Antagonistic peptide technology for functional dissection of CLE peptides revisited

Imaging plant cell walls using fluorescent stains: The beauty is in the details

Plants continuously face various environmental stressors throughout their lifetime. To be able to grow and adapt in different environments, they developed specialized tissues that allowed them to maintain a protected yet interconnected body. These tissues undergo specific primary and secondary cell wall modifications that are essential to ensure normal plant growth, adaptation and successful land colonization. The composition of cell walls can vary among different plant species, organs and tissues. The ability to remodel their cell walls is fundamental for plants to be able to cope with multiple biotic and abiotic stressors. A better understanding of the changes taking place in plant cell walls may help identify and develop new strategies as well as tools to enhance plants' survival under environmental stresses or prevent pathogen attack. Since the invention of microscopy, numerous imaging techniques have been developed to determine the composition and dynamics of plant cell walls during normal growth and in response to environmental stimuli. In this review, we discuss the main advances in imaging plant cell walls, with a particular focus on fluorescent stains for different cell wall components and their compatibility with tissue clearing techniques. Lay Description: Plants are continuously subjected to various environmental stresses during their lifespan. They evolved specialized tissues that thrive in different environments, enabling them to maintain a protected yet interconnected body. Such tissues undergo distinct primary and secondary cell wall alterations essential to normal plant growth, their adaptability and successful land colonization. Cell wall composition may differ among various plant species, organs and even tissues. To deal with various biotic and abiotic stresses, plants must have the capacity to remodel their cell walls. Gaining insight into changes that take place in plant cell walls will help identify and create novel tools and strategies to improve plants' ability to withstand environmental challenges. Multiple imaging techniques have been developed since the introduction of microscopy to analyse the composition and dynamics of plant cell walls during growth and in response to environmental changes. Advancements in plant tissue cleaning procedures and their compatibility with cell wall stains have significantly enhanced our ability to perform high-resolution cell wall imaging. At the same time, several factors influence the effectiveness of cleaning and staining plant specimens, as well as the time necessary for the process, including the specimen's size, thickness, tissue complexity and the presence of autofluorescence. In this review, we will discuss the major advances in imaging plant cell walls, with a particular emphasis on fluorescent stains for diverse cell wall components and their compatibility with tissue clearing techniques. We hope that this review will assist readers in selecting the most appropriate stain or combination of stains to highlight specific cell wall components of interest.

Nitrogen-Activated CLV3/ESR-Related 4 (CLE4) Regulates Shoot, Root, and Stolon Growth in Potato

In potato, high levels of nitrogen (N) can lead to excessive vegetative growth at the expense of tuber development, resulting in lower yield and poor-quality tubers. We found that Solanum tuberosum CLE4 (StCLE4) is expressed most strongly in the roots grown in N-rich media, and it positively regulates potato root growth under N-deficient conditions. We noted that StCLE4 functions as a negative regulator of normal shoot apex development similar to CLV3 in Arabidopsis. Transcriptomic analysis revealed that overexpression of StCLE4 resulted in the repression of the StIT1 gene, a regulator of potato tuber initiation. StCLE4-overexpressing stolons were converted into branches, that were similar to a mild phenotype of the it1 (identity of tuber 1) mutant. We also found that NIN-like proteins, key regulators of nitrate signaling bind to the regulatory sequence of StIT1 in a yeast one-hybrid assay. Taken together, our findings suggest that StCLE4 regulates shoot, root, and stolon growth in potato.

Phloem development

The evolution of the plant vascular system is a key process in Earth history because it enabled plants to conquer land and transform the terrestrial surface. Among the vascular tissues, the phloem is particularly intriguing because of its complex functionality. In angiosperms, its principal components are the sieve elements, which transport phloem sap, and their neighboring companion cells. Together, they form a functional unit that sustains sap loading, transport, and unloading. The developmental trajectory of sieve elements is unique among plant cell types because it entails selective organelle degradation including enucleation. Meticulous analyses of primary, so-called protophloem in the Arabidopsis thaliana root meristem have revealed key steps in protophloem sieve element formation at single-cell resolution. A transcription factor cascade connects specification with differentiation and also orchestrates phloem pole patterning via noncell-autonomous action of sieve element-derived effectors. Reminiscent of vascular tissue patterning in secondary growth, these involve receptor kinase pathways, whose antagonists guide the progression of sieve element differentiation. Receptor kinase pathways may also safeguard phloem formation by maintaining the developmental plasticity of neighboring cell files. Our current understanding of protophloem development in the A. thaliana root has reached sufficient detail to instruct molecular-level investigation of phloem formation in other organs.

