Membrane-bound RLCKs LIP1 and LIP2 are essential male factors controlling male-female attraction in Arabidopsis

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.

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.

Genome-wide identification and characterization of Calcium-Dependent Protein Kinase (CDPK) gene family in autotetraploid cultivated alfalfa (Medicago sativa subsp. sativa) and expression analysis under abiotic stresses

BACKGROUND

Calcium-dependent protein kinases (CDPKs), play multiple roles in plant development, growth and response to bio- or abiotic stresses. Calmodulin-like domains typically contain four EF-hand motifs for Ca²⁺ binding. The CDPK gene family can be divided into four subgroups in Arabidopsis, and it has been identified in many plants, such as rice, tomato, but has not been investigated in alfalfa (Medicago sativa subsp. sativa) yet.

RESULTS

In our study, 38 non-redundant MsCDPK genes were identified from the "XinJiangDaYe" alfalfa genome. They can be divided into four subgroups which is the same as in Arabidopsis and Medicago truncatula, and there were 15, 12,10 and 1 in CDPK I, II, III and IV, respectively. RNA-seq analysis revealed tissue-specificity of 38 MsCDPK genes. After researching the transcriptome data, we found these 38 MsCDPK members responsive to drought, salt, and cold stress treatments. Further analysis showed that the expression of almost all the MsCDPKs is regulated by abiotic stresses. In addition, we chose MsCDPK03, MsCDPK26, MsCDPK31 and MsCDPK36 for RT-qPCR validation which was from CDPK I-IV subgroups respectively. The result showed that the expression of these four genes was significantly induced by drought, salt and cold treatments. The subcellular location experiment showed that these four proteins were all located in nucleus.

CONCLUSION

In our study, we identified 38 distinct MsCDPK genes within the alfalfa genome, which were classified into four groups. We conducted a comprehensive analysis of various gene features, including physicochemical properties, phylogenetic relationships, exon-intron structures, conserved motifs, chromosomal locations, gene duplication events, cis-regulatory elements, 3D structures, and tissue-specific expression patterns, as well as responses to drought, salt, and cold stresses. These results also provide a solid foundation for further investigations into the functions of MsCDPKs aimed at improving drought tolerance in autotetraploid cultivated alfalfa through genetic engineering.

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.

Receptor-like cytoplasmic kinases: orchestrating plant cellular communication

The receptor-like kinase (RLK) family of receptors and the associated receptor-like cytoplasmic kinases (RLCKs) have expanded in plants because of selective pressure from environmental stress and evolving pathogens. RLCKs link pathogen perception to activation of coping mechanisms. RLK-RLCK modules regulate hormone synthesis and responses, reactive oxygen species (ROS) production, Ca2+ signaling, activation of mitogen-activated protein kinase (MAPK), and immune gene expression, all of which contribute to immunity. Some RLCKs integrate responses from multiple receptors recognizing distinct ligands. RLKs/RLCKs and nucleotide-binding domain, leucine-rich repeats (NLRs) were found to synergize, demonstrating the intertwined genetic network in plant immunity. Studies in arabidopsis (Arabidopsis thaliana) have provided paradigms about RLCK functions, but a lack of understanding of crop RLCKs undermines their application. In this review, we summarize current understanding of the diverse functions of RLCKs, based on model systems and observations in crop species, and the emerging role of RLCKs in pathogen and abiotic stress response signaling.

Arabidopsis class A S-acyl transferases modify the pollen receptors LIP1 and PRK1 to regulate pollen tube guidance

Protein S-acylation catalyzed by protein S-acyl transferases (PATs) is a reversible lipid modification regulating protein targeting, stability, and interaction profiles. PATs are encoded by large gene families in plants, and many proteins including receptor-like cytoplasmic kinases (RLCKs) and receptor-like kinases (RLKs) are subject to S-acylation. However, few PATs have been assigned substrates, and few S-acylated proteins have known upstream enzymes. We report that Arabidopsis (Arabidopsis thaliana) class A PATs redundantly mediate pollen tube guidance and participate in the S-acylation of POLLEN RECEPTOR KINASE1 (PRK1) and LOST IN POLLEN TUBE GUIDANCE1 (LIP1), a critical RLK or RLCK for pollen tube guidance, respectively. PAT1, PAT2, PAT3, PAT4, and PAT8, collectively named PENTAPAT for simplicity, are enriched in pollen and show similar subcellular distribution. Functional loss of PENTAPAT reduces seed set due to male gametophytic defects. Specifically, pentapat pollen tubes are compromised in directional growth. We determine that PRK1 and LIP1 interact with PENTAPAT, and their S-acylation is reduced in pentapat pollen. The plasma membrane (PM) association of LIP1 is reduced in pentapat pollen, whereas point mutations reducing PRK1 S-acylation affect its affinity with its interacting proteins. Our results suggest a key role of S-acylation in pollen tube guidance through modulating PM receptor complexes.

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.

The zinc finger protein DHHC09 S-acylates the kinase STRK1 to regulate H2O2 homeostasis and promote salt tolerance in rice

Soil salinity results in oxidative stress and heavy losses to crop production. The S-acylated protein SALT TOLERANCE RECEPTOR-LIKE CYTOPLASMIC KINASE 1 (STRK1) phosphorylates and activates CATALASE C (CatC) to improve rice (Oryza sativa L.) salt tolerance, but the molecular mechanism underlying its S-acylation involved in salt signal transduction awaits elucidation. Here, we show that the DHHC-type zinc finger protein DHHC09 S-acylates STRK1 at Cys5, Cys10, and Cys14 and promotes salt and oxidative stress tolerance by enhancing rice H2O2-scavenging capacity. This modification determines STRK1 targeting to the plasma membrane or lipid nanodomains and is required for its function. DHHC09 promotes salt signaling from STRK1 to CatC via transphosphorylation, and its deficiency impairs salt signal transduction. Our findings demonstrate that DHHC09 S-acylates and anchors STRK1 to the plasma membrane to promote salt signaling from STRK1 to CatC, thereby regulating H2O2 homeostasis and improving salt stress tolerance in rice. Moreover, overexpression of DHHC09 in rice mitigates grain yield loss under salt stress. Together, these results shed light on the mechanism underlying the role of S-acylation in RLK/RLCK-mediated salt signal transduction and provide a strategy for breeding highly salt-tolerant rice.

