Arabidopsis pollen tube integrity and sperm release are regulated by RALF-mediated signaling

Comparative analysis of rapid alkalinization factor peptide-triggered plant immunity in citrus and closely related species

Rapid alkalinization factors (RALFs) are plant-derived, cysteine-rich small peptides that play crucial roles in plant development and immunity. However, their function in citrus immunity remains unexplored. This study systematically identified RALF family members in sweet orange, a cultivated citrus species, and five closely related species known for their strong resistance to citrus diseases. Most species contained 13 RALF genes, and their corresponding RALF orthologs exhibited minimal variation in gene structure, sequence identity, conserved domains, and motifs. However, differences in cis-acting elements within promoter regions led to differences in their expression patterns under biotic stress. Compared with sweet orange, citrus-related species exhibited a stronger and more widespread induction of RALF genes. Additionally, RALF peptides, particularly RALF11, triggered more robust immune responses in these species, including ROS bursts, mitogen-activated protein kinase activation, and the upregulation of defense-related genes. Furthermore, the basal expression levels of CrRLK1Ls, the RALF receptor homologs, were higher in citrus-related species than in sweet orange. Amplifying the RALF-CrRLK1L pathway through CrRLK1L overexpression and RALF peptide application significantly enhanced sweet orange resistance to citrus canker. These findings suggest that citrus-related species have endured selective pressure, leading to greater promoter sequence diversity, which is conducive to fine-tune gene expression in response to environmental challenges.

The tri-molecular interaction controlling plant cell structure

The plant cell wall is a dynamic envelope crucial for cell structure. Recent insights highlight the pivotal roles of RAPID ALKALINIZATION FACTOR (RALF) peptides, LEUCINE-RICH REPEAT EXTENSINS (LRX), and the FERONIA (FER) receptor kinase in maintaining cell wall integrity. This tri-molecular complex, along with other membrane receptors, modulates cell wall mechanics through interactions with de-methylesterified homogalacturonan (HG) in a feedback-controlled manner along with other intra-cellular responses. Through the characterization of this complex, critical questions have emerged regarding the mechanistic details of RALF-induced HG modulation, the mechanosensing role of FER, and the structural roles of extensins in modulating cell wall dynamics. This review underscores the intricate feedback mechanisms involved in the maintenance of cell wall integrity and cellular growth dynamics, offering strategies to enhance crop productivity.

Sexual reproduction in land plants: an evolutionary perspective

KEY MESSAGE

We link key aspects of land plant reproductive evolution and detail how successive molecular changes leading to novel tissues and organs require co-evolution of communication systems between tissues. The transition of water-dependent reproduction of algae to mechanisms with very limited water dependence in many land plant lineages allowed plants to colonize diverse terrestrial environments, leading to the vast variety of extant plant species. The emergence of modified cell types, novel tissues, and organs enabled this transition; their origin is associated with the co-evolution of novel or adapted molecular communication systems and gene regulatory networks. In the light of an increasing number of genome sequences in combination with the establishment of novel genetic model organisms from diverse green plant lineages, our knowledge and understanding about the origin and evolution of individual traits that arose in a concerted way increases steadily. For example, novel members of gene families in signaling pathways emerged for communication between gametes and gametophytes with additional tissues surrounding the gametes. Here, we provide a comprehensive overview on the origin and evolution of reproductive novelties such as pollen grains, immobile sperms, ovules and seeds, carpels, gamete/gametophytic communication systems, double fertilization, and the molecular mechanisms that have arisen anew or have been co-opted during evolution, including but not limited to the incorporation of phytohormones, reactive oxygen species and redox signaling as well as small RNAs in regulatory modules that contributed to the evolution of land plant sexual reproduction.

Jack of all trades: crosstalk between FERONIA signaling and hormone pathways

The receptor kinase FERONIA (FER) is a multifaceted regulator of plant growth, development, reproduction, and stress responses. FER is functionally connected to many plant hormones in diverse biological processes. This review summarizes the current understanding of the interplay between FER and phytohormones, with a focus on abscisic acid, ethylene, jasmonic acid, auxin, and brassinosteroid. The mutual regulation between FER and plant hormones happens at multiple levels including ligands, receptors, and downstream signaling components. Plant hormones can regulate the expression of genes encoding FER and its ligands RAPID ALKALINIZATION FACTORs (RALFs) as well as the abundance and kinase activity of FER proteins. On the other hand, FER can regulate hormone biosynthesis, transport, perception, and downstream signaling components such as transcription factors. Evidence of the crosstalk between FER and phytohormones is also emerging in crop species. Despite the rapid progress made in this field, more mechanistic studies are still needed to gain a comprehensive understanding of the FER-phytohormone crosstalk. Future research prospects and potential approaches are also discussed in this review.

Ingenious Male-Female Communication Ensures Successful Double Fertilization in Angiosperms

The colonization of land by plants marked a pivotal transformation in terrestrial ecosystems. In order to adapt to the terrestrial environment, angiosperms, which dominate the terrestrial flora with around 300,000 species, have evolved sophisticated mechanisms for sexual reproduction involving intricate interactions between male and female structures, starting from pollen deposition on the stigma and culminating in double fertilization within the ovule. The pollen tube plays a crucial role by navigating through female tissues to deliver sperm cells. The molecular intricacies of these male-female interactions, involving numerous signaling pathways and regulatory proteins, have been extensively studied over the past two decades. This review summarizes recent findings on the regulatory mechanisms of these male-female interactions in angiosperms. We aim to provide a comprehensive understanding of plant reproductive biology and highlight the implications of these mechanisms for crop improvement and the development of new agricultural technologies.

Root-derived small peptides: Key regulators of plant development, stress resilience, and nutrient acquisition

Small peptides (SPs), emerging as crucial signaling molecules in plants, regulate diverse processes such as plant development, stress tolerance, and nutrient acquisition. Consisting of fewer than 100 amino acids, SPs are classified into two main groups: precursor-derived SPs and small open reading frame (sORF)-encoded SPs, including miRNA-encoded SPs. SPs are secreted from various plant parts, with root-derived SPs playing particularly significant roles in stress tolerance and nutrient uptake. Even at low concentrations, root-derived SPs are highly effective signaling molecules that influence the distribution and effects of phytohormones, particularly auxin. For instance, under low phosphorus conditions, CLAVATA3/Embryo-Surrounding Region-Related (CLE/CLV), a root-derived SP, enhances root apical meristem differentiation and root architecture to improve phosphate acquisition. By interacting with CLV2 and PEPR2 receptors, it modulates auxin-related pathways, directing root morphology changes to optimize nutrient uptake. During nitrogen (N) starvation, root-derived SPs are transported to the shoot, where they interact with leucine-rich repeat receptor kinases (LRR-RKs) to alleviate nitrogen deficiency. Similarly, C-terminally Encoded Peptides (CEPs) are involved in primary root growth and N-acquisition responses. Despite the identification of many SPs, countless others remain to be discovered, and the functions of those identified so far remain elusive. This review focuses on the functions of root-derived SPs, such as CLE, CEP, RALF, RGF, PSK, PSY, and DVL, and discusses the receptor-mediated signaling pathways involved. Additionally, it explores the roles of SPs in root architecture, plant development, and their metabolic functions in nutrient signaling.

