Expression-based and co-localization detection of arabinogalactan protein 6 and arabinogalactan protein 11 interactors in Arabidopsis pollen and pollen tubes

Determining the Role of OsAGP6P in Anther Development Within the Arabinogalactan Peptide Family of Rice (Oryza sativa)

Arabinogalactan proteins (AGPs) are complex proteoglycans present in plant cell walls across the kingdom. They play crucial roles in biological functions throughout the plant life cycle. In this study, we identified 43 gene members of the AG peptide (an AGP subfamily) within the rice genome, detailing their structure, protein-conserved domains, and motif compositions for the first time. We also examined the expression patterns of these genes across 18 tissues and organs, especially the different parts of the flower (anthers, pollen, pistil, sperm cells, and egg cells). Interestingly, the expression of some AG peptides is mainly present in the pollen grain. Transcription data and GUS staining confirmed that OsAGP6P-a member of the AG peptide gene family-is expressed in the stamen during pollen development stages 11-14, which are critical for maturation as microspores form after meiosis of pollen mother cells. It became noticeable from stage 11, when exine formation occurred-specifically at stage 12, when the intine began to develop. The overexpression of this gene in rice decreased the seed-setting rate (from 91.5% to 30.5%) and plant height (by 21.9%) but increased the tillering number (by 34.1%). These results indicate that AGP6P contributes to the development and fertility of pollen, making it a valuable gene target for future genetic manipulation of plant sterility through gene overexpression or editing.

AtFH5 recruits and transports the arabinogalactan protein AGP23 to maintain the tip growth of pollen tube

Actin cytoskeleton drives the targeted transport of cell wall components to sustain the tip growth of pollen tubes for double fertilization; however, the underlying mechanism remains largely unknown. Arabidopsis formin 5 (AtFH5), an actin-nucleating protein, localizes at secretory vesicles and mediates actin polymerization-based vesicle trafficking in pollen. Here, we demonstrate that AtFH5 determines the recruitment and transport of cell wall components in AtFH5-labeled vesicles during the tip growth of pollen tubes. Through a screen of interacting proteins of AtFH5, we identify many cell wall-related proteins, with arabinogalactan protein 23 (AGP23) occupying the highest frequency. AtFH5 interacts with AGP23 via its N-terminal extracellular domain (ECD) and jointly regulate the pollen germination and tube growth process. Further observations reveal that AGP23 co-localizes with AtFH5 at moving vesicles, germination sites, and pollen tube tips, suggesting that AGP23 is delivered by AtFH5-labeled vesicles. Deletion of the ECD of AtFH5 interrupts the dynamic localization and cell-wall connection of AGP23 in pollen grains and tubes. Cytological and genetic evidence shows that AGP23 and AtFH5 work in the same pathway to modulate cell wall composition. Together, our data uncover a role of formin in directing the sorting and deposition of cell wall components via secretory vesicle trafficking during pollen germination and tube growth.

Potent pollen gene regulation by DNA glycosylases in maize

Although DNA methylation primarily represses TEs, it also represses select genes that are methylated in plant body tissues but demethylated by DNA glycosylases (DNGs) in endosperm or pollen. Either one of two DNGs, MATERNAL DEREPRESSION OF R1 (MDR1) or DNG102, is essential for pollen viability in maize. Using single-pollen mRNA sequencing on pollen-segregating mutations in both genes, we identify 58 candidate DNG target genes that account for 11.1% of the wild-type transcriptome but are silent or barely detectable in other tissues. They are unusual in their tendency to lack introns but even more so in their TE-like methylation (teM) in coding DNA. The majority have predicted functions in cell wall modification, and they likely support the rapid tip growth characteristic of pollen tubes. These results suggest a critical role for DNA methylation and demethylation in regulating maize genes with the potential for extremely high expression in pollen but constitutive silencing elsewhere.

Potent pollen gene regulation by DNA glycosylases in maize

Although DNA methylation primarily represses TEs, it also represses select genes that are methylated in plant body tissues but demethylated by DNA glycosylases (DNGs) in endosperm or pollen. Activity of either one of two DNGs, MDR1 or DNG102, is essential for pollen viability in maize. Using single-pollen mRNA sequencing on pollen segregating mutations in both genes, we identified 58 candidate DNG target genes that account for 11.1% of the wild-type transcriptome but are silent or barely detectable in the plant body (sporophyte). They are unusual in their tendency to lack introns but even more so in their having TE-like methylation in their CDS. The majority have predicted functions in cell wall modification, and they likely support the rapid tip growth characteristic of pollen tubes. These results suggest a critical role for DNA methylation and demethylation in regulating maize genes with potential for extremely high expression in pollen but constitutive silencing elsewhere.

Lineage-specific gene duplication and expansion of DUF1216 gene family in Brassicaceae

Proteins containing domain of unknown function (DUF) are prevalent in eukaryotic genome. The DUF1216 proteins possess a conserved DUF1216 domain resembling to the mediator protein of Arabidopsis RNA polymerase II transcriptional subunit-like protein. The DUF1216 family are specifically existed in Brassicaceae, however, no comprehensive evolutionary analysis of DUF1216 genes have been performed. We performed a first comprehensive genome-wide analysis of DUF1216 proteins in Brassicaceae. Totally 284 DUF1216 genes were identified in 27 Brassicaceae species and classified into four subfamilies on the basis of phylogenetic analysis. The analysis of gene structure and conserved motifs revealed that DUF1216 genes within the same subfamily exhibited similar intron/exon patterns and motif composition. The majority members of DUF1216 genes contain a signal peptide in the N-terminal, and the ninth position of the signal peptide in most DUF1216 is cysteine. Synteny analysis revealed that segmental duplication is a major mechanism for expanding of DUF1216 genes in Brassica oleracea, Brassica juncea, Brassica napus, Lepidium meyneii, and Brassica carinata, while in Arabidopsis thaliana and Capsella rubella, tandem duplication plays a major role in the expansion of the DUF1216 gene family. The analysis of Ka/Ks (non-synonymous substitution rate/synonymous substitution rate) ratios for DUF1216 paralogous indicated that most of gene pairs underwent purifying selection. DUF1216 genes displayed a specifically high expression in reproductive tissues in most Brassicaceae species, while its expression in Brassica juncea was specifically high in root. Our studies offered new insights into the phylogenetic relationships, gene structures and expressional patterns of DUF1216 members in Brassicaceae, which provides a foundation for future functional analysis.

Development and genetic regulation of pollen intine in Arabidopsis and rice

Pollen intine serves as a protective layer situated between the pollen exine and the plasma membrane. It performs essential functions during pollen development, including maintaining the morphological structure of the pollen, preventing the loss of pollen contents, and facilitating pollen germination. The formation of the intine layer commences at the bicellular pollen stage. Pectin, cellulose, hemicellulose and structural proteins are the key constituents of the pollen intine. In Arabidopsis and rice, numerous regulatory factors associated with polysaccharide metabolism and material transport have been identified, which regulate intine development. In this review, we elucidate the developmental processes of the pollen wall and provide a concise summary of the research advancements in the development and genetic regulation of the pollen intine in Arabidopsis and rice. A comprehensive understanding of intine development and regulation is crucial for unraveling the genetic network underlying intine development in higher plants.

The triticale mature pollen and stigma proteomes - assembling the proteins for a productive encounter

Triticeae crops are major contributors to global food production and ensuring their capacity to reproduce and generate seeds is critical. However, despite their importance our knowledge of the proteins underlying Triticeae reproduction is severely lacking and this is not only true of pollen and stigma development, but also of their pivotal interaction. When the pollen grain and stigma are brought together they have each accumulated the proteins required for their intended meeting and accordingly studying their mature proteomes is bound to reveal proteins involved in their diverse and complex interactions. Using triticale as a Triticeae representative, gel-free shotgun proteomics was used to identify 11,533 and 2977 mature stigma and pollen proteins respectively. These datasets, by far the largest to date, provide unprecedented insights into the proteins participating in Triticeae pollen and stigma development and interactions. The study of the Triticeae stigma has been particularly neglected. To begin filling this knowledge gap, a developmental iTRAQ analysis was performed revealing 647 proteins displaying differential abundance as the stigma matures in preparation for pollination. An in-depth comparison to an equivalent Brassicaceae analysis divulged both conservation and diversification in the makeup and function of proteins involved in the pollen and stigma encounter. SIGNIFICANCE: Successful pollination brings together the mature pollen and stigma thus initiating an intricate series of molecular processes vital to crop reproduction. In the Triticeae crops (e.g. wheat, barley, rye, triticale) there persists a vast deficit in our knowledge of the proteins involved which needs to be addressed if we are to face the many upcoming challenges to crop production such as those associated with climate change. At maturity, both the pollen and stigma have acquired the protein complement necessary for their forthcoming encounter and investigating their proteomes will inevitably provide unprecedented insights into the proteins enabling their interactions. By combining the analysis of the most comprehensive Triticeae pollen and stigma global proteome datasets to date with developmental iTRAQ investigations, proteins implicated in the different phases of pollen-stigma interaction enabling pollen adhesion, recognition, hydration, germination and tube growth, as well as those underlying stigma development were revealed. Extensive comparisons between equivalent Triticeae and Brassiceae datasets highlighted both the conservation of biological processes in line with the shared goal of activating the pollen grain and promoting pollen tube invasion of the pistil to effect fertilization, as well as the significant distinctions in their proteomes consistent with the considerable differences in their biochemistry, physiology and morphology.

