Diversification of the C-TERMINALLY ENCODED PEPTIDE (CEP) gene family in angiosperms, and evolution of plant-family specific CEP genes

CEPs suppress auxin signaling but promote cytokinin signaling to inhibit root growth in Arabidopsis

C-terminally encoded peptides (CEPs) are peptide hormones that function as mobile signals coordinating crucial developmental programs in plants. Previous studies have revealed that CEPs exert negative regulation on root development through interaction with CEP receptors (CEPRs), CEP DOWNSTREAMs (CEPDs), the cytokinin receptor ARABIDOPSIS HISTIDINE KINASE (AHKs) and the transcriptional repressor Auxin/Indole-3-Acetic Acid (AUX/IAA). However, the precise molecular mechanisms underlying CEPs-mediated regulation of root development via auxin and cytokinin signaling pathways still necessitate further detailed investigation. In this study, we examined prior research and elucidated the underlying molecular mechanisms. The results showed that both synthetic AtCEPs and overexpression of AtCEP5 markedly supressed primary root elongation and lateral root (LR) formation in Arabidopsis. Molecular biology and genetics elucidated how CEPs inhibit root growth by suppressing auxin signaling while promoting cytokinin signaling. In summary, this study elucidated the inhibitory effects of AtCEPs on Arabidopsis root growth and provided insights into their potential molecular mechanisms, thus enhancing our comprehension of CEP-mediated regulation of plant growth and development.

A Genome-Wide Analysis of the CEP Gene Family in Cotton and a Functional Study of GhCEP46-D05 in Plant Development

C-TERMINALLY ENCODED PEPTIDEs (CEPs) are a class of peptide hormones that have been shown in previous studies to play an important role in regulating the development and response to abiotic stress in model plants. However, their role in cotton is not well understood. In this study, we identified 54, 59, 34, and 35 CEP genes from Gossypium hirsutum (2n = 4x = 52, AD1), G. barbadense (AD2), G. arboreum (2n = 2X = 26, A2), and G. raimondii (2n = 2X = 26, D5), respectively. Sequence alignment and phylogenetic analyses indicate that cotton CEP proteins can be categorized into two subgroups based on the differentiation of their CEP domain. Chromosomal distribution and collinearity analyses show that most of the cotton CEP genes are situated in gene clusters, suggesting that segmental duplication may be a critical factor in CEP gene expansion. Expression pattern analyses showed that cotton CEP genes are widely expressed throughout the plant, with some genes exhibiting specific expression patterns. Ectopic expression of GhCEP46-D05 in Arabidopsis led to a significant reduction in both root length and seed size, resulting in a dwarf phenotype. Similarly, overexpression of GhCEP46-D05 in cotton resulted in reduced internode length and plant height. These findings provide a foundation for further investigation into the function of cotton CEP genes and their potential role in cotton breeding.

Identification of CEP peptides encoded by the tobacco (Nicotiana tabacum) genome and characterization of their roles in osmotic and salt stress responses

Members of the CEP (C-terminally Encoded Peptide) gene family have been shown to be involved in various developmental processes and stress responses in plants. In order to understand the roles of CEP peptides in stress response, a comprehensive bioinformatics approach was employed to identify NtCEP genes in tobacco (Nicotiana tabacum L.) and to analyze their potential roles in stress responses. Totally 21 NtCEP proteins were identified and categorized into two subgroups based on their CEP domains. Expression changes of the NtCEP genes in response to various abiotic stresses were analyzed via qRT-PCR and the results showed that a number of NtCEPs were significant up-regulated under drought, salinity, or temperature stress conditions. Furthermore, application of synthesized peptides derived from NtCEP5, NtCEP13, NtCEP14, and NtCEP17 enhanced plant tolerance to different salt stress treatments. NtCEP5, NtCEP9 and NtCEP14, and NtCEP17 peptides were able to promote osmotic tolerance of tobacco plants. The results from this study suggest that NtCEP peptides may serve as important signaling molecules in tobacco's response to abiotic stresses.

CEP hormones at the nexus of nutrient acquisition and allocation, root development, and plant-microbe interactions

A growing understanding is emerging of the roles of peptide hormones in local and long-distance signalling that coordinates plant growth and development as well as responses to the environment. C-TERMINALLY ENCODED PEPTIDE (CEP) signalling triggered by its interaction with CEP RECEPTOR 1 (CEPR1) is known to play roles in systemic nitrogen (N) demand signalling, legume nodulation, and root system architecture. Recent research provides further insight into how CEP signalling operates, which involves diverse downstream targets and interactions with other hormone pathways. Additionally, there is emerging evidence of CEP signalling playing roles in N allocation, root responses to carbon levels, the uptake of other soil nutrients such as phosphorus and sulfur, root responses to arbuscular mycorrhizal fungi, plant immunity, and reproductive development. These findings suggest that CEP signalling more broadly coordinates growth across the whole plant in response to diverse environmental cues. Moreover, CEP signalling and function appear to be conserved in angiosperms. We review recent advances in CEP biology with a focus on soil nutrient uptake, root system architecture and organogenesis, and roles in plant-microbe interactions. Furthermore, we address knowledge gaps and future directions in this research field.

C-terminally encoded peptides act as signals to increase cotton root nitrate uptake under nonuniform salinity

Soil salinity is often heterogeneous in saline fields. Nonuniform root salinity increases nitrate uptake into cotton (Gossypium hirsutum) root portions exposed to low salinity, which may be regulated by root portions exposed to high salinity through a systemic long-distance signaling mechanism. However, the signals transmitted between shoots and roots and their precise molecular mechanisms for regulating nitrate uptake remain unknown. Here, we showed that nonuniform root salinity treatment using split-root systems increases the expression of C-TERMINALLY ENCODED PEPTIDE (GhCEP) genes in high-saline-treated root portions. GhCEP peptides originating in high-saline-treated root portions act as ascending long-distance mobile signals transported to the shoots to promote the expression of CEP DOWNSTREAM (GhCEPD) genes by inducing the expression of CEP receptor (GhCEPR) genes. The shoot-derived GhCEPD polypeptides act as descending mobile signals transported to the roots through the phloem, increasing the expression of nitrate transport genes NITRATE TRANSPORTER 1.1 (GhNRT1.1), GhNRT2.1, and GhNRT1.5 in nonsaline-treated root portions, thereby increasing nitrate uptake in the nonsaline-treated root portions. This study indicates that GhCEP and GhCEPD signals are transported between roots and shoots to increase nitrate uptake in cotton, and the transport from the nonsaline root side is in response to nonuniform root salinity distribution.

Root-Knot-Nematode-Encoded CEPs Increase Nitrogen Assimilation

C-terminally encoded peptides (CEPs) are plant developmental signals that regulate growth and adaptive responses to nitrogen stress conditions. These small signal peptides are common to all vascular plants, and intriguingly have been characterized in some plant parasitic nematodes. Here, we sought to discover the breadth of root-knot nematode (RKN)-encoded CEP-like peptides and define the potential roles of these signals in the plant-nematode interaction, focusing on peptide activity altering plant root phenotypes and nitrogen uptake and assimilation. A comprehensive bioinformatic screen identified 61 CEP-like sequences encoded within the genomes of six root-knot nematode (RKN; Meloidogyne spp.) species. Exogenous application of an RKN CEP-like peptide altered A. thaliana and M. truncatula root phenotypes including reduced lateral root number in M. truncatula and inhibited primary root length in A. thaliana. To define the role of RKN CEP-like peptides, we applied exogenous RKN CEP and demonstrated increases in plant nitrogen uptake through the upregulation of nitrate transporter gene expression in roots and increased 15N/14N in nematode-formed root galls. Further, we also identified enhanced nematode metabolic processes following CEP application. These results support a model of parasite-induced changes in host metabolism and inform endogenous pathways to regulate plant nitrogen assimilation.