Plant Peptide Ligands as Temporal and Spatial Regulators

Throughout the life cycle of a plant, numerous responses need to be carefully regulated to ensure proper development and appropriate responses to external stimuli, and plant hormones play a crucial role in this regulation. Since the early 1990s, there has been expansive research elucidating the central role that peptide ligands play as intrinsic short- and long-distance communicators during development and as regulators of phenotypic plasticity. In this review, we focus on recently discovered mechanisms that ensure correct spatial and temporal cellular responses triggered by peptide ligands and provide examples of how peptide processing proteins and apoplastic conditions can regulate peptide activity in a timely manner.

A comprehensive all-in-one CRISPR toolbox for large-scale screens in plants

Clustered regularly interspaced short palindromic repeats (CRISPR)-associated nuclease (Cas) technologies facilitate routine genome engineering of one or a few genes at a time. However, large-scale CRISPR screens with guide RNA libraries remain challenging in plants. Here, we have developed a comprehensive all-in-one CRISPR toolbox for Cas9-based genome editing, cytosine base editing, adenine base editing (ABE), Cas12a-based genome editing and ABE, and CRISPR-Act3.0-based gene activation in both monocot and dicot plants. We evaluated all-in-one T-DNA expression vectors in rice (Oryza sativa, monocot) and tomato (Solanum lycopersicum, dicot) protoplasts, demonstrating their broad and reliable applicability. To showcase the applications of these vectors in CRISPR screens, we constructed guide RNA (gRNA) pools for testing in rice protoplasts, establishing a high-throughput approach to select high-activity gRNAs. Additionally, we demonstrated the efficacy of sgRNA library screening for targeted mutagenesis of ACETOLACTATE SYNTHASE in rice, recovering novel candidate alleles for herbicide resistance. Furthermore, we carried out a CRISPR activation screen in Arabidopsis thaliana, rapidly identifying potent gRNAs for FLOWERING LOCUS T activation that confer an early-flowering phenotype. This toolbox contains 61 versatile all-in-one vectors encompassing nearly all commonly used CRISPR technologies. It will facilitate large-scale genetic screens for loss-of-function or gain-of-function studies, presenting numerous promising applications in plants.

Cell wall remodeling during plant regeneration

Plant regeneration is the process during which differentiated tissues or cells can reverse or alter their developmental trajectory to repair damaged tissues or form new organs. In the plant regeneration process, the cell wall not only functions as a foundational barrier and scaffold supporting plant cells but also influences cell fates and identities. Cell wall remodeling involves the selective degradation of certain cell wall components or the integration of new components. Recently, accumulating evidence has underscored the importance of cell wall remodeling in plant regeneration. Wounding signals, transmitted by transcription factors, trigger the expressions of genes responsible for cell wall loosening, which is essential for tissue repair. In de novo organ regeneration and somatic embryogenesis, phytohormones orchestrate a transcriptional regulatory network to induce cell wall remodeling, which promotes cell fate reprogramming and organ formation. This review summarizes the effects of cell wall remodeling on various regenerative processes and provides novel insights into the future research of uncharacterized roles of cell wall in plant regeneration.

Modulation of morphogenesis and metabolism by plant cell biomechanics: from model plants to traditional herbs

The quality of traditional herbs depends on organ morphogenesis and the accumulation of active pharmaceutical ingredients. While recent research highlights the significance of cell mechanobiology in model plant morphogenesis, our understanding of mechanical signal initiation and transduction in traditional herbs remains incomplete. Recent studies reveal a close correlation between cell wall (CW) biosynthesis and active ingredient production, yet the role of cell mechanics in balancing morphogenesis and secondary metabolism is often overlooked. This review explores how the cell wall, plasma membrane, cytoskeleton, and vacuole collaborate to regulate cell mechanics and respond to mechanical changes. We propose CW biosynthesis as a hub in connecting cell mechanics with secondary metabolism and emphasize that understanding the relationship between mechanical remodeling and secondary metabolism could provide new insights into plant cell mechanobiology and the breeding of high-quality herbs.

Comprehensive review of plant small signaling peptides: From stress adaptation mechanisms to practical solutions for crop resilience

Small signaling peptides (SSPs), short proteins of fewer than 100 amino acids, serve as pivotal signaling molecules with diverse structural features, post-translational modifications, and functional roles. They regulate various aspects of plant growth and development by modulating specific cellular signaling pathways. Research has shown that many SSPs are essential for mediating responses to environmental stresses. This review presents the structure, characteristics, and classification of plant SSPs and elucidates their roles in resistance signaling pathways through interactions with their specific receptors. We then summarize recent findings on the biological functions and regulatory mechanisms of SSPs in response to both biotic and abiotic stresses. Finally, we discuss the potential applications and future prospects of these peptides in plant protection. This review offers valuable insights for enhancing plant resilience to environmental stress and advancing sustainable agricultural practices, while also providing key references and perspectives to accelerate research on SSPs in plants.

Polarized subcellular activation of Rho proteins by specific ROPGEFs drives pollen germination in Arabidopsis thaliana

During plant fertilization, excess male gametes compete for a limited number of female gametes. The dormant male gametophyte, encapsulated in the pollen grain, consists of two sperm cells enclosed in a vegetative cell. After reaching the stigma of a compatible flower, quick and efficient germination of the vegetative cell to a tip-growing pollen tube is crucial to ensure fertilization success. Rho of Plants (ROP) signaling and their activating ROP Guanine Nucleotide Exchange Factors (ROPGEFs) are essential for initiating polar growth processes in multiple cell types. However, which ROPGEFs activate pollen germination is unknown. We investigated the role of ROPGEFs in initiating pollen germination and the required cell polarity establishment. Of the five pollen-expressed ROPGEFs, we found that GEF8, GEF9, and GEF12 are required for pollen germination and male fertilization success, as gef8;gef9;gef12 triple mutants showed almost complete loss of pollen germination in vitro and had a reduced allele transmission rate. Live-cell imaging and spatiotemporal analysis of subcellular protein distribution showed that GEF8, GEF9, and GEF11, but not GEF12, displayed transient polar protein accumulations at the future site of pollen germination minutes before pollen germination, demonstrating specific roles for GEF8 and GEF9 during the initiation of pollen germination. Furthermore, this novel GEF accumulation appears in a biphasic temporal manner and can shift its location laterally. We showed that the C-terminal domain of GEF8 and GEF9 confers their protein accumulation and demonstrated that GEFs locally activate ROPs and alter Ca2+ levels, which is required for pollen tube germination. We demonstrated that not all GEFs act redundantly during pollen germination, and we described for the first time a polar domain with spatiotemporal flexibility, which is crucial for the de novo establishment of a polar growth domain within a cell and, thus, for pollen function and fertilization success.