Type II arabinogalactans initiated by hydroxyproline-O-galactosyltransferases play important roles in pollen-pistil interactions

Arabinogalactan-proteins (AGPs) are hydroxyproline-rich glycoproteins containing a high sugar content and are widely distributed in the plant kingdom. AGPs have long been suggested to play important roles in sexual plant reproduction. The synthesis of their complex carbohydrates is initiated by a family of hydroxyproline galactosyltransferase (Hyp-GALT) enzymes which add the first galactose to Hyp residues in the protein backbone. Eight Hyp-GALT enzymes have been identified so far, and in the present work a mutant affecting five of these enzymes (galt2galt5galt7galt8galt9) was analyzed regarding the reproductive process. The galt25789 mutant presented a low seed set, and reciprocal crosses indicated a significant female gametophytic contribution to this mutant phenotype. Mutant ovules revealed abnormal callose accumulation inside the embryo sac and integument defects at the micropylar region culminating in defects in pollen tube reception. In addition, immunolocalization and biochemical analyses allowed the detection of a reduction in the amount of glucuronic acid in mutant ovary AGPs. Dramatically low amounts of high-molecular-weight Hyp-O-glycosides obtained following size exclusion chromatography of base-hydrolyzed mutant AGPs compared to the wild type indicated the presence of underglycosylated AGPs in the galt25789 mutant, while the monosaccharide composition of these Hyp-O-glycosides displayed no significant changes compared to the wild-type Hyp-O-glycosides. The present work demonstrates the functional importance of the carbohydrate moieties of AGPs in ovule development and pollen-pistil interactions.

I Choose You: Selecting Accurate Reference Genes for qPCR Expression Analysis in Reproductive Tissues in Arabidopsis thaliana

Quantitative real-time polymerase chain reaction (qPCR) is a widely used method to analyse the gene expression pattern in the reproductive tissues along with detecting gene levels in mutant backgrounds. This technique requires stable reference genes to normalise the expression level of target genes. Nonetheless, a considerable number of publications continue to present qPCR results normalised to a single reference gene and, to our knowledge, no comparative evaluation of multiple reference genes has been carried out in specific reproductive tissues of Arabidopsis thaliana. Herein, we assessed the expression stability levels of ten candidate reference genes (UBC9, ACT7, GAPC-2, RCE1, PP2AA3, TUA2, SAC52, YLS8, SAMDC and HIS3.3) in two conditional sets: one across flower development and the other using inflorescences from different genotypes. The stability analysis was performed using the RefFinder tool, which combines four statistical algorithms (geNorm, NormFinder, BestKeeper and the comparative ΔCt method). Our results showed that RCE1, SAC52 and TUA2 had the most stable expression in different flower developmental stages while YLS8, HIS3.3 and ACT7 were the top-ranking reference genes for normalisation in mutant studies. Furthermore, we validated our results by analysing the expression pattern of genes involved in reproduction and examining the expression of these genes in published mutant backgrounds. Overall, we provided a pool of appropriate reference genes for expression studies in reproductive tissues of A. thaliana, which will facilitate further gene expression studies in this context. More importantly, we presented a framework that will promote a consistent and accurate analysis of gene expression in any scientific field. Simultaneously, we highlighted the relevance of clearly defining and describing the experimental conditions associated with qPCR to improve scientific reproducibility.

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.

TaEXPB5 functions as a gene related to pollen development in thermo-sensitive male-sterility wheat with Aegilops kotschyi cytoplasm

The thermo-sensitive cytoplasmic male-sterility line with Aegilops kotschyi cytoplasm (K-TCMS) is completely male sterile under low temperature (< 18 ℃) during Zadoks growth stages 45-52, whereas its fertility can be restored under hot temperature (≥ 20 ℃). The K-TCMS line may facilitate hybrid breeding and hybrid wheat production. Therefore, to elucidate the molecular mechanisms of its male sterility/fertility conversion, we conducted the association analysis of proteins and transcript expression to screen fertility related genes using RNA-seq, iTRAQ, and PRM-based assay. A gene encoding expansin protein in wheat, TaEXPB5, was isolated in K-TCMS line KTM3315A, which upregulated expression in the fertility anthers. Subcellular localization analysis suggested that TaEXPB5 protein localized to nucleus and cell wall. The silencing of TaEXPB5 displayed pollen abortion and the declination of fertility. Further, cytological investigation indicated that the silencing of TaEXPB5 induced the early degradation of tapetum and abnormal development of pollen wall. These results implied that TaEXPB5 may be essential for anther or pollen development and male fertility of KTM3315A. These findings provide a novel insight into molecular mechanism of fertility conversion for thermo-sensitive cytoplasmic male-sterility wheat, and contribute to the molecular breeding of hybrid wheat in the future.

AT-HOOK MOTIF NUCLEAR LOCALIZED (AHL) proteins of ancient origin radiate new functions

AHL (AT-HOOK MOTIF NUCLEAR LOCALIZED) protein is an important transcription factor in plants that regulates a wide range of biological process. It is considered to have evolved from an independent PPC domain in prokaryotes to a complete protein in modern plants. AT-hook motif and PPC conserved domains are the main functional domains of AHL. Since the discovery of AHL, their evolution and function have been continuously studied. The AHL gene family has been identified in multiple species and the functions of several members of the gene family have been studied. Here, we summarize the evolution and structural characteristics of AHL genes, and emphasize their biological functions. This review will provide a basis for further functional study and crop breeding.

Arabinogalactan Proteins: Focus on the Role in Cellulose Synthesis and Deposition during Plant Cell Wall Biogenesis

Arabinogalactan proteins (AGPs) belong to a family of glycoproteins that are widely present in plants. AGPs are mostly composed of a protein backbone decorated with complex carbohydrate side chains and are usually anchored to the plasma membrane or secreted extracellularly. A trickle of compelling biochemical and genetic evidence has demonstrated that AGPs make exciting candidates for a multitude of vital activities related to plant growth and development. However, because of the diversity of AGPs, functional redundancy of AGP family members, and blunt-force research tools, the precise functions of AGPs and their mechanisms of action remain elusive. In this review, we put together the current knowledge about the characteristics, classification, and identification of AGPs and make a summary of the biological functions of AGPs in multiple phases of plant reproduction and developmental processes. In addition, we especially discuss deeply the potential mechanisms for AGP action in different biological processes via their impacts on cellulose synthesis and deposition based on previous studies. Particularly, five hypothetical models that may explain the AGP involvement in cellulose synthesis and deposition during plant cell wall biogenesis are proposed. AGPs open a new avenue for understanding cellulose synthesis and deposition in plants.

A mutation in the promoter of the arabinogalactan protein 7-like gene PcAGP7-1 affects cell morphogenesis and brassinolide content in pear (Pyrus communis L.) stems

Dwarfing rootstocks and dwarf cultivars are urgently needed for modern pear cultivation. However, germplasm resources for dwarfing pear are limited, and the underlying mechanisms remain unclear. We previously showed that dwarfism in pear is controlled by the single dominant gene PcDw (Dwarf). We report here that the expression of PcAGP7-1 (ARABINOGALACTAN PROTEIN 7-1), a key candidate gene for PcDw, is significantly higher in dwarf-type pear plants because of a mutation in an E-box in the promoter. Electrophoretic mobility shift assays and transient infiltration showed that the transcription factors PcBZR1 and PcBZR2 could directly bind to the E-box of the PcAGP7-1 promoter and repress transcription. Moreover, transgenic pear lines overexpressing PcAGP7-1 exhibited obvious dwarf phenotypes, whereas RNA interference pear lines for PcAGP7-1 were taller than controls. PcAGP7-1 overexpression also enhanced cell wall thickness, affected cell morphogenesis, and reduced brassinolide (BL) content, which inhibited BR signaling via a negative feedback loop, resulting in further dwarfing. Overall, we identified a dwarfing mechanism in perennial woody plants involving the BL-BZR/BES-AGP-BL regulatory module. Our findings provide insight into the molecular mechanism of plant dwarfism and suggest strategies for the molecular breeding of dwarf pear cultivars.

