The YDA-MKK4/MKK5-MPK3/MPK6 Cascade Functions Downstream of the RGF1-RGI Ligand-Receptor Pair in Regulating Mitotic Activity in Root Apical Meristem

A close-up of regulatory networks and signaling pathways of MKK5 in biotic and abiotic stresses

Mitogen-activated protein Kinase Kinase 5 (MKK5) is a central hub in the complex phosphorylation chain reaction of the Mitogen-activated protein kinases (MAPK) cascade, regulating plant responses to biotic and abiotic stresses. This review manuscript aims to provide a comprehensive analysis of the regulatory mechanism of the MKK5 involved in stress adaptation. This review will delve into the intricate post-transcriptional and post-translational modifications of the MKK5, discussing how they affect its expression, activity, and subcellular localization in response to stress signals. We also discuss the integration of the MKK5 into complex signaling pathways, orchestrating plant immunity against pathogens and its modulating role in regulating abiotic stresses, such as: drought, cold, heat, and salinity, through the phytohormonal signaling pathways. Furthermore, we highlight potential applications of the MKK5 for engineering stress-resilient crops and provide future perspectives that may pave the way for future studies. This review manuscript aims to provide valuable insights into the mechanisms underlying MKK5 regulation, bridge the gap from numerous previous findings, and offer a firm base in the knowledge of MKK5, its regulating roles, and its involvement in environmental stress regulation.

Hydrogen Peroxide Signaling in the Maintenance of Plant Root Apical Meristem Activity

Hydrogen peroxide (H2O2) is a prevalent reactive oxygen species (ROS) found in cells and takes a central role in plant development and stress adaptation. The root apical meristem (RAM) has evolved strong plasticity to adapt to complex and changing environmental conditions. Recent advances have made great progress in explaining the mechanism of key factors, such as auxin, WUSCHEL-RELATED HOMEOBOX 5 (WOX5), PLETHORA (PLT), SHORTROOT (SHR), and SCARECROW (SCR), in the regulation of RAM activity maintenance. H2O2 functions as an emerging signaling molecule to control the quiescent center (QC) specification and stem cell niche (SCN) activity. Auxin is a key signal for the regulation of RAM maintenance, which largely depends on the formation of auxin regional gradients. H2O2 regulates the auxin gradients by the modulation of intercellular transport. H2O2 also modulates the expression of WOX5, PLTs, SHR, and SCR to maintain RAM activity. The present review is dedicated to summarizing the key factors in the regulation of RAM activity and discussing the signaling transduction of H2O2 in the maintenance of RAM activity. H2O2 is a significant signal for plant development and environmental adaptation.

Sulfated peptides and their receptors: Key regulators of plant development and stress adaptation

Four distinct types of sulfated peptides have been identified in Arabidopsis thaliana. These peptides play crucial roles in regulating plant development and stress adaptation. Recent studies have revealed that Xanthomonas and Meloidogyne can secrete plant-like sulfated peptides, exploiting the plant sulfated peptide signaling pathway to suppress plant immunity. Over the past three decades, receptors for these four types of sulfated peptides have been identified, all of which belong to the leucine-rich repeat receptor-like protein kinase subfamily. A number of regulatory proteins have been demonstrated to play important roles in their corresponding signal transduction pathways. In this review, we comprehensively summarize the discoveries of sulfated peptides and their receptors, mainly in Arabidopsis thaliana. We also discuss their known biological functions in plant development and stress adaptation. Finally, we put forward a number of questions for reference in future studies.

StMAPKK5 Positively Regulates Response to Drought and Salt Stress in Potato

MAPKKs, as one of the main members of the mitogen-activated protein kinase (MAPK) cascade pathway, are located in the middle of the cascade and are involved in many physiological processes of plant growth and development, as well as stress tolerance. Previous studies have found that StMAPKK5 is responsive to drought and salt stress. To further investigate the function and regulatory mechanism of StMAPKK5 in potato stress response, potato variety 'Atlantic' was subjected to drought and NaCl treatments, and the expression of the StMAPKK5 gene was detected by qRT-PCR. StMAPKK5 overexpression and RNA interference-mediated StMAPKK5 knockdown potato plants were constructed. The relative water content, superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities, as well as proline (Pro) and malondialdehyde (MDA) contents of plant leaves, were also assayed under drought and NaCl stress. The StMAPKK5 interacting proteins were identified and validated by yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC). The results showed that the expression of StMAPKK5 was significantly up-regulated under drought and NaCl stress conditions. The StMAPKK5 protein was localized in the nucleus, cytoplasm, and cell membrane. The expression of StMAPKK5 affected the relative water content, the enzymatic activities of SOD, CAT, and POD, and the proline and MDA contents of potatoes under drought and salt stress conditions. These results suggest that StMAPKK5 plays a significant role in regulating drought and salt tolerance in potato crop. Yeast two-hybrid (Y2H) screening identified four interacting proteins: StMYB19, StZFP8, StPUB-like, and StSKIP19. BiFC confirmed the authenticity of the interactions. These findings suggest that StMAPKK5 is crucial for potato growth, development, and response to adversity.

Evolution of reactive oxygen species cellular targets for plant development

Reactive oxygen species (ROS) are the key players in regulating developmental processes of plants. Plants have evolved a large array of gene families to facilitate the ROS-regulated developmental process in roots and leaves. However, the cellular targets of ROS during plant evolutionary development are still elusive. Here, we found early evolution and large expansions of protein families such as mitogen-activated protein kinases (MAPK) in the evolutionarily important plant lineages. We review the recent advances in interactions among ROS, phytohormones, gasotransmitters, and protein kinases. We propose that these signaling molecules act in concert to maintain cellular ROS homeostasis in developmental processes of root and leaf to ensure the fine-tuning of plant growth for better adaptation to the changing climate.