Peptides and Reactive Oxygen Species Regulate Root Development

Like phytohormones, peptide hormones participate in many cellular processes, participate in intercellular communications, and are involved in signal transmission. The system of intercellular communications based on peptide-receptor interactions plays a critical role in the development and functioning of plants. One of the most important molecules are reactive oxygen species (ROS). ROS participate in signaling processes and intercellular communications, including the development of the root system. ROS are recognized as active regulators of cell division and differentiation, which depend on the oxidation-reduction balance. The stem cell niche and the size of the root meristem are maintained by the intercellular interactions and signaling networks of peptide hormone and ROS. Therefore, peptides and ROS can interact with each other both directly and indirectly and function as regulators of cellular processes. Peptides and ROS regulate cell division and stem cell differentiation through a negative feedback mechanism. In this review, we focused on the molecular mechanisms regulating the development of the main root, lateral roots, and nodules, in which peptides and ROS participate.

Genome-wide exploration and characterization of the RALFs and analysis of its role in peanut (Arachis hypogaea L.)

BACKGROUND

Rapid alkalinization factors (RALFs) are small peptides hormones that regulate plant growth and stress responses. Although RALFs have been identified in a broad range of land plant species, their roles in peanuts (Arachis hypogaea L.) remain largely unexplored.

RESULT

A total of 24 AhRALF genes we identified in the peanut genome and classified them into three clades through phylogenetic analysis. Whole genome duplication (WGD) or segmental duplication primarily drives the expansion of AhRALFs. Gene transcription analysis revealed that two genes from clade II (AhRALF1 and AhRALF12) and three from clade III (AhRALF8, AhRALF10, and AhRALF21) are highly expressed across 18 different tissues. Notably, AhRALF11 and AhRALF24, paralogous genes from clade II, are specifically expressed in immature buds and flowers. Additionally, AhRALF1, AhRALF12, AhRALF8, and AhRALF21 exhibited elevated expression under aluminum (Al) stress. Functional analysis of AhRALF1 confirmed its secretory function and inhibitory effect on root growth in Arabidopsis. Moreover, AhRALF1-silenced plants displayed reduced tolerance to Al stress, with altered antioxidant enzyme activities and increased oxidative damage.

CONCLUSION

This study provides a comprehensive analysis of the AhRALF gene family in peanut, highlighting their roles in growth regulation and stress responses. The function of AhRALF1 in enhancing peanut tolerance to Al stress was preliminary revealed. Our findings provide valuable insights into the roles of AhRALFs in peanuts and lay the groundwork for future functional studies and breeding programs.

Decoding small peptides: Regulators of plant growth and stress resilience

Small peptides (SPs) are pivotal signaling molecules that play essential roles in the precise regulation of plant growth, development, and stress responses. Recent advancements in sequencing technologies, bioinformatics approaches, and biochemical and molecular techniques have significantly enhanced the accuracy of SP identification, unveiling their diverse biological functions in plants. This review provides a comprehensive overview of the characteristics and methodologies for identifying SPs in plants. It highlights recent discoveries regarding the biological roles and signaling pathways of SPs in regulating plant growth, development, and plant-microbial interactions, as well as their contributions to plant resilience under various environmental stresses, including abiotic stress, nutrient deficiencies, and biotic challenges. Additionally, we discuss current insights into the potential applications of SPs and outline future research directions aimed at leveraging these molecules to enhance plant adaptation to environmental challenges. By integrating recent findings, this review lays a foundation for advancing the understanding and utilization of SPs to improve plant resilience and productivity.

FERONIA: A Malectin-Domain Receptor Kinase with Intricate Signaling Mechanisms and Profound Importance to Plant Wellness

Plants have evolved elaborate signaling networks, believed to be necessitated by the diversity and complexity of their morphology, developmental and reproductive strategies, and the need to cope with an ever-changing environment from which they are rooted and cannot escape. Their receptor-like kinase superfamilies, with members numbering in the hundreds to more than a thousand, exemplify how plants have evolved their signaling versatility. FERONIA (FER) receptor kinase from model Arabidopsis is a member of the Malectin-domain receptor kinase family conserved among plants. FER has a perplexingly broad functional range, impacting growth to reproduction throughout the plant life cycle, and survival when encountering biotic and abiotic stressors from the environment, such as pathogens and climatic adversity. Efforts to understand FER signaling have brought to light novel signaling strategies at the continuum of the plant cell wall and plasma membrane, and a network of cytoplasmic and nuclear pathways that together support its biological roles. The discussion here focuses on the cell surface mechanisms, including a sugar-peptide interaction-driven liquid-liquid phase separation process along the cell wall-plasma membrane interface and a plasma membrane-linked signaling node comprised of FER, a glycosylphosphatidylinositol-anchored protein, the RHO GTPase molecular switch and a generator for reactive oxygen species (ROS). The emerging recognition of how the broader FER-related receptor kinase family could impact plant wellness and agricultural productivity is also discussed.

Transcriptome dynamics in the Arabidopsis male germline during pollen-pistil interactions

When pollen lands on a receptive stigma, it germinates and extends a tube inside the transmitting tissue of the pistil to deliver the sperm cells for double fertilization. The growth of the pollen tube triggers significant alterations in its gene expression. The extent to which these changes occur in the vegetative cell or extend to the sperm cells transported by the tube is unclear but important to determine since sperm cells are believed to acquire a competency for fertilization during pollen-pistil interactions. To address these questions, we compared the transcriptomes of Arabidopsis thaliana sperm cells and vegetative nuclei isolated from mature pollen grains with those isolated from in vitro-grown pollen tubes. Importantly, we also compared transcriptomes of sperm cells obtained from pollen tubes grown under semi-in vivo conditions where tubes passed through a pistil section. Our data show that extensive transcriptomic changes occur in sperm cells during pollen tube growth, some of which are elicited only as sperms are carried through the pistil. Their analysis reveals a host of previously unidentified transcripts that may facilitate sperm maturation and gamete fusion. The vegetative cell undergoes even more extensive transcriptomic reprogramming during pollen tube growth, mainly through the upregulation of genes associated with pollen tube growth and vesicle-mediated transport. Interestingly, ATAC-seq data show that the promoters of genes upregulated in sperm during pollen tube growth are already accessible in sperm chromatin of mature pollen grains, suggesting pre-configured promoter accessibility. This study's expression data can be further explored here: https://bar.utoronto.ca/eFP-Seq_Browser/.