Glucuronidation of type II arabinogalactan polysaccharides function in sexual reproduction of Arabidopsis

Arabinogalactan proteins (AGPs) are complex, hyperglycosylated plant cell wall proteins with little known about the biological roles of their glycan moieties in sexual reproduction. Here, we report that GLCAT14A, GLCAT14B, and GLCAT14C, three enzymes responsible for the addition of glucuronic acid residues to AGPs, function in pollen development, polytubey block, and normal embryo development in Arabidopsis. Using biochemical and immunolabeling techniques, we demonstrated that the loss of function of the GLCAT14A, GLCAT14B, and GLCAT14C genes resulted in disorganization of the reticulate structure of the exine wall, abnormal development of the intine layer, and collapse of pollen grains in glcat14a/b and glcat14a/b/c mutants. Synchronous development between locules within the same anther was also lost in some glcat14a/b/c stamens. In addition, we observed excessive attraction of pollen tubes targeting glcat14a/b/c ovules, indicating that the polytubey block mechanism was compromised. Monosaccharide composition analysis revealed significant reductions in all sugars in glcat14a/b and glcat14a/b/c mutants except for arabinose and galactose, while immunolabeling showed decreased amounts of AGP sugar epitopes recognized by glcat14a/b and glcat14a/b/c mutants compared with the wild type. This work demonstrates the important roles that AG glucuronidation plays in Arabidopsis sexual reproduction and reproductive development.

Functional characterization of hydroxyproline-O-galactosyltransferases for Arabidopsis arabinogalactan-protein synthesis

BACKGROUND

Arabinogalactan-proteins (AGPs) are structurally complex hydroxyproline-rich cell wall glycoproteins ubiquitous in the plant kingdom. AGPs biosynthesis involves a series of post-translational modifications including the addition of type II arabinogalactans to non-contiguous Hyp residues. To date, eight Hyp-galactosyltransferases (Hyp-GALTs; GALT2-GALT9) belonging to CAZy GT31, are known to catalyze the addition of the first galactose residues to AGP protein backbones and enable subsequent AGP glycosylation. The extent of genetic redundancy, however, remains to be elucidated for the Hyp-GALT gene family.

RESULTS

To examine their gene redundancy and functions, we generated various multiple gene knock-outs, including a triple mutant (galt5 galt8 galt9), two quadruple mutants (galt2 galt5 galt7 galt8, galt2 galt5 galt7 galt9), and one quintuple mutant (galt2 galt5 galt7 galt8 galt9), and comprehensively examined their biochemical and physiological phenotypes. The key findings include: AGP precipitations with β-Yariv reagent showed that GALT2, GALT5, GALT7, GALT8 and GALT9 act redundantly with respect to AGP glycosylation in cauline and rosette leaves, while the activity of GALT7, GALT8 and GALT9 dominate in the stem, silique and flowers. Monosaccharide composition analysis showed that galactose was decreased in the silique and root AGPs of the Hyp-GALT mutants. TEM analysis of 25789 quintuple mutant stems indicated cell wall defects coincident with the observed developmental and growth impairment in these Hyp-GALT mutants. Correlated with expression patterns, galt2, galt5, galt7, galt8, and galt9 display equal additive effects on insensitivity to β-Yariv-induced growth inhibition, silique length, plant height, and pollen viability. Interestingly, galt7, galt8, and galt9 contributed more to primary root growth and root tip swelling under salt stress, whereas galt2 and galt5 played more important roles in seed morphology, germination defects and seed set. Pollen defects likely contributed to the reduced seed set in these mutants.

CONCLUSION

Additive and pleiotropic effects of GALT2, GALT5, GALT7, GALT8 and GALT9 on vegetative and reproductive growth phenotypes were teased apart via generation of different combinations of Hyp-GALT knock-out mutants. Taken together, the generation of higher order Hyp-GALT mutants demonstrate the functional importance of AG polysaccharides decorating the AGPs with respect to various aspects of plant growth and development.

Signaling at Physical Barriers during Pollen-Pistil Interactions

In angiosperms, double fertilization requires pollen tubes to transport non-motile sperm to distant egg cells housed in a specialized female structure known as the pistil, mediating the ultimate fusion between male and female gametes. During this journey, the pollen tube encounters numerous physical barriers that must be mechanically circumvented, including the penetration of the stigmatic papillae, style, transmitting tract, and synergid cells as well as the ultimate fusion of sperm cells to the egg or central cell. Additionally, the pollen tube must maintain structural integrity in these compact environments, while responding to positional guidance cues that lead the pollen tube to its destination. Here, we discuss the nature of these physical barriers as well as efforts to genetically and cellularly identify the factors that allow pollen tubes to successfully, specifically, and quickly circumnavigate them.

FLA14 is required for pollen development and preventing premature pollen germination under high humidity in Arabidopsis

BACKGROUND

As an important subfamily of arabinogalactan proteins (AGPs), fasciclin-like AGPs (FLAs) contribute to various aspects of growth, development and adaptation, yet their function remains largely elusive. Despite the diversity of FLAs, only two members, Arabidopsis FLA3 and rice MTR1, are reported to be involved in sexual reproduction. In this study, another Arabidopsis FLA-encoding gene, FLA14, was identified, and its role was investigated.

RESULTS

Arabidopsis FLA14 was found to be a pollen grain-specific gene. Expression results from fusion with green fluorescent protein showed that FLA14 was localized along the cell membrane and in Hechtian strands. A loss-of-function mutant of FLA14 showed no discernible defects during male gametogenesis, but precocious pollen germination occurred inside the mature anthers under high moisture conditions. Overexpression of FLA14 caused 39.2% abnormal pollen grains with a shrunken and withered appearance, leading to largely reduced fertility with short mature siliques and lower seed set. Cytological and ultramicroscopic observation showed that ectopic expression of FLA14 caused disruption at the uninucleate stage, resulting in either collapsed pollen with absent intine or pollen of normal appearance but with a thickened intine.

CONCLUSIONS

Taken together, our data suggest a role for FLA14 in pollen development and preventing premature pollen germination inside the anthers under high relative humidity in Arabidopsis.

Spatial and Temporal Distribution of Arabinogalactan Proteins during Larix decidua Mill. Male Gametophyte and Ovule Interaction

The role of ArabinoGalactan Proteins (AGPs) in the sexual reproduction of gymnosperms is not as well documented as that of angiosperms. In earlier studies, we demonstrated that AGPs play important roles during ovule differentiation in Larix decidua Mill. The presented results encouraged us to carry out further studies focused on the functions of these unique glycoproteins during pollen/pollen tube and ovule interactions in Larix. We identified and analyzed the localization of AGPs epitopes by JIM4, JIM8, JIM13 and LM2 antibodies (Abs) in male gametophytes and ovule tissue during pollination, the progamic phase, and after fertilization and in vitro growing pollen tubes. Our results indicated that (1) AGPs recognized by JIM4 Abs play an essential role in the interaction of male gametophytes and ovules because their appearance in ovule cells is induced by physical contact between reproductive partners; (2) after pollination, AGPs are secreted from the pollen cytoplasm into the pollen wall and contact the extracellular matrix of stigmatic tip cells followed by micropylar canal cells; (3) AGPs synthesized in nucellus cells before pollen grain germination are secreted during pollen tube growth into the extracellular matrix, where they can directly interact with male gametophytes; (4) in vitro cultured pollen tube AGPs labeled with LM2 Abs participate in the germination of pollen grain, while AGPs recognized by JIM8 Abs are essential for pollen tube tip growth.