Insights into pollen-stigma recognition: self-incompatibility mechanisms serve as interspecies barriers in Brassicaceae?

A new study provides a comprehensive molecular mechanism that controls interspecific incompatibility of self-incompatible (SI) plants in the Brassicaceae. This finding points to a potentially promising path to break interspecific barriers and achieve introgression of desirable traits into crops from distant species among SI crops in the Brassicaceae.

Two aspartic proteases, BnaAP36s and BnaAP39s, regulate pollen tube guidance in Brassica napus

Pollen tube (PT) growth towards the micropyle is critical for successful double fertilization. However, the mechanism of micropyle-directed PT growth is still unclear in Brassica napus. In this study, two aspartate proteases, BnaAP36s and BnaAP39s, were identified in B. napus. BnaAP36s and BnaAP39s were localized to the plasma membrane. The homologues of BnaAP36 and BnaAP39 were highly expressed in flower organs, especially in the anther. Sextuple and double mutants of BnaAP36s and BnaAP39s were then generated using CRISPR/Cas9 technology. Compared to WT, the seed-set of cr-bnaap36 and cr-bnaap39 mutants was reduced by 50% and 60%, respectively. The reduction in seed-set was also found when cr-bnaap36 and cr-bnaap39 were used as the female parent in a reciprocal cross assay. Like WT, cr-bnaap36 and cr-bnaap39 pollen were able to germinate and the relative PTs were able to elongate in style. Approximately 36% and 33% of cr-bnaap36 and cr-bnaap39 PTs, respectively, failed to grow towards the micropyle, indicating that BnaAP36s and BnaAP39s are essential for micropyle-directed PT growth. Furthermore, Alexander's staining showed that 10% of cr-bnaap39 pollen grains were aborted, but not cr-bnaap36, suggesting that BnaAP39s may also affect microspore development. These results suggest that BnaAP36s and BnaAP39s play a critical role in the growth of micropyle-directed PTs in B. napus.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01377-1.

Peptides/receptors signaling during plant fertilization

Double fertilization is a unique and particularly complicated process for the generation alternation of angiosperms. Sperm cells of angiosperms lose the motility compared with that of gymnosperms. The sperm cells are passively carried and transported by the pollen tube for a long journey before targeting the ovule. Two sperm cells are released at the cleft between the egg and the central cell and fused with two female gametes to produce a zygote and endosperm, respectively, to accomplish the so-called double fertilization process. In this process, extensive communication and interaction occur between the male (pollen or pollen tube) and the female (ovule). It is suggested that small peptides and receptor kinases play critical roles in orchestrating this cell-cell communication. Here, we illuminate the understanding of phases in the process, such as pollen-stigma recognition, the hydration and germination of pollen grains, the growth, guidance, and rupture of tubes, the release of sperm cells, and the fusion of gametes, by reviewing increasing data recently. The roles of peptides and receptor kinases in signaling mechanisms underlying cell-cell communication were focused on, and directions of future studies were perspected in this review.

Regulatory dynamics of gene expression in the developing male gametophyte of Arabidopsis

Sexual reproduction in angiosperms requires the production and delivery of two male gametes by a three-celled haploid male gametophyte. This demands synchronized gene expression in a short developmental window to ensure double fertilization and seed set. While transcriptomic changes in developing pollen are known for Arabidopsis, no studies have integrated RNA and proteomic data in this model. Further, the role of alternative splicing has not been fully addressed, yet post-transcriptional and post-translational regulation may have a key role in gene expression dynamics during microgametogenesis. We have refined and substantially updated global transcriptomic and proteomic changes in developing pollen for two Arabidopsis accessions. Despite the superiority of RNA-seq over microarray-based platforms, we demonstrate high reproducibility and comparability. We identify thousands of long non-coding RNAs as potential regulators of pollen development, hundreds of changes in alternative splicing and provide insight into mRNA translation rate and storage in developing pollen. Our analysis delivers an integrated perspective of gene expression dynamics in developing Arabidopsis pollen and a foundation for studying the role of alternative splicing in this model.

The phospholipid flippase ALA3 regulates pollen tube growth and guidance in Arabidopsis

Pollen tube guidance regulates the growth direction and ovule targeting of pollen tubes in pistils, which is crucial for the completion of sexual reproduction in flowering plants. The Arabidopsis (Arabidopsis thaliana) pollen-specific receptor kinase (PRK) family members PRK3 and PRK6 are specifically tip-localized and essential for pollen tube growth and guidance. However, the mechanisms controlling the polar localization of PRKs at the pollen tube tip are unclear. The Arabidopsis P4-ATPase ALA3 helps establish the polar localization of apical phosphatidylserine (PS) in pollen tubes. Here, we discovered that loss of ALA3 function caused pollen tube defects in growth and ovule targeting and significantly affected the polar localization pattern of PRK3 and PRK6. Both PRK3 and PRK6 contain two polybasic clusters in the intracellular juxtamembrane domain, and they bound to PS in vitro. PRK3 and PRK6 with polybasic cluster mutations showed reduced or abolished binding to PS and altered polar localization patterns, and they failed to effectively complement the pollen tube-related phenotypes of prk mutants. These results suggest that ALA3 influences the precise localization of PRK3, PRK6, and other PRKs by regulating the distribution of PS, which plays a key role in regulating pollen tube growth and guidance.