Antagonistic systemin receptors integrate the activation and attenuation of systemic wound signaling in tomato

Pattern recognition receptor (PRR)-mediated perception of damage-associated molecular patterns (DAMPs) triggers the first line of inducible defenses in both plants and animals. Compared with animals, plants are sessile and regularly encounter physical damage by biotic and abiotic factors. A longstanding problem concerns how plants achieve a balance between wound defense response and normal growth, avoiding overcommitment to catastrophic defense. Here, we report that two antagonistic systemin receptors, SYR1 and SYR2, of the wound peptide hormone systemin in tomato act in a ligand-concentration-dependent manner to regulate immune homeostasis. Whereas SYR1 acts as a high-affinity receptor to initiate systemin signaling, SYR2 functions as a low-affinity receptor to attenuate systemin signaling. The expression of systemin and SYR2, but not SYR1, is upregulated upon SYR1 activation. Our findings provide a mechanistic explanation for how plants appropriately respond to tissue damage based on PRR-mediated perception of DAMP concentrations and have implications for uncoupling defense-growth trade-offs.

Screening methods for thermotolerance in pollen

Plant reproduction is highly susceptible to temperature stress. The development of the male gametophyte in particular represents a critical element in the reproductive cycle with high sensitivity to elevated temperatures. Various methods have been used to test the effect of temperature stress on pollen performance or to determine the degree of susceptibility of given species and genotypes. The information gained informs the development of new crop varieties suited to grow under warmer conditions arising through climate change and facilitates predicting the behaviour of natural populations under these conditions. The characterization of pollen performance typically employs the terms 'pollen viability' and 'pollen vigour', which, however, are not necessarily used consistently across studies. Pollen viability is a nominal parameter and is often assayed relying on cellular features as proxy to infer the capability of pollen grains to germinate and complete double fertilization. Alternatively, pollen germination can be determined through in vitro growth assays, or by monitoring the ability of pollen tubes to complete different progamic steps in vivo (ability to reach an ovule, release sperm cells, lead to seed set). Pollen vigour is an ordinal parameter that describes pollen tube growth rate or the efficiency of pollen tube growth as inferred by its morphology or growth pattern. To ensure consistent and relevant terminology, this review defines these terms and summarizes the methodologies used to assess them.

Pollen tube-expressed RUPO forms a complex with OsMTD2 and OsRALF17 and OsRALF19 peptides in rice (Oryza sativa)

Pollen tubes are crucial for angiosperm plants, as they deliver sperm gametes for the essential process of double fertilization. Understanding the molecular mechanisms behind pollen tube germination and growth is critical; however, these processes remain partially elucidated in monocot cereal crops. Rapid Alkalinization Factor (RALF), a small peptide of about 5 kDa, binds to the CrRLK1L receptor and plays a role in various plant physiological processes, including reproduction and tip growth. Recently, we reported that OsRALF17 and OsRALF19 binds to the OsMTD2, pollen specific CrRLK1L member, and regulates pollen tube growth. In this study, we demonstrate that Ruptured Pollen tube (RUPO), another CrRLK1L member, is also a putative receptor for OsRALF17 and OsRALF19, and propose the formation of a receptor complex with OsMTD2. In tobacco epidermal cells, OsMTD2 and RUPO were co-localized at both the plasma membrane (PM) and the nuclear membrane. Additionally, we generated a RUPO-tagged line driven by its native promoter to visualize subcellular localization during pollen tube growth. RUPO localizes a tip-enriched distribution, with intense fluorescence at the tip's PM and cytoplasm in pollen tube. Upon treatment with synthetic OsRALF17M and OsRALF19M peptides, a reduction in the signal near the PM was observed, suggesting a potential response to these peptides. Our data support the role of RUPO as a candidate receptor for OsRALF17 and OsRALF19 in rice pollen tubes, thereby suggesting a novel mechanism for these RALFs in regulating pollen tube function. Additionally, we observed a significant delay in pollen tube burst time upon treatment with synthetic OsRALF17M and synthetic OsRALF19M. We propose that investigating this phenomenon may provide further insights into the specific signaling pathways mediated by these RALFs.

SlTDF1: A key regulator of tapetum degradation and pollen development in tomato

Pollen formation and development during the life cycle of flowering plant are crucial for maintaining reproductive and genetic diversity. In this study, an R2R3MYB family transcription factor, SlTDF1 (SlMYB35), was predominantly expressed in stamens. Repressed expression of SlTDF1 results in a delay in the degradation of the anther tapetum in tomatoes, which in turn leads to the formation of abnormal pollen, including a reduction in the number of single-fruit seeds and fertility when compared to wild-type plants. Analysis of paraffin sections demonstrated that SlTDF1 is a crucial factor in the maturation of tomato pollen. Further analysis of the transcriptomic data revealed that downregulation of the SlTDF1 gene significantly suppressed the expression of genes related to sugar metabolism and anther development. The findings of this study indicated that SlTDF1 plays a pivotal role in regulating tomato pollen development. Moreover, these findings provide a genetic resource for male sterility in tomato plants.

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.

CLAVATA3 INSENSITIVE RECEPTOR KINASEs regulate lateral root initiation and spacing in Arabidopsis

The root system architecture is very critical for plants to adapt to ever-changing environmental stimulations and is largely affected by lateral roots (LRs). Therefore, how plants regulate LR initiation and spacing is a key point for root system development. Previous studies have shown that RECEPTOR-LIKE KINASE 7 (RLK7) and its ligand TARGET OF LBD SIXTEEN 2 (TOLS2) control the initiation and spacing of LRs. However, the molecular mechanism underlying the perception and transduction of the TOLS2 signal by RLK7 remains to be elucidated. In this study, we explored whether CLAVATA3 INSENSITIVE RECEPTOR KINASEs (CIKs) are critical signaling components during Arabidopsis (Arabidopsis thaliana) LR development by investigating phenotypes of cik mutants and examining interactions between CIKs and members of the RLK7-mediated signaling pathway. Our results showed that high-order cik mutants generated more LRs because of more LR initiation and defective LR spacing. The cik mutants showed reduced sensitivity to applied TOLS2 peptides. TOLS2 application enhanced the interactions between CIKs and RLK7 and the RLK7-dependent phosphorylation of CIKs. In addition, overexpression of transcription factor PUCHI and constitutive activation of MITOGEN-ACTIVATED PROTEIN KINASE KINASE 4 (MKK4) and MKK5 partially rescued the spacing defects of LRs in cik and rlk7-3 mutants. Moreover, we discovered that auxin maximum in pericycle cells altered subcellular localization of CIKs to determine lateral root founder cells. These findings revealed that CIKs and RLK7 function together to perceive the TOLS2 signal and regulate LR initiation and spacing through the MKK4/5-MPK3/6-PUCHI cascade.