Comprehensive analysis of MAPK gene family in upland cotton (Gossypium hirsutum) and functional characterization of GhMPK31 in regulating defense response to insect infestation

KEY MESSAGE

The transcriptomic, phenotypic and metabolomic analysis of transgenic plants overexpressing GhMPK31 in upland cotton revealed the regulation of H2O2 burst and the synthesis of defensive metabolites by GhMPK31. Mitogen-activated protein kinases (MAPKs) are a crucial class of protein kinases, which play an essential role in various biological processes in plants. Upland cotton (G. hirsutum) is the most widely cultivated cotton species with high economic value. To gain a better understanding of the role of the MAPK gene family, we conducted a comprehensive analysis of the MAPK gene family in cotton. In this study, a total of 55 GhMPK genes were identified from the whole genome of G. hirsutum. Through an investigation of the expression patterns under diverse stress conditions, we discovered that the majority of GhMPK family members demonstrated robust responses to abiotic stress, pathogen stress and pest stress. Furthermore, the overexpression of GhMPK31 in cotton leaves led to a hypersensitive response (HR)-like cell death phenotype and impaired the defense capability of cotton against herbivorous insects. Transcriptome and metabolomics data analysis showed that overexpression of GhMPK31 enhanced the expression of H2O2-related genes and reduced the accumulation of defensive related metabolites. The direct evidence of GhMPK31 interacting with GhRBOHB (H2O2-generating protein) were found by Y2H, BiFC, and LCI. Therefore, we propose that the increase of H2O2 content caused by overexpression of GhMPK31 resulted in HR-like cell death in cotton leaves while reducing the accumulation of defensive metabolites, ultimately leading to a decrease in the defense ability of cotton against herbivorous insects. This study provides valuable insights into the function of MAPK genes in plant resistance to herbivorous insects.

Cultivating potential: Harnessing plant stem cells for agricultural crop improvement

Meristems are stem cell-containing structures that produce all plant organs and are therefore important targets for crop improvement. Developmental regulators control the balance and rate of cell divisions within the meristem. Altering these regulators impacts meristem architecture and, as a consequence, plant form. In this review, we discuss genes involved in regulating the shoot apical meristem, inflorescence meristem, axillary meristem, root apical meristem, and vascular cambium in plants. We highlight several examples showing how crop breeders have manipulated developmental regulators to modify meristem growth and alter crop traits such as inflorescence size and branching patterns. Plant transformation techniques are another innovation related to plant meristem research because they make crop genome engineering possible. We discuss recent advances on plant transformation made possible by studying genes controlling meristem development. Finally, we conclude with discussions about how meristem research can contribute to crop improvement in the coming decades.

MYB44 regulates PTI by promoting the expression of EIN2 and MPK3/6 in Arabidopsis

The plant signaling pathway that regulates pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) involves mitogen-activated protein kinase (MAPK) cascades that comprise sequential activation of several protein kinases and the ensuing phosphorylation of MAPKs, which activate transcription factors (TFs) to promote downstream defense responses. To identify plant TFs that regulate MAPKs, we investigated TF-defective mutants of Arabidopsis thaliana and identified MYB44 as an essential constituent of the PTI pathway. MYB44 confers resistance against the bacterial pathogen Pseudomonas syringae by cooperating with MPK3 and MPK6. Under PAMP treatment, MYB44 binds to the promoters of MPK3 and MPK6 to activate their expression, leading to phosphorylation of MPK3 and MPK6 proteins. In turn, phosphorylated MPK3 and MPK6 phosphorylate MYB44 in a functionally redundant manner, thus enabling MYB44 to activate MPK3 and MPK6 expression and further activate downstream defense responses. Activation of defense responses has also been attributed to activation of EIN2 transcription by MYB44, which has previously been shown to affect PAMP recognition and PTI development. AtMYB44 thus functions as an integral component of the PTI pathway by connecting transcriptional and posttranscriptional regulation of the MPK3/6 cascade.

PSW1, an LRR receptor kinase, regulates pod size in peanut

Pod size is a key agronomic trait that greatly determines peanut yield, the regulatory genes and molecular mechanisms that controlling peanut pod size are still unclear. Here, we used quantitative trait locus analysis to identify a peanut pod size regulator, POD SIZE/WEIGHT1 (PSW1), and characterized the associated gene and protein. PSW1 encoded leucine-rich repeat receptor-like kinase (LRR-RLK) and positively regulated pod stemness. Mechanistically, this allele harbouring a 12-bp insertion in the promoter and a point mutation in the coding region of PSW1 causing a serine-to-isoleucine (S618I) substitution substantially increased mRNA abundance and the binding affinity of PSW1 for BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE 1 (BAK1). Notably, PSW1HapII (super-large pod allele of PSW1) expression led to up-regulation of a positive regulator of pod stemness PLETHORA 1 (PLT1), thereby resulting in larger pod size. Moreover, overexpression of PSW1HapII increased seed/fruit size in multiple plant species. Our work thus discovers a conserved function of PSW1 that controls pod size and provides a valuable genetic resource for breeding high-yield crops.

Genome-Wide Identification of the MPK Gene Family and Expression Analysis under Low-Temperature Stress in the Banana

Mitogen-activated protein kinases (MAPKs and MPKs) are important in the process of resisting plant stress. In this study, 21, 12, 18, 16, and 10 MPKs were identified from Musa acuminata, Musa balbisiana, Musa itinerans, Musa schizocarpa, and Musa textilis, respectively. These MPKs were divided into Group A, B, C, and D. Phylogenetic analysis revealed that this difference in number was due to the gene shrinkage of the Group B subfamily of Musa balbisiana and Musa textilis. KEGG annotations revealed that K14512, which is involved in plant hormone signal transduction and the plant-pathogen interaction, was the most conserved pathway of the MPKs. The results of promoter cis-acting element prediction and focTR4 (Fusarium oxysporum f. sp. cubense tropical race 4) transcriptome expression analysis preliminarily confirmed that MPKs were relevant to plant hormone and biotic stress, respectively. The expression of MPKs in Group A was significantly upregulated at 4 °C, and dramatically, the MPKs in the root were affected by low temperature. miR172, miR319, miR395, miR398, and miR399 may be the miRNAs that regulate MPKs during low-temperature stress, with miR172 being the most critical. miRNA prediction and qRT-PCR results indicated that miR172 may negatively regulate MPKs. Therefore, we deduced that MPKs might coordinate with miR172 to participate in the process of the resistance to low-temperature stress in the roots of the banana. This study will provide a theoretical basis for further analysis of the mechanism of MPKs under low-temperature stress of bananas, and this study could be applied to molecular breeding of bananas in the future.

