Two Mitogen-Activated Protein Kinases, MPK3 and MPK6, Are Required for Funicular Guidance of Pollen Tubes in Arabidopsis

The TRAPPC8/TRS85 subunit of the Arabidopsis TRAPPIII tethering complex regulates endoplasmic reticulum function and autophagy

Transport protein particle (TRAPP) tethering complexes are known for their function as Rab GTPase exchange factors. Two versions of the complex are considered functionally separate: TRAPPII, an activator of the Rab11 family (RabA in plants) GTPases that function in post-Golgi sorting, and TRAPPIII, activating Rab1 family (RabD in plants) members that regulate endoplasmic reticulum (ER)-to-Golgi trafficking and autophagy. In Arabidopsis (Arabidopsis thaliana), the TRAPPIII complex has been identified and its subunit composition established, but little is known about its functions. Here, we found that binary subunit interactions of the plant TRAPPIII complex are analogous to those of metazoan TRAPPIII, with the 2 large subunits TRAPPC8 and TRAPPC11 linking the TRAPP core and the small C12 to C13 dimer. To gain insight into the functions of TRAPPIII in plants, we characterized 2 A. thaliana trappc8 mutants. These mutants display abnormalities in plant morphology, particularly in flower and seed development. They also exhibit autophagic defects, a constitutive ER stress response, and elevated levels of the ER lipid dolichol (Dol), which is an indispensable cofactor in protein glycosylation. These results indicate that plant TRAPPC8 is involved in multiple cellular trafficking events and suggest a link between ER stress responses and Dol levels.

Getting attached to membranes-How plant signaling networks employ PtdIns(4,5)P2

In eukaryotes, a small subset of membrane lipids, the phosphoinositides (PIs), exert regulatory effects on membrane-associated processes with profound impact on the organism, and PIs are relevant also for the physiology and development of plants. The PI, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) has emerged as an important regulatory player in plants, and in recent years this lipid has received substantial attention. This Update Review focuses on our current understanding of how PtdIns(4,5)P2 exerts its regulatory functions, how biosynthesis and degradation of this important regulatory lipid are controlled, and how PtdIns(4,5)P2 is linked to upstream and downstream elements within plant signalling networks.

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.

Genome-wide identification and analysis of abiotic stress responsiveness of the mitogen-activated protein kinase gene family in Medicago sativa L

BACKGROUND

The mitogen-activated protein kinase (MAPK) cascade is crucial cell signal transduction mechanism that plays an important role in plant growth and development, metabolism, and stress responses. The MAPK cascade includes three protein kinases, MAPK, MAPKK, and MAPKKK. The three protein kinases mediate signaling to downstream response molecules by sequential phosphorylation. The MAPK gene family has been identified and analyzed in many plants, however it has not been investigated in alfalfa.

RESULTS

In this study, Medicago sativa MAPK genes (referred to as MsMAPKs) were identified in the tetraploid alfalfa genome. Eighty MsMAPKs were divided into four groups, with eight in group A, 21 in group B, 21 in group C and 30 in group D. Analysis of the basic structures of the MsMAPKs revealed presence of a conserved TXY motif. Groups A, B and C contained a TEY motif, while group D contained a TDY motif. RNA-seq analysis revealed tissue-specificity of two MsMAPKs and tissue-wide expression of 35 MsMAPKs. Further analysis identified MsMAPK members responsive to drought, salt, and cold stress conditions. Two MsMAPKs (MsMAPK70 and MsMAPK75) responds to salt and cold stresses; two MsMAPKs (MsMAPK60 and MsMAPK73) responds to cold and drought stresses; four MsMAPKs (MsMAPK1, MsMAPK33, MsMAPK64 and MsMAPK71) responds to salt and drought stresses; and two MsMAPKs (MsMAPK5 and MsMAPK7) responded to all three stresses.

CONCLUSION

This study comprehensively identified and analysed the alfalfa MAPK gene family. Candidate genes related to abiotic stresses were screened by analysing the RNA-seq data. The results provide key information for further analysis of alfalfa MAPK gene functions and improvement of stress tolerance.

Comparative transcriptome analysis reveals the impact of daily temperature difference on male sterility in photo-thermo-sensitive male sterile wheat

BACKGROUND

Photo-thermo-sensitive male sterility (PTMS), which refers to the male sterility triggered by variations in photoperiod and temperature, is a crucial element in the wheat two-line hybrid system. The development of safe production and efficient propagation for male sterile lines holds utmost importance in two-line hybrid wheat. Under the stable photoperiod condition, PTMS is mainly induced by high or low temperatures in wheat, but the effect of daily temperature difference (DTD) on the fertility conversion of PTMS lines has not been reported. Here, three BS type PTMS lines including BS108, BS138, and BS366, as well as a control wheat variety J411 were used to analyze the correlation between fertility and DTD using differentially sowing tests, photo-thermo-control experiments, and transcriptome sequencing.

RESULTS

The differentially sowing tests suggested that the optimal sowing time for safe seed production of the three PTMS lines was from October 5th to 25th in Dengzhou, China. Under the condition of 12 h 12 °C, the PTMS lines were greatly affected by DTD and exhibited complete male sterility at a temperature difference of 15 °C. Furthermore, under different temperature difference conditions, a total of 20,677 differentially expressed genes (DEGs) were obtained using RNA sequencing. Moreover, through weighted gene co-expression network analysis (WGCNA) and KEGG enrichment analysis, the identified DEGs had a close association with "starch and sucrose metabolism", "phenylpropanoid biosynthesis", "MAPK signaling pathway-plant", "flavonoid biosynthesis", and "cutin, and suberine and wax biosynthesis". qRT-PCR analysis showed the expression levels of core genes related to KEGG pathways significantly decreased at a temperature difference of 15 ° C. Finally, we constructed a transcriptome mediated network of temperature difference affecting male sterility.

CONCLUSIONS

The findings provide important theoretical insights into the correlation between temperature difference and male sterility, providing guidance for the identification and selection of more secure and effective PTMS lines.