The emerging role of cysteine-rich peptides in pollen-pistil interactions

Unlike early land plants, flowering plants have evolved a pollen tube that transports a pair of non-motile sperm cells to the female gametophyte. This process, known as siphonogamy, was first observed in gymnosperms and later became prevalent in angiosperms. However, the precise molecular mechanisms underlying the male-female interactions remain enigmatic. From the landing of the pollen grain on the stigma to gamete fusion, the male part needs to pass various tests: how does the stigma distinguish between compatible and incompatible pollen? what mechanisms guide the pollen tube towards the ovule? what factors trigger pollen tube rupture? how is polyspermy prevented? and how does the sperm cell ultimately reach the egg? Successful male-female communication is essential for surmounting these challenges, with cysteine-rich peptides (CRPs) playing a pivotal role in this dialogue. In this review, we summarize the characteristics of four distinct classes of CRPs, systematically review recent progress in the role of CRPs in four crucial stages of pollination and fertilization, consider potential applications of this knowledge in crop breeding, and conclude by suggesting avenues for future research.

Arabidopsis RALF4 Rapidly Halts Pollen Tube Growth by Increasing ROS and Decreasing Calcium Cytoplasmic Tip Levels

In recent years, the rapid alkalinization factor (RALF) family of cysteine-rich peptides has been reported to be crucial for several plant signaling mechanisms, including cell growth, plant immunity and fertilization. RALF4 and RALF19 (RALF4/19) pollen peptides redundantly regulate the pollen tube integrity and growth through binding to their receptors ANXUR1/2 (ANX1/2) and Buddha's Paper Seal 1 and 2 (BUPS1/2), members of the Catharanthus roseus RLK1-like (CrRLK1L) family, and, thus, are essential for plant fertilization. However, the signaling mechanisms at the cellular level that follow these binding events remain unclear. In this study, we show that the addition of synthetic peptide RALF4 rapidly halts pollen tube growth along with the excessive deposition of plasma membrane and cell wall material at the tip. The ratiometric imaging of genetically encoded ROS and Ca2+ sensors-expressing pollen tubes shows that RALF4 treatment modulates the cytoplasmic levels of reactive oxygen species (ROS) and calcium (Ca2+) in opposite ways at the tip. Thus, we propose that pollen RALF4/19 peptides bind ANX1/2 and BUPS1/2 to regulate ROS and calcium homeostasis to ensure proper cell wall integrity and control of pollen tube growth.

The Role of Female and Male Genes in Regulating Pollen Tube Guidance in Flowering Plants

In flowering plants, fertilization is a complex process governed by precise communication between the male and female gametophytes. This review focuses on the roles of various female gametophyte cells-synergid, central, and egg cells-in facilitating pollen tube guidance and ensuring successful fertilization. Synergid cells play a crucial role in attracting the pollen tube, while the central cell influences the direction of pollen tube growth, and the egg cell is responsible for preventing polyspermy, ensuring correct fertilization. The review also examines the role of the pollen tube in this communication, highlighting the mechanisms involved in its growth regulation, including the importance of pollen tube receptors, signal transduction pathways, cell wall dynamics, and ion homeostasis. The Ca2+ concentration gradient is identified as a key factor in guiding pollen tube growth toward the ovule. Moreover, the review briefly compares these communication processes in angiosperms with those in non-flowering plants, such as mosses, ferns, and early gymnosperms, providing evolutionary insights into gametophytic signaling. Overall, this review synthesizes the current understanding of male-female gametophyte interactions and outlines future directions for research in plant reproductive biology.

Peptide Signals Regulate Nitrogen Deficiency Adaptation of Dicotyledonous Model Plants

Nitrogen is a crucial macroelement essential for plant growth and development. In Arabidopsis Thaliana, classical phytohormones such as auxin and cytokinin orchestrate local and systemic signalling networks coordinate plant growth and development in response to nitrogen deficiency. Nowadays, emerging signalling pathways involving small peptides like CLAVATA3/EMBRYO SURROUNDINGR REGION (CLE) and C-TERMINALLY ENCODED PEPTIDE (CEP) and their corresponding kinase receptors, also regulate Arabidopsis' adaptation to nitrogen scarcity. Unlike Arabidopsis, which adapts to nitrogen deficiency by changing root development, legumes have the unique ability to form nitrogen-fixing root nodules through symbiotic interactions with soil rhizobia. During the symbiotic nodulation in Medicago, CLE and CEP peptides and their receptors consist of an autoregulatory network governing the number of nodules in accordance with the soil nitrogen level. Additionally, other plant peptides, such as phytosulfokine (PSK) and root meristem growth factors (RGF), have been identified as new regulators of leguminous root nodule development under nitrogen-limited condition. However, the precise mechanism by which these peptides coordinate nitrogen deficiency response and the development of nitrogen-fixing organs remains to be fully elucidated. This review summarises the adaptive strategies of dicotyledons to nitrogen deficiency, with a particular focus on the regulation of Medicago nitrogen-fixing nodule development by the peptides.

Genome-Wide Identification of the RALF Gene Family and Expression Pattern Analysis in Zea mays (L.) under Abiotic Stresses

Rapid Alkalization Factor (RALF) is a signaling molecule in plants that plays a crucial role in growth and development, reproductive processes, and responses to both biotic and abiotic stresses. Although RALF peptides have been characterized in Arabidopsis and rice, a comprehensive bioinformatics analysis of the ZmRALF gene family in maize is still lacking. In this study, we identified 20 RALF genes in the maize genome. Sequence alignment revealed significant structural variation among the ZmRALF family genes. Phylogenetic analysis indicates that RALF proteins from Arabidopsis, rice, and maize can be classified into four distinct clades. Duplication events suggest that the expansion of the RALF gene family in maize primarily relies on whole-genome duplication. ZmRALF genes are widely expressed across various tissues; ZmRALF1/15/18/19 are highly expressed in roots, while ZmRALF6/11/14/16 are predominantly expressed in anthers. RNA-seq and RT-qPCR demonstrated that the expression levels of ZmRALF7, ZmRALF9, and ZmRALF13 were significantly up-regulated and down-regulated in response to PEG and NaCl stresses, respectively. Overall, our study provides new insights into the role of the RALF gene family in abiotic stress.

