation by ERECTA family and CLAVATA3 signaling pathways in Arabidopsis

The shoot apical meristem (SAM) gives rise to the aboveground organs of plants. The size of the SAM is relatively constant due to the balance between stem cell replenishment and cell recruitment into new organs. In angiosperms, the transcription factor WUSCHEL (WUS) promotes stem cell proliferation in the central zone of the SAM. WUS forms a negative feedback loop with a signaling pathway activated by CLAVATA3 (CLV3). In the periphery of the SAM, the ERECTA family receptors (ERfs) constrain WUS and CLV3 expression. Here, we show that four ligands of ERfs redundantly inhibit the expression of these two genes. Transcriptome analysis confirmed that WUS and CLV3 are the main targets of ERf signaling and uncovered new ones. Analysis of promoter reporters indicated that the WUS expression domain mostly overlaps with the CLV3 domain and does not shift along the apical-basal axis in clv3 mutants. Our three-dimensional mathematical model captured gene expression distributions at the single-cell level under various perturbed conditions. Based on our findings, CLV3 regulates cellular levels of WUS mostly through autocrine signaling, and ERfs regulate the spatial expression of WUS, preventing its encroachment into the peripheral zone.

CLAVATA signaling in plant-environment interactions

Plants must rapidly and dynamically adapt to changes in their environment. Upon sensing environmental signals, plants convert them into cellular signals, which elicit physiological or developmental changes that allow them to respond to various abiotic and biotic cues. Because plants can be simultaneously exposed to multiple environmental cues, signal integration between plant cells, tissues, and organs is necessary to induce specific responses. Recently, CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) peptides and their cognate CLAVATA-type receptors received increased attention for their roles in plant-environment interactions. CLE peptides are mobile signaling molecules, many of which are induced by a variety of biotic and abiotic stimuli. Secreted CLE peptides are perceived by receptor complexes on the surface of their target cells, which often include the leucine-rich repeat receptor-like kinase CLAVATA1. Receptor activation then results in cell-type and/or environment-specific responses. This review summarizes our current understanding of the diverse roles of environment-regulated CLE peptides in modulating plant responses to environmental cues. We highlight how CLE signals regulate plant physiology by fine-tuning plant-microbe interactions, nutrient homeostasis, and carbon allocation. Finally, we describe the role of CLAVATA receptors in the perception of environment-induced CLE signals and discuss how diverse CLE-CLAVATA signaling modules may integrate environmental signals with plant physiology and development.

Peptide signaling through leucine-rich repeat receptor kinases: insight into land plant evolution

Multicellular organisms need mechanisms for communication between cells so that they can fulfill their purpose in the organism as a whole. Over the last two decades, several small post-translationally modified peptides (PTMPs) have been identified as components of cell-to-cell signaling modules in flowering plants. Such peptides most often influence growth and development of organs not universally conserved among land plants. PTMPs have been matched to subfamily XI leucine-rich repeat receptor-like kinases with > 20 repeats. Phylogenetic analyses, facilitated by recently published genomic sequences of non-flowering plants, have identified seven clades of such receptors with a history back to the common ancestor of bryophytes and vascular plants. This raises a number of questions: When did peptide signaling arise during land plant evolution? Have orthologous peptide-receptor pairs preserved their biological functions? Has peptide signaling contributed to major innovations, such as stomata, vasculature, roots, seeds, and flowers? Using genomic, genetic, biochemical, and structural data and non-angiosperm model species, it is now possible to address these questions. The vast number of peptides that have not yet found their partners suggests furthermore that we have far more to learn about peptide signaling in the coming decades.

Deeper genomic insights into tomato CLE genes repertoire identify new active peptides

Background

In eukaryotes, cell-to-cell communication relies on the activity of small signaling peptides. In plant genomes, many hundreds of genes encode for such short peptide signals. However, only few of them are functionally characterized and due to the small gene size and high sequence variability, the comprehensive identification of such peptide-encoded genes is challenging. The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-RELATED (CLE) gene family encodes for short peptides that have a role in plant meristem maintenance, vascular patterning and responses to environment. The full repertoire of CLE genes and the role of CLE signaling in tomato (Solanum lycopersicum)- one of the most important crop plants- has not yet been fully studied.

Results

By using a combined approach, we performed a genome-wide identification of CLE genes using the current tomato genome version SL 4.0. We identified 52 SlCLE genes, including 37 new non annotated before. By analyzing publicly available RNAseq datasets we could confirm the expression of 28 new SlCLE genes. We found that SlCLEs are often expressed in a tissue-, organ- or condition-specific manner. Our analysis shows an interesting gene diversification within the SlCLE family that seems to be a result of gene duplication events. Finally, we could show a biological activity of selected SlCLE peptides in the root growth arrest that was SlCLV2-dependent.

Conclusions

Our improved combined approach revealed 37 new SlCLE genes. These findings are crucial for better understanding of the CLE signaling in tomato. Our phylogenetic analysis pinpoints the closest homologs of Arabidopsis CLE genes in tomato genome and can give a hint about the function of newly identified SlCLEs. The strategy described here can be used to identify more precisely additional short genes in plant genomes. Finally, our work suggests that the mechanism of root-active CLE peptide perception is conserved between Arabidopsis and tomato. In conclusion, our work paves the way to further research on the CLE-dependent circuits modulating tomato development and physiological responses.