Peptidome and Transcriptome Analysis of Plant Peptides Involved in Bipolaris maydis Infection of Maize

Southern corn leaf blight (SCLB) caused by Bipolaris maydis threatens maize growth and yield worldwide. In this study, TMT-labeled comparative peptidomic analysis was established between infected and uninfected maize leaf samples using liquid-chromatography-coupled tandem mass spectrometry. The results were further compared and integrated with transcriptome data under the same experimental conditions. Plant peptidomic analysis identified 455 and 502 differentially expressed peptides (DEPs) in infected maize leaves on day 1 and day 5, respectively. A total of 262 common DEPs were identified in both cases. Bioinformatic analysis indicated that the precursor proteins of DEPs are associated with many pathways generated by SCLB-induced pathological changes. The expression profiles of plant peptides and genes in maize plants were considerably altered after B. maydis infection. These findings provide new insights into the molecular mechanisms of SCLB pathogenesis and offer a basis for the development of maize genotypes with SCLB resistance.

pH-dependent CLE peptide perception permits phloem differentiation in Arabidopsis roots

The plant vasculature delivers phloem sap to the growth apices of sink organs, the meristems, via the interconnected sieve elements of the protophloem.^1,2,3 In the A. thaliana root meristem, the stem cells form two files of protophloem sieve elements (PPSEs), whose timely differentiation requires a set of positive genetic regulators. In corresponding loss-of-function mutants, signaling of secreted CLAVATA3/EMBRYO SURROUNDING REGION 45 (CLE45) peptide through the BARELY ANY MERISTEM 3 (BAM3) receptor is hyperactive and interferes with PPSE differentiation. This can be mimicked by an external CLE45 application to wild type. Because developing PPSEs express CLE45-BAM3 pathway components from early on until terminal differentiation, it remains unclear how they escape the autocrine inhibitory CLE45 signal. Here, we report that the wild type becomes insensitive to CLE45 treatment on neutral to alkaline pH media, as well as upon simultaneous treatment with a specific proton pump inhibitor at a standard pH of 5.7. We find that these observations can be explained by neither pH-dependent CLE45 uptake nor pH-dependent CLE45 charge. Moreover, pH-dependent perception specifically requires the CLE45 R4 residue and is not observed for the redundant PPSE-specific CLE25 and CLE26 peptides. Finally, pH-dependent CLE45 response in developing PPSEs as opposed to pH-independent response in neighboring cell files indicates that late-developing PPSEs can no longer sense CLE45. This is consistent with an apoplastic acidic to alkaline pH gradient we observed along developing PPSE cell files. In summary, we conclude that developing PPSEs self-organize their transition to differentiation by desensitizing themselves against autocrine CLE45 signaling through an apoplastic pH increase.

Functional characterization of C-TERMINALLY ENCODED PEPTIDE (CEP) family in Brassica rapa L

The small regulatory C-TERMINALLY ENCODED PEPTIDE (CEP) peptide family plays crucial roles in plant growth and stress response. However, little is known about this peptide family in Brassica species. Here, we performed a systematic analysis to identify the putative Brassica rapa L. CEP (BrCEP) gene family. In total, 27 BrCEP genes were identified and they were classified into four subgroups based on the CEP motifs similarity. BrCEP genes displayed distinct expression patterns in response to both developmental and several environmental signals, suggesting their broad roles during Brassica rapa development. Furthuremore, the synthetic BrCEP3 peptide accelerated Brassica rapa primary root growth in a hydrogen peroxide (H₂O₂) and Ca²⁺ dependent manner. In summary, our work will provide fundamental insights into the physiological function of CEP peptides during Brassica rapa development.