The receptor kinase OsANX limits precocious flowering and inflorescence over-branching and maintains pollen tube integrity in rice

CrRLK1L subfamily members are involved in diverse growth- and development-related processes in Arabidopsis. However, the functions of their counterparts in rice are unknown. Here, OsANX expression was detected in developing inflorescences, mature pollen grains, and growing pollen tubes, and it was localized to the plasma membrane in pollen grains and tobacco epidermal cells. Homozygous osanx progeny could not be segregated from the CRISPR/Cas9-edited mutants osanx-c1+/- and osanx-c2+/-, and such progeny were segregated only occasionally from osanx-c3+/-. Further, all three alleles showed osanx male but not female gamete transmission defects, in line with premature pollen tube rupture in osanx-c3. Additionally, osanx-c3 exhibited precocious flowering, excessively branched inflorescences, and an extremely low seed setting rate of 1.4 %, while osanx-c2+/- and osanx-c3+/- had no obvious defects in inflorescence development or the seed setting rate compared to wild-type Nipponbare (Nip). Consistent with this, the complemented line pPS1:OsANX-GFP/osanx-c2 (PSC), in which the lack of OsANX expression was inflorescence-specific, showed slightly earlier flowering and overly-branched panicles. Multiple inflorescence meristem transition-related and inflorescence architecture-related genes were expressed at higher levels in osanx-c3 than in Nip; thus, they may partially account for the aforementioned mutant phenotypes. Our findings broaden our understanding of the biological functions of OsANX in rice.

Pollen-expressed RLCKs control pollen tube burst

In angiosperms, the pollen tube enters the receptive synergid cell, where it ruptures to release its cytoplasm along with two sperm cells. This interaction is complex, and the exact signal transducers that trigger the bursting of pollen tubes are not well understood. In this study, we identify three homologous receptor-like cytoplasmic kinases (RLCKs) expressed in pollen tubes of Arabidopsis, Delayed Burst 1/2/3 (DEB1/2/3), which play a crucial role in this process. These genes produce proteins localized on the plasma membrane, and their knockout causes delayed pollen tube burst and entrance of additional pollen tubes into the embryo sac due to fertilization recovery. We show that DEBs interact with the Ca2+ pump ACA9, influencing the dynamics of cytoplasmic Ca2+ in pollen tubes through phosphorylation. These results highlight the importance of DEBs as key signal transducers and the critical function of the DEB-ACA9 axis in timely pollen tube burst in synergids.

Pull the fuzes: Processing protein precursors to generate apoplastic danger signals for triggering plant immunity

The apoplast is one of the first cellular compartments outside the plasma membrane encountered by phytopathogenic microbes in the early stages of plant tissue invasion. Plants have developed sophisticated surveillance mechanisms to sense danger events at the cell surface and promptly activate immunity. However, a fine tuning of the activation of immune pathways is necessary to mount a robust and effective defense response. Several endogenous proteins and enzymes are synthesized as inactive precursors, and their post-translational processing has emerged as a critical mechanism for triggering alarms in the apoplast. In this review, we focus on the precursors of phytocytokines, cell wall remodeling enzymes, and proteases. The physiological events that convert inactive precursors into immunomodulatory active peptides or enzymes are described. This review also explores the functional synergies among phytocytokines, cell wall damage-associated molecular patterns, and remodeling, highlighting their roles in boosting extracellular immunity and reinforcing defenses against pests.

Arabidopsis pollen prolyl-hydroxylases P4H4/6 are relevant for correct hydroxylation and secretion of LRX11 in pollen tubes

Major constituents of the plant cell walls are structural proteins that belong to the hydroxyproline-rich glycoprotein (HRGP) family. Leucine-rich repeat extensin (LRX) proteins contain a leucine-rich domain and a C-terminal domain with repetitive Ser-Pro3-5 motifs that are potentially to be O-glycosylated. It has been demonstrated that pollen-specific LRX8-LRX11 from Arabidopsis thaliana are necessary to maintain the integrity of the pollen tube cell wall during polarized growth. In HRGPs, including classical extensins (EXTs), and probably in LRXs, proline residues are converted to hydroxyproline by prolyl-4-hydroxylases (P4Hs), thus defining novel O-glycosylation sites. In this context, we aimed to determine whether hydroxylation and subsequent O-glycosylation of Arabidopsis pollen LRXs are necessary for their proper function and cell wall localization in pollen tubes. We hypothesized that pollen-expressed P4H4 and P4H6 catalyze the hydroxylation of the proline units present in Ser-Pro3-5 motifs of LRX8-LRX11. Here, we show that the p4h4-1 p4h6-1 double mutant exhibits a reduction in pollen germination rates and a slight reduction in pollen tube length. Pollen germination is also inhibited by P4H inhibitors, suggesting that prolyl hydroxylation is required for pollen tube development. Plants expressing pLRX11::LRX11-GFP in the p4h4-1 p4h6-1 background show partial re-localization of LRX11-green fluorescent protein (GFP) from the pollen tube tip apoplast to the cytoplasm. Finally, immunoprecipitation-tandem mass spectrometry analysis revealed a decrease in oxidized prolines (hydroxyprolines) in LRX11-GFP in the p4h4-1 p4h6-1 background compared with lrx11 plants expressing pLRX11::LRX11-GFP. Taken together, these results suggest that P4H4 and P4H6 are required for pollen germination and for proper hydroxylation of LRX11 necessary for its localization in the cell wall of pollen tubes.

Small Peptides: Orchestrators of Plant Growth and Developmental Processes

Small peptides (SPs), ranging from 5 to 100 amino acids, play integral roles in plants due to their diverse functions. Despite their low abundance and small molecular weight, SPs intricately regulate critical aspects of plant life, including cell division, growth, differentiation, flowering, fruiting, maturation, and stress responses. As vital mediators of intercellular signaling, SPs have garnered significant attention in plant biology research. This comprehensive review delves into SPs' structure, classification, and identification, providing a detailed understanding of their significance. Additionally, we summarize recent findings on the biological functions and signaling pathways of prominent SPs that regulate plant growth and development. This review also offers a perspective on future research directions in peptide signaling pathways.

FERONIA: A Receptor Kinase at the Core of a Global Signaling Network

Initially identified as a key regulator of female fertility in Arabidopsis, the FERONIA (FER) receptor kinase is now recognized as crucial for almost all aspects of plant growth and survival. FER partners with a glycosylphosphatidylinositol-anchored protein of the LLG family to act as coreceptors on the cell surface. The FER-LLG coreceptor interacts with different RAPID ALKALINIZATION FACTOR (RALF) peptide ligands to function in various growth and developmental processes and to respond to challenges from the environment. The RALF-FER-LLG signaling modules interact with molecules in the cell wall, cell membrane, cytoplasm, and nucleus and mediate an interwoven signaling network. Multiple FER-LLG modules, each anchored by FER or a FER-related receptor kinase, have been studied, illustrating the functional diversity and the mechanistic complexity of the FER family signaling modules. The challenges going forward are to distill from this complexity the unifying schemes where possible and attain precision and refinement in the knowledge of critical details upon which future investigations can be built. By focusing on the extensively characterized FER, this review provides foundational information to guide the next phase of research on FER in model as well as crop species and potential applications for improving plant growth and resilience.

Not just signals: RALFs as cell wall-structuring peptides

Rapid alkalinization factors (RALFs) have long been known to act as signaling molecules in plant cells, but whether they affect cell wall (CW) patterning and expansion remains unclear. Very recent advances in tip-growing cells showed that positively charged RALFs affect key attributes of the structural components of the nascent CW.