A MAPK signaling cascade regulates the fusaric acid-induced cell death in Arabidopsis

Mycotoxin contamination of foods and feeds is a global problem. Fusaric acid (FA) is a mycotoxin produced by Fusarium species that are phytopathogens of many economically important plant species. FA can cause programmed cell death (PCD) in several plant species. However, the signaling mechanisms of FA-induced cell death in plants are largely unknown. Here we showed that FA induced cell death in the model plant Arabidopsis thaliana, and MPK3/6 phosphorylation was triggered by FA in Arabidopsis. Both the acid nature and the radical of FA are required for its activity in inducing MPK3/6 activation and cell death. Expression of the constitutively active MKK5DD resulted in the activation of MPK3/6 and promoted the FA-induced cell death. Our work demonstrates that the MKK5-MPK3/6 cascade positively regulates FA-induced cell death in Arabidopsis and also provides insight into the mechanisms of how cell death is induced by FA in plants.

Unveiling the secrets of non-coding RNA-encoded peptides in plants: A comprehensive review of mining methods and research progress

Non-coding RNAs (ncRNAs) are not conventionally involved in protein encoding. However, recent findings indicate that ncRNAs possess the capacity to code for proteins or peptides. These ncRNA-encoded peptides (ncPEPs) are vital for diverse plant life processes and exhibit significant potential value. Despite their importance, research on plant ncPEPs is limited, with only a few studies conducted and less information on the underlying mechanisms, and the field remains in its nascent stage. This manuscript provides a comprehensive overview of ncPEPs mining methods in plants, focusing on prediction, identification, and functional analysis. We discuss the strengths and weaknesses of various techniques, identify future research directions in the ncPEPs domain, and elucidate the biological functions and agricultural application prospects of plant ncPEPs. By highlighting the immense potential and research value of ncPEPs, we aim to lay a solid foundation for more in-depth studies in plant science.

Dual regulations of cell cycle regulator DPa by auxin in Arabidopsis root distal stem cell maintenance

The plant hormone auxin plays a key role to maintain root stem cell identity which is essential for root development. However, the molecular mechanism by which auxin regulates root distal stem cell (DSC) identity is not well understood. In this study, we revealed that the cell cycle factor DPa is a vital regulator in the maintenance of root DSC identity through multiple auxin signaling cascades. On the one hand, auxin positively regulates the transcription of DPa via AUXIN RESPONSE FACTOR 7 and ARF19. On the other hand, auxin enhances the protein stability of DPa through MITOGEN-ACTIVATED PROTEIN KINASE 3 (MPK3)/MPK6-mediated phosphorylation. Consistently, mutation of the identified three threonine residues (Thr10, Thr25, and Thr227) of DPa to nonphosphorylated form alanine (DPa3A) highly decreased the phosphorylation level of DPa, which decreased its protein stability and affected the maintenance of root DSC identity. Taken together, this study provides insight into the molecular mechanism of how auxin regulates root distal stem cell identity through the dual regulations of DPa at both transcriptional and posttranslational levels.

Dynamic GOLVEN-ROOT GROWTH FACTOR 1 INSENSITIVE signaling in the root cap mediates root gravitropism

Throughout the exploration of the soil, roots interact with their environment and adapt to different conditions. Directional root growth is guided by asymmetric molecular patterns but how these become established or are dynamically regulated is poorly understood. Asymmetric gradients of the phytohormone auxin are established during root gravitropism, mainly through directional transport mediated by polarized auxin transporters. Upon gravistimulation, PIN-FORMED2 (PIN2) is differentially distributed and accumulates at the lower root side to facilitate asymmetric auxin transport up to the elongation zone where it inhibits cell elongation. GOLVEN (GLV) peptides function in gravitropism by affecting PIN2 abundance in epidermal cells. In addition, GLV signaling through ROOT GROWTH FACTOR 1 INSENSITIVE (RGI) receptors regulates root apical meristem maintenance. Here, we show that GLV-RGI signaling in these 2 processes in Arabidopsis (Arabidopsis thaliana) can be mapped to different cells in the root tip and that, in the case of gravitropism, it operates mainly in the lateral root cap (LRC) to maintain PIN2 levels at the plasma membrane (PM). Furthermore, we found that GLV signaling upregulates the phosphorylation level of PIN2 in an RGI-dependent manner. In addition, we demonstrated that the RGI5 receptor is asymmetrically distributed in the LRC and accumulates in the lower side of the LRC after gravistimulation. Asymmetric GLV-RGI signaling in the root cap likely accounts for differential PIN2 abundance at the PM to temporarily support auxin transport up to the elongation zone, thereby representing an additional level of control on the asymmetrical auxin flux to mediate differential growth of the root.

Comprehensive genome-wide identification and functional characterization of MAPK cascade gene families in Nelumbo

Mitogen-activated protein kinase (MAPK) cascade signaling pathway plays pivotal roles in various plant biological processes. However, systematic study of MAPK cascade gene families is yet to be conducted in lotus. Herein, 198 putative MAPK genes, including 152 MAP3Ks, 15 MKKs, and 31 MPKs genes were identified in Nelumbo. Segmental duplication was identified as the predominant factor driving MAPK cascade gene family expansion in lotus. MAPK cascade genes in N. nucifera and N. lutea shared high degree of sequence homologies, with 84, 9, and 19 homologous MAP3K, MKK, and MPK gene pairs being detected between the two species, respectively, with most genes predominantly undergoing purifying selection. Gene expression profiling indicated that NnMAPK cascade genes were extensively involved in plant development and submergence stress response. Co-expression analysis revealed potential interaction between transcription factors (TFs) and NnMAPK cascade genes in various biological processes. NnMKK showed predicted interactions with multiple NnMAP3K or NnMPK proteins, which suggested that functional diversity of MAPK cascade genes could be as a result of their complex protein interaction mechanisms. This first systematic analysis of MAPK cascade families in lotus provides deeper insights into their evolutionary dynamics and functional properties, which potentially could be crucial for lotus genetic improvement.

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.