CRISPR-Cas9 multiplex genome editing of the hydroxyproline-O-galactosyltransferase gene family alters arabinogalactan-protein glycosylation and function in Arabidopsis

BACKGROUND

Arabinogalactan-proteins (AGPs) are a class of hydroxyproline-rich proteins (HRGPs) that are heavily glycosylated (> 90%) with type II arabinogalactans (AGs). AGPs are implicated in various plant growth and development processes including cell expansion, somatic embryogenesis, root and stem growth, salt tolerance, hormone signaling, male and female gametophyte development, and defense. To date, eight Hyp-O-galactosyltransferases (GALT2-6, HPGT1-3) have been identified; these enzymes are responsible for adding the first sugar, galactose, onto AGPs. Due to gene redundancy among the GALTs, single or double galt genetic knockout mutants are often not sufficient to fully reveal their biological functions.

RESULTS

Here, we report the successful application of CRISPR-Cas9 gene editing/multiplexing technology to generate higher-order knockout mutants of five members of the GALT gene family (GALT2-6). AGPs analysis of higher-order galt mutants (galt2 galt5, galt3 galt4 galt6, and galt2 galt3 galt4 galt5 gal6) demonstrated significantly less glycosylated AGPs in rosette leaves, stems, and siliques compared to the corresponding wild-type organs. Monosaccharide composition analysis of AGPs isolated from rosette leaves revealed significant decreases in arabinose and galactose in all the higher-order galt mutants. Phenotypic analyses revealed that mutation of two or more GALT genes was able to overcome the growth inhibitory effect of β-D-Gal-Yariv reagent, which specifically binds to β-1,3-galactan backbones on AGPs. In addition, the galt2 galt3 galt4 galt5 gal6 mutant exhibited reduced overall growth, impaired root growth, abnormal pollen, shorter siliques, and reduced seed set. Reciprocal crossing experiments demonstrated that galt2 galt3 galt4 galt5 gal6 mutants had defects in the female gametophyte which were responsible for reduced seed set.

CONCLUSIONS

Our CRISPR/Cas9 gene editing/multiplexing approach provides a simpler and faster way to generate higher-order mutants for functional characterization compared to conventional genetic crossing of T-DNA mutant lines. Higher-order galt mutants produced and characterized in this study provide insight into the relationship between sugar decorations and the various biological functions attributed to AGPs in plants.

Glycosylphosphatidylinositol-anchored proteins mediate the interactions between pollen/pollen tube and pistil tissues

In flowering plants, pollen germination on the stigma and pollen tube growth in pistil tissues are critical for sexual plant reproduction, which are involved in the interactions between pollen/pollen tube and pistil tissues. GPI-anchored proteins (GPI-APs) are located on the external surface of the plasma membrane and function in various processes of sexual plant reproduction. The evidences suggest that GPI-APs participate in endosome machinery, Ca²⁺ oscillations, the development of the transmitting tract, the maintenance of the integrity of pollen tube, the enhancement of interactions of the receptor-like kinase (RLK) and ligand, and guidance of the growth of pollen tube, and so on. In this review, we will summarize the recent progress on the roles of GPI-APs in the interactions between pollen/pollen tube and pistil tissues during pollination, such as pollen germination on the stigma, pollen tube growth in the transmitting tract, pollen tube guidance to the ovule, and pollen tube reception in the embryo sac. We will also discuss the future outlook of GPI-APs in the interactions between pollen/pollen tube and pistil tissues.

Insights Into the Molecular Evolution of AT-Hook Motif Nuclear Localization Genes in Brassica napus

AT-hook motif nuclear localization (AHL) proteins belong to a family of transcription factors, and play important roles in plant growth and development and response to various stresses through protein-DNA and protein-protein interactions. To better understand the Brassica napus AHL gene family, AHL genes in B. napus and related species were analyzed. Using Arabidopsis as a reference, 122 AHL gene family members were first identified in B. napus. According to the phylogenetic tree and gene organization, the BnaAHLs were classified into two clades (Clade-A and Clade-B) and three types (Type-I, Type-II, and Type-III). Gene organization and motif distribution analysis suggested that the AHL gene family is relatively conserved during evolution. These BnaAHLs are unevenly distributed on 38 chromosomes and expanded by whole-genome duplication (WGD) or segmental duplication. And large-scale loss events have also occurred in evolution. All types of BnaAHLs are subject to purification or neutral selection, while some positive selection sites are also identified in Type-II and Type-III groups. At the same time, the purification effect of Type-I members are stronger than that of the others. In addition, RNA-seq data and cis-acting element analysis also suggested that the BnaAHLs play important roles in B. napus growth and development, as well as in response to some abiotic and biotic stresses. Protein-protein interaction analysis identified some important BnaAHL-binding proteins, which also play key roles in plant growth and development. This study is helpful to fully understand the origin and evolution of the AHL gene in B. napus, and lays the foundation for their functional studies.

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.

Arabidopsis thaliana Pollen Tube Culture for Multi-omics Studies

Pollen tubes have been key models to study plant cell wall elongation. Arabidopsis thaliana, although small, is a nice model, easy to grow and with a large set of studies to simplify result integration and interpretation. Pollen tubes may be used for gene expression essays, but also for biochemical characterization of the cell wall composition. However, pollen tube culture methods though seemingly straightforward have often a multitude of small technical details crucial for success, quickly deterring the more inexperienced and setting back experiments for months at the time. Here we propose a detailed method to set up easily a pollen tube culture routine in any lab, with a minimal set of equipment, to isolate and process pollen tubes for gene expression and/or cell wall biochemistry studies.

PbrPCCP1 mediates the PbrTTS1 signaling to control pollen tube growth in pear

In plants, genes containing the C2 domain have been identified and play a crucial role in many key physiological processes. One hundred and sixty-six genes containing a C2 domain were identified in pear and 38 genes contained multiple C2 domains. Whole genome duplication and tandem duplication events were the major forces driving the C2 superfamily expansion, and C2 superfamily members have evolved under negative selection. There were 104 C2 genes expressed during pollen tube growth. Here, we identified Pbr028378.1 containing the C2 domain from pear and named it PbrPCCP1. PbrPCCP1 was localized in the plasma membrane and mainly expressed in pollen. PbrPCCP1 interacted with PbrTTS1, which contained a Cys-rich C-terminal domain, and promoted pollen tube growth. The Pollen ole e I domain of PbrTTS1 was responsible for its interaction. Additionally, pollen tube growth was inhibited and the promoting effect of PbrTTS1 was attenuated when PbrPCCP1 expression level was knocked-down by antisense oligonucleotides. The qRT-PCR results indicated that PbrPCCP1 and PbrTTS1 expression levels were consistently present in the style after pollination, and their expression levels were up-regulated within 24 h. This implied that they could co-regulate pollen tube growth when the pollen tube grew in the pistil.

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.

Evolution of Cell Wall Polymers in Tip-Growing Land Plant Gametophytes: Composition, Distribution, Functional Aspects and Their Remodeling

During evolution of land plants, the first colonizing species presented leafy-dominant gametophytes, found in non-vascular plants (bryophytes). Today, bryophytes include liverworts, mosses, and hornworts. In the first seedless vascular plants (lycophytes), the sporophytic stage of life started to be predominant. In the seed producing plants, gymnosperms and angiosperms , the gametophytic stage is restricted to reproduction. In mosses and ferns, the haploid spores germinate and form a protonema, which develops into a leafy gametophyte producing rhizoids for anchorage, water and nutrient uptakes. The basal gymnosperms (cycads and Ginkgo) reproduce by zooidogamy. Their pollen grains develop a multi-branched pollen tube that penetrates the nucellus and releases flagellated sperm cells that swim to the egg cell. The pollen grain of other gymnosperms (conifers and gnetophytes) as well as angiosperms germinates and produces a pollen tube that directly delivers the sperm cells to the ovule (siphonogamy). These different gametophytes, which are short or long-lived structures, share a common tip-growing mode of cell expansion. Tip-growth requires a massive cell wall deposition to promote cell elongation, but also a tight spatial and temporal control of the cell wall remodeling in order to modulate the mechanical properties of the cell wall. The growth rate of these cells is very variable depending on the structure and the species, ranging from very slow (protonemata, rhizoids, and some gymnosperm pollen tubes), to a slow to fast-growth in other gymnosperms and angiosperms. In addition, the structural diversity of the female counterparts in angiosperms (dry, semi-dry vs wet stigmas, short vs long, solid vs hollow styles) will impact the speed and efficiency of sperm delivery. As the evolution and diversity of the cell wall polysaccharides accompanied the diversification of cell wall structural proteins and remodeling enzymes, this review focuses on our current knowledge on the biochemistry, the distribution and remodeling of the main cell wall polymers (including cellulose, hemicelluloses, pectins, callose, arabinogalactan-proteins and extensins), during the tip-expansion of gametophytes from bryophytes, pteridophytes (lycophytes and monilophytes), gymnosperms and the monocot and eudicot angiosperms.