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.

Peptide REF1 is a local wound signal promoting plant regeneration

Plants frequently encounter wounding and have evolved an extraordinary regenerative capacity to heal the wounds. However, the wound signal that triggers regenerative responses has not been identified. Here, through characterization of a tomato mutant defective in both wound-induced defense and regeneration, we demonstrate that in tomato, a plant elicitor peptide (Pep), REGENERATION FACTOR1 (REF1), acts as a systemin-independent local wound signal that primarily regulates local defense responses and regenerative responses in response to wounding. We further identified PEPR1/2 ORTHOLOG RECEPTOR-LIKE KINASE1 (PORK1) as the receptor perceiving REF1 signal for plant regeneration. REF1-PORK1-mediated signaling promotes regeneration via activating WOUND-INDUCED DEDIFFERENTIATION 1 (WIND1), a master regulator of wound-induced cellular reprogramming in plants. Thus, REF1-PORK1 signaling represents a conserved phytocytokine pathway to initiate, amplify, and stabilize a signaling cascade that orchestrates wound-triggered organ regeneration. Application of REF1 provides a simple method to boost the regeneration and transformation efficiency of recalcitrant crops.

Small but powerful: RALF peptides in plant adaptive and developmental responses

Plants live in a highly dynamic environment and require to rapidly respond to a plethora of environmental stimuli, so that to maintain their optimal growth and development. A small plant peptide, rapid alkalization factor (RALF), can rapidly increase the pH value of the extracellular matrix in plant cells. RALFs always function with its corresponding receptors. Mechanistically, effective amount of RALF is induced and released at the critical period of plant growth and development or under different external environmental factors. Recent studies also highlighted the role of RALF peptides as important regulators in plant intercellular communications, as well as their operation in signal perception and as ligands for different receptor kinases on the surface of the plasma membrane, to integrate various environmental cues. In this context, understanding the fine-print of above processes may be essential to solve the problems of crop adaptation to various harsh environments under current climate trends scenarios, by genetic means. This paper summarizes the current knowledge about the structure and diversity of RALF peptides and their roles in plant development and response to stresses, highlighting unanswered questions and problems to be solved.

Rice small secreted peptide, OsRALF26, recognized by FERONIA-like receptor 1 induces immunity in rice and Arabidopsis

Rapid alkalinization factors (RALFs), belonging to a family of small secreted peptides, have been considered as important signaling molecules in diverse biological processes, including immunity. Current studies on RALF-modulated immunity mainly focus on Arabidopsis, but little is reported in crop plants. The rice immune receptor XA21 confers immunity to the bacterial blight pathogen, Xanthomonas oryzae pv. oryzae (Xoo). Here, we pursued functional characterization of rice RALF26 (OsRALF26) up-regulated by Xoo during XA21-mediated immune response. When applied exogenously as a recombinant peptide, OsRALF26 induced a series of immune responses, including pathogenesis-related genes (PRs) induction, reactive oxygen species (ROS) production, and callose deposition in rice and/or Arabidopsis. Transgenic rice and Arabidopsis overexpressing OsRALF26 exhibited significantly enhanced resistance to Xoo and Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), respectively. In yeast two-hybrid, pull-down assays, and co-immunoprecipitation analyses, rice FER-like receptor 1 (OsFLR1) was identified as a receptor of OsRALF26. Transient expression of OsFLR1 in Nicotiana benthamiana leaves displayed significantly increased ROS production and callose deposition after OsRALF26 treatment. Together, we propose that OsRALF26 induced by Xoo in an XA21-dependent manner is perceived by OsFLR1 and may play a novel role in the enforcement of XA21-mediated immunity.

The RALF signaling pathway regulates cell wall integrity during pollen tube growth in maize

Autocrine signaling pathways regulated by RAPID ALKALINIZATION FACTORs (RALFs) control cell wall integrity during pollen tube germination and growth in Arabidopsis (Arabidopsis thaliana). To investigate the role of pollen-specific RALFs in another plant species, we combined gene expression data with phylogenetic and biochemical studies to identify candidate orthologs in maize (Zea mays). We show that Clade IB ZmRALF2/3 mutations, but not Clade III ZmRALF1/5 mutations, cause cell wall instability in the sub-apical region of the growing pollen tube. ZmRALF2/3 are mainly located in the cell wall and are partially able to complement the pollen germination defect of their Arabidopsis orthologs AtRALF4/19. Mutations in ZmRALF2/3 compromise pectin distribution patterns leading to altered cell wall organization and thickness culminating in pollen tube burst. Clade IB, but not Clade III ZmRALFs, strongly interact as ligands with the pollen-specific Catharanthus roseus RLK1-like (CrRLK1L) receptor kinases Z. mays FERONIA-like (ZmFERL) 4/7/9, LORELEI-like glycosylphosphatidylinositol-anchor (LLG) proteins Z. mays LLG 1 and 2 (ZmLLG1/2), and Z. mays pollen extension-like (PEX) cell wall proteins ZmPEX2/4. Notably, ZmFERL4 outcompetes ZmLLG2 and ZmPEX2 outcompetes ZmFERL4 for ZmRALF2 binding. Based on these data, we suggest that Clade IB RALFs act in a dual role as cell wall components and extracellular sensors to regulate cell wall integrity and thickness during pollen tube growth in maize and probably other plants.

Small but mighty: Peptides regulating abiotic stress responses in plants

Throughout evolution, plants have developed strategies to confront and alleviate the detrimental impacts of abiotic stresses on their growth and development. The combat strategies involve intricate molecular networks and a spectrum of early and late stress-responsive pathways. Plant peptides, consisting of fewer than 100 amino acid residues, are at the forefront of these responses, serving as pivotal signalling molecules. These peptides, with roles similar to phytohormones, intricately regulate plant growth, development and facilitate essential cell-to-cell communications. Numerous studies underscore the significant role of these small peptides in coordinating diverse signalling events triggered by environmental challenges. Originating from the proteolytic processing of larger protein precursors or directly translated from small open reading frames, including microRNA (miRNA) encoded peptides from primary miRNA, these peptides exert their biological functions through binding with membrane-embedded receptor-like kinases. This interaction initiates downstream cellular signalling cascades, often involving major phytohormones or reactive oxygen species-mediated mechanisms. Despite these advances, the precise modes of action for numerous other small peptides remain to be fully elucidated. In this review, we delve into the dynamics of stress physiology, mainly focusing on the roles of major small signalling peptides, shedding light on their significance in the face of changing environmental conditions.

