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

Peptide hormones in plants

Peptide hormones are defined as small secreted polypeptide-based intercellular communication signal molecules. Such peptide hormones are encoded by nuclear genes, and often go through proteolytic processing of preproproteins and post-translational modifications. Most peptide hormones are secreted out of the cell to interact with membrane-associated receptors in neighboring cells, and subsequently activate signal transductions, leading to changes in gene expression and cellular responses. Since the discovery of the first plant peptide hormone, systemin, in tomato in 1991, putative peptide hormones have continuously been identified in different plant species, showing their importance in both short- and long-range signal transductions. The roles of peptide hormones are implicated in, but not limited to, processes such as self-incompatibility, pollination, fertilization, embryogenesis, endosperm development, stem cell regulation, plant architecture, tissue differentiation, organogenesis, dehiscence, senescence, plant-pathogen and plant-insect interactions, and stress responses. This article, collectively written by researchers in this field, aims to provide a general overview for the discoveries, functions, chemical natures, transcriptional regulations, and post-translational modifications of peptide hormones in plants. We also updated recent discoveries in receptor kinases underlying the peptide hormone sensing and down-stream signal pathways. Future prospective and challenges will also be discussed at the end of the article.

Understanding the Crucial Role of Phosphate and Iron Availability in Regulating Root Nodule Symbiosis

The symbiosis between legumes and nitrogen-fixing bacteria (rhizobia) is instrumental in sustaining the nitrogen cycle and providing fixed nitrogen to the food chain. Both partners must maintain an efficient nutrient exchange to ensure a successful symbiosis. This mini-review highlights the intricate phosphate and iron uptake and homeostasis processes taking place in legumes during their interactions with rhizobia. The coordination of transport and homeostasis of these nutrients in host plants and rhizobia ensures an efficient nitrogen fixation process and nutrient use. We discuss the genetic machinery controlling the uptake and homeostasis of these nutrients in the absence of rhizobia and under symbiotic conditions with this soil bacterium. We also highlight the genetic impact of the availability of phosphate and iron to coordinate the activation of the genetic programs that allow legumes to engage in symbiosis with rhizobia. Finally, we discuss how the transcription factor phosphate starvation response might be a crucial genetic element to integrate the plant's needs of nitrogen, iron and phosphate while interacting with rhizobia. Understanding the coordination of the iron and phosphate uptake and homeostasis can lead us to better harness the ecological benefits of the legume-rhizobia symbiosis, even under adverse environmental conditions.

CEP signaling coordinates plant immunity with nitrogen status

Plant endogenous signaling peptides shape growth, development and adaptations to biotic and abiotic stress. Here, we identify C-TERMINALLY ENCODED PEPTIDEs (CEPs) as immune-modulatory phytocytokines in Arabidopsis thaliana. Our data reveals that CEPs induce immune outputs and are required to mount resistance against the leaf-infecting bacterial pathogen Pseudomonas syringae pv. tomato. We show that effective immunity requires CEP perception by tissue-specific CEP RECEPTOR 1 (CEPR1) and CEPR2. Moreover, we identify the related RECEPTOR-LIKE KINASE 7 (RLK7) as a CEP4-specific CEP receptor contributing to CEP-mediated immunity, suggesting a complex interplay of multiple CEP ligands and receptors in different tissues during biotic stress. CEPs have a known role in the regulation of root growth and systemic nitrogen (N)-demand signaling. We provide evidence that CEPs and their receptors promote immunity in an N status-dependent manner, suggesting a previously unknown molecular crosstalk between plant nutrition and cell surface immunity. We propose that CEPs and their receptors are central regulators for the adaptation of biotic stress responses to plant-available resources.

GmbZIP4a/b Positively Regulate Nodule Number by Affecting Cytokinin Biosynthesis in Glycine max