Protein S-acyltransferases and acyl protein thioesterases, regulation executors of protein S-acylation in plants

Protein S-acylation, also known as palmitoylation, is an important lipid post-translational modification of proteins in eukaryotes. S-acylation plays critical roles in a variety of protein functions involved in plant development and responses to abiotic and biotic stresses. The status of S-acylation on proteins is dynamic and reversible, which is catalyzed by protein S-acyltransferases (PATs) and reversed by acyl protein thioesterases. The cycle of S-acylation and de-S-acylation provides a molecular mechanism for membrane-associated proteins to undergo cycling and trafficking between different cell compartments and thus works as a switch to initiate or terminate particular signaling transductions on the membrane surface. In plants, thousands of proteins have been identified to be S-acylated through proteomics. Many S-acylated proteins and quite a few PAT-substrate pairs have been functionally characterized. A recently characterized acyl protein thioesterases family, ABAPT family proteins in Arabidopsis, has provided new insights into the de-S-acylation process. However, our understanding of the regulatory mechanisms controlling the S-acylation and de-S-acylation process is surprisingly incomplete. In this review, we discuss how protein S-acylation level is regulated with the focus on catalyzing enzymes in plants. We also propose the challenges and potential developments for the understanding of the regulatory mechanisms controlling protein S-acylation in plants.

EXPLICIT-Kinase: A gene expression predictor for dissecting the functions of the Arabidopsis kinome

Protein kinases regulate virtually all cellular processes, but it remains challenging to determine the functions of all protein kinases, collectively called the "kinome", in any species. We developed a computational approach called EXPLICIT-Kinase to predict the functions of the Arabidopsis kinome. Because the activities of many kinases can be regulated transcriptionally, their gene expression patterns provide clues to their functions. A universal gene expression predictor for Arabidopsis was constructed to predict the expression of 30,172 non-kinase genes based on the expression of 994 kinases. The model reconstituted highly accurate transcriptomes for diverse Arabidopsis samples. It identified the significant kinases as predictor kinases for predicting the expression of Arabidopsis genes and pathways. Strikingly, these predictor kinases were often regulators of related pathways, as exemplified by those involved in cytokinesis, tissue development, and stress responses. Comparative analyses revealed that portions of these predictor kinases are shared and conserved between Arabidopsis and maize. As an example, we identified a conserved predictor kinase, RAF6, from a stomatal movement module. We verified that RAF6 regulates stomatal closure. It can directly interact with SLAC1, a key anion channel for stomatal closure, and modulate its channel activity. Our approach enables a systematic dissection of the functions of the Arabidopsis kinome.

Screening DHHCs of S-acylated proteins using an OsDHHC cDNA library and bimolecular fluorescence complementation in rice

S-acylation is an important lipid modification that primarily involves DHHC proteins (DHHCs) and associated S-acylated proteins. No DHHC-S-acylated protein pair has been reported so far in rice (Oryza sativa L.) and the molecular mechanisms underlying S-acylation in plants are largely unknown. We constructed an OsDHHC cDNA library for screening corresponding pairs of DHHCs and S-acylated proteins using bimolecular fluorescence complementation assays. Five DHHC-S-acylated protein pairs (OsDHHC30-OsCBL2, OsDHHC30-OsCBL3, OsDHHC18-OsNOA1, OsDHHC13-OsNAC9, and OsDHHC14-GSD1) were identified in rice. Among the pairs, OsCBL2 and OsCBL3 were S-acylated by OsDHHC30 in yeast and rice. The localization of OsCBL2 and OsCBL3 in the endomembrane depended on S-acylation mediated by OsDHHC30. Meanwhile, all four OsDHHCs screened complemented the thermosensitive phenotype of an akr1 yeast mutant, and their DHHC motifs were required for S-acyltransferase activity. Overexpression of OsDHHC30 in rice plants improved their salt and oxidative tolerance. Together, these results contribute to our understanding of the molecular mechanism underlying S-acylation in plants.

A receptor-like cytoplasmic kinase PCRK2 undergoes ubiquitination and proteasomal degradation

Receptor-like cytoplasmic kinase (RLCK) subfamily VII members are involved in diverse biological processes, like reproduction, immunity, growth and development. Ubiquitination and proteasomal degradation of a RLCK VII member, BOTRYTIS-INDUCED KINASE1 (BIK1) play important roles in regulating immune signaling. It remains largely unknown whether most other RLCK VII members undergo ubiquitination and proteasomal degradation. Here, we select the 6-member RLCK VII-4 to examine the potential proteasomal degradation of its members. We find that three closely related RLCK VII-4 members, PBL38 (AvrPphB SUSCEPTIBLE1-LIKE38), PCRK1 (PTI-COMPROMISED RECEPTOR-LIKE CYTOPLASMIC KINASE1), and PCRK2 are under proteasomal control, while the other members in this group are not. Moreover, we demonstrate that PCRK2 undergoes ubiquitination and proteasomal in a kinase activity-dependent manner. However, the plasma membrane (PM) localization of PCRK2 is not required for its degradation. Our work suggests that many other RLCK VII members may undergo ubiquitination and proteasomal degradation to modulate their homeostasis and cellular functions.