Rice kinase OsMRLK63 contributes to drought tolerance by regulating reactive oxygen species production

Drought is a major adverse environmental factor that plants face in nature but the molecular mechanism by which plants transduce stress signals and further endow themselves with tolerance remains unclear. Malectin/malectin-like domains containing receptor-like kinases (MRLKs) have been proposed to act as receptors in multiple biological signaling pathways, but limited studies show their roles in drought-stress signaling and tolerance. In this study, we demonstrate OsMRLK63 in rice (Oryza sativa L.) functions in drought tolerance by acting as the receptor of 2 rapid alkalization factors, OsRALF45 and OsRALF46. We show OsMRLK63 is a typical receptor-like kinase that positively regulates drought tolerance and reactive oxygen species (ROS) production. OsMRLK63 interacts with and phosphorylates several nicotinamide adenine dinucleotide phosphate (NADPH) oxidases with the primarily phosphorylated site at Ser26 in the N-terminal of RESPIRATORY BURST OXIDASE HOMOLOGUE A (OsRbohA). The application of the 2 small signal peptides (OsRALF45/46) on rice can greatly alleviate the dehydration of plants induced by mimic drought. This function depends on the existence of OsMRLK63 and the NADPH oxidase-dependent ROS production. The 2 RALFs interact with OsMRLK63 by binding to its extracellular domain, suggesting they may act as drought/dehydration signal sensors for the OsMRLK63-mediated process. Our study reveals a OsRALF45/46-OsMRLK63-OsRbohs module which contributes to drought-stress signaling and tolerance in rice.

Genome wide characterization and expression analysis of CrRLK1L gene family in wheat unravels their roles in development and stress-specific responses

Catharanthus roseus receptor-like kinase 1-like (CrRLK1L) genes encode a subfamily of receptor-like kinases (RLK) that regulate diverse processes during plant growth, development, and stress responses. The first CrRLK1L was identified from the Catharanthus roseus, commonly known as Madagascar periwinkle. Subsequently, CrRLK1L gene families have been characterized in many plants. The genome of T. aestivum encodes 15 CrRLK1L genes with 43 paralogous copies, with three homeologs each, except for -2-D and -7-A, which are absent. Chromosomal localization analysis revealed a markedly uneven distribution of CrRLK1L genes across seven different chromosomes, with chromosome 4 housing the highest number of genes, while chromosome 6 lacked any CrRLK1L genes. Tissue-specific gene expression analysis revealed distinct expression patterns among the gene family members, with certain members exhibiting increased expression in reproductive tissues. Gene expression analysis in response to various abiotic and biotic stress conditions unveiled differential regulation of gene family members. Cold stress induces CrRLK1Ls -4-B and -15-A while downregulating -3-A and -7B. Drought stress upregulates -9D, contrasting with the downregulation of -7D. CrRLK1L-15-B and -15-D were highly induced in response to 1 hr of heat, and combined drought and heat stress, whereas -10-B is downregulated. Similarly, in response to NaCl stress, only CrRLK1L1 homeologs were induced. Fusarium graminearum and Claviceps purpurea inoculation induces homeologs of CrRLK1L-6 and -7. The analysis of cis-acting elements in the promoter regions identified elements crucial for plant growth and developmental processes. This comprehensive genome-wide analysis and expression study provides valuable insights into the essential functions of CrRLK1L members in wheat.

Signalling between the sexes during pollen tube reception

Plant reproduction is a complex, highly-coordinated process in which a single, male germ cell grows through the maternal reproductive tissues to reach and fertilise the egg cell. Focussing on Arabidopsis thaliana, we review signalling between male and female partners which is important throughout the pollen tube journey, especially during pollen tube reception at the ovule. Numerous receptor kinases and their coreceptors are implicated in signal perception in both the pollen tube and synergid cells at the ovule entrance, and several specific peptide and carbohydrate ligands for these receptors have recently been identified. Clarifying the interplay between these signals and the downstream responses they instigate presents a challenge for future research and may help to illuminate broader principles of plant cell-cell communication.

A self-regulatory cell-wall-sensing module at cell edges controls plant growth

Morphogenesis of multicellular organs requires coordination of cellular growth. In plants, cell growth is determined by turgor pressure and the mechanical properties of the cell wall, which also glues cells together. Because plants have to integrate tissue-scale mechanical stresses arising through growth in a fixed tissue topology, they need to monitor cell wall mechanical status and adapt growth accordingly. Molecular factors have been identified, but whether cell geometry contributes to wall sensing is unknown. Here we propose that plant cell edges act as cell-wall-sensing domains during growth. We describe two Receptor-Like Proteins, RLP4 and RLP4-L1, which occupy a unique polarity domain at cell edges established through a targeted secretory transport pathway. We show that RLP4s associate with the cell wall at edges via their extracellular domain, respond to changes in cell wall mechanics and contribute to directional growth control in Arabidopsis.

Synergid cell calcium oscillations refine understanding of FERONIA/LORELEI signaling during interspecific hybridization

KEY MESSAGE

Pollen tubes from closely related species and mutants lacking pollen tube MYB transcription factors are able to initiate FER/LRE-dependent synergid cell calcium oscillations. Reproductive isolation leads to the evolution of new species; however, the molecular mechanisms that maintain reproductive barriers between sympatric species are not well defined. In flowering plants, sperm cells are immotile and are delivered to female gametes by the pollen grain. After landing on the stigmatic surface, the pollen grain germinates a polarized extension, the pollen tube, into floral tissue. After growing via polar extension to the female gametes and shuttling its cargo of sperm cells through its cytoplasm, the pollen tube signals its arrival and identity to synergid cells that flank the egg. If signaling is successful, the pollen tube and receptive synergid cell burst, and sperm cells are released for fusion with female gametes. To better understand cell-cell recognition during reproduction and how reproductive barriers are maintained between closely related species, pollen tube-initiated synergid cell calcium ion dynamics were examined during interspecific crosses. It was observed that interspecific pollen tubes successfully trigger synergid cell calcium oscillations-a hallmark of reproductive success-but signaling fails downstream of key signaling genes and sperm are not released. This work further defines pollen tube-synergid cell signaling as a critical block to interspecific hybridization and suggests that the FERONIA/LORELEI signaling mechanism plays multiple parallel roles during pollen tube reception.

Defective pollen tube tip growth induces neo-polyploid infertility

Genome duplication (generating polyploids) is an engine of novelty in eukaryotic evolution and a promising crop improvement tool. Yet newly formed polyploids often have low fertility. Here we report that a severe fertility-compromising defect in pollen tube tip growth arises in new polyploids of Arabidopsis arenosa. Pollen tubes of newly polyploid A. arenosa grow slowly, have aberrant anatomy and disrupted physiology, often burst prematurely, and have altered gene expression. These phenotypes recover in evolved polyploids. We also show that gametophytic (pollen tube) genotypes of two tip-growth genes under selection in natural tetraploid A. arenosa are strongly associated with pollen tube performance in the tetraploid. Our work establishes pollen tube tip growth as an important fertility challenge for neo-polyploid plants and provides insights into a naturally evolved multigenic solution.