Glycosylphosphatidylinositol-Anchored Proteins in Arabidopsis and One of Their Common Roles in Signaling Transduction

Diverse proteins are found modified with glycosylphosphatidylinositol (GPI) at their carboxyl terminus in eukaryotes, which allows them to associate with membrane lipid bilayers and anchor on the external surface of the plasma membrane. GPI-anchored proteins (GPI-APs) play crucial roles in various processes, and more and more GPI-APs have been identified and studied. In this review, previous genomic and proteomic predictions of GPI-APs in Arabidopsis have been updated, which reveal their high abundance and complexity. From studies of individual GPI-APs in Arabidopsis, certain GPI-APs have been found associated with partner receptor-like kinases (RLKs), targeting RLKs to their subcellular localization and helping to recognize extracellular signaling polypeptide ligands. Interestingly, the association might also be involved in ligand selection. The analyses suggest that GPI-APs are essential and widely involved in signal transduction through association with RLKs.

Natural variation among Arabidopsis thaliana accessions in tolerance to high magnesium supply

High magnesium (Mg2+) in some extreme serpentine soils or semi-arid regions is an important factor affecting crop growth and development. Specific loci that form the genetic framework underlying high Mg2+ homeostasis, however, are not well understood. By using GWA mapping on 388 accessions of Arabidopsis thaliana selected from a worldwide collection and genotyped at approximately 250,00 SNPs, we successfully identified 109 and 74 putative genetic regions associated in nutrient traits under normal (1,000 µM) and high Mg2+ (10,000 µM), respectively. Above 90% SNPs associated with nutrients including Mg2+ and only two SNPs shared between normal and high Mg2+. A single strong peak of SNPs associated with Ca concentration corresponding to candidate gene At1g60420 ARABIDOPSIS NUCLEOREDOXIN (AtNRX1) under high Mg2+ was further determined. Compared with wildtype, mutants of Atnrx1-1 and Atnrx1-2 supplied with high Mg2+ had higher Ca concentrations in the plant, and higher cytosolic Ca2+ concentrations during root elongation, as well as higher fresh weight and lateral-root number. This suggests that AtNRX1 was a critical gene negatively regulating Ca uptake under high Mg2+ conditions. The discovery could help to breed/select crops that can adapt to high-Mg2+ soils such as serpentine soils (high ratio of Mg2+: Ca2+) or Mars soil with high levels of magnesium sulfate.

Immunolocalization of AGPs and Pectins in Quercus suber Gametophytic Structures

The arabinogalactan proteins (AGPs) are highly glycosylated proteins, ubiquitous in plants that have been linked to numerous aspects of sexual reproduction in several plant species, including the monoecious tree species Quercus suber. AGPs are found in cell membranes and cell walls of all types of tissues, including reproductive cells and organs. Pectins are cell wall components that also have been shown to change in composition and quantity during the maturations of the male and female gametophyte in cork oak. These findings were only possible to reveal, due to the histological study of AGP and pectins epitopes by immunolabeling. The immunofluorescence microscopy technique uses antibodies linked to fluorophores and relies on the specificity of the antibody binding to its antigen, labeling the epitope with a fluorescent dye.In the method presented here, we explore the immunolocalization technique performed in male and female flowers of Quercus suber, using London Resin (LR-White) as the embedding medium, after vacuum fixation with formaldehyde/glutaraldehyde. An extensive description of all the aspects of this technique is provided, from the plant material developmental stages selection to the critical analysis of results performed, continuously supported by troubleshooting recommendations.

The distinct functions of two classical arabinogalactan proteins BcMF8 and BcMF18 during pollen wall development in Brassica campestris

Arabinogalactan proteins (AGPs) are extensively glycosylated hydroxyproline-rich glycoproteins ubiquitous in all plant tissues and cells. AtAGP6 and AtAGP11, the only two functionally known pollen-specific classical AGP encoding genes in Arabidopsis, are reported to have redundant functions in microspore development. BcMF18 and BcMF8 isolated from Brassica campestris are the orthologues of AtAGP6 and AtAGP11, respectively. In contrast to the functional redundancy of AtAGP6 and AtAGP11, single-gene disruption of BcMF8 led to deformed pollen grains with abnormal intine development and ectopic aperture formation in B. campestris. Here, we further explored the action of BcMF18 and its relationship with BcMF8. BcMF18 was specifically expressed in pollen during the late stages of microspore development. Antisense RNA transgenic lines with BcMF18 reduction resulted in aberrant pollen grains with abnormal cellulose distribution, lacking intine, cytoplasm and nuclei. Transgenic plants with repressive expression of both BcMF8 and BcMF18 showed a hybrid phenotype, expressing a mixture of the phenotypes of the single gene knockdown plant lines. In addition, we identified functional diversity between BcMF18/BcMF8 and AtAGP6/AtAGP11, mainly reflected by the specific contribution of BcMF18 and BcMF8 to pollen wall formation. These results suggest that, unlike the orthologous genes AtAGP6 and AtAGP11 in Arabidopsis, BcMF18 and BcMF8 are both integral to pollen biogenesis in B. campestris, acting through independent pathways during microspore development.

Insights into the Evolution of Hydroxyproline-Rich Glycoproteins from 1000 Plant Transcriptomes

The carbohydrate-rich cell walls of land plants and algae have been the focus of much interest given the value of cell wall-based products to our current and future economies. Hydroxyproline-rich glycoproteins (HRGPs), a major group of wall glycoproteins, play important roles in plant growth and development, yet little is known about how they have evolved in parallel with the polysaccharide components of walls. We investigate the origins and evolution of the HRGP superfamily, which is commonly divided into three major multigene families: the arabinogalactan proteins (AGPs), extensins (EXTs), and proline-rich proteins. Using motif and amino acid bias, a newly developed bioinformatics pipeline, we identified HRGPs in sequences from the 1000 Plants transcriptome project (www.onekp.com). Our analyses provide new insights into the evolution of HRGPs across major evolutionary milestones, including the transition to land and the early radiation of angiosperms. Significantly, data mining reveals the origin of glycosylphosphatidylinositol (GPI)-anchored AGPs in green algae and a 3- to 4-fold increase in GPI-AGPs in liverworts and mosses. The first detection of cross-linking (CL)-EXTs is observed in bryophytes, which suggests that CL-EXTs arose though the juxtaposition of preexisting SPn EXT glycomotifs with refined Y-based motifs. We also detected the loss of CL-EXT in a few lineages, including the grass family (Poaceae), that have a cell wall composition distinct from other monocots and eudicots. A key challenge in HRGP research is tracking individual HRGPs throughout evolution. Using the 1000 Plants output, we were able to find putative orthologs of Arabidopsis pollen-specific GPI-AGPs in basal eudicots.

Gene Mining for Proline Based Signaling Proteins in Cell Wall of Arabidopsis thaliana

The cell wall (CW) as a first line of defense against biotic and abiotic stresses is of primary importance in plant biology. The proteins associated with cell walls play a significant role in determining a plant's sustainability to adverse environmental conditions. In this work, the genes encoding cell wall proteins (CWPs) in Arabidopsis were identified and functionally classified using geneMANIA and GENEVESTIGATOR with published microarrays data. This yielded 1605 genes, out of which 58 genes encoded proline-rich proteins (PRPs) and glycine-rich proteins (GRPs). Here, we have focused on the cellular compartmentalization, biological processes, and molecular functioning of proline-rich CWPs along with their expression at different plant developmental stages. The mined genes were categorized into five classes on the basis of the type of PRPs encoded in the cell wall of Arabidopsis thaliana. We review the domain structure and function of each class of protein, many with respect to the developmental stages of the plant. We have then used networks, hierarchical clustering and correlations to analyze co-expression, co-localization, genetic, and physical interactions and shared protein domains of these PRPs. This has given us further insight into these functionally important CWPs and identified a number of potentially new cell-wall related proteins in A. thaliana.