CEP peptide and cytokinin pathways converge on CEPD glutaredoxins to inhibit root growth

C-TERMINALLY ENCODED PEPTIDE (CEP) and cytokinin hormones act over short and long distances to control plant responses to environmental cues. CEP and cytokinin pathway mutants share phenotypes, however, it is not known if these pathways intersect. We show that CEP and cytokinin signalling converge on CEP DOWNSTREAM (CEPD) glutaredoxins to inhibit primary root growth. CEP inhibition of root growth was impaired in mutants defective in trans-zeatin (tZ)-type cytokinin biosynthesis, transport, perception, and output. Concordantly, mutants affected in CEP RECEPTOR 1 showed reduced root growth inhibition in response to tZ, and altered levels of tZ-type cytokinins. Grafting and organ-specific hormone treatments showed that tZ-mediated root growth inhibition involved CEPD activity in roots. By contrast, root growth inhibition by CEP depended on shoot CEPD function. The results demonstrate that CEP and cytokinin pathways intersect, and utilise signalling circuits in separate organs involving common glutaredoxin genes to coordinate root growth.

The Medicago SymCEP7 hormone increases nodule number via shoots without compromising lateral root number

Legumes acquire soil nutrients through nitrogen-fixing root nodules and lateral roots. To balance the costs and benefits of nodulation, legumes negatively control root nodule number by autoregulatory and hormonal pathways. How legumes simultaneously coordinate root nodule and lateral root development to procure nutrients remains poorly understood. In Medicago (Medicago truncatula), a subset of mature C-TERMINALLY ENCODED PEPTIDE (CEP) hormones can systemically promote nodule number, but all CEP hormones tested to date negatively regulate lateral root number. Here we showed that Medicago CEP7 produces a mature peptide, SymCEP7, that promotes nodulation from the shoot without compromising lateral root number. Rhizobial inoculation induced CEP7 in the susceptible root nodulation zone in a Nod factor-dependent manner, and, in contrast to other CEP genes, its transcription level was elevated in the ethylene signaling mutant sickle. Using mass spectrometry, fluorescence microscopy and expression analysis, we demonstrated that SymCEP7 activity requires the COMPACT ROOT ARCHITECTURE 2 receptor and activates the shoot-to-root systemic effector, miR2111. Shoot-applied SymCEP7 rapidly promoted nodule number in the pM to nM range at concentrations up to five orders of magnitude lower than effects mediated by root-applied SymCEP7. Shoot-applied SymCEP7 also promoted nodule number in White Clover (Trifolium repens) and Lotus (Lotus japonicus), which suggests that this biological function may be evolutionarily conserved. We propose that SymCEP7 acts in the Medicago shoot to counter balance the autoregulation pathways induced rapidly by rhizobia to enable nodulation without compromising lateral root growth, thus promoting the acquisition of nutrients other than nitrogen to support their growth.

Molecular Mechanisms of Regulation of Root Development by Plant Peptides

Peptides perform many functions, participating in the regulation of cell differentiation, regulating plant growth and development, and also involved in the response to stress factors and in antimicrobial defense. Peptides are an important class biomolecules for intercellular communication and in the transmission of various signals. The intercellular communication system based on the ligand-receptor bond is one of the most important molecular bases for creating complex multicellular organisms. Peptide-mediated intercellular communication plays a critical role in the coordination and determination of cellular functions in plants. The intercellular communication system based on the receptor-ligand is one of the most important molecular foundations for creating complex multicellular organisms. Peptide-mediated intercellular communication plays a critical role in the coordination and determination of cellular functions in plants. The identification of peptide hormones, their interaction with receptors, and the molecular mechanisms of peptide functioning are important for understanding the mechanisms of both intercellular communications and for regulating plant development. In this review, we drew attention to some peptides involved in the regulation of root development, which implement this regulation by the mechanism of a negative feedback loop.

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.

A rulebook for peptide control of legume-microbe endosymbioses

Plants engage in mutually beneficial relationships with microbes, such as arbuscular mycorrhizal fungi or nitrogen-fixing rhizobia, for optimized nutrient acquisition. In return, the microbial symbionts receive photosynthetic carbon from the plant. Both symbioses are regulated by the plant nutrient status, indicating the existence of signaling pathways that allow the host to fine-tune its interactions with the beneficial microbes depending on its nutrient requirements. Peptide hormones coordinate a plethora of developmental and physiological processes and, recently, various peptide families have gained special attention as systemic and local regulators of plant-microbe interactions and nutrient homeostasis. In this review, we identify five 'rules' or guiding principles that govern peptide function during symbiotic plant-microbe interactions, and highlight possible points of integration with nutrient acquisition pathways.

Arbuscular mycorrhizal symbiosis enhances tomato lateral root formation by modulating CEP2 peptide expression

Plant lateral root (LR) growth usually is stimulated by arbuscular mycorrhizal (AM) symbiosis. However, the molecular mechanism is still unclear. We used gene expression analysis, peptide treatment and virus-induced gene alteration assays to demonstrate that C-terminally encoded peptide (CEP2) expression in tomato was downregulated during AM symbiosis to mitigate its negative effect on LR formation through an auxin-related pathway. We showed that enhanced LR density and downregulated CEP2 expression were observed during mycorrhizal symbiosis. Synthetic CEP2 peptide treatment reduced LR density and impaired the expression of genes involved in indole-3-butyric acid (IBA, the precursor of IAA) to IAA conversion, auxin polar transport and the LR-related signaling pathway; however, application of IBA or synthetic auxin 1-naphthaleneacetic acid (NAA) to the roots may rescue both defective LR formation and reduced gene expression. CEP receptor 1 (CEPR1) might be the receptor of CEP2 because its knockdown plants did not respond to CEP2 treatment. Most importantly, the LR density of CEP2 overexpression or knockdown plants could not be further increased by AM inoculation, suggesting that CEP2 was critical for AM-induced LR formation. These results indicated that AM symbiosis may regulate root development by modulating CEP2, which affects the auxin-related pathway.

Progress in the Self-Regulation System in Legume Nodule Development-AON (Autoregulation of Nodulation)

The formation and development of legumes nodules requires a lot of energy. Legumes must strictly control the number and activity of nodules to ensure efficient energy distribution. The AON system can limit the number of rhizobia infections and nodule numbers through the systemic signal pathway network that the aboveground and belowground parts participate in together. It can also promote the formation of nodules when plants are deficient in nitrogen. The currently known AON pathway includes four parts: soil NO₃⁻ signal and Rhizobium signal recognition and transmission, CLE-SUNN is the negative regulation pathway, CEP-CRA2 is the positive regulation pathway and the miR2111/TML module regulates nodule formation and development. In order to ensure the biological function of this important approach, plants use a variety of plant hormones, polypeptides, receptor kinases, transcription factors and miRNAs for signal transmission and transcriptional regulation. This review summarizes and discusses the research progress of the AON pathway in Legume nodule development.