Arabidopsis EXTRA-LARGE G PROTEIN 1 (XLG1) functions together with XLG2 and XLG3 in PAMP-triggered MAPK activation and immunity

Pattern-triggered immunity (PTI) is an essential strategy used by plants to deploy broad-spectrum resistance against pathogen attacks. Heterotrimeric G proteins have been reported to contribute to PTI. Of the three non-canonical EXTRA-LARGE G PROTEINs (XLGs) in Arabidopsis thaliana, XLG2 and XLG3 were shown to positively regulate immunity, but XLG1 was not considered to function in defense, based on the analysis of a weak xlg1 allele. In this study, we characterized the xlg1 xlg2 xlg3 triple knockout mutants generated from an xlg1 knockout allele. The strong xlg1 xlg2 xlg3 triple mutants compromised pathogen-associated molecular pattern (PAMP)-triggered activation of mitogen-activated protein kinases (MAPKs) and resistance to pathogen infection. The three XLGs interacted with MAPK cascade proteins involved in defense signaling, including the MAPK kinase kinases MAPKKK3 and MAPKKK5, the MAPK kinases MKK4 and MKK5, and the MAPKs MPK3 and MPK6. Expressing a constitutively active form of MKK4 restored MAPK activation and partially recovered the compromised disease resistance seen in the strong xlg1 xlg2 xlg3 triple mutant. Furthermore, mutations of all three XLGs largely restored the phenotype of the autoimmunity mutant bak1-interacting receptor-like kinase 1. Our study reveals that all three XLGs function redundantly in PAMP-triggered MAPK activation and plant immunity.

Auxin promotes hypocotyl elongation by enhancing BZR1 nuclear accumulation in Arabidopsis

Auxin and brassinosteroids (BRs) are two major growth-promoting phytohormones that shape hypocotyl elongation; however, the cross-talk between auxin and BR in this process is not fully understood. In this study, we found that auxin-induced hypocotyl elongation is dependent on brassinazole-resistant 1 (BZR1), a core BR signaling component. Auxin promotes BZR1 nuclear accumulation in hypocotyl cells, a process dependent on mitogen-activated protein kinase 3 (MPK3) and MPK6, which are both activated by auxin and whose encoding genes are highly expressed in hypocotyls. We determined that MPK3/MPK6 phosphorylate and reduce the protein stability of general regulatory factor 4 (GRF4), a member of the 14-3-3 family of proteins that retain BZR1 in the cytoplasm. In summary, this study reveals the molecular mechanism by which auxin promotes hypocotyl elongation by enhancing BZR1 nuclear accumulation via MPK3/MPK6-regulated GRF4 protein stability.

Comparative transcriptomics analysis of developing peanut (Arachis hypogaea L.) pods reveals candidate genes affecting peanut seed size

Pod size is one of the most important agronomic features of peanuts, which directly affects peanut yield. Studies on the regulation mechanism underpinning pod size in cultivated peanuts remain hitherto limited compared to model plant systems. To better understand the molecular elements that underpin peanut pod development, we conducted a comprehensive analysis of chronological transcriptomics during pod development in four peanut accessions with similar genetic backgrounds, but varying pod sizes. Several plant transcription factors, phytohormones, and the mitogen-activated protein kinase (MAPK) signaling pathways were significantly enriched among differentially expressed genes (DEGs) at five consecutive developmental stages, revealing an eclectic range of candidate genes, including PNC, YUC, and IAA that regulate auxin synthesis and metabolism, CYCD and CYCU that regulate cell differentiation and proliferation, and GASA that regulates seed size and pod elongation via gibberellin pathway. It is plausible that MPK3 promotes integument cell division and regulates mitotic activity through phosphorylation, and the interactions between these genes form a network of molecular pathways that affect peanut pod size. Furthermore, two variant sites, GCP4 and RPPL1, were identified which are stable at the QTL interval for seed size attributes and function in plant cell tissue microtubule nucleation. These findings may facilitate the identification of candidate genes that regulate pod size and impart yield improvement in cultivated peanuts.

Genome-wide identification and characterization of the MAPKKK, MKK, and MPK families in Chinese elite maize inbred line Huangzaosi

Mitogen-activated protein kinase (MAPK or MPK) cascades consist of three protein kinase components, MAPK kinase kinases (MAPKKKs), MAPK kinases (MKKs and MPKs), which are indispensable for various plant physiological processes. The functions of MAPK families have been extensively studied in maize (Zea mays L.) and other plant species, but little is known about MAPK families in the elite Chinese maize line Huangzaosi (hzs). In this study, we observed that overall performance of Huangzaosi was substantially better than that of B73 under drought conditions at the seedling and V16 stages with a favorable root/canopy ratio. In silico analyses identified 72, 10, and 24 MAPKKKs, MKKs, and MPKs, respectively, in Huangzaosi. Examinations of phylogenetic relationships among Arabidopsis thaliana (L.) Heynh., rice (Oryza sativa L.), and maize (lines B73 and hzs), gene structures, conserved protein motifs, and chromosomal locations revealed their evolutionary relationships. The basal gene expression levels and tissue specificities of all three MAPK families in hzs reflected the diversity in the MAPK functions related to growth and development. The quantitative real-time polymerase chain reaction (qPCR) assay indicated that certain MAPK genes with high basal expression levels in the primary and crown roots responded differentially to drought between B73 and hzs, suggesting that these genes may contribute to their distinct drought tolerance at different developmental stages. The important information regarding the evolution and expression of hzs MAPK family members generated in this study provides a new avenue for the better understanding on the regulatory mechanism of MAPK cascade in the core inbred line hzs, which may be useful to guide the development of new maize cultivars with desirable traits (e.g., drought resistance).

Arabidopsis MAPKK kinases YODA, MAPKKK3, and MAPKKK5 are functionally redundant in development and immunity

Three MAPK cascade components in Arabidopsis, YDA (MAPKKK4) and MAPKKK3/5, function redundantly in multiple developmental processes and immunity and regulate floral organ abscission.