Systematic Annotation Reveals CEP Function in Tomato Root Development and Abiotic Stress Response

Tomato (Solanum lycopersicum) is one of the most important vegetable crops worldwide; however, environmental stressors severely restrict tomato growth and yield. Therefore, it is of great interest to discover novel regulators to improve tomato growth and environmental stress adaptions. Here, we applied a comprehensive bioinformatics approach to identify putative tomato C-TERMINALLY ENCODED PEPTIDE (CEP) genes and to explore their potential physiological function in tomato root development and abiotic stress responses. A total of 17 tomato CEP genes were identified and grouped into two subgroups based on the similarity of CEP motifs. The public RNA-Seq data revealed that tomato CEP genes displayed a diverse expression pattern in tomato tissues. Additionally, CEP genes expression was differentially regulated by nitrate or ammonium status in roots and shoots, respectively. The differences in expression levels of CEP genes induced by nitrogen indicate a potential involvement of CEPs in tomato nitrogen acquisition. The synthetic CEP peptides promoted tomato primary root growth, which requires nitric oxide (NO) and calcium signaling. Furthermore, we also revealed that CEP peptides improved tomato root resistance to salinity. Overall, our work will contribute to provide novel genetic breeding strategies for tomato cultivation under adverse environments.

Genome-wide identification reveals the function of CEP peptide in cucumber root development

C-Terminally Encoded (CEP) peptides are crucial plant growth regulators. Nevertheless, their physiological roles in cucumber (Cucumis sativus L.), an essential worldwide economical vegetable, remains untapped. In this study, 6 cucumber CEP (CsCEP) genes were identified. A comprehensive analysis showed that the CsCEP proteins displayed conserved characteristics to the identified CEP protein members in other species. CsCEP genes expression levels were variant in cucumber tissues, and were also differentially induced by several environmental factors, suggesting distinct and overlapping roles of CsCEPs in various cucumber developmental processes. We further revealed that synthetic CsCEP4 peptide promoted cucumber primary root growth in a reactive oxygen species (ROS) dependent manner. Overall, our work will provide fundamental insights into the crucial physiological roles of small bioactive peptides during cucumber root development.

WOX going on: CLE peptides in plant development

The development of plant tissues requires cell-cell communication facilitated by chemical and peptide hormones, including small signaling peptides in the CLAVATA3/EMBRYO-SURROUNDING REGION (CLE) family. The paradigmatic CLE signaling peptide CLAVATA3 regulates the size of the shoot apical meristem and the expression of the stem cell-promoting WUSCHEL transcription factor through an unknown mechanism. This review discusses recent advances in CLE signaling, showing that CLE pathways are conserved in bryophytes, that CLE peptides in Arabidopsis thaliana regulate stem cell identity and cell division in root tissues, and connections to auxin biosynthesis in regulating flower and leaf development. These advances shed light on potential WUSCHEL family-independent aspects of CLE signaling and the overlap between CLE and auxin signaling.

Metaphloem development in the Arabidopsis root tip

The phloem transport network is a major evolutionary innovation that enabled plants to dominate terrestrial ecosystems. In the growth apices, the meristems, apical stem cells continuously produce early 'protophloem'. This is easily observed in Arabidopsis root meristems, in which the differentiation of individual protophloem sieve element precursors into interconnected conducting sieve tubes is laid out in a spatio-temporal gradient. The mature protophloem eventually collapses as the neighboring metaphloem takes over its function further distal from the stem cell niche. Compared with protophloem, metaphloem ontogenesis is poorly characterized, primarily because its visualization is challenging. Here, we describe the improved TetSee protocol to investigate metaphloem development in Arabidopsis root tips in combination with a set of molecular markers. We found that mature metaphloem sieve elements are only observed in the late post-meristematic root, although their specification is initiated as soon as protophloem sieve elements enucleate. Moreover, unlike protophloem sieve elements, metaphloem sieve elements only differentiate once they have fully elongated. Finally, our results suggest that metaphloem differentiation is not directly controlled by protophloem-derived cues but rather follows a distinct, robust developmental trajectory.