Application of Synthetic Peptide CEP1 Increases Nutrient Uptake Rates Along Plant Roots

The root system of a plant provides vital functions including resource uptake, storage, and anchorage in soil. The uptake of macro-nutrients like nitrogen (N), phosphorus (P), potassium (K), and sulphur (S) from the soil is critical for plant growth and development. Small signaling peptide (SSP) hormones are best known as potent regulators of plant growth and development with a few also known to have specialized roles in macronutrient utilization. Here we describe a high throughput phenotyping platform for testing SSP effects on root uptake of multiple nutrients. The SSP, CEP1 (C-TERMINALLY ENCODED PEPTIDE) enhanced nitrate uptake rate per unit root length in Medicago truncatula plants deprived of N in the high-affinity transport range. Single structural variants of M. truncatula and Arabidopsis thaliana specific CEP1 peptides, MtCEP1D1:hyp4,11 and AtCEP1:hyp4,11, enhanced uptake not only of nitrate, but also phosphate and sulfate in both model plant species. Transcriptome analysis of Medicago roots treated with different MtCEP1 encoded peptide domains revealed that hundreds of genes respond to these peptides, including several nitrate transporters and a sulfate transporter that may mediate the uptake of these macronutrients downstream of CEP1 signaling. Likewise, several putative signaling pathway genes including LEUCINE-RICH REPEAT RECPTOR-LIKE KINASES and Myb domain containing transcription factors, were induced in roots by CEP1 treatment. Thus, a scalable method has been developed for screening synthetic peptides of potential use in agriculture, with CEP1 shown to be one such peptide.

Plant protein-coding gene families: Their origin and evolution

Steady advances in genome sequencing methods have provided valuable insights into the evolutionary processes of several gene families in plants. At the core of plant biodiversity is an extensive genetic diversity with functional divergence and expansion of genes across gene families, representing unique phenomena. The evolution of gene families underpins the evolutionary history and development of plants and is the subject of this review. We discuss the implications of the molecular evolution of gene families in plants, as well as the potential contributions, challenges, and strategies associated with investigating phenotypic alterations to explain the origin of plants and their tolerance to environmental stresses.

Evolution of RGF/GLV/CLEL Peptide Hormones and Their Roles in Land Plant Growth and Regulation

Rooting is a key innovation during plant terrestrialization. RGFs/GLVs/CLELs are a family of secreted peptides, playing key roles in root stem cell niche maintenance and pattern formation. The origin of this peptide family is not well characterized. RGFs and their receptor genes, RGIs, were investigated comprehensively using phylogenetic and genetic analyses. We identified 203 RGF genes from 24 plant species, representing a variety of land plant lineages. We found that the RGF genes originate from land plants and expand via multiple duplication events. The lineage-specific RGF duplicates are retained due to their regulatory divergence, while a majority of RGFs experienced strong purifying selection in most land plants. Functional analysis indicated that RGFs and their receptor genes, RGIs, isolated from liverwort, tomato, and maize possess similar biological functions with their counterparts from Arabidopsis in root development. RGFs and RGIs are likely coevolved in land plants. Our studies shed light on the origin and functional conservation of this important peptide family in plant root development.

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.

SiCEP3, a C-terminally encoded peptide from Setaria italica, promotes ABA import and signaling

C-terminally encoded peptides (CEPs) are small peptides, typically post-translationally modified, and highly conserved in many species. CEPs are known to inhibit plant growth and development, but the mechanisms are not well understood. In this study, 14 CEPs were identified in Setaria italica and divided into two groups. The transcripts of most SiCEPs were more abundant in roots than in other detected tissues. SiCEP3, SiCEP4, and SiCEP5 were also highly expressed in panicles. Moreover, expression of all SiCEPs was induced by abiotic stresses and phytohormones. SiCEP3 overexpression and application of synthetic SiCEP3 both inhibited seedling growth. In the presence of abscisic acid (ABA), growth inhibition and ABA content in seedlings increased with the concentration of SiCEP3. Transcripts encoding eight ABA transporters and six ABA receptors were induced or repressed by synthetic SiCEP3, ABA, and their combination. Further analysis using loss-of-function mutants of Arabidopsis genes functioning as ABA transporters, receptors, and in the biosynthesis and degradation of ABA revealed that SiCEP3 promoted ABA import at least via NRT1.2 (NITRATE TRANSPORTER 1.2) and ABCG40 (ATP-BINDING CASSETTE G40). In addition, SiCEP3, ABA, or their combination inhibited the kinase activities of CEP receptors AtCEPR1/2. Taken together, our results indicated that the CEP-CEPR module mediates ABA signaling by regulating ABA transporters and ABA receptors in planta.

A new method to visualize CEP hormone-CEP receptor interactions in vascular tissue in vivo

C-TERMINALLY ENCODED PEPTIDEs (CEPs) control diverse responses in plants including root development, root system architecture, nitrogen demand signalling, and nutrient allocation that influences yield, and there is evidence that different ligands impart different phenotypic responses. Thus, there is a need for a simple method that identifies bona fide CEP hormone-receptor pairings in vivo and examines whether different CEP family peptides bind the same receptor. We used formaldehyde or photoactivation to cross-link fluorescently tagged group 1 or group 2 CEPs to receptors in semi-purified Medicago truncatula or Arabidopsis thaliana leaf vascular tissues to verify that COMPACT ROOT ARCHITECTURE 2 (CRA2) is the Medicago CEP receptor, and to investigate whether sequence diversity within the CEP family influences receptor binding. Formaldehyde cross-linked the fluorescein isothiocyanate (FITC)-tagged Medicago group 1 CEP (MtCEP1) to wild-type Medicago or Arabidopsis vascular tissue cells, but not to the CEP receptor mutants, cra2 or cepr1. Binding competition showed that unlabelled MtCEP1 displaces FITC-MtCEP1 from CRA2. In contrast, the group 2 CEP, FITC-AtCEP14, bound to vascular tissue independently of CEPR1 or CRA2, and AtCEP14 did not complete with FITC-MtCEP1 to bind CEP receptors. The binding of a photoactivatable FITC-MtCEP1 to the periphery of Medicago vascular cells suggested that CRA2 localizes to the plasma membrane. We separated and visualized a fluorescent 105 kDa protein corresponding to the photo-cross-linked FITC-MtCEP1-CRA2 complex using SDS-PAGE. Mass spectrometry identified CRA2-specific peptides in this protein band. The results indicate that FITC-MtCEP1 binds to CRA2, MtCRA2 and AtCEPR1 are functionally equivalent, and the binding specificities of group 1 and group 2 CEPs are distinct. Using formaldehyde or photoactivated cross-linking of biologically active, fluorescently tagged ligands may find wider utility by identifying CEP-CEP receptor pairings in diverse plants.