Overlapping functions of YDA and MAPKKK3/MAPKKK5 upstream of MPK3/MPK6 in plant immunity and growth/development

Arabidopsis MITOGEN-ACTIVATED PROTEIN KINASE3 (MAPK3 or MPK3) and MPK6 play important signaling roles in plant immunity and growth/development. MAPK KINASE4 (MKK4) and MKK5 function redundantly upstream of MPK3 and MPK6 in these processes. YODA (YDA), also known as MAPK KINASE KINASE4 (MAPKKK4), is upstream of MKK4/MKK5 and forms a complete MAPK cascade (YDA-MKK4/MKK5-MPK3/MPK6) in regulating plant growth and development. In plant immunity, MAPKKK3 and MAPKKK5 function redundantly upstream of the same MKK4/MKK5-MPK3/MPK6 module. However, the residual activation of MPK3/MPK6 in the mapkkk3 mapkkk5 double mutant in response to flg22 pathogen-associated molecular pattern (PAMP) treatment suggests the presence of additional MAPKKK(s) in this MAPK cascade in signaling plant immunity. To investigate whether YDA is also involved in plant immunity, we attempted to generate mapkkk3 mapkkk5 yda triple mutants. However, it was not possible to recover one of the double mutant combinations (mapkkk5 yda) or the triple mutant (mapkkk3 mapkkk5 yda) due to a failure of embryogenesis. Using the clustered regularly interspaced short palindromic repeats (CRISPR) - CRISPR-associated protein 9 (Cas9) approach, we generated weak, N-terminal deletion alleles of YDA, yda-del, in a mapkkk3 mapkkk5 background. PAMP-triggered MPK3/MPK6 activation was further reduced in the mapkkk3 mapkkk5 yda-del mutant, and the triple mutant was more susceptible to pathogen infection, suggesting YDA also plays an important role in plant immune signaling. In addition, MAPKKK5 and, to a lesser extent, MAPKKK3 were found to contribute to gamete function and embryogenesis, together with YDA. While the double homozygous mapkkk3 yda mutant showed the same growth and development defects as the yda single mutant, mapkkk5 yda double mutant and mapkkk3 mapkkk5 yda triple mutants were embryo lethal, similar to the mpk3 mpk6 double mutants. These results demonstrate that YDA, MAPKKK3, and MAPKKK5 have overlapping functions upstream of the MKK4/MKK5-MPK3/MPK6 module in both plant immunity and growth/development.

Regulation of pattern-triggered immunity and growth by phytocytokines

Endogenous signalling peptides play diverse roles during plant growth, development and stress responses. Research in recent years has unravelled peptides with previously known growth-regulatory function as immune-modulatory agents that fine-tune pattern-triggered immunity (PTI). Moreover, peptides that are long known as endogenous danger signals were recently implicated in growth and development. In analogy to metazoan systems these peptides are referred to as phytocytokines. In this review we will highlight recent progress made on our understanding of phytocytokines simultaneously regulating growth and PTI which shows the complex interplay of peptide signalling pathways regulating multiple aspects of a plant's life.

RGI-GOLVEN signaling promotes cell surface immune receptor abundance to regulate plant immunity

Plant immune responses must be tightly controlled for proper allocation of resources for growth and development. In plants, endogenous signaling peptides regulate developmental and growth‐related processes. Recent research indicates that some of these peptides also have regulatory functions in the control of plant immune responses. This classifies these peptides as phytocytokines as they show analogies with metazoan cytokines. However, the mechanistic basis for phytocytokine‐mediated regulation of plant immunity remains largely elusive. Here, we identify GOLVEN2 (GLV2) peptides as phytocytokines in Arabidopsis thaliana. GLV2 signaling enhances sensitivity of plants to elicitation with immunogenic bacterial elicitors and contributes to resistance against virulent bacterial pathogens. GLV2 is perceived by ROOT MERISTEM GROWTH FACTOR 1 INSENSITIVE (RGI) receptors. RGI mutants show reduced elicitor sensitivity and enhanced susceptibility to bacterial infection. RGI3 forms ligand‐induced complexes with the pattern recognition receptor (PRR) FLAGELLIN SENSITIVE 2 (FLS2), suggesting that RGIs are part of PRR signaling platforms. GLV2‐RGI signaling promotes PRR abundance independent of transcriptional regulation and controls plant immunity via a previously undescribed mechanism of phytocytokine activity.

RGI-GOLVEN signaling promotes cell surface immune receptor abundance to regulate plant immunity

Plant immune responses must be tightly controlled for proper allocation of resources for growth and development. In plants, endogenous signaling peptides regulate developmental and growth-related processes. Recent research indicates that some of these peptides also have regulatory functions in the control of plant immune responses. This classifies these peptides as phytocytokines as they show analogies with metazoan cytokines. However, the mechanistic basis for phytocytokine-mediated regulation of plant immunity remains largely elusive. Here, we identify GOLVEN2 (GLV2) peptides as phytocytokines in Arabidopsis thaliana. GLV2 signaling enhances sensitivity of plants to elicitation with immunogenic bacterial elicitors and contributes to resistance against virulent bacterial pathogens. GLV2 is perceived by ROOT MERISTEM GROWTH FACTOR 1 INSENSITIVE (RGI) receptors. RGI mutants show reduced elicitor sensitivity and enhanced susceptibility to bacterial infection. RGI3 forms ligand-induced complexes with the pattern recognition receptor (PRR) FLAGELLIN SENSITIVE 2 (FLS2), suggesting that RGIs are part of PRR signaling platforms. GLV2-RGI signaling promotes PRR abundance independent of transcriptional regulation and controls plant immunity via a previously undescribed mechanism of phytocytokine activity.

Essential roles of SERKs in the ROOT MERISTEM GROWTH FACTOR-mediated signaling pathway

ROOT MERISTEM GROWTH FACTORs (RGFs), a group of peptide hormones, play key roles in root apical meristem development. In Arabidopsis (Arabidopsis thaliana), there are 11 members of RGFs, in which at least RGF1, RGF2, and RGF3 are expressed at the root tip and are involved in root stem cell niche maintenance. RGFs are perceived by five functionally redundant receptor-like protein kinases, RGF1 INSENSITIVE 1 (RGI1) to RGI5, to maintain the expression of two downstream APETALA 2 (AP2) transcription factor genes, PLETHORA 1 (PLT1) and PLT2, and to stabilize PLT2. RGI1 to RGI3 were also named RGF RECEPTOR 1 (RGFR1) to RGFR3, respectively. Although previous studies have suggested that BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) and its paralogs, SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASEs (SERKs), may act as coreceptors of RGIs, comprehensive genetic and biochemical analyses have not been well documented. Here, we report that single, double, and triple mutants of SERKs show various degrees of short root phenotypes and insensitivity to exogenously applied RGF1. The interaction between RGIs and BAK1 and their mutual phosphorylation are RGF1 dependent. We also found that RGF1-induced MAPK activation relies on both RGIs and SERKs. We demonstrate that RGIs play redundant roles in regulating root apical meristem development. Therefore, we genetically and biochemically substantiated that SERKs, as coreceptors, play essential roles in the RGF1-mediated signaling pathway.