Sporophytic control of tapetal development and pollen fertility by a mitogen-activated protein kinase cascade in rice

Tapetum, the innermost layer of the anther wall, provides essential nutrients and materials for pollen development. Timely degradation of anther tapetal cells is a prerequisite for normal pollen development in flowering plants. Tapetal cells facilitate male gametogenesis by providing cellular contents after highly coordinated programmed cell death (PCD). Tapetal development is regulated by a transcriptional network. However, the signaling pathway(s) involved in this process are poorly understood. In this study, we report that a mitogen-activated protein kinase (MAPK) cascade composed of OsYDA1/OsYDA2-OsMKK4-OsMPK6 plays an important role in tapetal development and male gametophyte fertility. Loss of function of this MAPK cascade leads to anther indehiscence, enlarged tapetum, and aborted pollen grains. Tapetal cells in osmkk4 and osmpk6 mutants exhibit an increased presence of lipid body-like structures within the cytoplasm, which is accompanied by a delayed occurrence of PCD. Expression of a constitutively active version of OsMPK6 (CA-OsMPK6) can rescue the pollen defects in osmkk4 mutants, confirming that OsMPK6 functions downstream of OsMKK4 in this pathway. Genetic crosses also demonstrated that the MAPK cascade sporophyticly regulates pollen development. Our study reveals a novel function of rice MAPK cascade in plant male reproductive biology.

Evidence for dual targeting control of Arabidopsis 6-phosphogluconate dehydrogenase isoforms by N-terminal phosphorylation

The oxidative pentose-phosphate pathway (OPPP) retrieves NADPH from glucose-6-phosphate, which is important in chloroplasts at night and in plastids of heterotrophic tissues. We previously studied how OPPP enzymes may transiently locate to peroxisomes, but how this is achieved for the third enzyme remained unclear. By extending our genetic approach, we demonstrated that Arabidopsis isoform 6-phosphogluconate dehydrogenase 2 (PGD2) is indispensable in peroxisomes during fertilization, and investigated why all PGD-reporter fusions show a mostly cytosolic pattern. A previously published interaction of a plant PGD with thioredoxin m was confirmed using Trxm2 for yeast two-hybrid (Y2H) and bimolecular fluorescent complementation (BiFC) assays, and medial reporter fusions (with both ends accessible) proved to be beneficial for studying peroxisomal targeting of PGD2. Of special importance were phosphomimetic changes at Thr6, resulting in a clear targeting switch to peroxisomes, while a similar change at position Ser7 in PGD1 conferred plastid import. Apparently, efficient subcellular localization can be achieved by activating an unknown kinase, either early after or during translation. N-terminal phosphorylation of PGD2 interfered with dimerization in the cytosol, thus allowing accessibility of the C-terminal peroxisomal targeting signal (PTS1). Notably, we identified amino acid positions that are conserved among plant PGD homologues, with PTS1 motifs first appearing in ferns, suggesting a functional link to fertilization during the evolution of seed plants.

MAPK20-mediated ATG6 phosphorylation is critical for pollen development in Solanum lycopersicum L

In flowering plants, male gametogenesis is tightly regulated by numerous genes. Mitogen-activated protein kinase (MAPK) plays a critical role in plant development and stress response, while its role in plant reproductive development is largely unclear. The present study demonstrated MAPK20 phosphorylation of ATG6 to mediate pollen development and germination in tomato (Solanum lycopersicum L.). MAPK20 was preferentially expressed in the stamen of tomato, and mutation of MAPK20 resulted in abnormal pollen grains and inhibited pollen viability and germination. MAPK20 interaction with ATG6 mediated the formation of autophagosomes. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed that ATG6 was phosphorylated by MAPK20 at Ser-265. Mutation of ATG6 in wild-type (WT) or in MAPK20 overexpression plants resulted in malformed and inviable pollens. Meanwhile, the number of autophagosomes in mapk20 and atg6 mutants was significantly lower than that of WT plants. Our results suggest that MAPK20-mediated ATG6 phosphorylation and autophagosome formation are critical for pollen development and germination.

Identification and expression analysis of MAPK cascade gene family in foxtail millet (Setaria italica)

The mitogen-activated protein kinase (MAPK) cascade pathway is a highly conserved plant cell signaling pathway that plays an important role in plant growth and development and stress response. Currently, MAPK cascade genes have been identified and reported in a variety of plants including Arabidopsis thaliana, Oryza sativa, and Triticum aestivum, but have not been identified in foxtail millet (Setaria italica). In this study, a total of 93 MAPK cascade genes, including 15 SiMAPKs, 10 SiMAPKKs and 68 SiMAPKKKs genes, were identified by genome-wide analysis of foxtail millet, and these genes were distributed on nine chromosomes of foxtail millet. Using phylogenetic analysis, we divided the SiMAPKs and SiMAPKKs into four subgroups, respectively, and the SiMAPKKKs into three subgroups (Raf, ZIK, and MEKK). Whole-genome duplication analysis revealed that there are 14 duplication pairs in the MAPK cascade family in foxtail millet, and they are expanded by segmental replication events. Results from quantitative real-time PCR (qRT-PCR) revealed that the expression levels of most SiMAPKs and SiMAPKKs were changed under both exogenous hormone and abiotic stress treatments, with SiMAPK3 and SiMAPKK4-2 being induced under almost all treatments, while the expression of SiMAPKK5 was repressed. In a nutshell, this study will shed some light on the evolution of MAPK cascade genes and the functional mechanisms underlying MAPK cascade genes in response to hormonal and abiotic stress signaling pathways in foxtail millet (Setaria italica).

Rice Mitogen-Activated Protein Kinase regulates serotonin accumulation and interacts with cell cycle regulators under prolonged UV-B exposure

Stress conditions such as UV-B exposure activates MAPKs in Arabidopsis and rice. UV-B radiation is hazardous to plant as it causes photosystem disruption, DNA damage and ROS generation. Here we report its effect on biological pathways by studying the global changes in transcript profile in rice seedling exposed to UV-B radiation for 1 h and 16 h. Short UV-B exposure (1 h) led to moderate changes, while a drastic change in transcript landscape was observed after long term UV-B exposure (16 h) in rice seedlings. Prolonged UV-B exposure negatively impacts the expression of cell cycle regulating genes and several other metabolic pathways in developing seedlings. MAP kinase signaling cascade gets activated upon UV-B exposure similar to reports in Arabidopsis indicating conservation of its function in both dicot and monocot. Expression analysis in inducible overexpression transgenic lines of MPK3 and MPK6 shows higher transcript abundance of phytoalexin biosynthesis gene like Oryzalexin D synthase and Momilactone A synthase, along with serotonin biosynthesis genes. An accumulation of serotonin was observed upon UV-B exposure and its abundance positively correlates with the MPK3 and MPK6 transcript level in the respective over-expression lines. Interestingly, multiple cell cycle inhibitor proteins including WEE1 and SMR1 interact with MPK3 and MPK6 thus, implying a major role of this pathway in cell cycle regulation under stress condition. Overall overexpression of MPK3 and MPK6 found to be detrimental for rice as overexpression lines shows higher cell death and compromised tolerance to UV-B.