SUPPRESSOR OF FRIGIDA (SUF4) Supports Gamete Fusion via Regulating Arabidopsis EC1 Gene Expression

The EGG CELL1 (EC1) gene family of Arabidopsis (Arabidopsis thaliana) comprises five members that are specifically expressed in the egg cell and redundantly control gamete fusion during double fertilization. We investigated the activity of all five EC1 promoters in promoter-deletion studies and identified SUF4 (SUPPRESSOR OF FRIGIDA4), a C₂H₂ transcription factor, as a direct regulator of the EC1 gene expression. In particular, we demonstrated that SUF4 binds to all five Arabidopsis EC1 promoters, thus regulating their expression. The down-regulation of SUF4 in homozygous suf4-1 ovules results in reduced EC1 expression and delayed sperm fusion, which can be rescued by expressing SUF4-β-glucuronidase under the control of the SUF4 promoter. To identify more gene products able to regulate EC1 expression together with SUF4, we performed coexpression studies that led to the identification of MOM1 (MORPHEUS' MOLECULE1), a component of a silencing mechanism that is independent of DNA methylation marks. In mom1-3 ovules, both SUF4 and EC1 genes are down-regulated, and EC1 genes show higher levels of histone 3 lysine-9 acetylation, suggesting that MOM1 contributes to the regulation of SUF4 and EC1 gene expression.

Structure-Activity Relation of AMOR Sugar Molecule That Activates Pollen-Tubes for Ovular Guidance

Successful fertilization in flowering plants depends on the precise directional growth control of pollen tube through the female pistil tissue toward the female gametophyte contained in the ovule for delivery of nonmotile sperm cells. Cys-rich peptides LUREs secreted from the synergid cells on either side of the egg cell act as ovular attractants of pollen tubes. Competency control by the pistil is crucial for the response of pollen tubes to these ovular attractants. We recently reported that ovular 4-O-methyl-glucuronosyl arabinogalactan (AMOR) induces competency of the pollen tube to respond to ovular attractant LURE peptides in Torenia fournieri. The beta isomer of the terminal disaccharide 4-O-methyl-glucuronosyl galactose was essential and sufficient for the competency induction. However, critical and noncritical structures in the disaccharide have not been dissected deeply. Herein, we report the synthesis of new AMOR analogs and the structure-activity relationships for AMOR activity in the presence of these synthesized analogs. Removal of 4-O-methyl group or -COOH from the glucuronosyl residue of the disaccharide dramatically reduces AMOR activity. The pyranose backbone of the second sugar of disaccharide is essential for the activity but not hydroxy groups. The role of beta isomer of the disaccharide 4-Me-GlcA-β(1,6)-Gal is very specific for competency control, as there was no difference in effect among the sugar analogs tested for pollen germination. This study represents the first structure-activity relationship study, to our knowledge, of a sugar molecule involved in plant reproduction, which opens a way for modification of the molecule without loss of activity.

In Favor of Establishment: Regulation of Chromatid Cohesion in Plants

In eukaryotic organisms, the correct regulation of sister chromatid cohesion, whereby sister chromatids are paired and held together, is essential for accurate segregation of the sister chromatids and homologous chromosomes into daughter cells during mitosis and meiosis, respectively. Sister chromatid cohesion requires a cohesin complex comprised of structural maintenance of chromosome adenosine triphosphatases and accessory proteins that regulate the association of the complex with chromosomes or that are involved in the establishment or release of cohesion. The cohesin complex also plays important roles in the repair of DNA double-strand breaks, regulation of gene expression and chromosome condensation. In this review, we summarize progress in understanding cohesion dynamics in plants, with the aim of uncovering differences at specific stages. We also highlight dissimilarities between plants and other eukaryotes with respect to the key players involved in the achievement of cohesion, pointing out areas that require further study.

WSA206, a paralog of duplicated MPF2-like MADS-box family is recruited in fertility function in Withania

Best known for their implication in calyx inflation, MPF2-like genes pertinent to the STMADS11 clade of the MADS-box family exert their functions in leaf development, flowering time, inflorescence architecture and floral reversion to just name but a few. However, our knowledge about their involvement in fertility function remained obscure. Therefore the major thrust of this study was to probe the recruitment of WSA206 (MPF2-like) protein in fertility function. The WSA206 functions were revealed by knocking down and overexpressing this protein in Withania somnifera. The WSA206 promoter functions were defined by stable integration in Arabidopsis using GUS tag. The interactions of WSA206 were investigated by screening Arabidopsis Oligo-dT yeast library and YFP-split analysis. WSA206 knockdown plants revealed fewer flowers, abortion in seed set, reduction in pollen number and deformed non-viable pollen in comparison with wild type counterparts. Overexpression of WSA206 in Withania generated more berries/seeds and healthier viable pollen grains. Remarkably, along with fertility control, the impairment in calyx inflation in knockdown Withania plants and extraordinary growth of sepals in overexpression lines is observed. Thus, fertility and calyx inflation are tightly coupled traits under the control of WSA206. Coding sequence revealed SNP mutations from arginine to lysine as well as a leucine-rich motif duplication at the C-terminus, a characteristic feature of pollen specific and fertility function proteins. The protein-protein interaction spectrum of WSA206 comprises 40% of those MADS and non-MADS-box proteins implicated in floral/anther expression and embryogenesis. Predominant WSA206 promoter:GUS expression accrued in the anthers/pollen may be attributed to of the presence of GAAATTGTTA pollen specific proximal motifs along with several other anther specific homotypic cis-clusters. MPF2-like protein WSA206 through interactions with MADS-box and non-MADS-domain proteins confers male fertility in Withania regulated by anther/pollen specific promoter motif GAAATTGTTA.

BnPME is progressively induced after microspore reprogramming to embryogenesis, correlating with pectin de-esterification and cell differentiation in Brassica napus

BACKGROUND

Pectins are one of the main components of plant cell walls. They are secreted to the wall as highly methylesterified forms that can be de-esterified by pectin methylesterases (PMEs). The degree of methylesterification of pectins changes during development, PMEs are involved in the cell wall remodeling that occurs during diverse plant developmental processes. Nevertheless, the functional meaning of pectin-related wall remodeling in different cell types and processes remains unclear. In vivo, the microspore follows the gametophytic pathway and differentiates to form the pollen grain. In vitro, the microspore can be reprogrammed by stress treatments becoming a totipotent cell that starts to proliferate and follows the embryogenic pathway, a process known as microspore embryogenesis.

RESULTS

To investigate if the change of developmental programme of the microspore towards embryogenesis involves changes in pectin esterification levels, which would cause the cell wall remodeling during the process, in the present study, dynamics of PME expression and degrees of pectin esterification have been analysed during microspore embryogenesis and compared with the gametophytic development, in Brassica napus. A multidisciplinary approach has been adopted including BnPME gene expression analysis by quantitative RT-PCR, fluorescence in situ hybridization, immuno-dot-blot and immunofluorescence with JIM5 and JIM7 antibodies to reveal low and highly-methylesterified pectins. The results showed that cell differentiation at advanced developmental stages involved induction of BnPME expression and pectin de-esterification, processes that were also detected in zygotic embryos, providing additional evidence that microspore embryogenesis mimics zygotic embryogenesis. By contrast, early microspore embryogenesis, totipotency and proliferation were associated with low expression of BnPME and high levels of esterified pectins.

CONCLUSIONS

The results show that the change of developmental programme of the microspore involves changes in pectin esterification associated with proliferation and differentiation events, which may cause the cell wall remodeling during the process. The findings indicate pectin-related modifications in the cell wall during microspore embryogenesis, providing new insights into the role of pectin esterification and cell wall configuration in microspore totipotency, embryogenesis induction and progression.

Importance of organellar proteins, protein translocation and vesicle transport routes for pollen development and function

Protein translocation. Cellular homeostasis strongly depends on proper distribution of proteins within cells and insertion of membrane proteins into the destined membranes. The latter is mediated by organellar protein translocation and the complex vesicle transport system. Considering the importance of protein transport machineries in general it is foreseen that these processes are essential for pollen function and development. However, the information available in this context is very scarce because of the current focus on deciphering the fundamental principles of protein transport at the molecular level. Here we review the significance of protein transport machineries for pollen development on the basis of pollen-specific organellar proteins as well as of genetic studies utilizing mutants of known organellar proteins. In many cases these mutants exhibit morphological alterations highlighting the requirement of efficient protein transport and translocation in pollen. Furthermore, expression patterns of genes coding for translocon subunits and vesicle transport factors in Arabidopsis thaliana are summarized. We conclude that with the exception of the translocation systems in plastids-the composition and significance of the individual transport systems are equally important in pollen as in other cell types. Apparently for plastids only a minimal translocon, composed of only few subunits, exists in the envelope membranes during maturation of pollen. However, only one of the various transport systems known from thylakoids seems to be required for the function of the "simple thylakoid system" existing in pollen plastids. In turn, the vesicle transport system is as complex as seen for other cell types as it is essential, e.g., for pollen tube formation.