Signaling Peptides Regulating Abiotic Stress Responses in Plants

As sessile organisms, plants are exposed to constantly changing environments that are often stressful for their growth and development. To cope with these stresses, plants have evolved complex and sophisticated stress-responsive signaling pathways regulating the expression of transcription factors and biosynthesis of osmolytes that confer tolerance to plants. Signaling peptides acting like phytohormones control various aspects of plant growth and development via cell-cell communication networks. These peptides are typically recognized by membrane-embedded receptor-like kinases, inducing activation of cellular signaling to control plant growth and development. Recent studies have revealed that several signaling peptides play important roles in plant responses to abiotic stress. In this mini review, we provide recent findings on the roles and signaling pathways of peptides that are involved in coordinating plant responses to abiotic stresses, such as dehydration, high salinity, reactive oxygen species, and heat. We also discuss recent developments in signaling peptides that play a role in plant adaptation responses to nutrient deficiency stress, focusing on nitrogen and phosphate deficiency responses.

Multigene editing reveals that MtCEP1/2/12 redundantly control lateral root and nodule number in Medicago truncatula

The multimember CEP (C-terminally Encoded Peptide) gene family is a complex group that is involved in various physiological activities in plants. Previous studies demonstrated that MtCEP1 and MtCEP7 control lateral root formation or nodulation, but these studies were based only on gain of function or artificial miRNA (amiRNA)/RNAi approaches, never knockout mutants. Moreover, an efficient multigene editing toolkit is not currently available for Medicago truncatula. Our quantitative reverse transcription-PCR data showed that MtCEP1, 2, 4, 5, 6, 7, 8, 9, 12, and 13 were up-regulated under nitrogen starvation conditions and that MtCEP1, 2, 7, 9, and 12 were induced by rhizobial inoculation. Treatment with synthetic MtCEP peptides of MtCEP1, 2, 4, 5, 6, 8, and 12 repressed lateral root emergence and promoted nodulation in the R108 wild type but not in the cra2 mutant. We optimized CRISPR/Cas9 [clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9] genome editing system for M. truncatula, and thus created single mutants of MtCEP1, 2, 4, 6, and 12 and the double mutants Mtcep1/2C and Mtcep5/8C; however, these mutants did not exhibit significant differences from R108. Furthermore, a triple mutant Mtcep1/2/12C and a quintuple mutant Mtcep1/2/5/8/12C were generated and exhibited more lateral roots and fewer nodules than R108. Overall, MtCEP1, 2, and 12 were confirmed to be redundantly important in the control of lateral root number and nodulation. Moreover, the CRISPR/Cas9-based multigene editing protocol provides an additional tool for research on the model legume M. truncatula, which is highly efficient at multigene mutant generation.

Nitrogen Systemic Signaling: From Symbiotic Nodulation to Root Acquisition

Plant nutrient acquisition is tightly regulated by resource availability and metabolic needs, implying the existence of communication between roots and shoots to ensure their integration at the whole-plant level. Here, we focus on systemic signaling pathways controlling nitrogen (N) nutrition, achieved both by the root import of mineral N and, in legume plants, through atmospheric N fixation by symbiotic bacteria inside dedicated root nodules. We explore features conserved between systemic pathways repressing or enhancing symbiotic N fixation and the regulation of mineral N acquisition by roots, as well as their integration with other environmental factors, such as phosphate, light, and CO₂ availability.

C-terminally encoded peptides (CEPs) are potential mediators of abiotic stress response in plants

Intracellular signaling is a critical determinant of the normal growth and development of plants. Signaling peptides, also known as peptide hormones, along with classical phytohormones, are the significant players of plant intracellular signaling. C-terminally encoded peptide (CEP), a 15-amino acid post-translationally peptide identified in Arabidopsis, plays a pivotal role in lateral root formation, nodulation, and act as long-distance root to shoot signaling molecule in N-starvation conditions. Expression of CEP gene members in Arabidopsis is perturbed by nitrogen starvation; however, not much is known regarding their role in other abiotic stress conditions. To gain a comprehensive insight into CEP biology, we identified CEP genes across diverse plant genera (Glycine max, Sorghum bicolor, Brassica rapa, Zea mays, and Oryza sativa) using bioinformatics tools. In silico promoter analysis revealed that CEP gene promoters show an abundance of abiotic stress-responsive elements suggesting a possible role of CEPs in abiotic stress signaling. Spatial and temporal expression patterns of CEP via RNA seq and microarray revealed that various CEP genes are transcriptionally regulated in response to abiotic stresses. Validation of rice CEP genes expression by qRT-PCR showed that OsCEP1, OsCEP8, OsCEP9, and OsCEP10 were highly upregulated in response to different abiotic stress conditions. Our findings suggest these CEP genes might be important mediators of the abiotic stress response and warrant further overexpression/knockout studies to delineate their precise role in abiotic stress response.

Nitrate regulation of lateral root and root hair development in plants

Nitrogen (N) is one of the most important macronutrients for plant growth and development. However, the concentration and distribution of N varies in soil due to a variety of environmental factors. In response, higher plants have evolved a developmentally flexible root system to efficiently take up N under N-limited conditions. Over the past decade, significant progress has been made in understanding this form of plant 'root-foraging' behavior, which is controlled by both a local and a long-distance systemic nitrate signaling pathway. In this review, we focus on the key components of nitrate perception, signaling, and transduction and its role in lateral root development. We also highlight recent findings on the molecular mechanisms of the nitrate systemic signaling pathway, including small signaling peptides involved in long-distance shoot-root communication. Furthermore, we summarize the transcription factor networks responsible for nitrate-dependent lateral root and root hair development.

The NIN transcription factor coordinates CEP and CLE signaling peptides that regulate nodulation antagonistically

Legumes tightly regulate nodule number to balance the cost of supporting symbiotic rhizobia with the benefits of nitrogen fixation. C-terminally Encoded Peptides (CEPs) and CLAVATA3-like (CLE) peptides positively and negatively regulate nodulation, respectively, through independent systemic pathways, but how these regulations are coordinated remains unknown. Here, we show that rhizobia, Nod Factors, and cytokinins induce a symbiosis-specific CEP gene, MtCEP7, which positively regulates rhizobial infection. Via grafting and split root studies, we reveal that MtCEP7 increases nodule number systemically through the MtCRA2 receptor. MtCEP7 and MtCLE13 expression in rhizobia-inoculated roots rely on the MtCRE1 cytokinin receptor and on the MtNIN transcription factor. MtNIN binds and transactivates MtCEP7 and MtCLE13, and a NIN Binding Site (NBS) identified within the proximal MtCEP7 promoter is required for its symbiotic activation. Overall, these results demonstrate that a cytokinin-MtCRE1-MtNIN regulatory module coordinates the expression of two antagonistic, symbiosis-related, peptide hormones from different families to fine-tune nodule number.

CLE and CEP peptides regulate rhizobial symbiosis in legumes to balance the benefits of nitrogen fixation with the metabolic costs of nodule production. Here Laffont et al. show that cytokinin and bacterial Nod factors induce Medicago CEP7 which acts antagonistically to CLE13 to fine-tune nodulation.