Transcriptional regulations of pollen tube reception are associated with the fertility of the ginger species Zingiber zerumbet and Zingiber corallinum

Zingiber zerumbet and Zingiber corallinum are economically valuable species in the genus Zingiber. While Z. corallinum is sexually active, Z. zerumbet adopts clonal propagation, although it has the potential for sexual reproduction. It is unclear so far at which step during the sexual reproduction of Z. zerumbet inhibition occurs, and what are the regulatory mechanisms underlying this inhibition. Here, by comparing with the fertile species Z. corallinum using microscopy-based methods, we show that rare differences were observed in Z. zerumbet up to the point when the pollen tubes invaded the ovules. However, a significantly higher percentage of ovules still contained intact pollen tubes 24 h after pollination, suggesting pollen tube rupture was impaired in this species. Further RNA-seq analysis generated accordant results, showing that the transcription of ANX and FER, as well as genes for the partners in the same complexes (e.g., BUPS and LRE, respectively), and those putative peptide signals (e.g., RALF34), were timely activated in Z. corallinum, which ensured the pollen tubes being able to grow, reorient to ovules, and receipt by embryo sacs. In Z. zerumbet, genes for these complexes were cooperatively suppressed, which would result in the maintenance of PT integrity due to the disruption of RALF34-ANX/BUPS signaling in PT and the failure of PT reception by an active synergid due to the insufficiency of the synergid-harbored FER/LRE complex. Taking the results from the cytological and RNA-seq studies together, a model is proposed to illustrate the possible regulation mechanisms in Z. zerumbet and Z. corallinum, in which the regulations for pollen tube rupture and reception are proposed as the barrier for sexual reproduction in Z. zerumbet.

Revisiting the role of MAPK signalling pathway in plants and its manipulation for crop improvement

The mitogen-activated protein kinase (MAPK) pathway is an important signalling event associated with every aspect of plant growth, development, yield, abiotic and biotic stress adaptation. Being a central metabolic pathway, it is a vital target for manipulation for crop improvement. In this review, we have summarised recent advancements in understanding involvement of MAPK signalling in modulating abiotic and biotic stress tolerance, architecture and yield of plants. MAPK signalling cross talks with reactive oxygen species (ROS) and abscisic acid (ABA) signalling events in bringing about abiotic stress adaptation in plants. The intricate involvement of MAPK pathway with plant's pathogen defence ability has also been identified. Further, recent research findings point towards participation of MAPK signalling in shaping plant architecture and yield. These make MAPK pathway an important target for crop improvement and we discuss here various strategies to tweak MAPK signalling components for designing future crops with improved physiology and phenotypes.

Gene co-expression modules behind the three-pistil formation in wheat

Multi-pistil trait in wheat is of great potential value in plant development research and crop breeding. Our previous studies identified the Pis1 locus that causes three pistils in wheat by genetic mapping using multiple DNA marker systems. However, there are still 26 candidate genes on the locus, and the causal gene remains to be found. In this study, we aimed to approach the molecular mechanism of multi-pistil formation. Comparative RNA sequencing (RNA-Seq) during the pistil formation was undertaken in four wheat lines: a three-pistil mutant TP, a single-pistil TILLING mutant of TP (SP), a three-pistil near-isogenic line CM28TP with the background of cultivar Chunmai 28 (CM28), and CM28. Electron microscopic analysis specified probable developmental stages of young spikes for the three-pistil formation. mRNA sequencing in the young spikes of the four lines represented 253 down-regulated genes and 98 up-regulated genes in both three-pistil lines, which included six potential genes for ovary development. Weighted gene co-expression analysis represented three-pistil trait-associated transcription factor-like genes, among which one hub gene, ARF5, was the most highlighted. ARF5 is on the Pis1 locus and an orthologue of MONOPTEROS which mediates tissue development in Arabidopsis. qRT-PCR validation implies that the deficiency of ARF5 underlies the three-pistil formation in wheat.

The female germ unit is essential for pollen tube funicular guidance in Arabidopsis thaliana

In angiosperm, two immotile sperm cells are delivered to the female gametes for fertilization by a pollen tube, which perceives guidance cues from ovules at least at two critical sites, micropyle for short-distance guidance and funiculus for comparably longer distance guidance. Compared with the great progress in understanding pollen tube micropylar guidance, little is known about the signaling for funicular guidance. Here, we show that funiculus plays an important role in pollen tube guidance and report that female gametophyte (FG) plays a critical role in funicular guidance by analysis of a 3-dehydroquinate synthase (DHQS) mutant. Loss function of DHQS in FG interrupts pollen tube funicular guidance, suggesting that the guiding signal is generated from FG. We show the evidence that the capacity of funicular guidance is established during FG functional specification after the establishment of cell identity. Specific expression of DHQS in the synergid cells, central cells, or egg cells can rescue funicular guidance defect in dhqs/+, indicating all the female germ unit cells are involved in the funicular guidance. The finding reveals that the attracting signal of pollen tube funicular guidance was generated at a site and stage manner and provides novel clue to locate and search for the signal.

Quantitative Phosphoproteomic Analysis Reveals Potential Regulatory Mechanisms of Early Fruit Enlargement in Pecan (Carya illinoinensis)

Pecan (Carya illinoinensis) is a popular tree nut. Its fruit development undergoes slow growth, rapid expansion, core hardening, and kernel maturation stages. However, little is known about how pecan initiates fruit development and enlargement after pollination. In this study, we performed the first large-scale identification of potential phosphorylation sites and proteins at early development of pecan fruit by a label-free phosphoproteomic quantification technique. A total of 2155 phosphosites were identified from 1953 phosphopeptides covering 1311 phosphoproteins in unpollinated pistils and fruits at 5 and 9 weeks after pollination. Of these, 699 nonredundant phosphoproteins were differentially phosphorylated (DP). Furthermore, the phosphorylation intensity of DP proteins in brassinolide (BR) and auxin signaling were analyzed, and the function of CiBZR1 was investigated. Ectopic expression of CiBZR1 resulted in BR response phenotypes with curled leaves and fruit, while enlarged seed size in Arabidopsis. Subcellular localization and transcriptional activation activity assay demonstrated that CiBZR1 distributed in both the nucleus and cytoplasm with transcriptional activity. When two phosphosites mutated, CiBZR1^S201P,S205G moved to the nucleus completely, while the transcriptional activity remained unchanged. Taken together, our data reveal extensive phosphoproteins and lay a foundation to comprehensively dissect the potential post-translational regulation mechanism of early development of pecan fruit.