Processing and Formation of Bioactive CLE40 Peptide Are Controlled by Posttranslational Proline Hydroxylation

Small posttranslationally modified signaling peptides are proteolytically derived from larger precursor proteins and subject to several additional steps of modification, including Pro hydroxylation, Hyp glycosylation, and/or Tyr sulfation. The processing proteases and the relevance of posttranslational modifications for peptide biogenesis and activity are largely unknown. In this study these questions were addressed for the Clavata3/Endosperm Surrounding Region (CLE) peptide CLE40, a peptide regulator of stem cell differentiation in the Arabidopsis (Arabidopsis thaliana) root meristem. We identify three subtilases (SBT1.4, SBT1.7, and SBT4.13) that cleave the CLE40 precursor redundantly at two sites. C-terminal processing releases the mature peptide from its precursor and is thus required for signal biogenesis. SBT-mediated cleavage at a second site within the mature peptide attenuates the signal. The second cleavage is prevented by Pro hydroxylation, resulting in the formation of mature and bioactive CLE40 in planta. Our data reveal a role for posttranslational modification by Pro hydroxylation in the regulation of CLE40 formation and activity.

Peptide Signaling Pathways in Vascular Differentiation

CLE peptide and related signaling pathways take up prominent roles in the development of both vascular tissues, xylem and phloem.

The Systemin Signaling Cascade As Derived from Time Course Analyses of the Systemin-responsive Phosphoproteome

Systemin is a small peptide with important functions in plant wound response signaling. Although the transcriptional responses of systemin action are well described, the signaling cascades involved in systemin perception and signal transduction at the protein level are poorly understood. Here we used a tomato cell suspension culture system to profile phosphoproteomic responses induced by systemin and its inactive Thr17Ala analog, allowing us to reconstruct a systemin-specific kinase/phosphatase signaling network. Our time-course analysis revealed early phosphorylation events at the plasma membrane, such as dephosphorylation of H+-ATPase, rapid phosphorylation of NADPH-oxidase and Ca2+-ATPase. Later responses involved transient phosphorylation of small GTPases, vesicle trafficking proteins and transcription factors. Based on a correlation analysis of systemin-induced phosphorylation profiles, we predicted substrate candidates for 44 early systemin-responsive kinases, which includes receptor kinases and downstream kinases such as MAP kinases, as well as nine phosphatases. We propose a regulatory module in which H+-ATPase LHA1 is rapidly de-phosphorylated at its C-terminal regulatory residue T955 by phosphatase PLL5, resulting in the alkalization of the growth medium within 2 mins of systemin treatment. We found the MAP kinase MPK2 to have increased phosphorylation level at its activating TEY-motif at 15 min post-treatment. The predicted interaction of MPK2 with LHA1 was confirmed by in vitro kinase assays, suggesting that the H+-ATPase LHA1 is re-activated by MPK2 later in the systemin response. Our data set provides a resource of proteomic events involved in systemin signaling that will be valuable for further in-depth functional studies in elucidation of systemin signaling cascades.

Nutrient-Responsive Small Signaling Peptides and Their Influence on the Root System Architecture

The root system architecture (RSA) of plants is highly dependent on the surrounding nutrient environment. The uptake of essential nutrients triggers various signaling cascades and fluctuations in plant hormones to elicit physical changes in RSA. These pathways may involve signaling components known as small signaling peptides (SSPs), which have been implicated in a variety of plant developmental processes. This review discusses known nutrient-responsive SSPs with a focus on several subclasses that have been shown to play roles in root development. Most functionally well-characterized cases of SSP-mediated changes in RSA are found in responses to nitrogen (N) and phosphorus (P) availability, but other nutrients have also been known to affect the expression of SSP-encoding genes. These nutrient-responsive SSPs may interact downstream with leucine-rich repeat receptor kinases (LRR-RKs) to modulate hormone signaling and cellular processes impacting plant root development. SSPs responsive to multiple nutrient cues potentially act as mediators of crosstalk between the signaling pathways. Study of SSP pathways is complicated because of functional redundancy within peptide and receptor families and due to their functionality partly associated with post-translational modifications; however, as genomic research and techniques progress, novel SSP-encoding genes have been identified in many plant species. Understanding and characterizing the roles of SSPs influencing the root phenotypes will help elucidate the processes that plants use to optimize nutrient acquisition in the environment.