CEP receptor signalling controls root system architecture in Arabidopsis and Medicago

Root system architecture (RSA) influences the effectiveness of resources acquisition from soils but the genetic networks that control RSA remain largely unclear. We used rhizoboxes, X-ray computed tomography, grafting, auxin transport measurements and hormone quantification to demonstrate that Arabidopsis and Medicago CEP (C-TERMINALLY ENCODED PEPTIDE)-CEP RECEPTOR signalling controls RSA, the gravitropic set-point angle (GSA) of lateral roots (LRs), auxin levels and auxin transport. We showed that soil-grown Arabidopsis and Medicago CEP receptor mutants have a narrower RSA, which results from a steeper LR GSA. Grafting showed that CEPR1 in the shoot controls GSA. CEP receptor mutants exhibited an increase in rootward auxin transport and elevated shoot auxin levels. Consistently, the application of auxin to wild-type shoots induced a steeper GSA and auxin transport inhibitors counteracted the CEP receptor mutant's steep GSA phenotype. Concordantly, CEP peptides increased GSA and inhibited rootward auxin transport in wild-type but not in CEP receptor mutants. The results indicated that CEP-CEP receptor-dependent signalling outputs in Arabidopsis and Medicago control overall RSA, LR GSA, shoot auxin levels and rootward auxin transport. We propose that manipulating CEP signalling strength or CEP receptor downstream targets may provide means to alter RSA.

The Peptide Hormone Receptor CEPR1 Functions in the Reproductive Tissue to Control Seed Size and Yield

The interaction of C-TERMINALLY ENCODED PEPTIDES (CEPs) with CEP RECEPTOR1 (CEPR1) controls root growth and development, as well as nitrate uptake, but has no known role in determining yield. We used physiological, microscopic, molecular, and grafting approaches to demonstrate a reproductive tissue-specific role for CEPR1 in controlling yield and seed size. Independent Arabidopsis (Arabidopsis thaliana) cepr1 null mutants showed disproportionately large reductions in yield and seed size relative to their decreased vegetative growth. These yield defects correlated with compromised reproductive development predominantly in female tissues, as well as chlorosis, and the accumulation of anthocyanins in cepr1 reproductive tissues. The thinning of competing reproductive organs to improve source-to-sink ratios in cepr1, along with reciprocal bolt-grafting experiments, demonstrated that CEPR1 acts locally in the reproductive bolt to control yield and seed size. CEPR1 is expressed throughout the vasculature of reproductive organs, including in the chalazal seed coat, but not in other seed tissues. This expression pattern implies that CEPR1 controls yield and seed size from the maternal tissue. The complementation of cepr1 mutants with transgenic CEPR1 rescued the yield and other phenotypes. Transcriptional analyses of cepr1 bolts showed alterations in the expression levels of several genes of the CEP-CEPR1 and nitrogen homeostasis pathways. This transcriptional profile was consistent with cepr1 bolts being nitrogen deficient and with a reproductive tissue-specific function for CEP-CEPR1 signaling. The results reveal a local role for CEPR1 in the maternal reproductive tissue in determining seed size and yield, likely via the control of nitrogen delivery to the reproductive sinks.

Hormone-like peptides and small coding genes in plant stress signaling and development

Recent works have shed light on the long-distance interorgan signaling by which hormone-like peptides precisely regulate physiological effects in a manner similar to phytohormones. Many such peptides have already been identified in the primary model plant, Arabidopsis thaliana. In addition, Arabidopsis genome reanalysis revealed over 7000 novel candidate small coding genes, some of which are likely to be associated with hormone-like peptides. Hormone-like peptides have also been reported to play critical roles in interorgan communications during morphogenesis and stress responses. In this review, we focus on the functional roles of hormone-like peptides and small coding genes in cell-to-cell and/or long-distance communications during plant stress signaling and development and discuss the evolutionary conservation of these peptides among plants.

CEP3 levels affect starvation-related growth responses of the primary root

CEPs (C-TERMINALLY ENCODED PEPTIDEs) inhibit Arabidopsis primary root growth by unknown mechanisms. We investigated how CEP3 levels control primary root growth. CEP3 peptide application decreased cell division, S-phase cell number, root meristematic cell number, and meristem zone (MZ) size in a dose- and CEP RECEPTOR1-dependent manner. Grafting showed that CEP3-dependent growth inhibition requires root and shoot CEPR1. CEP3 induced mitotic quiescence in MZ cells significantly faster than that induced by nutrient limitation alone. CEP3 also inhibited the restoration of S-phase to mitotically quiescence cells by nutrient resupply without quantitatively reducing TARGET OF RAPAMYCIN (TOR) kinase activity. In contrast, cep3-1 had an increased meristem size and S-phase cell number under nitrogen (N)-limited conditions, but not under N-sufficient conditions. Furthermore, cep3-1 meristematic cells remained in S-phase longer than wild-type cells during a sustained carbon (C) and N limitation. RNA sequencing showed that CEP3 peptide down-regulated genes involved in S-phase entry, cell wall and ribosome biogenesis, DNA replication, and meristem expansion, and up-regulated genes involved in catabolic processes and proteins and peptides that negatively control meristem expansion and root growth. Many of these genes were reciprocally regulated in cep3-1. The results suggest that raising CEP3 induces starvation-related responses that curtail primary root growth under severe nutrient limitation.

Polyproline II Helix as a Recognition Motif of Plant Peptide Hormones and Flagellin Peptide flg22

Background:

Plant peptide hormones play a crucial role in plant growth and development. A group of these peptide hormones are signaling peptides with 5 - 23 amino acids. Flagellin peptide (flg22) also elicits an immune response in plants. The functions are expressed through recognition of the peptide hormones and flg22. This recognition relies on membrane localized receptor kinases with extracellular leucine rich repeats (LRR-RKs). The structures of plant peptide hormones - AtPep1, IDA, IDL1, RGFs 1- 3, TDIF/CLE41 - and of flg22 complexed with LRR domains of corresponding LRRRKs and co-receptors SERKs have been determined. However, their structures are well not analyzed and characterized in detail. The structures of PIP, CEP, CIF, and HypSys are still unknown.

Objective:

Our motivation is to clarify structural features of these plant, small peptides and Flg22 in their bound states.

Methods:

In this article, we performed secondary structure assignments and HELFIT analyses (calculating helix axis, pitch, radius, residues per turn, and handedness) based on the atomic coordinates from the crystal structures of AtPep1, IDA, IDL1, RGFs 1- 3, TDIF/CLE41 - and of flg22. We also performed sequence analysis of the families of PIP, CEP, CIF, and HypSys in order to predict their secondary structures.

Results:

Following AtPep1 with 23 residues adopts two left handed polyproline helices (PPIIs) with six and four residues. IDA, IDL1, RGFs 1 - 2, and TDIF/CLE41 with 12 or 13 residues adopt a four residue PPII; RGF3 adopts two PPIIs with four residues. Flg22 with 22 residues also adopts a six residue PPII. The other peptide hormones – PIP, CEP, CIF, and HypSys – that are rich in proline or hydroxyproline presumably prefer PPII.

Conclusion:

The present analysis indicates that PPII helix in the plant small peptide hormones and in flg22 is crucial for recognition of the LRR domains in receptors.