Genetic and evolutionary dissection of melatonin response signaling facilitates the regulation of plant growth and stress responses

The expansion of gene families during evolution could generate functional diversity among their members to regulate plant growth and development. Melatonin, a phylogenetically ancient molecule, is vital for many aspects of a plant's life. Understanding the functional diversity of the molecular players involved in melatonin biosynthesis, signaling, and metabolism will facilitate the regulation of plant phenotypes. However, the molecular mechanism of melatonin response signaling elements in regulating this network still has many challenges. Here, we provide an in-depth analysis of the functional diversity and evolution of molecular components in melatonin signaling pathway. Genetic analysis of multiple mutants in plant species will shed light on the role of gene families in melatonin regulatory pathways. Phylogenetic analysis of these genes was performed, which will facilitate the identification of melatonin-related genes for future study. Based on the abovementioned signal networks, the mechanism of these genes was summarized to provide reference for studying the regulatory mechanism of melatonin in plant phenotypes. We hope that this work will facilitate melatonin research in higher plants and finely tuned spatio-temporal regulation of melatonin signaling.

Genome-Wide Characterization of the Mitogen-Activated Protein Kinase Gene Family and Their Expression Patterns in Response to Drought and Colletotrichum Gloeosporioides in Walnut (Juglans regia)

Mitogen-activated protein kinases (MAPKs) are a family of Ser/Thr (serine/threonine) protein kinases that play very important roles in plant responses to biotic and abiotic stressors. However, the MAPK gene family in the important crop walnut (Juglans regia L.) has been less well studied compared with other species. We discovered 25 JrMAPK members in the Juglans genome in this study. The JrMAPK gene family was separated into four subfamilies based on phylogenetic analysis, and members of the same subgroup had similar motifs and exons/introns. A variety of cis-acting elements, mainly related to the light response, growth and development, stress response, and hormone responses, were detected in the JrMAPK gene promoters. Collinearity analysis showed that purification selection was the main driving force in JrMAPK gene evolution, and segmental and tandem duplications played key roles in the expansion of the JrMAPK gene family. The RNA-Seq (RNA Sequencing) results indicated that many of the JrMAPK genes were expressed in response to different levels of Colletotrichum gloeosporioides infection. JrMAPK1, JrMAPK3, JrMAPK4, JrMAPK5, JrMAPK6, JrMAPK7, JrMAPK9, JrMAPK11, JrMAPK12, JrMAPK13, JrMAPK17, JrMAPK19, JrMAPK20, and JrMAPK21 were upregulated at the transcriptional level in response to the drought stress treatment. The results of this study will help in further investigations of the evolutionary history and biological functions of the MAPK gene family in walnut.

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

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

Biohardening of Banana cv. Karpooravalli (ABB; Pisang Awak) With Bacillus velezensis YEBBR6 Promotes Plant Growth and Reprograms the Innate Immune Response Against Fusarium oxysporum f.sp. cubense

Induction of innate immune response and growth promotion in banana by B. velezensis against Foc.

MAP3Kε1/2 Interact with MOB1A/1B and Play Important Roles in Control of Pollen Germination through Crosstalk with JA Signaling in Arabidopsis

Restriction of pollen germination before the pollen grain is pollinated to stigma is essential for successful fertilization in angiosperms. However, the mechanisms underlying the process remain poorly understood. Here, we report functional characterization of the MAPKKK kinases, MAP3Kε1 and MAP3Kε2, involve in control of pollen germination in Arabidopsis. The two genes were expressed in different tissues with higher expression levels in the tricellular pollen grains. The map3kε1 map3kε2 double mutation caused abnormal callose accumulation, increasing level of JA and precocious pollen germination, resulting in significantly reduced seed set. Furthermore, the map3kε1 map3kε2 double mutations obviously upregulated the expression levels of genes in JA biosynthesis and signaling. The MAP3Kε1/2 interacted with MOB1A/1B which shared homology with the core components of Hippo singling pathway in yeast. The Arabidopsis mob1a mob1b mutant also exhibited a similar phenotype of precocious pollen germination to that in map3kε1 map3kε2 mutants. Taken together, these results suggested that the MAP3Kεs interacted with MOB1s and played important role in restriction of the precocious pollen germination, possibly through crosstalk with JA signaling and influencing callose accumulation in Arabidopsis.

MAP kinase cascades in plant development and immune signaling

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

LARP6C orchestrates posttranscriptional reprogramming of gene expression during hydration to promote pollen tube guidance

Increasing evidence suggests that posttranscriptional regulation is a key player in the transition between mature pollen and the progamic phase (from pollination to fertilization). Nonetheless, the actors in this messenger RNA (mRNA)-based gene expression reprogramming are poorly understood. We demonstrate that the evolutionarily conserved RNA-binding protein LARP6C is necessary for the transition from dry pollen to pollen tubes and the guided growth of pollen tubes towards the ovule in Arabidopsis thaliana. In dry pollen, LARP6C binds to transcripts encoding proteins that function in lipid synthesis and homeostasis, vesicular trafficking, and polarized cell growth. LARP6C also forms cytoplasmic granules that contain the poly(A) binding protein and possibly represent storage sites for translationally silent mRNAs. In pollen tubes, the loss of LARP6C negatively affects the quantities and distribution of storage lipids, as well as vesicular trafficking. In Nicotiana benthamiana leaf cells and in planta, analysis of reporter mRNAs designed from the LARP6C target MGD2 provided evidence that LARP6C can shift from a repressor to an activator of translation when the pollen grain enters the progamic phase. We propose that LARP6C orchestrates the timely posttranscriptional regulation of a subset of mRNAs in pollen during the transition from the quiescent to active state and along the progamic phase to promote male fertilization in plants.

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.