A Decade of Pollen Phosphoproteomics

Angiosperm mature pollen represents a quiescent stage with a desiccated cytoplasm surrounded by a tough cell wall, which is resistant to the suboptimal environmental conditions and carries the genetic information in an intact stage to the female gametophyte. Post pollination, pollen grains are rehydrated, activated, and a rapid pollen tube growth starts, which is accompanied by a notable metabolic activity, synthesis of novel proteins, and a mutual communication with female reproductive tissues. Several angiosperm species (Arabidopsis thaliana, tobacco, maize, and kiwifruit) were subjected to phosphoproteomic studies of their male gametophyte developmental stages, mostly mature pollen grains. The aim of this review is to compare the available phosphoproteomic studies and to highlight the common phosphoproteins and regulatory trends in the studied species. Moreover, the pollen phosphoproteome was compared with root hair phosphoproteome to pinpoint the common proteins taking part in their tip growth, which share the same cellular mechanisms.

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.

The role of diverse LURE-type cysteine-rich peptides as signaling molecules in plant reproduction

In angiosperm sexual reproduction, the male pollen tube undergoes a series of interactions with female tissues. For efficient growth and precise guidance, the pollen tube perceives extracellular ligands. In recent decades, various types of secreted cysteine-rich peptides (CRPs) have been identified as peptide ligands that regulate diverse angiosperm reproduction processes, including pollen tube germination, growth, guidance, and rupture. Notably, in two distant core eudicot plants, multiple LURE-type CRPs were found to be secreted from egg-accompanying synergid cells, and these CRPs act as a cocktail of pollen tube attractants for the final step of pollen tube guidance. LURE-type CRPs have species-preferential activity, even among close relatives, and exhibit remarkably divergent molecular evolution with conserved cysteine frameworks, demonstrating that they play a key role in species recognition in pollen tube guidance. In this review, I focus on "reproductive CRPs," particularly LURE-type CRPs, which underlie common but species-specific mechanisms in angiosperm sexual reproduction, and discuss their action, functional regulation, receptors, and evolution.

AtLURE1/PRK6-mediated signaling promotes conspecific micropylar pollen tube guidance

Reproductive isolation is a prerequisite to form and maintain a new species. Multiple prezygotic and postzygotic reproductive isolation barriers have been reported in plants. In the model plant, Arabidopsis thaliana conspecific pollen tube precedence controlled by AtLURE1/PRK6-mediated signaling has been recently reported as a major prezygotic reproductive isolation barrier. By accelerating emergence of own pollen tubes from the transmitting tract, A. thaliana ovules promote self-fertilization and thus prevent fertilization by a different species. Taking advantage of a septuple atlure1null mutant, we now report on the role of AtLURE1/PRK6-mediated signaling for micropylar pollen tube guidance. Compared with wild-type (WT) ovules, atlure1null ovules displayed remarkably reduced micropylar pollen tube attraction efficiencies in modified semi-in vivo A. thaliana ovule targeting assays. However, when prk6 mutant pollen tubes were applied, atlure1null ovules showed micropylar attraction efficiencies comparable to that of WT ovules. These findings indicate that AtLURE1/PRK6-mediated signaling regulates micropylar pollen tube attraction in addition to promoting emergence of own pollen tubes from the transmitting tract. Moreover, semi-in vivo ovule targeting competition assays with the same amount of pollen grains from both A. thaliana and Arabidopsis lyrata showed that A. thaliana WT and xiuqiu mutant ovules are mainly targeted by own pollen tubes and that atlure1null mutant ovules are also entered to a large extent by A. lyrata pollen tubes. Taken together, we report that AtLURE1/PRK6-mediated signaling promotes conspecific micropylar pollen tube attraction representing an additional prezygotic isolation barrier.

Function of Small Peptides During Male-Female Crosstalk in Plants

Plant peptides secreted as signal molecular to trigger cell-to-cell signaling are indispensable for plant growth and development. Successful sexual reproduction in plants requires extensive communication between male and female gametophytes, their gametes, and with the surrounding sporophytic tissues. In the past decade, it has been well-documented that small peptides participate in many important reproductive processes such as self-incompatibility, pollen tube growth, pollen tube guidance, and gamete interaction. Here, we provide a comprehensive overview of the peptides regulating the processes of male-female crosstalk in plant, aiming at systematizing the knowledge on the sexual reproduction, and signaling of plant peptides in future.

Genome-Wide Identification and Characterization of Calcium-Dependent Protein Kinase (CDPK) and CDPK-Related Kinase (CRK) Gene Families in Medicago truncatula

Calcium-dependent protein kinase (CDPK or CPK) and CDPK-related kinase (CRK) play an important role in plant growth, development, and adaptation to environmental stresses. However, their gene families had been yet inadequately investigated in Medicago truncatula. In this study, six MtCRK genes were computationally identified, they were classified into five groups with MtCDPKs based on phylogenetic relationships. Six pairs of segmental duplications were observed in MtCDPK and MtCRK genes and the Ka/Ks ratio, an indicator of selection pressure, was below 0.310, indicating that these gene pairs underwent strong purifying selection. Cis-acting elements of morphogenesis, multiple hormone responses, and abiotic stresses were predicted in the promoter region. The spatial expression of MtCDPKs and MtCRKs displays diversity. The expression of MtCDPKs and MtCRKs could be regulated by various stresses. MtCDPK4, 14, 16, 22, and MtCRK6 harbor both N-myristoylation site and palmitoylation site and were anchored on plasma membrane, while MtCDPK7, 9, and 15 contain no or only one N-acylation site and were distributed in cytosol and nucleus, suggesting that the N-terminal acylation sites play a key role in subcellular localization of MtCDPKs and MtCRKs. In summary, comprehensive characterization of MtCDPKs and MtCRKs provide a subset of candidate genes for further functional analysis and genetic improvement against drought, cold, salt and biotic stress.