CEP-CEPR1 signalling inhibits the sucrose-dependent enhancement of lateral root growth

Lateral root (LR) proliferation is a major determinant of soil nutrient uptake. How resource allocation controls the extent of LR growth remains unresolved. We used genetic, physiological, transcriptomic, and grafting approaches to define a role for C-TERMINALLY ENCODED PEPTIDE RECEPTOR 1 (CEPR1) in controlling sucrose-dependent LR growth. CEPR1 inhibited LR growth in response to applied sucrose, other metabolizable sugars, and elevated light intensity. Pathways through CEPR1 restricted LR growth by reducing LR meristem size and the length of mature LR cells. RNA-sequencing of wild-type (WT) and cepr1-1 roots with or without sucrose treatment revealed an intersection of CEP-CEPR1 signalling with the sucrose transcriptional response. Sucrose up-regulated several CEP genes, supporting a specific role for CEP-CEPR1 in the response to sucrose. Moreover, genes with basally perturbed expression in cepr1-1 overlap with WT sucrose-responsive genes significantly. We found that exogenous CEP inhibited LR growth via CEPR1 by reducing LR meristem size and mature cell length. This result is consistent with CEP-CEPR1 acting to curtail the extent of sucrose-dependent LR growth. Reciprocal grafting indicates that LR growth inhibition requires CEPR1 in both the roots and shoots. Our results reveal a new role for CEP-CEPR1 signalling in controlling LR growth in response to sucrose.

The LaCEP1 peptide modulates cluster root morphology in Lupinus albus

White lupin cluster roots are specialized brush-like root structures that are formed in some species under phosphorus (P)-deficient conditions. They intensely secrete protons and organic acid anions for solubilization and acquisition of sparingly soluble phosphates. Phytohormones and sucrose modulate cluster root number, but the molecular mechanisms of cluster root formation have been elusive. Here, a novel peptide phytohormone was identified that affects cluster root development. It belongs to the C-TERMINALLY-ENCODED PEPTIDE (CEP) family. Members of that family arrest root growth and modulate branching in model species. LaCEP1 was highly expressed in the pre-emergence zone of clusters. Over-expression of the gene encoding the LaCEP1 propeptide resulted in moderate inhibition of cluster root formation. The primary and lateral root lengths of lupin were little affected by the overexpression, but LaCEP1 reduced cluster rootlet and root hair elongation. Addition of a 15-mer core peptide derived from LaCEP1 similarly altered root morphology and modified cluster activity, suggesting that a core sequence of the propeptide is functionally sufficient. Stable overexpression in Arabidopsis confirmed the LaCEP1 function in root growth inhibition across species. Taken together, the root inhibitory effects of the LaCEP1 phytohormone suggest a role as of a regulatory module involved in cluster root development in white lupin.

Look Closely, the Beautiful May Be Small: Precursor-Derived Peptides in Plants

During the past decade, a flurry of research focusing on the role of peptides as short- and long-distance signaling molecules in plant cell communication has been undertaken. Here, we focus on peptides derived from nonfunctional precursors, and we address several key questions regarding peptide signaling. We provide an overview of the regulatory steps involved in producing a biologically active peptide ligand that can bind its corresponding receptor(s) and discuss how this binding and subsequent activation lead to specific cellular outputs. We discuss different experimental approaches that can be used to match peptide ligands with their receptors. Lastly, we explore how peptides evolved from basic signaling units regulating essential processes in plants to more complex signaling systems as new adaptive traits developed and how nonplant organisms exploit this signaling machinery by producing peptide mimics.

Proteolysis and nitrogen: emerging insights

Nitrogen (N) is a core component of fertilizers used in modern agriculture to increase yields and thus to help feed a growing global population. However, this comes at a cost to the environment, through run-off of excess N as a result of poor N-use efficiency (NUE) by crops. An obvious remedy to this problem would therefore be the improvement of NUE, which requires advancing our understanding on N homeostasis, sensing, and uptake. Proteolytic pathways are linked to N homeostasis as they recycle proteins that contain N and carbon; however, emerging data suggest that their functions extend beyond this simple recycling. Here, we highlight roles of proteolytic pathways in non-symbiotic and symbiotic N uptake and in systemic N sensing. We also offer a novel view in which we suggest that proteolytic pathways have roles in N homeostasis that differ from their accepted function in recycling.

The Importance of Conserved Serine for C-Terminally Encoded Peptides Function Exertion in Apple

BACKGROUND

The C-terminally encoded peptide (CEP) family has been shown to play vital roles in plant growth. Although a genome-wide analysis of this family has been performed in Arabidopsis, little is known regarding CEPs in apple (Malus domestica).

METHODS

Here, a comprehensive bioinformatics approach was applied to identify MdCEPs in apple, and 12 MdCEP genes were identified and distributed on 6 chromosomes.

RESULTS

MdCEP1 peptide had an inhibitory effect on root growth of apple seedlings, indicating that MdCEP1 played a negative role in root development. In addition, the serine and glycine residues remained conserved within the CEP domains, and MdCEP1 lost its function after mutation of these two key amino acids, suggesting that Ser¹⁰ and Gly¹⁴ residues are crucial for MdCEPs-mediated root growth of apple. Encouragingly, multiple sequence alignment of 273 CEP domains showed that Ser¹⁰ residue was evolutionarily conserved in monocot and eudicot plants. MdCEP derivative (Ser to Cys) lost the ability to inhibit the root growth of Nicotiana benthamiana, Setaria italic, Samolous parviflorus, and Raphanus sativus L. and up-regulate the NO_3^- importer gene NRT2.1.

CONCLUSION

Taken together, Ser¹⁰ residue is crucial for CEP function exertion in higher land plants, at least in apple.

Regulation of plant peptide hormones and growth factors by post-translational modification

The number, diversity and significance of peptides as regulators of cellular differentiation, growth, development and defence of plants has long been underestimated. Peptides have now emerged as an important class of signals for cell-to-cell communication over short distances, and also for long-range signalling. We refer to these signalling molecules as peptide growth factors and peptide hormones, respectively. As compared to remarkable progress with respect to the mechanisms of peptide perception and signal transduction, the biogenesis of signalling peptides is still in its infancy. This review focuses on the biogenesis and activity of small post-translationally modified peptides. These peptides are derived from inactive pre-pro-peptides of approximately 70-120 amino acids. Multiple post-translational modifications (PTMs) may be required for peptide maturation and activation, including proteolytic processing, tyrosine sulfation, proline hydroxylation and hydroxyproline glycosylation. While many of the enzymes responsible for these modifications have been identified, their impact on peptide activity and signalling is not fully understood. These PTMs may or may not be required for bioactivity, they may inactivate the peptide or modify its signalling specificity, they may affect peptide stability or targeting, or its binding affinity with the receptor. In the present review, we will first introduce the peptides that undergo PTMs and for which these PTMs were shown to be functionally relevant. We will then discuss the different types of PTMs and the impact they have on peptide activity and plant growth and development. We conclude with an outlook on the open questions that need to be addressed in future research.