MicroRNA Omics Analysis of Camellia sinesis Pollen Tubes in Response to Low-Temperature and Nitric Oxide

Nitric oxide (NO) as a momentous signal molecule participates in plant reproductive development and responds to various abiotic stresses. Here, the inhibitory effects of the NO-dominated signal network on the pollen tube growth of Camellia sinensis under low temperature (LT) were studied by microRNA (miRNA) omics analysis. The results showed that 77 and 71 differentially expressed miRNAs (DEMs) were induced by LT and NO treatment, respectively. Gene ontology (GO) analysis showed that DEM target genes related to microtubules and actin were enriched uniquely under LT treatment, while DEM target genes related to redox process were enriched uniquely under NO treatment. In addition, the target genes of miRNA co-regulated by LT and NO are only located on the cell membrane and cell wall, and most of them are enriched in metal ion binding and/or transport and cell wall organization. Furthermore, DEM and its target genes related to metal ion binding/transport, redox process, actin, cell wall organization and carbohydrate metabolism were identified and quantified by functional analysis and qRT-PCR. In conclusion, miRNA omics analysis provides a complex signal network regulated by NO-mediated miRNA, which changes cell structure and component distribution by adjusting Ca²⁺ gradient, thus affecting the polar growth of the C. sinensis pollen tube tip under LT.

Reproduction Multitasking: The Male Gametophyte

The gametophyte represents the sexual phase in the alternation of generations in plants; the other, nonsexual phase is the sporophyte. Here, we review the evolutionary origins of the male gametophyte among land plants and, in particular, its ontogenesis in flowering plants. The highly reduced male gametophyte of angiosperm plants is a two- or three-celled pollen grain. Its task is the production of two male gametes and their transport to the female gametophyte, the embryo sac, where double fertilization takes place. We describe two phases of pollen ontogenesis-a developmental phase leading to the differentiation of the male germline and the formation of a mature pollen grain and a functional phase representing the pollen tube growth, beginning with the landing of the pollen grain on the stigma and ending with double fertilization. We highlight recent advances in the complex regulatory mechanisms involved, including posttranscriptional regulation and transcript storage, intracellular metabolic signaling, pollen cell wall structure and synthesis, protein secretion, and phased cell-cell communication within the reproductive tissues.

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.

Mitogen-activated protein kinase 6 negatively regulates secondary wall biosynthesis by modulating MYB46 protein stability in Arabidopsis thaliana

The R2R3-MYB transcription factor MYB46 functions as a master switch for secondary cell wall biosynthesis, ensuring the exquisite expression of the secondary wall biosynthetic genes in the tissues where secondary walls are critical for growth and development. At the same time, suppression of its function is needed when/where formation of secondary walls is not desirable. Little is known about how this opposing control of secondary cell wall formation is achieved. We used both transient and transgenic expression of MYB46 and mitogen-activated protein kinase 6 (MPK6) to investigate the molecular mechanism of the post-translational regulation of MYB46. We show that MYB46 is phosphorylated by MPK6, leading to site specific phosphorylation-dependent degradation of MYB46 by the ubiquitin-mediated proteasome pathway. In addition, the MPK6-mediated MYB46 phosphorylation was found to regulate in planta secondary wall forming function of MYB46. Furthermore, we provide experimental evidences that MYB83, a paralog of MYB46, is not regulated by MPK6. The coupling of MPK signaling to MYB46 function provides insights into the tissue- and/or condition-specific activity of MYB46 for secondary wall biosynthesis.

MaMAPK3-MaICE1-MaPOD P7 pathway, a positive regulator of cold tolerance in banana

BACKGROUND

Banana is a tropical fruit with a high economic impact worldwide. Cold stress greatly affects the development and production of banana.

RESULTS

In the present study, we investigated the functions of MaMAPK3 and MaICE1 involved in cold tolerance of banana. The effect of RNAi of MaMAPK3 on Dajiao (Musa spp. 'Dajiao'; ABB Group) cold tolerance was evaluated. The leaves of the MaMAPK3 RNAi transgenic plants showed wilting and severe necrotic symptoms, while the wide-type (WT) plants remained normal after cold exposure. RNAi of MaMAPK3 significantly changed the expressions of the cold-responsive genes, and the oxidoreductase activity was significantly changed in WT plants, while no changes in transgenic plants were observed. MaICE1 interacted with MaMAPK3, and the expression level of MaICE1 was significantly decreased in MaMAPK3 RNAi transgenic plants. Over-expression of MaICE1 in Cavendish banana (Musa spp. AAA group) indicated that the cold resistance of transgenic plants was superior to that of the WT plants. The POD P7 gene was significantly up-regulated in MaICE1-overexpressing transgenic plants compared with WT plants, and the POD P7 was proved to interact with MaICE1.

CONCLUSIONS

Taken together, our work provided new and solid evidence that MaMAPK3-MaICE1-MaPOD P7 pathway positively improved the cold tolerance in monocotyledon banana, shedding light on molecular breeding for the cold-tolerant banana or other agricultural species.

Plant cell cycle regulators: Mitogen-activated protein kinase, a new regulating switch?

Cell cycle is essential for the maintenance of genetic material and continuity of a species. Its regulation involves a complex interplay between multiple proteins with diverse molecular functions such as the kinases, transcription factors, proteases and phosphatases. Every step of this cycle requires a certain combination of these protein regulators which paves the way for the next stage. It is now evident that plants have their own unique features in the context of cell cycle regulation. Cell cycle in plants is not only necessary for maintenance of its physio-morphological parameter but it also regulates traits important for mankind like grain or fruit size. This makes it even more important to understand how plants regulate its cell cycle amidst various a/biotic stresses it is subjected to during its lifetime. The association of MAPK signaling pathways with every major developmental and stress response pathways in plants raises the question of its potential role in cell cycle regulation. There are number of cell cycle regulating proteins with putative sites for MAPK phosphorylation. The MAPK signaling pathway may directly or in a parallel pathway regulate the plant cell cycle. Unraveling the role of MAPK in cell cycle will open up new arenas to explore.