A Complex Journey: Cell Wall Remodeling, Interactions, and Integrity During Pollen Tube Growth

In flowering plants, pollen tubes undergo a journey that starts in the stigma and ends in the ovule with the delivery of the sperm cells to achieve double fertilization. The pollen cell wall plays an essential role to accomplish all the steps required for the successful delivery of the male gametes. This extended path involves female tissue recognition, rapid hydration and germination, polar growth, and a tight regulation of cell wall synthesis and modification, as its properties change not only along the pollen tube but also in response to guidance cues inside the pistil. In this review, we focus on the most recent advances in elucidating the molecular mechanisms involved in the regulation of cell wall synthesis and modification during pollen germination, pollen tube growth, and rupture.

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.

Genetic variation in ZmTIP1 contributes to root hair elongation and drought tolerance in maize

Drought is a major abiotic stress that threatens maize production globally. A previous genome-wide association study identified a significant association between the natural variation of ZmTIP1 and the drought tolerance of maize seedlings. Here, we report on comprehensive genetic and functional analysis, indicating that ZmTIP1, which encodes a functional S-acyltransferase, plays a positive role in regulating the length of root hairs and the level of drought tolerance in maize. We show that enhancing ZmTIP1 expression in transgenic Arabidopsis and maize increased root hair length, as well as plant tolerance to water deficit. In contrast, ZmTIP1 transposon-insertional mutants displayed the opposite phenotype. A calcium-dependent protein kinase, ZmCPK9, was identified as a substrate protein of ZmTIP1, and ZmTIP1-mediated palmitoylation of two cysteine residues facilitated the ZmCPK9 PM association. The results of this research enrich our knowledge about ZmTIP1-mediated protein S-acylation modifications in relation to the regulation of root hair elongation and drought tolerance. Additionally, the identification of a favourable allele of ZmTIP1 also provides a valuable genetic resource or selection target for the genetic improvement of maize.

Fertilization in flowering plants: an odyssey of sperm cell delivery

In light of the available discoveries in the field, this review manuscript discusses on plant reproduction mechanism and molecular players involved in the process. Sperm cells in angiosperms are immotile and are physically distant to the female gametophytes (FG). To secure the production of the next generation, plants have devised a clever approach by which the two sperm cells in each pollen are safely delivered to the female gametophyte where two fertilization events occur (by each sperm cell fertilizing an egg cell and central cell) to give rise to embryo and endosperm. Each of the successfully fertilized ovules later develops into a seed. Sets of macromolecules play roles in pollen tube (PT) guidance, from the stigma, through the transmitting tract and funiculus to the micropylar end of the ovule. Other sets of genetic players are involved in PT reception and in its rupture after it enters the ovule, and yet other sets of genes function in gametic fusion. Angiosperms have come long way from primitive reproductive structure development to today's sophisticated, diverse, and in most cases flamboyant organ. In this review, we will be discussing on the intricate yet complex molecular mechanism of double fertilization and how it might have been shaped by the evolutionary forces focusing particularly on the model plant Arabidopsis.

Obtaining Mutant Pollen for Phenotypic Analysis and Pollen Tube Dual Staining

Mutant phenotype observation is the most useful and important method to study which biological process a gene-of-interest is involved in. In flowering plants, excessive pollen grains land and germinate on the stigma, then pollen tubes grow through the transmitting tract to reach the ovules, eventually enter the micropyle to complete double fertilization. First, for mutants whose homozygotes could not be obtained due to pollen tube defects, it is difficult to observe the defect phenotype since the pollen grains of different genotypes are mixed together. Here, we provide a detailed protocol to pick out mutant pollen grains from the heterozygous mutant plants in Arabidopsis thaliana. By using this method, we could obtain sufficient mutant pollen grains for phenotypic analysis. Second, it is difficult to compare the pollen/pollen tube behavior of two different genotypes/species in vivo in a same pistil. Here, we develop a new dual staining method which combines GUS staining with aniline blue staining. By using this method, we can analyze the competence of the two different pollen tubes in the same pistil.

Cysteine-rich peptides promote interspecific genetic isolation in Arabidopsis

Reproductive isolation is a prerequisite for speciation. Failure of communication between female tissues of the pistil and paternal pollen tubes imposes hybridization barriers in flowering plants. Arabidopsis thaliana LURE1 (AtLURE1) peptides and their male receptor PRK6 aid attraction of the growing pollen tube to the ovule. Here, we report that the knockout of the entire AtLURE1 gene family did not affect fertility, indicating that AtLURE1-PRK6-mediated signaling is not required for successful fertilization within one Arabidopsis species. AtLURE1s instead function as pollen tube emergence accelerators that favor conspecific pollen over pollen from other species and thus promote reproductive isolation. We also identified maternal peptides XIUQIU1 to -4, which attract pollen tubes regardless of species. Cooperation between ovule attraction and pollen tube growth acceleration favors conspecific fertilization and promotes reproductive isolation.

To preserve or to destroy, that is the question: the role of the cell wall integrity pathway in pollen tube growth

In plants, cell-shape is defined by the cell wall, a complex network of polymers located outside the plasma membrane. During cell growth, cell wall properties have to be adjusted, assuring cell expansion without compromising cell integrity. Plasma membrane-located receptors sense cell wall properties, transducing extracellular signals into intracellular cascades through the cell wall integrity (CWI) pathway that, in turn, leads to adjustments in the regulation and composition of the cell wall. Using pollen tube growth as a single celled model system, we describe the importance of RAPID ALKALINIZATION FACTOR (RALF) peptides as sensors of cell wall integrity. RALF peptides can mediate the communication between cell wall components and plasma membrane-localized receptor-like kinases (RLKs) of the CrRLK1L family. The subsequent activation of intracellular pathways regulates H⁺, Ca²⁺, and ROS levels in the cell and apoplast, thereby modulating cell wall integrity. Interestingly, the RALF-CrRLK1L module and some of the components working up- and downstream of the RLK is conserved in many other developmental and physiological signaling processes.