Impact of Plant Peptides on Symbiotic Nodule Development and Functioning

Ribosomally synthesized peptides have wide ranges of functions in plants being, for example, signal molecules, transporters, alkaloids, or antimicrobial agents. Legumes are an unprecedented rich source of peptides, which are used to control the symbiosis of these plants with the nitrogen-fixing Rhizobium bacteria. Here, we discuss the function and the evolution of these peptides playing an important role in the formation or functioning of the symbiotic organs, the root nodules. We distinguish peptides that can be either cell-autonomous or secreted short-range or long-range signals, carrying messages in or between plant cells or that can act as effectors interacting with the symbiotic bacteria. Peptides are further classified according to the stage of the symbiotic process where they act. Several peptide classes, including RALF, DLV, ENOD40, and others, control Rhizobium infection and the initiation of cell divisions and the formation of nodule primordia. CLE and CEP peptides are implicated in systemic and local control of nodule initiation during autoregulation of nodulation and in response to the nutritional demands of the plant. Still other peptides act at later stages of the symbiosis. The PSK peptide is thought to be involved in the suppression of immunity in nodules and the nodule-specific cysteine-rich, GRP, and SNARP (LEED..PEED) peptide families are essential in the functioning of the nitrogen fixing root nodules. The NCRs and possibly also the GRP and SNARPs are targeted to the endosymbionts and play essential roles in the terminal differentiation of these bacteria.

CEP peptide hormones: key players in orchestrating nitrogen-demand signalling, root nodulation, and lateral root development

Secreted peptide hormones play pivotal roles in plant growth and development. So far, CEPs (C-TERMINALLY ENCODED PEPTIDEs) have been shown to act through CEP receptors (CEPRs) to control nitrogen (N)-demand signalling, nodulation, and lateral root development. Secreted CEP peptides can enter the xylem stream to act as long-distance signals, but evidence also exists for CEPs acting in local circuits. Recently, CEP peptide species varying in sequence, length, and post-translational modifications have been identified. A more comprehensive understanding of CEP biology requires insight into the in planta function of CEP genes, CEP peptide biogenesis, the components of CEP signalling cascades and, finally, how CEP peptide length, amino-acid composition, and post-translational modifications affect biological activity. In this review, we highlight recent studies that have advanced our understanding in these key areas and discuss some future directions.

In Silico Prediction of Ligand-Binding Sites of Plant Receptor Kinases Using Conservation Mapping

Plasma membrane-bound plant receptor-like kinases (RLKs) can be categorized based on their ligand-binding extracellular domain. The largest group encompasses RLKs having ectodomains with leucine-rich repeats (LRRs). The LRR-RLKs can further be assigned to classes mainly based on the number of LRRs. Many of the receptors of the classes X and XI with more than 20 LRRs are activated by small secreted peptide ligands. To understand how peptide signaling works, it is of interest to identify the amino acids of the receptor that are directly involved in ligand interaction. Such residues have most likely been conserved over evolutionary time and can therefore be predicted to be conserved in receptor orthologues of different plant species. Here we present an in silico method to identify such residues. This involves a simplified method for identification of orthologues and a web-based program for identifying the most conserved amino acids aside from the leucines that structure the ectodomain. The method has been validated for the LRR-RLKs HAESA (HAE) and PHYTOSULFOKINE RECEPTOR1 (PSKR1) for which conservation-mapping results closely matched recent structure-based identification of ligand and co-receptor-interacting residues.

Molecular mimicry modulates plant host responses to pathogens

BACKGROUND

Pathogens often secrete molecules that mimic those present in the plant host. Recent studies indicate that some of these molecules mimic plant hormones required for development and immunity.

SCOPE AND CONCLUSION

This Viewpoint reviews the literature on microbial molecules produced by plant pathogens that functionally mimic molecules present in the plant host. This article includes examples from nematodes, bacteria and fungi with emphasis on RaxX, a microbial protein produced by the bacterial pathogen Xanthomonas oryzae pv. oryzae. RaxX mimics a plant peptide hormone, PSY (plant peptide containing sulphated tyrosine). The rice immune receptor XA21 detects sulphated RaxX but not the endogenous peptide PSY. Studies of the RaxX/XA21 system have provided insight into both host and pathogen biology and offered a framework for future work directed at understanding how XA21 and the PSY receptor(s) can be differentially activated by RaxX and endogenous PSY peptides.

Genome-Wide Identification of Medicago Peptides Involved in Macronutrient Responses and Nodulation

Growing evidence indicates that small, secreted peptides (SSPs) play critical roles in legume growth and development, yet the annotation of SSP-coding genes is far from complete. Systematic reannotation of the Medicago truncatula genome identified 1,970 homologs of established SSP gene families and an additional 2,455 genes that are potentially novel SSPs, previously unreported in the literature. The expression patterns of known and putative SSP genes based on 144 RNA sequencing data sets covering various stages of macronutrient deficiencies and symbiotic interactions with rhizobia and mycorrhiza were investigated. Focusing on those known or suspected to act via receptor-mediated signaling, 240 nutrient-responsive and 365 nodulation-responsive Signaling-SSPs were identified, greatly expanding the number of SSP gene families potentially involved in acclimation to nutrient deficiencies and nodulation. Synthetic peptide applications were shown to alter root growth and nodulation phenotypes, revealing additional regulators of legume nutrient acquisition. Our results constitute a powerful resource enabling further investigations of specific SSP functions via peptide treatment and reverse genetics.

Diverse Peptide Hormones Affecting Root Growth Identified in the Medicago truncatula Secreted Peptidome

Multigene families encoding diverse secreted peptide hormones play important roles in plant development. A need exists to efficiently elucidate the structures and post-translational-modifications of these difficult-to-isolate peptide hormones in planta so that their biological functions can be determined. A mass spectrometry and bioinformatics approach was developed to comprehensively analyze the secreted peptidome of Medicago hairy root cultures and xylem sap. We identified 759 spectra corresponding to the secreted products of twelve peptide hormones including four CEP (C-TERMINALLY ENCODED PEPTIDE), two CLE (CLV3/ENDOSPERM SURROUNDING REGION RELATED) and six XAP (XYLEM SAP ASSOCIATED PEPTIDE) peptides. The MtCEP1, MtCEP2, MtCEP5 and MtCEP8 peptides identified differed in post-translational-modifications. Most were hydroxylated at conserved proline residues but some MtCEP1 derivatives were tri-arabinosylated. In addition, many CEP peptides possessed unexpected N- and C-terminal extensions. The pattern of these extensions suggested roles for endo- and exoproteases in CEP peptide maturation. Longer than expected, hydroxylated and homogeneously modified mono- and tri-arabinosylated CEP peptides corresponding to their in vivo structures were chemically synthesized to probe the effect of these post-translational-modifications on function. The ability of CEP peptides to elevate root nodule number was increased by hydroxylation at key positions. MtCEP1 peptides with N-terminal extensions or with tri-arabinosylation modification, however, were unable to impart increased nodulation. The MtCLE5 and MtCLE17 peptides identified were of precise size, and inhibited main root growth and increased lateral root number. Six XAP peptides, each beginning with a conserved DY sulfation motif, were identified including MtXAP1a, MtXAP1b, MtXAP1c, MtXAP3, MtXAP5 and MtXAP7. MtXAP1a and MtXAP5 inhibited lateral root emergence. Transcriptional analyses demonstrated peptide hormone gene expression in the root vasculature and tip. Since hairy roots can be induced on many plants, their corresponding root cultures may represent ideal source materials to efficiently identify diverse peptide hormones in vivo in a broad range of species.