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

The mitotic activity of root apical meristem (RAM) is critical to primary root growth and development. Previous studies have identified the roles of ROOT GROWTH FACTOR 1 (RGF1), a peptide ligand, and its receptors, RGF1 INSENSITIVEs (RGIs), a clade of five leucine-rich-repeat receptor-like kinases, in promoting cell division in the RAM, which determines the primary root length. However, the downstream signaling components remain elusive. In this study, we identify a complete mitogen-activated protein kinase (MAPK or MPK) cascade, composed of YDA, MKK4/MKK5, and MPK3/MPK6, that functions downstream of the RGF1-RGI ligand-receptor pair. Similar to the rgi1/2/3/4/5 quintuple mutant, loss-of-function mutants of MPK3 and MPK6, MKK4 and MKK5, or YDA show a short-root phenotype, which is associated with reduced mitotic activity and lower expression of PLETHORA 1 (PLT1)/PLT2 in the RAM. Furthermore, MPK3/MPK6 activation in response to exogenous RGF1 treatment is impaired in the rgi1/2/3/4/5 quintuple, yda single, and mkk4 mkk5 double mutants. Epistatic analyses demonstrated that the expression of constitutively active MKK4, MKK5, or YDA driven by the RGI2 promoter can rescue the short-root phenotype of the rgi1/2/3/4/5 mutant. Taken together, these results suggest that the YDA-MKK4/MKK5-MPK3/MPK6 cascade functions downstream of the RGF1-RGI ligand-receptor pair and upstream of PLT1/PLT2 to modulate the stem cell population and primary root growth in Arabidopsis.

Stigma Receptivity Is Controlled by Functionally Redundant MAPK Pathway Components in Arabidopsis

In angiosperms, the process of pollination relies on species-specific interaction and signaling between the male (pollen) and female (pistil) counterparts where the interplay between several pollen and stigma proteins decides the fate of the pollen. In Brassicaceae, the dry stigmatic papillary cells control pollen germination by releasing resources only to compatible pollen thereby allowing pollen to hydrate and germinate. Despite the identification of a number of stigmatic proteins that facilitate pollination responses, the signaling mechanisms that regulate functions of these proteins have remained unknown. Here, we show that, in Arabidopsis, an extremely functionally redundant mitogen-activated protein kinase (MAPK) cascade is required for maintaining stigma receptivity to accept compatible pollen. Our genetic analyses demonstrate that in stigmas, five MAPK kinases (MKKs), MKK1/2/3/7/9 are required to transmit upstream signals to two MPKs, MPK3/4, to mediate compatible pollination. Compromised functions of these five MKKs in the quintuple mutant (mkk1/2/3RNAi/mkk7/9) phenocopied pollination defects observed in the mpk4RNAi/mpk3 double mutant. We further show that this MAPK nexus converges on Exo70A1, a previously identified stigma receptivity factor essential for pollination. Given that pollination is the crucial initial step during plant reproduction, understanding the mechanisms that govern successful pollination could lead to development of strategies to improve crop yield.

Stigma Receptivity Is Controlled by Functionally Redundant MAPK Pathway Components in Arabidopsis

In angiosperms, the process of pollination relies on species-specific interaction and signaling between the male (pollen) and female (pistil) counterparts where the interplay between several pollen and stigma proteins decides the fate of the pollen. In Brassicaceae, the dry stigmatic papillary cells control pollen germination by releasing resources only to compatible pollen thereby allowing pollen to hydrate and germinate. Despite the identification of a number of stigmatic proteins that facilitate pollination responses, the signaling mechanisms that regulate functions of these proteins have remained unknown. Here, we show that, in Arabidopsis, an extremely functionally redundant mitogen-activated protein kinase (MAPK) cascade is required for maintaining stigma receptivity to accept compatible pollen. Our genetic analyses demonstrate that in stigmas, five MAPK kinases (MKKs), MKK1/2/3/7/9 are required to transmit upstream signals to two MPKs, MPK3/4, to mediate compatible pollination. Compromised functions of these five MKKs in the quintuple mutant (mkk1/2/3RNAi/mkk7/9) phenocopied pollination defects observed in the mpk4RNAi/mpk3 double mutant. We further show that this MAPK nexus converges on Exo70A1, a previously identified stigma receptivity factor essential for pollination. Given that pollination is the crucial initial step during plant reproduction, understanding the mechanisms that govern successful pollination could lead to development of strategies to improve crop yield.

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.

MAPK cascade gene family in Camellia sinensis: In-silico identification, expression profiles and regulatory network analysis

BACKGROUND

Mitogen Activated Protein Kinase (MAPK) cascade is a fundamental pathway in organisms for signal transduction. Though it is well characterized in various plants, there is no systematic study of this cascade in tea.

RESULT

In this study, 5 genes of Mitogen Activated Protein Kinase Kinase (MKK) and 16 genes of Mitogen Activated Protein Kinase (MPK) in Camellia sinensis were found through a genome-wide search taking Arabidopsis thaliana as the reference genome. Also, phylogenetic relationships along with structural analysis which includes gene structure, location as well as protein conserved motifs and domains, were systematically examined and further, predictions were validated by the results. The plant species taken for comparative study clearly displayed segmental duplication, which was a significant candidate for MAPK cascade expansion. Also, functional interaction was carried out in C. sinensis based on the orthologous genes in Arabidopsis. The expression profiles linked to various stress treatments revealed wide involvement of MAPK and MAPKK genes from Tea in response to various abiotic factors. In addition, the expression of these genes was analysed in various tissues.

CONCLUSION

This study provides the targets for further comprehensive identification, functional study, and also contributed for a better understanding of the MAPK cascade regulatory network in C. sinensis.

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.

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.

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.

The Solanum chacoense Fertilization-Related Kinase 3 (ScFRK3) is involved in male and female gametophyte development

BACKGROUND

The Fertilization-related kinases (FRK) form a class that belongs to the MEKK subfamily of plant MAPKKKs. It was recently shown that FRK class kinases expanded during angiosperm evolution, reaching their maximum numbers in the lineage leading to solanaceous species and culminating in the Solanum genus where they account for more than 40% of the total MEKKs. The first members studied, ScFRK1 and ScFRK2 were shown to play a pivotal role in gametophyte development in the wild potato species Solanum chacoense.

RESULTS

ScFRK3 is also involved in gametophyte development. ScFRK3 is expressed in developing pollen and young ovules, reaching its highest level immediately after meiosis and during the mitosis steps in both gametophytes. Hence, three independent lines of ScFRK3 RNAi mutant plants showed decreased number of seeds per fruit. We also observed an important number of degenerated embryo sac in mature ovary. Analysis of ovule development showed that most embryo sac did not enter mitosis I in ScFRK3 RNAi mutant plants. Severe lethality was also observed during male gametophyte development, pollen being arrested before mitosis I, as observed in the female gametophyte. Obvious defects in vegetative organs were not observed, emphasizing the reproductive roles of the FRK class kinases. To isolate MAP kinases acting downstream of ScFRK3, a de novo S. chacoense transcriptome from male and female reproductive organs was assembled. Of the five ScMKKs and 16 ScMPKs retrieved, only the ScMKK3 interacted with ScFRK3, while only the ScMPK13 interacted with ScMKK3, leading to an apparent single three-tiered canonical MAP kinase cascade combination involving ScFRK3-ScMKK3-ScMPK13.