Peptide/receptor-like kinase-mediated signaling involved in male-female interactions

In flowering plants, extensive male-female interactions during pollen germination on the stigma, pollen tube growth and guidance in the transmitting tract, and pollen tube reception by the female gametophyte are required for successful double fertilization in which various signaling cascades are involved. Peptide/receptor-like kinase-mediated signaling has been found playing important roles in these male-female interactions. Here, we mainly summarized the progress made on the regulatory roles of peptide/receptor-like kinase-mediated signaling pathways in four critical stages during reproduction in higher plants.

Arabidopsis BIG1 and BIG5 are crucial for male gametophyte transmission

Arabidopsis contains five Brefeldin A-inhibited guanine nucleotide exchange factors (BIGs), which play a critical role in vesicle biogenesis for protein traffic from the Golgi to the plasma membrane. Biological processes regulated by BIG1-BIG4 are postulated to be distinct from those by BIG5. However, we show that the self-pollinated BIG1^+/- big5 silique do not produce homozygous seeds, and some pollen tubes from BIG1^+/- big5 anthers grew slowly in vitro and failed to target nearby ovules in vivo. We identified the big1 big5 homozygote from the progeny of BIG1^+/- big5 plants transformed with BIG5, whose expression is driven by a pollen-specific promoter pLat52, indicating that male gametophyte transmission is blocked in the double mutant. Confocal microscopy indicated that BIG1 and BIG5 are co-localized in trans Golgi network. Thus, our data indicate that BIG1 and BIG5 are crucial for male gametophyte transmission.

CoNekT: an open-source framework for comparative genomic and transcriptomic network analyses

The recent accumulation of gene expression data in the form of RNA sequencing creates unprecedented opportunities to study gene regulation and function. Furthermore, comparative analysis of the expression data from multiple species can elucidate which functional gene modules are conserved across species, allowing the study of the evolution of these modules. However, performing such comparative analyses on raw data is not feasible for many biologists. Here, we present CoNekT (Co-expression Network Toolkit), an open source web server, that contains user-friendly tools and interactive visualizations for comparative analyses of gene expression data and co-expression networks. These tools allow analysis and cross-species comparison of (i) gene expression profiles; (ii) co-expression networks; (iii) co-expressed clusters involved in specific biological processes; (iv) tissue-specific gene expression; and (v) expression profiles of gene families. To demonstrate these features, we constructed CoNekT-Plants for green alga, seed plants and flowering plants (Picea abies, Chlamydomonas reinhardtii, Vitis vinifera, Arabidopsis thaliana, Oryza sativa, Zea mays and Solanum lycopersicum) and thus provide a web-tool with the broadest available collection of plant phyla. CoNekT-Plants is freely available from http://conekt.plant.tools, while the CoNekT source code and documentation can be found at https://github.molgen.mpg.de/proost/CoNekT/.

LMM24 Encodes Receptor-Like Cytoplasmic Kinase 109, Which Regulates Cell Death and Defense Responses in Rice

Lesion mimic mutants are excellent models for research on molecular mechanisms of cell death and defense responses in rice. We identified a new rice lesion mimic mutant lmm24 from a mutant pool of indica rice cultivar “ZhongHui8015”. The LMM24 gene was identified by MutMap, and LMM24 was confirmed as a receptor-like cytoplasmic kinase 109 by amino acid sequence analysis. The lmm24 mutant displayed dark brown lesions in leaves and growth retardation that were not observed in wild-type ZH8015. The results of histochemical staining and TUNEL assays showed enhanced ROS accumulation and cell death in lmm24. Chloroplast degradation was observed in lmm24 leaves, with decreased expression of photosynthesis-related genes and increased expression of the senescence-induced STAYGREEN (SGR) gene and other senescence-associated genes. Furthermore, lmm24 exhibited enhanced resistance to rice blast fungus Magnaporthe oryzae (M. oryzae) and up-regulation of defense response genes. Our data demonstrate that LMM24 regulates cell death and defense responses in rice.

SERK Receptor-like Kinases Control Division Patterns of Vascular Precursors and Ground Tissue Stem Cells during Embryo Development in Arabidopsis

During embryo development, the vascular precursors and ground tissue stem cells divide to renew themselves and produce the vascular tissue, endodermal cells, and cortical cells. However, the molecular mechanisms regulating division of these stem cells have remained largely elusive. In this study, we show that loss of function of SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) genes results in aberrant embryo development. Fewer cortical, endodermal, and vascular cells are generated in the embryos of serk1 serk2 bak1 triple mutants. WUSCHEL-RELATED HOMEOBOX 5 (WOX5) is ectopically expressed in vascular cells of serk1 serk2 bak1 embryos. The first transverse division of vascular precursors in mid-globular embryos and second asymmetric division of ground tissue stem cells in early-heart embryos are abnormally altered to a longitudinal division. The embryo defects can be partially rescued by constitutively activated mitogen-activated protein kinase (MAPK) kinase kinase YODA (YDA) and MAPK kinase MKK5. Taken together, our results reveal that SERK-mediated signals regulate division patterns of vascular precursors and ground tissue stem cells, likely via the YDA-MKK4/5 cascade, during embryo development.

