The cytological changes of tobacco zygote and proembryo cells induced by beta-glucosyl Yariv reagent suggest the involvement of arabinogalactan proteins in cell division and cell plate formation

CsREV-CsTCP4-CsVND7 module shapes xylem patterns differentially between stem and leaf to enhance tea plant tolerance to drought

Cultivating drought-tolerant tea varieties enhances both yield and quality of tea plants in northern China. However, the mechanisms underlying their drought tolerance remain largely unknown. Here we identified a key regulator called CsREV, which differentially regulates xylem patterns between leaves and stems, thereby conferring drought tolerance in tea plants. When drought occurs, upregulation of CsREV activates the CsVND7a-dependent xylem vessel differentiation. However, when drought persists, the vessel differentiation is hindered as CsVND7a is downregulated by CsTCP4a. This, combined with the CsREV-promoted secondary-cell-wall thickness of xylem vessel, leads to the enhanced curling of leaves, a characteristic closely associated with plant drought tolerance. Notably, this inhibitory effect of CsTCP4a on CsVND7a expression is absent in stems, allowing stem xylem vessels to continuously differentiate. Overall, the CsREV-CsTCP4-CsVND7 module is differentially utilized to shape the xylem patterns in leaves and stems, potentially balancing water transportation and utilization to improve tea plant drought tolerance.

Applications of the Yariv reagent in polysaccharide analysis and plant physiology from theory to practice

Arabinogalactan (AG), a biologically active substance found abundantly in plants, is of significant interest in plant physiology due to its unique physicochemical properties. Yariv reagent, widely utilized in AG-II related applications, forms insoluble precipitates when bound to AG-II. This paper provides a comprehensive overview of the synthesis methods, physicochemical properties, and various dissociation methods of the Yariv reagent to enhance its utility in AG-II studies. Furthermore, the review explores the binding mechanisms and applications of the Yariv reagent, highlighting the advancements in studying the Yariv-AG complex in plant physiology. The aim of this review is to inspire new research ideas and foster novel applications of the Yariv reagent from synthesis to implementation.

Single-cell RNA sequencing analysis of the embryogenic callus clarifies the spatiotemporal developmental trajectories of the early somatic embryo in Dimocarpus longan

Plant embryogenic calli (ECs) can undergo somatic embryogenesis to regenerate plants. This process is mediated by regulatory factors, such as transcription factors and specifically expressed genes, but the precise molecular mechanisms underlying somatic embryogenesis at the single-cell level remain unclear. In this study, we performed high-resolution single-cell RNA sequencing analysis to determine the cellular changes in the EC of the woody plant species Dimocarpus longan (longan) and clarify the continuous cell differentiation trajectories at the transcriptome level. The highly heterogeneous cells in the EC were divided into 12 putative clusters (e.g., proliferating, meristematic, vascular, and epidermal cell clusters). We determined cluster-enriched expression marker genes and found that overexpression of the epidermal cell marker gene GDSL ESTERASE/LIPASE-1 inhibited the hydrolysis of triacylglycerol. In addition, the stability of autophagy was critical for the somatic embryogenesis of longan. The pseudo-timeline analysis elucidated the continuous cell differentiation trajectories from early embryonic cell division to vascular and epidermal cell differentiation during the somatic embryogenesis of longan. Moreover, key transcriptional regulators associated with cell fates were revealed. We found that ETHYLENE RESPONSIVE FACTOR 6 was characterized as a heat-sensitive factor that negatively regulates longan somatic embryogenesis under high-temperature stress conditions. The results of this study provide new spatiotemporal insights into cell division and differentiation during longan somatic embryogenesis at single-cell resolution.

Multi-omic characterization of the maize GPI synthesis mutant gwt1 with defects in kernel development

BACKGROUND

Glycosylphosphatidylinositol (GPI) and GPI-anchored proteins (GAPs) are important for cell wall formation and reproductive development in Arabidopsis. However, monocot counterparts that function in kernel endosperm development have yet to be discovered. Here, we performed a multi-omic analysis to explore the function of GPI related genes on kernel development in maize.

RESULTS

In maize, 48 counterparts of human GPI synthesis and lipid remodeling genes were identified, in which null mutation of the glucosaminyl-phosphatidylinositol O-acyltransferase1 gene, ZmGWT1, caused a kernel mutant (named gwt1) with defects in the basal endosperm transport layer (BETL). We performed plasma membrane (PM) proteomics to characterize the potential GAPs involved in kernel development. In total, 4,981 proteins were successfully identified in 10-DAP gwt1 kernels of mutant and wild-type (WT), including 1,638 membrane-anchored proteins with different posttranslational modifications. Forty-seven of the 256 predicted GAPs were differentially accumulated between gwt1 and WT. Two predicted BETL-specific GAPs (Zm00001d018837 and Zm00001d049834), which kept similar abundance at general proteome but with significantly decreased abundance at membrane proteome in gwt1 were highlighted.