Arabinogalactan proteins and pectin distribution during female gametogenesis in Quercus suber L

BACKGROUND AND AIMS

Quercus suber L. (cork oak) is one of the most important monoecious tree species in semi-arid regions of Southern Europe, with a high ecological value and economic potential. However, as a result of its long reproductive cycle, complex reproductive biology and recalcitrant seeds, conventional breeding is demanding. In its complex reproductive biology, little is known about the most important changes that occur during female gametogenesis. Arabinogalactan proteins (AGPs) and pectins are the main components of plant cell walls and have been reported to perform common functions in cell differentiation and organogenesis of reproductive plant structures. AGPs have been shown to serve as important molecules in several steps of the reproductive process in plants, working as signalling molecules, associated with the sporophyte-gametophyte transition, and pectins have been implicated in pollen-pistil interactions before double fertilization. In this study, the distribution of AGP and pectin epitopes was assessed during female gametogenesis.

METHODS

Immunofluorescence labelling of female flower cells was performed with a set of monoclonal antibodies (mAbs) directed to the carbohydrate moiety of AGPs (JIM8 and JIM13) and pectic homogalacturonans (HGs) (mAbs JIM5 and JIM7).

KEY RESULTS

The selective labelling obtained with AGP and pectin mAbs JIM8, JIM13, JIM5 and JIM7 during Q. suber female gametogenesis shows that AGPs and pectic HG can work as markers for mapping gametophytic cell differentiation in this species. Pectic HG showed different distribution patterns, depending on their levels of methyl esterification. Methyl-esterified HGs showed a uniform distribution in the overall female flower cells before fertilization and a more specific pattern after fertilization. A low methyl-ester pectin distribution pattern during the different developmental stages appears to be related to the pathway that pollen tubes follow to reach the embryo sac. AGPs showed a more sparse distribution in early stages of development, but specific labelling is shown in the synergids and their filiform apparatus.

CONCLUSIONS

The labelling obtained with anti-AGP and anti-pectin mAbs in Q. suber female flower cells showed a dynamic distribution of AGPs and pectic HGs, which may render these molecules useful molecular markers during female gametogenesis. Changes occurring during development will be determined in order to help describe cork oak ovule structural properties before and after fertilization, providing new insight to better understand Q. suber female gametogenesis.

Auxin Biosynthesis, Accumulation, Action and Transport are Involved in Stress-Induced Microspore Embryogenesis Initiation and Progression in Brassica napus

Isolated microspores are reprogrammed in vitro by stress, becoming totipotent cells and producing embryos and plants via a process known as microspore embryogenesis. Despite the abundance of data on auxin involvement in plant development and embryogenesis, no data are available regarding the dynamics of auxin concentration, cellular localization and the expression of biosynthesis genes during microspore embryogenesis. This work involved the analysis of auxin concentration and cellular accumulation; expression of TAA1 and NIT2 encoding enzymes of two auxin biosynthetic pathways; expression of the PIN1-like efflux carrier; and the effects of inhibition of auxin transport and action by N-1-naphthylphthalamic acid (NPA) and α-(p-chlorophenoxy) isobutyric acid (PCIB) during Brassica napus microspore embryogenesis. The results indicated de novo auxin synthesis after stress-induced microspore reprogramming and embryogenesis initiation, accompanying the first cell divisions. The progressive increase of auxin concentration during progression of embryogenesis correlated with the expression patterns of TAA1 and NIT2 genes of auxin biosynthetic pathways. Auxin was evenly distributed in early embryos, whereas in heart/torpedo embryos auxin was accumulated in apical and basal embryo regions. Auxin efflux carrier PIN1-like gene expression was induced in early multicellular embryos and increased at the globular/torpedo embryo stages. Inhibition of polar auxin transport (PAT) and action, by NPA and PCIB, impaired embryo development, indicating that PAT and auxin action are required for microspore embryo progression. NPA also modified auxin embryo accumulation patterns. These findings indicate that endogenous auxin biosynthesis, action and polar transport are required in stress-induced microspore reprogramming, embryogenesis initiation and progression.

A decade of pollen transcriptomics

KEY MESSAGE

Overview of pollen transcriptome studies. Pollen development is driven by gene expression, and knowledge of the molecular events underlying this process has undergone a quantum leap in the last decade through studies of the transcriptome. Here, we outline historical evidence for male haploid gene expression and review the wealth of pollen transcriptome data now available. Knowledge of the transcriptional capacity of pollen has progressed from genetic studies to the direct analysis of RNA and from gene-by-gene studies to analyses on a genomic scale. Microarray and/or RNA-seq data can now be accessed for all phases and cell types of developing pollen encompassing 10 different angiosperms. These growing resources have accelerated research and will undoubtedly inspire new directions and the application of system-based research into the mechanisms that govern the development, function and evolution of angiosperm pollen.

WsMAGO2, a duplicated MAGO NASHI protein with fertility attributes interacts with MPF2-like MADS-box proteins

WsMAGO2 a duplicated protein in Withania through interactions with MPF2-like proteins affects male fertility by producing fewer flowers and aborted non-viable pollens/seeds regulated by anther-specific GAATTTGTGA motif. The MAGO NASHIs are highly conserved genes that encode proteins known to be involved in RNA physiology and many other developmental processes including germ cell differentiation in animals. However, their structural and functional implications in plants as fertility function proteins remained fragmented. MAGO (shorter name of MAGO NASHI) proteins form heterodimers with MPF2-like MADS-box proteins which are recruited in calyx identity and male fertility in Solanaceous plants. Four MAGO genes namely WsMAGO1 and WsMAGO2 and TaMAGO1 and TaMAGO2 were isolated from Withania somnifera and Tubocapsicum anomalum, respectively. These genes have duplicated probably due to whole genome duplication event. Dysfunction of WsMAGO2 through double-stranded RNAi in Withania revealed suppression of RNA transcripts, non-viable pollens, fewer flowers and aborted non-viable seeds in the developing berry suggesting a role of this protein in many traits particularly male fertility. WsMAGO2 flaunted stronger yeast 2-hybrid interactions with MPF2-like proteins WSA206, WSB206 and TAB201 than other MAGO counterparts. The native transcripts of WsMAGO2 culminated in stamens and seed-bearing berries though other MAGO orthologs also exhibited expression albeit at lower level. Coding sequences of the two orthologs are highly conserved, but they differ substantially in their upstream promoter regions. Remarkably, WsMAGO2 promoter is enriched with many anther-specific cis-motifs common in fertility function genes promoters. Among them, disruption of GAATTTGTGA abolished YFP/GUS gene expression in anthers alluding towards its involvement in regulating expression of MAGO in anther. Our findings support a possible recruitment of WsMAGO2 in fertility trait in Withania. These genes have practical application in hybrid production through cytoplasmic male sterility maintenance for enhancement in crops yield.

Arabinogalactan proteins: rising attention from plant biologists

Key message: AGP update: plant reproduction. Arabinogalactan proteins (AGPs) are a large family of hydroxyproline-rich proteins, heavily glycosylated, ubiquitous in land plants, including basal angiosperms and also in many algae. They have been shown to serve as important molecules in several steps of the reproductive process in plants. Due to their special characteristics, such as high sugar content and their means of association with the membrane, they are often perceived as likely candidates for many different aspects of the reproductive process such as signalling molecules, cell identity determinants, morphogens, nutrient sources and support for pollen tube growth, among others. Nevertheless, the study of these proteins pose many difficulties when it comes to studying them individually. Most of the work done involved the use of the β-glucosyl Yariv reagent and antibodies that recognize the carbohydrate epitopes only. Recently, new approaches have been used to study AGPs largely based in the remarkable growing volume of microarray data made available. Either using older techniques or the most recent ones, a clearer picture is emerging for the functions and mode of action of these molecules in the plant reproductive processes. Here, we present an overview about the most important studies made in this area, focusing on the latest advances and the possibilities for future studies in the field.