Insights into secrets along the pollen tube pathway in need to be discovered

The process of plant fertilization provides an outstanding example of refined control of gene expression. During this elegant process, subtle communication occurs between neighboring cells, based on chemical signals, that induces cellular mechanisms of patterning and growth. Having faced an immediate issue of self-incompatibility responses, the pathway to fertilization starts once the stigmatic cells recognize a compatible pollen grain, and it continues with numerous players interacting to affect pollen tube growth and the puzzling process of navigation along the transmitting tract. The pollen tube goes through a guidance process that begins with a preovular stage (i.e. prior to the influence of the target ovule), with interactions with factors from the transmitting tissue. In the subsequent ovular-guidance stage a specific relationship develops between the pollen tube and its target ovule. This stage is divided into the funicular and micropylar guidance steps, with numerous receptors working in signalling cascades. Finally, just after the pollen tube has passed beyond the synergids, fusion of the gametes occurs and the developing seed-the ultimate aim of the process-will start to mature. In this paper, we review the existing knowledge of the crucial biological processes involved in pollen-pistil interactions that give rise to the new seed.

A Fruitful Journey: Pollen Tube Navigation from Germination to Fertilization

In flowering plants, pollen tubes undergo tip growth to deliver two nonmotile sperm to the ovule where they fuse with an egg and central cell to achieve double fertilization. This extended journey involves rapid growth and changes in gene activity that manage compatible interactions with at least seven different cell types. Nearly half of the genome is expressed in haploid pollen, which facilitates genetic analysis, even of essential genes. These unique attributes make pollen an ideal system with which to study plant cell-cell interactions, tip growth, cell migration, the modulation of cell wall integrity, and gene expression networks. We highlight the signaling systems required for pollen tube navigation and the potential roles of Ca²⁺ signals. The dynamics of pollen development make sexual reproduction highly sensitive to heat stress. Understanding this vulnerability may generate strategies to improve seed crop yields that are under threat from climate change.

Pollen tube integrity regulation in flowering plants: insights from molecular assemblies on the pollen tube surface

Contents Summary 687 I. Introduction 687 II. Pollen tube membrane-localized receptors coordinate cell integrity and sperm release 689 III. RALF peptides mediate autocrine and paracrine signaling 689 IV. ROS and ion channel signaling mediate intracellular response 690 V. Involvements from pollen tube cell wall components 690 VI. Concluding remarks 691 Acknowledgements 692 Author contributions 692 References 692 SUMMARY: Unlike in animals, sperm in flowering plants are immotile and they are embraced as passive cargoes by a pollen tube which embarks on a long journey in the pistil to deliver them to the female gametophyte for fertilization. How the pollen tube switches from a rapid polarized growth towards its target to an abrupt disintegration for sperm cell release inside the female gametophyte is puzzling. Recent studies have shown that members of the Catharanthus roseus RLK1-like (CrRLK1L) receptor kinase family and their ligands, 5-kDa cysteine-rich peptide rapid alkalinization factors (RALFs), engage in an intricate balancing act involving autocrine and paracrine signaling to maintain pollen tube growth and induce timely tube rupture at the spatially confined pollen tube-female gametophyte interface. Here, we review recent progress related to pollen tube integrity control, mainly focusing on the molecular understanding of signaling as well as intracellular signaling nodes in Arabidopsis. Some missing links and future perspectives are also discussed.

Friend or foe: Signaling mechanisms during double fertilization in flowering seed plants

Since the first description of double fertilization 120 years ago, the processes of pollen tube growth and guidance, sperm cell release inside the receptive synergid cell, as well as fusion of two sperm cells to the female gametes (egg and central cell) have been well documented in many flowering plants. Especially microscopic techniques, including live cell imaging, were used to visualize these processes. Molecular as well as genetic methods were applied to identify key players involved. However, compared to the first 11 decades since its discovery, the past decade has seen a tremendous advancement in our understanding of the molecular mechanisms regulating angiosperm fertilization. Whole signaling networks were elucidated including secreted ligands, corresponding receptors, intracellular interaction partners, and further downstream signaling events involved in the cross-talk between pollen tubes and their cargo with female reproductive cells. Biochemical and structural biological approaches are now increasingly contributing to our understanding of the different signaling processes required to distinguish between compatible and incompatible interaction partners. Here, we review the current knowledge about signaling mechanisms during above processes with a focus on the model plants Arabidopsis thaliana and Zea mays (maize). The analogy that many of the identified "reproductive signaling mechanisms" also act partly or fully in defense responses and/or cell death is also discussed.

The Long Journey of Pollen Tube in the Pistil

In non-cleistogamous plants, the male gametophyte, the pollen grain is immotile and exploits various agents, such as pollinators, wind, and even water, to arrive to a receptive stigma. The complex process of pollination involves a tubular structure, i.e., the pollen tube, which delivers the two sperm cells to the female gametophyte to enable double fertilization. The pollen tube has to penetrate the stigma, grow in the style tissues, pass through the septum, grow along the funiculus, and navigate to the micropyle of the ovule. It is a long journey for the pollen tube and its two sperm cells before they meet the female gametophyte, and it requires very accurate regulation to perform successful fertilization. In this review, we update the knowledge of molecular dialogues of pollen-pistil interaction, especially the progress of pollen tube activation and guidance, and give perspectives for future research.