Legumes have the capability to form nodules that facilitate symbiotic nitrogen fixation (SNF) with rhizobia. Given the substantial energy consumption during the process of SNF, legumes need to optimize nodule number in response to everchanging environmental scenarios. The TGACG BINDING FACTOR1/4 (TGA1/4) are key players in the basal immune response of plants. In this study, both β-glucuronidase staining and quantitative reverse transcription PCR (qRT-PCR) demonstrated that both GmbZIP4a and GmbZIP4b are inducible upon rhizobial inoculation. To investigate their roles further, we constructed gmbzip4a/b double mutants using CRISPR/Cas9 system. Nodulation assessments revealed that these double mutants displayed a reduction in the number of infection threads, which subsequently resulted in a decreased nodule number. However, the processes associated with nodule development including nodule fresh weight, structural characteristics, and nitrogenase activity, remained unaffected in the double mutants. Subsequent transcriptome analyses revealed that zeatin biosynthesis was downregulated in gmbzip4a/b mutants post rhizobial inoculation. Supporting these findings, genes associated with cytokinin (CTK) signaling pathway were upregulated in Williams 82 (Wm82), but this upregulation was not observed in the double mutants after rhizobial treatment. These results suggest that GmbZIP4a/b positively influences nodule formation by promoting the activation of the CTK signaling pathway during the early stages of nodule formation.

TML1 and TML2 synergistically regulate nodulation and affect arbuscular mycorrhiza in Medicago truncatula

Two symbiotic processes, nodulation and arbuscular mycorrhiza, are primarily controlled by the plant's need for nitrogen (N) and phosphorus (P), respectively. Autoregulation of nodulation (AON) and autoregulation of mycorrhizal symbiosis (AOM) both negatively regulate their respective processes and share multiple components-plants that make too many nodules usually have higher arbuscular mycorrhiza (AM) fungal root colonization. The protein TML (TOO MUCH LOVE) was shown to function in roots to maintain susceptibly to rhizobial infection under low N conditions and control nodule number through AON in Lotus japonicus. Medicago truncatula has two sequence homologs: MtTML1 and MtTML2. We report the generation of stable single and double mutants harboring multiple allelic variations in MtTML1 and MtTML2 using CRISPR-Cas9 targeted mutagenesis and screening of a transposon mutagenesis library. Plants containing single mutations in MtTML1 or MtTML2 produced two to three times the nodules of wild-type plants, whereas plants containing mutations in both genes displayed a synergistic effect, forming 20× more nodules compared to wild-type plants. Examination of expression and heterozygote effects suggests that genetic compensation may play a role in the observed synergy. Plants with mutations in both TMLs only showed mild increases in AM fungal root colonization at later timepoints in our experiments, suggesting that these genes may also play a minor role in AM symbiosis regulation. The mutants created will be useful tools to dissect the mechanism of synergistic action of MtTML1 and MtTML2 in M. truncatula symbiosis with beneficial microbes.

TML1 AND TML2 SYNERGISTICALLY REGULATE NODULATION AND AFFECT ARBUSCULAR MYCORRHIZA IN MEDICAGO TRUNCATULA

Two symbiotic processes, nodulation and arbuscular mycorrhiza, are primarily controlled by the plant's need for nitrogen (N) and phosphorus (P), respectively. Autoregulation of Nodulation (AON) and Autoregulation of Mycorrhization (AOM) both negatively regulate their respective processes and share multiple components - plants that make too many nodules usually have higher AM fungal root colonization. The protein TML (TOO MUCH LOVE) was shown to function in roots to maintain susceptibly to rhizobial infection under low N conditions and control nodule number through AON in Lotus japonicus . M. truncatula has two sequence homologs: Mt TML1 and Mt TML2. We report the generation of stable single and double mutants harboring multiple allelic variations in MtTML1 and MtTML2 using CRISPR-Cas9 targeted mutagenesis and screening of a transposon mutagenesis library. Plants containing single mutations in Mt TML1 or Mt TML2 produced 2-3 times the nodules of wild-type plants whereas plants containing mutations in both genes displayed a synergistic effect, forming 20x more nodules compared to wild type plants. Examination of expression and heterozygote effects suggest genetic compensation may play a role in the observed synergy. Plants with mutations in both TMLs only showed mild increases in AM fungal root colonization at later timepoints in our experiments, suggesting these genes may also play a minor role in AM symbiosis regulation. The mutants created will be useful tools to dissect the mechanism of synergistic action of Mt TML1 and Mt TML2 in M. truncatula symbiosis with beneficial microbes.