Shoot-to-root mobile polypeptides involved in systemic regulation of nitrogen acquisition

Plants uptake nitrogen (N) from the soil mainly in the form of nitrate. However, nitrate is often distributed heterogeneously in natural soil. Plants, therefore, have a systemic long-distance signalling mechanism by which N starvation on one side of the root leads to a compensatory N uptake on the other N-rich side^1,2. This systemic N acquisition response is triggered by a root-to-shoot mobile peptide hormone, C-TERMINALLY ENCODED PEPTIDE (CEP), originating from the N-starved roots^3,4, but the molecular nature of the descending shoot-to-root signal remains elusive. Here, we show that phloem-specific polypeptides that are induced in leaves upon perception of root-derived CEP act as descending long-distance mobile signals translocated to each root. These shoot-derived polypeptides, which we named CEP DOWNSTREAM 1 (CEPD1) and CEPD2, upregulate the expression of the nitrate transporter gene NRT2.1 in roots specifically when nitrate is present in the rhizosphere. Arabidopsis plants deficient in this pathway show impaired systemic N acquisition response accompanied with N-deficiency symptoms. These fundamental mechanistic insights should provide a conceptual framework for understanding systemic nutrient acquisition responses in plants.

Functional C-TERMINALLY ENCODED PEPTIDE (CEP) plant hormone domains evolved de novo in the plant parasite Rotylenchulus reniformis

Sedentary plant-parasitic nematodes (PPNs) induce and maintain an intimate relationship with their host, stimulating cells adjacent to root vascular tissue to re-differentiate into unique and metabolically active 'feeding sites'. The interaction between PPNs and their host is mediated by nematode effectors. We describe the discovery of a large and diverse family of effector genes, encoding C-TERMINALLY ENCODED PEPTIDE (CEP) plant hormone mimics (RrCEPs), in the syncytia-forming plant parasite Rotylenchulus reniformis. The particular attributes of RrCEPs distinguish them from all other CEPs, regardless of origin. Together with the distant phylogenetic relationship of R. reniformis to the only other CEP-encoding nematode genus identified to date (Meloidogyne), this suggests that CEPs probably evolved de novo in R. reniformis. We have characterized the first member of this large gene family (RrCEP1), demonstrating its significant up-regulation during the plant-nematode interaction and expression in the effector-producing pharyngeal gland cell. All internal CEP domains of multi-domain RrCEPs are followed by di-basic residues, suggesting a mechanism for cleavage. A synthetic peptide corresponding to RrCEP1 domain 1 is biologically active and capable of up-regulating plant nitrate transporter (AtNRT2.1) expression, whilst simultaneously reducing primary root elongation. When a non-CEP-containing, syncytia-forming PPN species (Heterodera schachtii) infects Arabidopsis in a CEP-rich environment, a smaller feeding site is produced. We hypothesize that CEPs of R. reniformis represent a two-fold adaptation to sustained biotrophy in this species: (i) increasing host nitrate uptake, whilst (ii) limiting the size of the syncytial feeding site produced.

Different Pathways Act Downstream of the CEP Peptide Receptor CRA2 to Regulate Lateral Root and Nodule Development

C-TERMINALLY ENCODED PEPTIDEs (CEPs) control root system architecture in a non-cell-autonomous manner. In Medicago truncatula, MtCEP1 affects root development by increasing nodule formation and inhibiting lateral root emergence by unknown pathways. Here, we show that the MtCEP1 peptide-dependent increase in nodulation requires the symbiotic signaling pathway and ETHYLENE INSENSITIVE2 (EIN2)/SICKLE (SKL), but acts independently of SUPER NUMERIC NODULES. MtCEP1-dependent inhibition of lateral root development acts through an EIN2-independent mechanism. MtCEP1 increases nodulation by promoting rhizobial infections, the developmental competency of roots for nodulation, the formation of fused nodules, and an increase in frequency of nodule development that initiates at proto-phloem poles. These phenotypes are similar to those of the ein2/skl mutant and support that MtCEP1 modulates EIN2-dependent symbiotic responses. Accordingly, MtCEP1 counteracts the reduction in nodulation induced by increasing ethylene precursor concentrations, and an ethylene synthesis inhibitor treatment antagonizes MtCEP1 root phenotypes. MtCEP1 also inhibits the development of EIN2-dependent pseudonodule formation. Finally, mutants affecting the COMPACT ROOT ARCHITECTURE2 (CRA2) receptor, which is closely related to the Arabidopsis CEP Receptor1, are unresponsive to MtCEP1 effects on lateral root and nodule formation, suggesting that CRA2 is a CEP peptide receptor mediating both organogenesis programs. In addition, an ethylene inhibitor treatment counteracts the cra2 nodulation phenotype. These results indicate that MtCEP1 and its likely receptor, CRA2, mediate nodulation and lateral root development through different pathways.

Root-to-shoot signalling: integration of diverse molecules, pathways and functions

Plant adaptive potential is critically dependent upon efficient communication and co-ordination of resource allocation and signalling between above- and below-ground plant parts. Plant roots act as gatekeepers that sense and encode information about soil physical, chemical and biological factors, converting them into a sophisticated network of signals propagated both within the root itself, and also between the root and shoot, to optimise plant performance for a specific set of conditions. In return, plant roots receive and decode reciprocal information coming from the shoot. The communication modes are highly diverse and include a broad range of physical (electric and hydraulic signals, propagating Ca2+ and ROS waves), chemical (assimilates, hormones, peptides and nutrients), and molecular (proteins and RNA) signals. Further, different signalling systems operate at very different timescales. It remains unclear whether some of these signalling systems operate in a priming mode(s), whereas others deliver more specific information about the nature of the signal, or whether they carry the same 'weight'. This review summarises the current knowledge of the above signalling mechanisms, and reveals their hierarchy, and highlights the importance of integration of these signalling components, to enable optimal plant functioning in a dynamic environment.

Overexpression of Peptide-Encoding OsCEP6.1 Results in Pleiotropic Effects on Growth in Rice (O. sativa)

Plant peptide hormones play an important role in regulating plant developmental programs via cell-to-cell communication in a non-cell autonomous manner. To characterize the biological relevance of C-TERMINALLY ENCODED PEPTIDE (CEP) genes in rice, we performed a genome-wide search against public databases using a bioinformatics approach and identified six additional CEP members. Expression analysis revealed a spatial-temporal pattern of OsCEP6.1 gene in different tissues and at different developmental stages of panicle. Interestingly, the expression level of the OsCEP6.1 was also significantly up-regulated by exogenous cytokinin. Application of a chemically synthesized 15-amino acid OsCEP6.1 peptide showed that OsCEP6.1 had a negative role in regulating root and seedling growth, which was further confirmed by transgenic lines. Furthermore, the constitutive expression of OsCEP6.1 was sufficient to lead to panicle architecture and grain size variations. Scanning electron microscopy analysis revealed that the phenotypic variation of OsCEP6.1 overexpression lines resulted from decreased cell size but not reduced cell number. Moreover, starch accumulation was not significantly affected. Taken together, these data suggest that the OsCEP6.1 peptide might be involved in regulating the development of panicles and grains in rice.