The CLE9/10 secretory peptide regulates stomatal and vascular development through distinct receptors

The frequency and orientation of cell division are regulated by intercellular signalling molecules; however, tissue-specific regulatory systems for cell divisions are only partially understood. Here, we report that the peptide hormone CLAVATA3/ESR-RELATED 9/10 (CLE9/10) regulates two different developmental processes, stomatal lineage development and xylem development, through two distinct receptor systems in Arabidopsis thaliana. We show that the receptor kinase HAESA-LIKE 1 (HSL1) is a CLE9/10 receptor that regulates stomatal lineage cell division, and BARELY NO MERISTEM (BAM) class receptor kinases are CLE9/10 receptors that regulate periclinal cell division of xylem precursor cells. Both HSL1 and BAM1 bind to CLE9/10, but only HSL1 recruits SOMATIC EMBRYOGENESIS RECEPTOR KINASES as co-receptors in the presence of CLE9/10, suggesting different signalling modes for these receptor systems.

A Collection of Mutants for CLE-Peptide-Encoding Genes in Arabidopsis Generated by CRISPR/Cas9-Mediated Gene Targeting

The ligand-receptor-mediated intercellular communication system plays important roles in coordinating developmental and physiological events in multicellular organisms. In plants, CLAVATA3/EMBRYO SURROUNDING REGION (CLE) peptides and their cognate receptors are thought to be involved in various aspects of the plant life cycle. Although the importance of this communication is broadly recognized, most CLE peptides are yet to be functionally characterized. A major problem in research on small signaling peptide-encoding genes is the limited number of loss-of-function mutants available due to their small gene size. CRISPR/Cas9-mediated gene targeting has the potential to overcome this problem, as it can be used to generate targeted mutations in essentially any gene, regardless of size. Here we generated a series of mutants of CLE-peptide-encoding genes. Newly generated clv3 and cle40 mutants reproduced the expected mutant phenotypes in the shoot apical meristem and root meristem, respectively. Our results show that CRISPR/Cas9-mediated gene targeting is a powerful tool for genetic analyses, even of small genes. We also report a novel mutant for CLE44 [which is thought to encode a tracheary elements differentiation inhibitory factor (TDIF)] and show that CLE44 contributes to vascular development. The bioresources presented here will be a powerful tool for further characterization of CLE peptides.

Fine-Tuning Development Through Antagonistic Peptides: An Emerging Theme

Peptide ligand-receptor kinase interactions have emerged as a key component of plant growth and development. Now, highly related small signaling peptides have been shown to act antagonistically on the same receptor kinase, providing new insights into how plants optimize developmental processes using competitive peptides.

CLE peptides and their signaling pathways in plant development

Cell-to-cell communication is crucial for the coherent functioning of multicellular organisms, and they have evolved intricate molecular mechanisms to achieve such communication. Small, secreted peptide hormones participate in cell-to-cell communication to regulate various physiological processes. One such family of plant peptide hormones is the CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-related (CLE) family, whose members play crucial roles in the differentiation of shoot and root meristems. Recent biochemical and genetic studies have characterized various CLE signaling modules, which include CLE peptides, transmembrane receptors, and downstream intracellular signaling components. CLE signaling systems are conserved across the plant kingdom but have divergent modes of action in various developmental processes in different species. Moreover, several CLE peptides play roles in symbiosis, parasitism, and responses to abiotic cues. Here we review recent studies that have provided new insights into the mechanisms of CLE signaling.

Conservation of the abscission signaling peptide IDA during Angiosperm evolution: withstanding genome duplications and gain and loss of the receptors HAE/HSL2

The peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), which signals through the leucine-rich repeat receptor-like kinases HAESA (HAE) and HAESA-LIKE2 (HSL2), controls different cell separation events in Arabidopsis thaliana. We hypothesize the involvement of this signaling module in abscission processes in other plant species even though they may shed other organs than A. thaliana. As the first step toward testing this hypothesis from an evolutionarily perspective we have identified genes encoding putative orthologs of IDA and its receptors by BLAST searches of publically available protein, nucleotide and genome databases for angiosperms. Genes encoding IDA or IDA-LIKE (IDL) peptides and HSL proteins were found in all investigated species, which were selected as to represent each angiosperm order with available genomic sequences. The 12 amino acids representing the bioactive peptide in A. thaliana have virtually been unchanged throughout the evolution of the angiosperms; however, the number of IDL and HSL genes varies between different orders and species. The phylogenetic analyses suggest that IDA, HSL2, and the related HSL1 gene, were present in the species that gave rise to the angiosperms. HAE has arisen from HSL1 after a genome duplication that took place after the monocot-eudicots split. HSL1 has also independently been duplicated in the monocots, while HSL2 has been lost in gingers (Zingiberales) and grasses (Poales). IDA has been duplicated in eudicots to give rise to functionally divergent IDL peptides. We postulate that the high number of IDL homologs present in the core eudicots is a result of multiple whole genome duplications (WGD). We substantiate the involvement of IDA and HAE/HSL2 homologs in abscission by providing gene expression data of different organ separation events from various species.