MicroRNAs as potent regulators in nitrogen and phosphorus signaling transduction and their applications

Nitrogen (N) and phosphorus (Pi) are essential macronutrients that affect plant growth and development by influencing the molecular, metabolic, biochemical, and physiological responses at the local and whole levels in plants. N and Pi stresses suppress the physiological activities of plants, resulting in agricultural productivity losses and severely threatening food security. Accordingly, plants have elaborated diverse strategies to cope with N and Pi stresses through maintaining N and Pi homeostasis. MicroRNAs (miRNAs) as potent regulators fine-tune N and Pi signaling transduction that are distinct and indivisible from each other. Specific signals, such as noncoding RNAs (ncRNAs), interact with miRNAs and add to the complexity of regulation. Elucidation of the mechanisms by which miRNAs regulate N and Pi signaling transduction aids in the breeding of plants with strong tolerance to N and Pi stresses and high N and Pi use efficiency by fine-tuning MIR genes or miRNAs. However, to date, there has been no detailed and systematic introduction and comparison of the functions of miRNAs in N and Pi signaling transduction from the perspective of miRNAs and their applications. Here, we summarized and discussed current advances in the involvement of miRNAs in N and Pi signaling transduction and highlighted that fine-tuning the MIR genes or miRNAs involved in maintaining N and Pi homeostasis might provide valuable sights for sustainable agriculture.

More than meets the eye: knowns and unknowns of the trafficking of small secreted proteins in Arabidopsis

Small proteins represent a significant portion of the cargo transported through plant secretory pathways, playing crucial roles in developmental processes, fertilization, and responses to environmental stresses. Despite the importance of small secreted proteins, substantial knowledge gaps persist regarding the regulatory mechanisms governing their trafficking along the secretory pathway, and their ultimate localization or destination. To address these gaps, we conducted a comprehensive literature review, focusing particularly on trafficking and localization of Arabidopsis small secreted proteins with potential biochemical and/or signaling roles in the extracellular space, typically those within the size range of 101-200 amino acids. Our investigation reveals that while at least six members of the 21 mentioned families have a confirmed extracellular localization, eight exhibit intracellular localization, including cytoplasmic, nuclear, and chloroplastic locations, despite the presence of N-terminal signal peptides. Further investigation into the trafficking and secretion mechanisms of small protein cargo could not only deepen our understanding of plant cell biology and physiology but also provide a foundation for genetic manipulation strategies leading to more efficient plant cultivation.

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 PEPTIDE (CEP) and cytokinin hormone signaling intersect to promote shallow lateral root angles

Root system architecture (RSA) influences the acquisition of heterogeneously dispersed soil nutrients. Cytokinin and C-TERMINALLY ENCODED PEPTIDE (CEP) hormones affect RSA, in part by controlling the angle of lateral root (LR) growth. Both hormone pathways converge on CEP DOWNSTREAM 1 (CEPD1) and CEPD2 to control primary root growth; however, a role for CEPDs in controlling the growth angle of LRs is unknown. Using phenotyping combined with genetic and grafting approaches, we show that CEP hormone-mediated shallower LR growth requires cytokinin biosynthesis and perception in roots via ARABIDOPSIS HISTIDINE KINASE 2 (AHK2) and AHK3. Consistently, cytokinin biosynthesis and ahk2,3 mutants phenocopied the steeper root phenotype of cep receptor 1 (cepr1) mutants on agar plates, and CEPR1 was required for trans-Zeatin (tZ)-type cytokinin-mediated shallower LR growth. In addition, the cepd1,2 mutant was less sensitive to CEP and tZ, and showed basally steeper LRs on agar plates. Cytokinin and CEP pathway mutants were grown in rhizoboxes to define the role of these pathways in controlling RSA. Only cytokinin receptor mutants and cepd1,2 partially phenocopied the steeper-rooted phenotype of cepr1 mutants. These results show that CEP and cytokinin signaling intersect to promote shallower LR growth, but additional components contribute to the cepr1 phenotype in soil.

Rhizobium symbiotic efficiency meets CEP signaling peptides

C-terminally encoded peptides (CEP) signaling peptides are drivers of systemic pathways regulating nitrogen (N) acquisition in different plants, from Arabidopsis to legumes, depending on mineral N availability (e.g. nitrate) and on the whole plant N demand. Recent studies in the Medicago truncatula model legume revealed how root-produced CEP peptides control the root competence for endosymbiosis with N fixing rhizobia soil bacteria through the activity of the Compact Root Architecture 2 (CRA2) CEP receptor in shoots. Among CEP genes, MtCEP7 was shown to be tightly linked to nodulation, and the dynamic temporal regulation of its expression reflects the plant ability to maintain a different symbiotic root competence window depending on the symbiotic efficiency of the rhizobium strain, as well as to reinitiate a new window of root competence for nodulation.

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.