CONCLUSIONS

The ScFRK3 MAPKKK is involved in a signaling cascade that regulates both male and female gamete development, and most probably act upstream of ScMKK3 and ScMPK13.

A stress-response-related inter-compartmental signalling pathway regulates embryonic cuticle integrity in Arabidopsis

The embryonic cuticle is necessary for normal seed development and seedling establishment in Arabidopsis. Although mutants with defective embryonic cuticles have been identified, neither the deposition of cuticle material, nor its regulation, has been described during embryogenesis. Here we use electron microscopy, cuticle staining and permeability assays to show that cuticle deposition initiates de novo in patches on globular embryos. By combining these techniques with genetics and gene expression analysis, we show that successful patch coalescence to form a continuous cuticle requires a signalling involving the endosperm-specific subtilisin protease ALE1 and the receptor kinases GSO1 and GSO2, which are expressed in the developing embryonic epidermis. Transcriptome analysis shows that this pathway regulates stress-related gene expression in seeds. Consistent with these findings we show genetically, and through activity analysis, that the stress-associated MPK6 protein acts downstream of GSO1 and GSO2 in the developing embryo. We propose that a stress-related signalling pathway has been hijacked in some angiosperm seeds through the recruitment of endosperm-specific components. Our work reveals the presence of an inter-compartmental dialogue between the endosperm and embryo that ensures the formation of an intact and functional cuticle around the developing embryo through an “auto-immune” type interaction.

Plant embryogenesis occurs deep within the tissues of the developing seed, and leads to the production of the mature embryo. In Arabidopsis and many other plant species embryo-derive structure (such as the cotyledons) are suddenly exposed to environmental stresses such as low humidity. In these species the embryonic cuticle provides a primary defence against environmental stress, and particularly dehydration, at germination. The formation of an intact and functional cuticle during embryogenesis is thus of key importance for seedling survival. Our work shows that a signalling pathway involving receptor-kinases expressed in the embryo epidermis, and a protease expressed in the endosperm tissue surrounding the embryo, is critical for ensuring the production of an intact cuticle. Furthermore, we show that a component of stress-related MAP-Kinase signalling in plants acts downstream in this pathway, possibly to mediate transcriptional responses characteristic of responses to stress. We propose that plants have redeployed a signalling pathway associated with stress resistance to ensure the formation of an intact embryonic cuticle prior to germination, and thus ensure seedling survival at germination.

A MAPK cascade downstream of IDA-HAE/HSL2 ligand-receptor pair in lateral root emergence

Lateral root (LR) emergence is a highly coordinated process involving precise cell-cell communication. Here, we show that MITOGEN-ACTIVATED PROTEIN KINASE3 (MPK3) and MPK6, and their upstream MAP-kinase kinases (MAPKKs), MKK4 and MKK5, function downstream of HAESA (HAE)/HAESA-LIKE2 (HSL2) and their ligand INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) during LR emergence. Loss of function of MKK4/MKK5 or MPK3/MPK6 results in restricted passage of the growing lateral root primordia (LRP) through the overlaying endodermal, cortical and epidermal cell layers, leading to reduced LR density. The MKK4/MKK5-MPK3/MPK6 module regulates the expression of cell wall remodelling genes in cells overlaying LRP and therefore controls pectin degradation in the middle lamella. Expression of constitutively active MKK4 or MKK5 driven by the HAE or HSL2 promoter fully rescues the LR emergence defect in the ida and hae hsl2 mutants. In addition, the MKK4/MKK5-MPK3/MPK6 module is indispensable in auxin-facilitated LR emergence. Our study provides insights into the auxin-governed and IDA-HAE/HLS2 ligand-receptor pair-mediated LR emergence process.

Regulation of pollen lipid body biogenesis by MAP kinases and downstream WRKY transcription factors in Arabidopsis

Signaling pathways that control the activities in non-photosynthetic plastids, important sites of plant metabolism, are largely unknown. Previously, we demonstrated that WRKY2 and WRKY34 transcription factors play an essential role in pollen development downstream of mitogen-activated protein kinase 3 (MPK3) and MPK6 in Arabidopsis. Here, we report that GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSLOCATOR 1 (GPT1) is a key target gene of WRKY2/WRKY34. GPT1 transports glucose-6-phosphate (Glc6P) into plastids for starch and/or fatty acid biosynthesis depending on the plant species. Loss of function of WRKY2/WRKY34 results in reduced GPT1 expression, and concomitantly, reduced accumulation of lipid bodies in mature pollen, which leads to compromised pollen viability, germination, pollen tube growth, and male transmission in Arabidopsis. Pollen-specific overexpression of GPT1 rescues the pollen defects of wrky2 wrky34 double mutant. Furthermore, gain-of-function activation of MPK3/MPK6 enhances GPT1 expression; whereas GPT1 expression is reduced in mkk4 mkk5 double mutant. Together, this study revealed a cytoplasmic/nuclear signaling pathway capable of coordinating the metabolic activities in plastids. High-level expression of GPT1 at late stages of pollen development drives Glc6P from cytosol into plastids, where Glc6P is used for fatty acid biosynthesis, an important step of lipid body biogenesis. The accumulation of lipid bodies during pollen maturation is essential to pollen fitness and successful reproduction.

Plastids are important sites of plant metabolism including fatty acid and starch biosynthesis. At present, how the activities in the plastids are coordinated with those in the cytoplasm and the signaling pathway(s) involved are largely unknown. Previously, we demonstrated that WRKY2 and WRKY34 transcription factors play an essential role in pollen development downstream of mitogen-activated protein kinase 3 (MPK3) and MPK6 in Arabidopsis. Here, we report that GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSLOCATOR 1 (GPT1) is a key target gene of WRKY2/WRKY34. GPT1 is localized on the membrane of plastids and transports glucose-6-phosphate (Glc6P) into plastids for starch and/or fatty acid biosynthesis depending on the plant species. Genetic analyses demonstrated that WRKY2/WRKY34 and their upstream MPK3/MPK6 are involved in regulating GPT1 expression, therefore, the accumulation of lipid bodies in mature pollen, which is critical to pollen viability, pollen germination, pollen tube growth, and male transmission in Arabidopsis. This study revealed a cytoplasmic/nuclear signaling pathway capable of coordinating the metabolic activities in plastids. High-level expression of GPT1 at late stages of pollen development drives Glc6P from cytosol into plastids, where Glc6P is used for fatty acid biosynthesis, an important step of lipid body biogenesis. The accumulation of lipid bodies during pollen maturation is essential to pollen fitness and successful reproduction.