Beyond the meristems: similarities in the CLAVATA3 and INFLORESCENCE DEFICIENT IN ABSCISSION peptide mediated signalling pathways

Plants form new organs throughout their lives; this requires a balance between cell proliferation and differentiation, and between the generation and loss of organs. To do this, plants must maintain a population of stem cells within the meristems, and at the same time, closely control the identity and position of cells at the meristem boundaries as they differentiate to new leaf or flower primordia. Once developed, organs may need to be shed, either as a controlled developmental decision-such as floral abscission after pollination, or as a response to disease, environmental stress, and predators. Cell wall degradation at specialized abscission zone (AZ) cells needs to occur for this to take place, but since there is little cell rearrangement in plants, cell separation events are also important for plant architecture. In this Opinion paper we discuss the role of two peptide ligand signalling systems that control stem cell homeostasis and cell separation, respectively. We draw parallels between the signalling pathways and explore on the commonalities of the downstream components activated and controlled by the signalling peptides. We provide evidence for AZ cells having a meristem identity and discuss the role of identical KNOTTED-LIKE HOMEOBOX (KNOX) transcription factors in meristem maintenance and abscission. Lastly we explore the evolutionary relationship between the pathways.

CLE19 expressed in the embryo regulates both cotyledon establishment and endosperm development in Arabidopsis

Embryo and endosperm development are two well co-ordinated developmental processes in seed formation; however, signals involved in embryo and endosperm interactions remain poorly understood. It has been shown before that CLAVATA3/ESR-RELATED 19 (CLE19) peptide is able to trigger root meristem consumption in a CLV2-dependent manner. In this study, the role of CLE19 in Arabidopsis seed development was explored using antagonistic peptide technology. CLE19 is expressed in the epidermal layers of the cotyledon primordia, hypocotyl, and root cap in the embryo. Transgenic plants carrying an antagonistic CLE19 G6T construct expressed under the control of CLE19 regulatory elements exhibited a dominant seed abortion phenotype, with defective cotyledon establishment in embryos and delayed nuclear proliferation and cellularization in endosperms. Ectopic expression of CLE19 G6T in Arabidopsis under the control of an endosperm-specific ALE1 promoter led to a similar defect in cotyledon establishment in embryos but without an evident effect on endosperm development. We therefore propose that CLE19 may act as a mobile peptide co-ordinating embryo and endosperm development.

Modulation of Arabidopsis and monocot root architecture by CLAVATA3/EMBRYO SURROUNDING REGION 26 peptide

Plant roots are important for a wide range of processes, including nutrient and water uptake, anchoring and mechanical support, storage functions, and as the major interface with the soil environment. Several small signalling peptides and receptor kinases have been shown to affect primary root growth, but very little is known about their role in lateral root development. In this context, the CLE family, a group of small signalling peptides that has been shown to affect a wide range of developmental processes, were the focus of this study. Here, the expression pattern during lateral root initiation for several CLE family members is explored and to what extent CLE1, CLE4, CLE7, CLE26, and CLE27, which show specific expression patterns in the root, are involved in regulating root architecture in Arabidopsis thaliana is assessed. Using chemically synthesized peptide variants, it was found that CLE26 plays an important role in regulating A. thaliana root architecture and interacts with auxin signalling. In addition, through alanine scanning and in silico structural modelling, key residues in the CLE26 peptide sequence that affect its activity are pinpointed. Finally, some interesting similarities and differences regarding the role of CLE26 in regulating monocot root architecture are presented.