Mitogen-Activated Protein Kinase and Substrate Identification in Plant Growth and Development

Mitogen-activated protein kinases (MAPKs) form tightly controlled signaling cascades that play essential roles in plant growth, development, and defense response. However, the molecular mechanisms underlying MAPK cascades are still very elusive, largely because of our poor understanding of how they relay the signals. The MAPK cascade is composed of MAPK, MAPKK, and MAPKKK. They transfer signals through the phosphorylation of MAPKKK, MAPKK, and MAPK in turn. MAPKs are organized into a complex network for efficient transmission of specific stimuli. This review summarizes the research progress in recent years on the classification and functions of MAPK cascades under various conditions in plants, especially the research status and general methods available for identifying MAPK substrates, and provides suggestions for future research directions.

MAP kinase cascades in plant development and immune signaling

Mitogen-activated protein kinase (MAPK) cascades are important signaling modules regulating diverse biological processes. During the past 20 years, much progress has been made on the functions of MAPK cascades in plants. This review summarizes the roles of MAPKs, known MAPK substrates, and our current understanding of MAPK cascades in plant development and innate immunity. In addition, recent findings on the molecular links connecting surface receptors to MAPK cascades and the mechanisms underlying MAPK signaling specificity are also discussed.

Two phylogenetically unrelated peptide-receptor modules jointly regulate lateral root initiation via a partially shared signaling pathway in Arabidopsis thaliana

Peptide-receptor signaling is an important system for intercellular communication, regulating many developmental processes. A single process can be controlled by several distinct signaling peptides. However, since peptide-receptor modules are usually studied separately, their mechanistic interactions remain largely unexplored. Two phylogenetically unrelated peptide-receptor modules, GLV6/GLV10-RGI and TOLS2/PIP2-RLK7, independently described as inhibitors of lateral root initiation, show striking similarities between their expression patterns and gain- and loss-of-function phenotypes, suggesting a common function during lateral root spacing and initiation. The GLV6/GLV10-RGI and TOLS2/PIP2-RLK7 modules trigger similar transcriptional changes, likely in part via WRKY transcription factors. Their overlapping set of response genes includes PUCHI and PLT5, both required for the effect of GLV6/10, as well as TOLS2, on lateral root initiation. Furthermore, both modules require the activity of MPK6 and can independently trigger MPK3/MPK6 phosphorylation. The GLV6/10 and TOLS2/PIP2 signaling pathways seem to converge in the activation of MPK3/MPK6, leading to the induction of a similar transcriptional response in the same target cells, thereby regulating lateral root initiation through a (partially) common mechanism. Convergence of signaling pathways downstream of phylogenetically unrelated peptide-receptor modules adds an additional, and hitherto unrecognized, level of complexity to intercellular communication networks in plants.

De novo and comparative transcriptomic analysis explain morphological differences in Panax notoginseng taproots

Background

Panax notoginseng (Burk.) F. H. Chen (PN) belonging to the genus Panax of family Araliaceae is widely used in traditional Chinese medicine to treat various diseases. PN taproot, as the most vital organ for the accumulation of bioactive components, presents a variable morphology (oval or long), even within the same environment. However, no related studies have yet explained the molecular mechanism of phenotypic differences. To investigate the cause of differences in the taproot phenotype, de novo and comparative transcriptomic analysis on PN taproot was performed.

Results

A total of 133,730,886 and 114,761,595 paired-end clean reads were obtained based on high-throughput sequencing from oval and long taproot samples, respectively. 121,955 unigenes with contig N50 = 1,774 bp were generated by using the de novo assembly transcriptome, 63,133 annotations were obtained with the BLAST. And then, 42 genes belong to class III peroxidase (PRX) gene family, 8 genes belong to L-Ascorbate peroxidase (APX) gene family, and 55 genes belong to a series of mitogen-activated protein kinase (MAPK) gene family were identified based on integrated annotation results. Differentially expressed genes analysis indicated substantial up-regulation of PnAPX3 and PnPRX45, which are related to reactive oxygen species metabolism, and the PnMPK3 gene, which is related to cell proliferation and plant root development, in long taproots compared with that in oval taproots. Furthermore, the determination results of real-time quantitative PCR, enzyme activity, and H₂O₂ content verified transcriptomic analysis results.

Conclusion

These results collectively demonstrate that reactive oxygen species (ROS) metabolism and the PnMPK3 gene may play vital roles in regulating the taproot phenotype of PN. This study provides further insights into the genetic mechanisms of phenotypic differences in other species of the genus Panax.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08283-w.

CLE peptides: critical regulators for stem cell maintenance in plants

Plant CLE peptides, which regulate stem cell maintenance in shoot and root meristems and in vascular bundles through LRR family receptor kinases, are novel, complex, and to some extent conserved. Over the past two decades, peptide ligands of the CLAVATA3 (CLV3) /Embryo Surrounding Region (CLE) family have been recognized as critical short- and long-distance communication signals in plants, especially for stem cell homeostasis, cell fate determination and physiological responses. Stem cells located at the shoot apical meristem (SAM), the root apical meristem (RAM) and the procambium divide and differentiate into specialized cells that form a variety of tissues such as epidermis, ground tissues, xylem and phloem. In the SAM of Arabidopsis (Arabidopsis thaliana), the CLV3 peptide restricts the number of stem cells via leucine-rich repeat (LRR)-type receptor kinases. In the RAM, root-active CLE peptides are critical negative regulators, while ROOT GROWTH FACTOR (RGF) peptides are positive regulators in stem cell maintenance. Among those root-active CLE peptides, CLE25 promotes, while CLE45 inhibits phloem differentiation. In vascular bundles, TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF)/CLE41/CLE44 promotes procambium cell division, and prevents xylem differentiation. Orthologs of CLV3 have been identified in liverwort (Marchantia polymorpha), tomato (Solanum lycopersicum), rice (Oryza sativa), maize (Zea mays) and lotus (Lotus japonicas), suggesting that CLV3 is an evolutionarily conserved signal in stem cell maintenance. However, functional characterization of endogenous CLE peptides and corresponding receptor kinases, and the downstream signal transduction has been challenging due to their genome-wide redundancies and rapid evolution.