A plant Bro1 domain protein BRAF regulates multivesicular body biogenesis and membrane protein homeostasis

Plant development, defense, and many physiological processes rely on the endosomal sorting complex required for transport (ESCRT) machinery to control the homeostasis of membrane proteins by selective vacuolar degradation. Although ESCRT core components are conserved among higher eukaryotes, the regulators that control the function of the ESCRT machinery remain elusive. We recently identified a plant-specific ESCRT component, FREE1, that is essential for multivesicular body/prevacuolar compartment (MVB/PVC) biogenesis and vacuolar sorting of membrane proteins. Here we identify a plant-specific Bro1-domain protein BRAF, which regulates FREE1 recruitment to the MVB/PVC membrane by competitively binding to the ESCRT-I component Vps23. Altogether, we have successfully identified a role for BRAF, whose function as a unique evolutionary ESCRT regulator in orchestrating intraluminal vesicle formation in MVB/PVCs and the sorting of membrane proteins for degradation in plants makes it an important regulatory mechanism underlying the ESCRT machinery in higher eukaryotes.

ZmSTK1 and ZmSTK2, encoding receptor-like cytoplasmic kinase, are involved in maize pollen development with additive effect

Pollen germination and pollen tube growth are important physiological processes of sexual reproduction of plants and also are involved in signal transduction. Our previous study reveals that ZmSTK1 and ZmSTK2 are two receptor-like cytoplasmic kinases (RLCK) homologs in Zea mays as members of receptor-like protein kinase (RLK) subfamily, sharing 86% identity at the amino acid level. Here, we report that ZmSTK1 and ZmSTK2, expressed at late stages of pollen development, regulate maize pollen development with additive effect. ZmSTK1 or ZmSTK2 mutation exhibited severe pollen transmission deficiency, which thus influenced pollen fertility. Moreover, the kinase domains of ZmSTKs were cross-interacted with C-terminus of enolases detected by co-immunoprecipitation (Co-IP) and yeast two-hybrid system (Y2H), respectively. Further, the detective ZmSTK1 or ZmSTK2 was associated with decreased activity of enolases and also reduced downstream metabolite contents, which enolases are involved in glycolytic pathway, such as phosphoenolpyruvate (PEP), pyruvate, ADP/ATP, starch, glucose, sucrose and fructose. This study reveals that ZmSTK1 and ZmSTK2 regulate maize pollen development and indirectly participate in glycolytic pathway.

CORNICHON sorting and regulation of GLR channels underlie pollen tube Ca2+ homeostasis

Compared to animals, evolution of plant calcium (Ca²⁺) physiology has led to a loss of proteins for influx and small ligand-operated control of cytosolic Ca²⁺, leaving many Ca²⁺ mechanisms unaccounted for. Here, we show a mechanism for sorting and activation of glutamate receptor-like channels (GLRs) by CORNICHON HOMOLOG (CNIH) proteins. Single mutants of pollen-expressed Arabidopsis thaliana GLRs (AtGLRs) showed growth and Ca²⁺ flux phenotypes expected for plasma membrane Ca²⁺ channels. However, higher-order mutants of AtGLR3.3 revealed phenotypes contradicting this assumption. These discrepancies could be explained by subcellular AtGLR localization, and we explored the implication of AtCNIHs in this sorting. We found that AtGLRs interact with AtCNIH pairs, yielding specific intracellular localizations. AtCNIHs further trigger AtGLR activity in mammalian cells without any ligand. These results reveal a regulatory mechanism underlying Ca²⁺ homeostasis by sorting and activation of AtGLRs by AtCNIHs.

N-myristoylation and S-acylation are common modifications of Ca2+ -regulated Arabidopsis kinases and are required for activation of the SLAC1 anion channel

N-myristoylation and S-acylation promote protein membrane association, allowing regulation of membrane proteins. However, how widespread this targeting mechanism is in plant signaling processes remains unknown. Through bioinformatics analyses, we determined that among plant protein kinase families, the occurrence of motifs indicative for dual lipidation by N-myristoylation and S-acylation is restricted to only five kinase families, including the Ca2+ -regulated CDPK-SnRK and CBL protein families. We demonstrated N-myristoylation of CDPK-SnRKs and CBLs by incorporation of radiolabeled myristic acid. We focused on CPK6 and CBL5 as model cases and examined the impact of dual lipidation on their function by fluorescence microscopy, electrophysiology and functional complementation of Arabidopsis mutants. We found that both lipid modifications were required for proper targeting of CBL5 and CPK6 to the plasma membrane. Moreover, we identified CBL5-CIPK11 complexes as phosphorylating and activating the guard cell anion channel SLAC1. SLAC1 activation by CPK6 or CBL5-CIPK11 was strictly dependent on dual lipid modification, and loss of CPK6 lipid modification prevented functional complementation of cpk3 cpk6 guard cell mutant phenotypes. Our findings establish the general importance of dual lipid modification for Ca2+ signaling processes, and demonstrate their requirement for guard cell anion channel regulation.

Receptor-Like Cytoplasmic Kinases: Central Players in Plant Receptor Kinase-Mediated Signaling

Receptor kinases (RKs) are of paramount importance in transmembrane signaling that governs plant reproduction, growth, development, and adaptation to diverse environmental conditions. Receptor-like cytoplasmic kinases (RLCKs), which lack extracellular ligand-binding domains, have emerged as a major class of signaling proteins that regulate plant cellular activities in response to biotic/abiotic stresses and endogenous extracellular signaling molecules. By associating with immune RKs, RLCKs regulate multiple downstream signaling nodes to orchestrate a complex array of defense responses against microbial pathogens. RLCKs also associate with RKs that perceive brassinosteroids and signaling peptides to coordinate growth, pollen tube guidance, embryonic and stomatal patterning, floral organ abscission, and abiotic stress responses. The activity and stability of RLCKs are dynamically regulated not only by RKs but also by other RLCK-associated proteins. Analyses of RLCK-associated components and substrates have suggested phosphorylation relays as a major mechanism underlying RK-mediated signaling.