The RAF-like mitogen-activated protein kinase kinase kinases RAF22 and RAF28 are required for the regulation of embryogenesis in Arabidopsis

In Arabidopsis, embryonic development follows a stereotypical pattern of cell division. Although many factors have been reported to participate in establishment of the proper embryonic pattern, the molecular mechanisms underlying pattern formation are unclear. In this study we showed that RAF22 and RAF28, two RAF-like mitogen-activated protein kinase kinase kinases (MAPKKKs) in Arabidopsis, are involved in the regulation of embryogenesis. The double knockout mutant of RAF22 and RAF28 was embryo lethal. A large proportion of the raf22^-/- raf28^+/- mutant embryos exhibited various defects, including disordered proembryo cell divisions, disruption of the bilaterally symmetrical structure, abnormally formative divisions of hypophysis and exaggerated suspensor growth. Whereas the kinase active form of RAF22 could complement these embryonic aberrant phenotypes, the kinase inactive form could not. The restrictive expression of the basal cell fate marker WOX8 in the abnormally dividing suspensor cells and the apical cell linage marker WOX2 in the abnormal proembryos indicated that apical and basal cell fates were unchanged in the abnormal embryos. The polar distribution of the auxin maxima and the PIN1 and PIN7 auxin transporters was markedly altered in the abnormal embryos. Our results suggest that RAF22 and RAF28 are important components of embryogenesis and that auxin polar transport may be involved in this regulation.

Evidence for a specific and critical role of mitogen-activated protein kinase 20 in uni-to-binucleate transition of microgametogenesis in tomato

Mitogen-activated protein kinases (MAPKs) regulate diverse aspects of plant growth. However, their potential role in reproductive development remains elusive. Here, we discovered an unique role of SlMPK20, a plant-specific group D MAPK, in pollen development in tomato. RNAi-mediated suppression of SlMPK20 or its knockout using CRISPR/Cas9 significantly reduced or completely abolished pollen viability, respectively, with no effects on maternal fertility. Cell biology and gene expression analyses established that SlMPK20 exerts its role specifically at the uni-to-binucleate transition during microgametogenesis. This assertion is based on the findings that the transgenic pollen was largely arrested at the binucleate stage with the appearance of subcellular abnormality at the middle uninucleate microspore stage; and SlMPK20 mRNA and SlMPK20-GUS signals were localized in the tetrads, uninuclear microspores and binuclear pollen grains but not in microspore mother cells or mature pollen grains. Transcriptomic and proteomic analyses revealed that knockout of SlMPK20 significantly reduced the expression of a large number of genes controlling sugar and auxin metabolism and signaling in anthers. Finally, protein-protein interaction assays identified SlMYB32 as a putative target protein of SlMPK20. We conclude that SlMPK20 specifically regulates post-meiotic pollen development through modulating sugar and auxin metabolism and signaling.

Cell and Developmental Biology of Plant Mitogen-Activated Protein Kinases

Plant mitogen-activated protein kinases (MAPKs) constitute a network of signaling cascades responsible for transducing extracellular stimuli and decoding them to dedicated cellular and developmental responses that shape the plant body. Over the last decade, we have accumulated information about how MAPK modules control the development of reproductive tissues and gametes and the embryogenic and postembryonic development of vegetative organs such as roots, root nodules, shoots, and leaves. Of key importance to understanding how MAPKs participate in developmental and environmental signaling is the characterization of their subcellular localization, their interactions with upstream signal perception mechanisms, and the means by which they target their substrates. In this review, we summarize the roles of MAPK signaling in the regulation of key plant developmental processes, and we survey what is known about the mechanisms guiding the subcellular compartmentalization of MAPK modules.

Multilayered signaling pathways for pollen tube growth and guidance

Sexual reproductive success is essential for the survival of all higher organisms. As the most prosperous and diverse group of land plants on earth, flowering plants evolved highly sophisticated fertilization mechanisms. To adapt to the terrestrial environment, a tubular structure pollen tube has been evolved to deliver the immobile sperm cells to the egg and central cell enclosed within the ovule. The pollen tube is generated from the vegetative cell of the pollen (male gametophyte), where two sperm cells are hosted. Pollen tube elongation in the maternal tissue and navigation to the ovule require intimate cell-cell interactions between the tube and female tissues. Questions on how the single-celled pollen tube accomplishes such task and how the female tissues accommodate the tube have attracted many plant biologists. Here, we review recent progresses and concepts in understanding the molecular mechanisms governing pollen tube growth and its interactions with the female tissues. We will also discuss the future perspective in this field.

How many receptor-like kinases are required to operate a pollen tube

Successful fertilization depends on active molecular dialogues that the male gametophyte can establish with the pistil and the female gametophyte. Pollen grains and stigmas must recognize each other; pollen tubes need to identify the pistil tissues they will penetrate, follow positional cues to exit the transmitting tract and finally, locate the ovules. These molecular dialogues directly affect pollen tube growth rate and orientation. Receptor-like kinases (RLKs) are natural candidates for the perception and decoding of extracellular signals and their transduction to downstream cytoplasmic interactors. Here, we update knowledge regarding how RLKs are involved in pollen tube growth, cell wall integrity and guidance. In addition, we use public data to build a pollen tube RLK interactome that might help direct experiments to elucidate the function of pollen RLKs and their associated proteins.

Chemical signaling for pollen tube guidance at a glance

Pollen tube guidance is a unique navigating system that is required for the successful sexual reproduction of plants. As plant sperm cells are non-motile and egg cells are embedded deep inside the female tissues, a pollen tube delivers the two sperm cells that it contains by growing towards the ovule, in which the egg cell resides. Pollen tube growth towards the ovule is precisely controlled and divided into two stages, preovular and ovular guidance. In this Cell Science at a Glance article and accompanying poster, we provide a comprehensive overview of pollen tube guidance and highlight some of the attractant peptides used during ovular guidance. We further discuss the precise one-to-one guidance system that exists in multi-ovular plants. The pollen tube-blocking system, which is mediated by male–female crosstalk communication, to avoid attraction of multiple pollen tubes, is also reviewed.