MKK4/5-MPK3/6 Cascade Regulates Agrobacterium-Mediated Transformation by Modulating Plant Immunity in Arabidopsis

Agrobacterium tumefaciens is a specialized plant pathogen that causes crown gall disease and is commonly used for Agrobacterium-mediated transformation. As a pathogen, Agrobacterium triggers plant immunity, which affects transformation. However, the signaling components and pathways in plant immunity to Agrobacterium remain elusive. We demonstrate that two Arabidopsis mitogen-activated protein kinase kinases (MAPKKs) MKK4/MKK5 and their downstream mitogen-activated protein kinases (MAPKs) MPK3/MPK6 play major roles in both Agrobacterium-triggered immunity and Agrobacterium-mediated transformation. Agrobacteria induce MPK3/MPK6 activity and the expression of plant defense response genes at a very early stage. This process is dependent on the MKK4/MKK5 function. The loss of the function of MKK4 and MKK5 or their downstream MPK3 and MPK6 abolishes plant immunity to agrobacteria and increases transformation frequency, whereas the activation of MKK4 and MKK5 enhances plant immunity and represses transformation. Global transcriptome analysis indicates that agrobacteria induce various plant defense pathways, including reactive oxygen species (ROS) production, ethylene (ET), and salicylic acid- (SA-) mediated defense responses, and that MKK4/MKK5 is essential for the induction of these pathways. The activation of MKK4 and MKK5 promotes ROS production and cell death during agrobacteria infection. Based on these results, we propose that the MKK4/5-MPK3/6 cascade is an essential signaling pathway regulating Agrobacterium-mediated transformation through the modulation of Agrobacterium-triggered plant immunity.

Identification and Characterization of Short Crown Root 8, a Temperature-Sensitive Mutant Associated with Crown Root Development in Rice

Crown roots are essential for plants to obtain water and nutrients, perceive environmental changes, and synthesize plant hormones. In this study, we identified and characterized short crown root 8 (scr8), which exhibited a defective phenotype of crown root and vegetative development. Temperature treatment showed that scr8 was sensitive to temperature and that the mutant phenotypes were rescued when grown under low temperature condition (20 °C). Histological and EdU staining analysis showed that the crown root formation was hampered and that the root meristem activity was decreased in scr8. With map-based cloning strategy, the SCR8 gene was fine-mapped to an interval of 126.4 kb on chromosome 8. Sequencing analysis revealed that the sequence variations were only found in LOC_Os08g14850, which encodes a CC-NBS-LRR protein. Expression and inoculation test analysis showed that the expression level of LOC_Os08g14850 was significantly decreased under low temperature (20 °C) and that the resistance to Xanthomonas oryzae pv. Oryzae (Xoo) was enhanced in scr8. These results indicated that LOC_Os08g14850 may be the candidate of SCR8 and that its mutation activated the plant defense response, resulting in a crown root growth defect.

Peptide Signaling during Plant Reproduction

Plant signaling peptides are involved in cell-cell communication networks and coordinate a wide range of plant growth and developmental processes. Signaling peptides generally bind to receptor-like kinases, inducing their dimerization with co-receptors for signaling activation to trigger cellular signaling and biological responses. Fertilization is an important life event in flowering plants, involving precise control of cell-cell communications between male and female tissues. Peptide-receptor-like kinase-mediated signaling plays an important role in male-female interactions for successful fertilization in flowering plants. Here, we describe the recent findings on the functions and signaling pathways of peptides and receptors involved in plant reproduction processes including pollen germination, pollen tube growth, pollen tube guidance to the embryo sac, and sperm cell reception in female tissues.

Root meristem growth factor RGF, a sulfated peptide hormone in plants

In recent decades, small secreted peptides have been recognized as a new class of intercellular signaling phytohormones in plants. Tyrosine sulfation plays crucial roles in peptide hormone bioactivities in plants. The Arabidopsis tyrosylprotein sulfotransferase mutant tpst-1 causes severe abnormalities in the root tip due to deficiency in the biosynthesis of all functional tyrosine-sulfated peptides. Root meristem growth factor RGF, a sulfated peptide hormone specifically expressed in the root tip, was found to complement tpst-1 root defects. This review summarizes the history of the identification of RGF, the characteristics of RGF, the identification of RGF receptors, and the target of RGF. In brief, RGF is a 13 amino acid sulfated peptide. The RGF peptide mutant rgf1,2,3 exhibited a reduced size of the root apical meristem, indicating that RGF maintains cell proliferation activity in the root apical meristem. RGF receptors were identified by comprehensive binding analysis with a custom-made receptor expression library. The RGF receptor mutant rgfr1,2,3 showed a phenotype of reduced root length due to a reduction in the root apical meristem and was insensitive to RGF. The signaling cascade through RGF-RGF receptor pairs regulates the gradient formation of PLETHORA (PLT), which is known as the master regulator of root formation. In the peptide mutant rgf1,2,3 and receptor mutant rgfr1,2,3, the gradient of PLT proteins disappeared, indicating that RGF defines the PLT protein gradient to ensure robust root growth and root development. Recent studies of the downstream signaling of RGF-RGF receptor pairs are also described in this review.

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.

Transcriptome and Metabolomic Analyses Reveal Regulatory Networks Controlling Maize Stomatal Development in Response to Blue Light

(1) Background: Blue light is important for the formation of maize stomata, but the signal network remains unclear. (2) Methods: We replaced red light with blue light in an experiment and provided a complementary regulatory network for the stomatal development of maize by using transcriptome and metabolomics analysis. (3) Results: Exposure to blue light led to 1296 differentially expressed genes and 419 differential metabolites. Transcriptome comparisons and correlation signaling network analysis detected 55 genes, and identified 6 genes that work in the regulation of the HY5 module and MAPK cascade, that interact with PTI1, COI1, MPK2, and MPK3, in response to the substitution of blue light in environmental adaptation and signaling transduction pathways. Metabolomics analysis showed that two genes involved in carotenoid biosynthesis and starch and sucrose metabolism participate in stomatal development. Their signaling sites located on the PHI1 and MPK2 sites of the MAPK cascade respond to blue light signaling. (4) Conclusions: Blue light remarkably changed the transcriptional signal transduction and metabolism of metabolites, and eight obtained genes worked in the HY5 module and MAPK cascade.