Arabidopsis AT-hook protein TEK positively regulates the expression of arabinogalactan proteins for Nexine formation

Nexine is a conserved layer of the pollen wall. We previously reported that the nexine layer is absent in the knockout mutant of Arabidopsis TRANSPOSABLE ELEMENT SILENCING VIA AT-HOOK (TEK) gene. In this study, we investigated the molecular regulatory functions of TEK in pollen development and identified the genes encoding Arabinogalactan proteins (AGPs) as direct targets of TEK, which are essential for nexine formation. Phenotypic similarity between tek and the TEK-SRDX transgenic lines suggest that TEK plays a role in transcriptional activation in anther development. Microarray analysis identified a total of 661 genes downregulated in tek, including four genes encoding AGPs, AGP6, AGP11, AGP23, and AGP40. Electrophoretic mobility shift assays showed that TEK could directly bind the nuclear matrix attachment region (MAR) and the promoter of AGP6. Chromatin immunoprecipitation followed by PCR analysis demonstrated that TEK is enriched in the promoters of the four AGP genes. Expression of AGP6 driven by the TEK promoter in tek partially rescued both nexine formation and plant fertility. These results indicate that TEK directly regulates AGP expression in the anther to control nexine layer formation. We also proposed that glycoproteins might be essential components of the nexine layer in the pollen wall.

Cell wall glycoproteins at interaction sites between parasitic giant dodder (Cuscuta reflexa) and its host Pelargonium zonale

The process of host plant penetration by parasitic dodder (genus Cuscuta) is accompanied by molecular and structural changes at the host/parasite interface. Recently, changes in pectin methyl esterification levels in the host cell walls abutting parasitic cells in established infection sites were reported. In addition to that, we show here that the composition of cell wall glycoproteins in Cuscuta-infected Pelargonium zonale undergoes substantial changes. While several arabinogalactan protein epitopes exhibit decreased abundances in the vicinity of the Cuscuta reflexa haustorium, extensins tend to increase in the infected areas.

Evaluation of the presence of arabinogalactan proteins and pectins during Quercus suber male gametogenesis

BACKGROUND AND AIMS

Quercus suber (cork oak) is a dominant tree of the Fagaceae in forests of the south-west Iberian Peninsula. It is monoecious with a long progamic phase that provides a comprehensive system for comparative studies in development and sexual reproduction. In this study the distribution of arabinogalactan protein (AGPs) and pectin epitopes in anthers of Q. suber was assessed to map these hydroxyproline-rich glycoproteins and the galacturonate-rich acidic polysaccharides during pollen development. Methods Immunolocalization in male flowers was performed with a set of monoclonal antibodies directed against the carbohydrate moiety that recognizes AGPs and pectins. To identify AGP genes involved in cork oak male flower development, a search was conducted for annotated AGP genes in the available transcriptome data of the Cork Oak EST Consortium database (www.corkoakdb.org).

KEY RESULTS

Ubiquitous labelling in all cell types was obtained with anti-homogalacturan antibodies for methyl-esterified pectins. In contrast, the antibody that labelled non-methyl-esterified homogalacturans had a preferential presence in microsporocyte cells walls at the beginning of pollen development. Intense labelling was obtained with anti-AGP antibodies both in the tapetum and in the intine wall near the pollen apertures and later in the generative cell wall and vegetative cell. Evaluation of the putative AGPs highly expressed in the male gametophyte was achieved by quantitative RT-PCR analysis in male and female cork oak flowers.

CONCLUSIONS

Four putative AGP genes were identified that are preferentially expressed in the male flower compared with the female flower. The putative Arabidopsis thaliana orthologues of these genes are associated with preferential expression in pollen, suggesting that the AGPs probably play a significant role in cork oak reproduction.

Back to the future with the AGP-Ca2+ flux capacitor

BACKGROUND

Arabinogalactan proteins (AGPs) are ubiquitous in green plants. AGPs comprise a widely varied group of hydroxyproline (Hyp)-rich cell surface glycoproteins (HRGPs). However, the more narrowly defined classical AGPs massively predominate and cover the plasma membrane. Extensive glycosylation by pendant polysaccharides O-linked to numerous Hyp residues like beads of a necklace creates a unique ionic compartment essential to a wide range of physiological processes including germination, cell extension and fertilization. The vital clue to a precise molecular function remained elusive until the recent isolation of small Hyp-arabinogalactan polysaccharide subunits; their structural elucidation by nuclear magentic resonance imaging, molecular simulations and direct experiment identified a 15-residue consensus subunit as a β-1,3-linked galactose trisaccharide with two short branched sidechains each with a single glucuronic acid residue that binds Ca(2+) when paired with its adjacent sidechain.

SCOPE

AGPs bind Ca(2+) (Kd ∼ 6 μm) at the plasma membrane (PM) at pH ∼5·5 but release it when auxin-dependent PM H(+)-ATPase generates a low periplasmic pH that dissociates AGP-Ca(2+) carboxylates (pka ∼3); the consequential large increase in free Ca(2+) drives entry into the cytosol via Ca(2+) channels that may be voltage gated. AGPs are thus arguably the primary source of cytosolic oscillatory Ca(2+) waves. This differs markedly from animals, in which cytosolic Ca(2+) originates mostly from internal stores such as the sarcoplasmic reticulum. In contrast, we propose that external dynamic Ca(2+) storage by a periplasmic AGP capacitor co-ordinates plant growth, typically involving exocytosis of AGPs and recycled Ca(2+), hence an AGP-Ca(2+) oscillator.

CONCLUSIONS

The novel concept of dynamic Ca(2+) recycling by an AGP-Ca(2+) oscillator solves the long-standing problem of a molecular-level function for classical AGPs and thus integrates three fields: AGPs, Ca(2+) signalling and auxin. This accounts for the involvement of AGPs in plant morphogenesis, including tropic and nastic movements.

Differential expression patterns of arabinogalactan proteins in Arabidopsis thaliana reproductive tissues

Arabinogalactan proteins (AGPs) are heavily glycosylated proteins existing in all members of the plant kingdom and are differentially distributed through distinctive developmental stages. Here, we showed the individual distributions of specific Arabidopsis AGPs: AGP1, AGP9, AGP12, AGP15, and AGP23, throughout reproductive tissues and indicated their possible roles in several reproductive processes. AGP genes specifically expressed in female tissues were identified using available microarray data. This selection was confirmed by promoter analysis using multiple green fluorescent protein fusions to a nuclear localization signal, β-glucuronidase fusions, and in situ hybridization as approaches to confirm the expression patterns of the AGPs. Promoter analysis allowed the detection of a specific and differential presence of these proteins along the pathway followed by the pollen tube during its journey to reach the egg and the central cell inside the embryo sac. AGP1 was expressed in the stigma, style, transmitting tract, and the chalazal and funiculus tissues of the ovules. AGP9 was present along the vasculature of the reproductive tissues and AGP12 was expressed in the stigmatic cells, chalazal and funiculus cells of the ovules, and in the septum. AGP15 was expressed in all pistil tissues, except in the transmitting tract, while AGP23 was specific to the pollen grain and pollen tube. The expression pattern of these AGPs provides new evidence for the detection of a subset of specific AGPs involved in plant reproductive processes, being of significance for this field of study. AGPs are prominent candidates for male-female communication during reproduction.

Changes in histone methylation and acetylation during microspore reprogramming to embryogenesis occur concomitantly with Bn HKMT and Bn HAT expression and are associated with cell totipotency, proliferation, and differentiation in Brassica napus

In response to stress treatments, microspores can be reprogrammed to become totipotent cells that follow an embryogenic pathway producing haploid and double-haploid embryos which are important biotechnological tools in plant breeding. Recent studies have revealed the involvement of DNA methylation in regulating this process, but no information is available on the role of histone modifications in microspore embryogenesis. Histone modifications are major epigenetic marks controlling gene expression during plant development and in response to environmental changes. Lysine methylation of histones, accomplished by histone lysine methyltransferases (HKMTs), can occur on different lysine residues, with histone H3K9 methylation being mainly associated with transcriptionally silenced regions. In contrast, histone H3 and H4 acetylation is carried out by histone acetyltransferases (HATs) and is associated with actively transcribed genes. In this work, we analyzed 3 different histone epigenetic marks: dimethylation of H3K9 (H3K9me2) and acetylation of H3 and H4 (H3Ac and H4Ac) during microspore embryogenesis in Brassica napus by Western blot and immunofluorescence assays. The expression patterns of histone methyltransferase BnHKMT and histone acetyltransferase BnHAT genes have also been analyzed by qPCR. Our results revealed different spatial and temporal distribution patterns for methylated and acetylated histone variants during microspore embryogenesis and their similarity with the expression profiles of BnHKMT and BnHAT, respectively. The data presented suggest the participation of H3K9me2 and HKMT in embryo cell differentiation and heterochromatinization events, whereas H3Ac, H4Ac, and HAT would be involved in transcriptional activation, totipotency, and proliferation events during cell reprogramming and embryo development.