Transcriptome and Metabolomic Analyses Reveal Regulatory Networks Controlling Maize Stomatal Development in Response to Blue Light

(1) Background: Blue light is important for the formation of maize stomata, but the signal network remains unclear. (2) Methods: We replaced red light with blue light in an experiment and provided a complementary regulatory network for the stomatal development of maize by using transcriptome and metabolomics analysis. (3) Results: Exposure to blue light led to 1296 differentially expressed genes and 419 differential metabolites. Transcriptome comparisons and correlation signaling network analysis detected 55 genes, and identified 6 genes that work in the regulation of the HY5 module and MAPK cascade, that interact with PTI1, COI1, MPK2, and MPK3, in response to the substitution of blue light in environmental adaptation and signaling transduction pathways. Metabolomics analysis showed that two genes involved in carotenoid biosynthesis and starch and sucrose metabolism participate in stomatal development. Their signaling sites located on the PHI1 and MPK2 sites of the MAPK cascade respond to blue light signaling. (4) Conclusions: Blue light remarkably changed the transcriptional signal transduction and metabolism of metabolites, and eight obtained genes worked in the HY5 module and MAPK cascade.

Recent advances in peptide signaling during Arabidopsis root development

Roots provide the plant with water and nutrients and anchor it in a substrate. Root development is controlled by plant hormones and various sets of transcription factors. Recently, various small peptides and their cognate receptors have been identified as controlling root development. Small peptides bind to membrane-localized receptor-like kinases, inducing their dimerization with co-receptor proteins for signaling activation and giving rise to cellular signaling outputs. Small peptides function as local and long-distance signaling molecules involved in cell-to-cell communication networks, coordinating root development. In this review, we survey recent advances in the peptide ligand-mediated signaling pathways involved in the control of root development in Arabidopsis. We describe the interconnection between peptide signaling and conventional phytohormone signaling. Additionally, we discuss the diversity of identified peptide-receptor interactions during plant root development.

Receptor-like Kinases in Root Development: Current Progress and Future Directions

Cell-to-cell and cell-to-environment communications are critical to the growth and development of plants. Cell surface-localized receptor-like kinases (RLKs) are mainly involved in sensing various extracellular signals to initiate their corresponding cellular responses. As important vegetative organs for higher plants to adapt to a terrestrial living situation, roots play a critical role for the survival of plants. It has been demonstrated that RLKs control many biological processes during root growth and development. In this review, we summarize several key regulatory processes during Arabidopsis root development in which RLKs play critical roles. We also put forward a number of relevant questions that are required to be explored in future studies.

The RGF/GLV/CLEL Family of Short Peptides Evolved Through Lineage-Specific Losses and Diversification and Yet Conserves Its Signaling Role Between Vascular Plants and Bryophytes

Short secreted plant peptides act as key signaling molecules and control a plethora of developmental and physiological processes. The ROOT GROWTH FACTOR (RGF)/GOLVEN (GLV)/CLE-Like (CLEL) family of peptides was discovered to be involved in root development in Arabidopsis thaliana. In contrast to active research efforts, which have been revealing receptors and downstream signaling components, little attention has been paid to evolutionary processes that shaped the RGF signaling system as we know it in angiosperms today. As a first step toward understanding how RGF signaling emerged and evolved, this study aimed to elucidate the phylogenetic distribution and functional conservation of RGF-like sequences. Using publicly available, genome and transcriptome data, RGF-like sequences were searched in 27 liverworts, 22 mosses, 8 hornworts, 23 lycophytes, 23 ferns, 38 gymnosperms, and 8 angiosperms. This led to the identification of more than four hundreds of RGF-like sequences in all major extant land plant lineages except for hornworts. Sequence comparisons within and between taxonomic groups identified lineage-specific characters. Notably, one of the two major RGF subgroups, represented by A. thaliana RGF6/GLV1/CLEL6, was found only in vascular plants. This subgroup, therefore, likely emerged in a common ancestor of vascular plants after its divergence from bryophytes. In bryophytes, our results infer independent losses of RGF-like sequences in mosses and hornworts. On the other hand, a single, highly similar RGF-like sequence is conserved in liverworts, including Marchantia polymorpha, a genetically tractable model species. When constitutively expressed, the M. polymorpha RGF-like sequence (MpRGF) affected plant development and growth both in A. thaliana and M. polymorpha. This suggests that MpRGF can exert known RGF-like effects and that MpRGF is under transcriptional control so that its potent activities are precisely controlled. These data suggest that RGFs are conserved as signaling molecules in both vascular plants and bryophytes and that lineage-specific diversification has increased sequence variations of RGFs. All together, our findings form a basis for further studies into RGF peptides and their receptors, which will contribute to our understandings of how peptide signaling pathways evolve.

The Roles of Peptide Hormones and Their Receptors during Plant Root Development

Peptide hormones play pivotal roles in many physiological processes through coordinating developmental and environmental cues among different cells. Peptide hormones are recognized by their receptors that convey signals to downstream targets and interact with multiple pathways to fine-tune plant growth. Extensive research has illustrated the mechanisms of peptides in shoots but functional studies of peptides in roots are scarce. Reactive oxygen species (ROS) are known to be involved in stress-related events. However, recent studies have shown that they are also associated with many processes that regulate plant development. Here, we focus on recent advances in understanding the relationships between peptide hormones and their receptors during root growth including outlines of how ROS are integrated with these networks.

Receptor-Like Protein Kinases Function Upstream of MAPKs in Regulating Plant Development

Mitogen-activated protein kinases (MAPKs) are a group of protein kinase broadly involved in various signal pathways in eukaryotes. In plants, MAPK cascades regulate growth, development, stress responses and immunity by perceiving signals from the upstream regulators and transmitting the phosphorylation signals to the downstream signaling components. To reveal the interactions between MAPK cascades and their upstream regulators is important for understanding the functional mechanisms of MAPKs in the life span of higher plants. Typical receptor-like protein kinases (RLKs) are plasma membrane-located to perceive endogenous or exogenous signal molecules in regulating plant growth, development and immunity. MAPK cascades bridge the extracellular signals and intracellular transcription factors in many RLK-mediated signaling pathways. This review focuses on the current findings that RLKs regulate plant development through MAPK cascades and discusses questions that are worth investigating in the near future.