CONCLUSIONS

Our results show the importance of GPI and GAPs for endosperm development and provide candidate genes for further investigation of the regulatory network in which ZmGWT1 participates.

Identification and expression profiles of xylogen-like arabinogalactan protein (XYLP) gene family in Phyllostachys edulis in different developmental tissues and under various abiotic stresses

Xylogen-like arabinogalactan protein (XYLP) is an atypical lipid transport protein. In this study, 23 Phyllostachys edulis XYLPs were identified, and their proteins contain characteristic structures of AGP and nsLTP domain. All PeXYLPs can be divided into four clades, and their genes were unevenly distributed on 11 chromosome scaffolds. Collinear analysis revealed that segmental duplication was the main driver for PeXYLP family expansion. The cis-acting elements presented in the promoter are involved in various regulations of PeXYLPs expression. G.O. annotation revealed that PeXYLPs are mainly interested in lipid transport and synthesis and primarily function at the plasma membrane. Transcriptome analysis revealed that PeXYLPs were spatiotemporally expressed and displayed significant variability during various tissue development. Besides that, some PeXYLPs also respond to multiple phytohormones and abiotic stresses. By semi-quantitative RT-PCR, the response of some PeXYLPs to MeJA was confirmed, and the proteins were shown to localize to the plasma membrane mainly. WGCNA in defined regions of fast-growing bamboo shoots revealed that 5 PeXYLPs in 4 gene co-expression modules showed a positive module-trait relationship with three fast-growing regions. This systematic analysis of the PeXYLP family will provide a foundation for further insight into the functions of individual PeXYLP in a specific tissue or organ development, phytohormone perception, and stress responses in the future.

Surface-localized glycoproteins act through class C ARFs to fine-tune gametophore initiation in Physcomitrium patens

Arabinogalactan proteins are functionally diverse cell wall structural glycoproteins that have been implicated in cell wall remodeling, although the mechanistic actions remain elusive. Here, we identify and characterize two AGP glycoproteins, SLEEPING BEAUTY (SB) and SB-like (SBL), that negatively regulate the gametophore bud initiation in Physcomitrium patens by dampening cell wall loosening/softening. Disruption of SB and SBL led to accelerated gametophore formation and altered cell wall compositions. The function of SB is glycosylation dependent and genetically connected with the class C auxin response factor (ARF) transcription factors PpARFC1B and PpARFC2. Transcriptomics profiling showed that SB upregulates PpARFC2, which in turn suppresses a range of cell wall-modifying genes that are required for cell wall loosening/softening. We further show that PpARFC2 binds directly to multiple AuxRE motifs on the cis-regulatory sequences of PECTIN METHYLESTERASE to suppress its expression. Hence, our results demonstrate a mechanism by which the SB modulates the strength of intracellular auxin signaling output, which is necessary to fine-tune the timing of gametophore initials formation.

CkREV regulates xylem vessel development in Caragana korshinskii in response to drought

Drought stress poses severe threat to the development and even the survival status of plants. Plants utilize various methods responding to drought, among which the forming of more well-developed xylem in leaf vein in woody plants deserves our attention. Herein, we report a transcription factor CkREV from HD-ZIP III family in Caragana korshinskii, which possesses significant functions in drought response by regulating xylem vessel development in leaf vein. Research reveal that in C. korshinskii the expression level of CkREV located in xylem vessel and adjacent cells will increase as the level of drought intensifies, and can directly induce the expression of CkLAX3, CkVND6, CkVND7, and CkPAL4 by binding to their promoter regions. In Arabidopsis thaliana, CkREV senses changes in drought stress signals and bidirectionally regulates the expression of related genes to control auxin polar transport, vessel differentiation, and synthesis of cell wall deposits, thereby significantly enhancing plant drought tolerance. In conclusion, our findings offer a novel understanding of the regulation of CkREV, a determinant of leaf adaxial side, on the secondary development of xylem vessels in leaf vein to enhance stress tolerance in woody plants.

Plant cytokinesis and the construction of new cell wall

Cytokinesis in plants is fundamentally different from that in animals and fungi. In plant cells, a cell plate forms through the fusion of cytokinetic vesicles and then develops into the new cell wall, partitioning the cytoplasm of the dividing cell. The formation of the cell plate entails multiple stages that involve highly orchestrated vesicle accumulation, fusion, and membrane maturation, which occur concurrently with the timely deposition of polysaccharides such as callose, cellulose, and cross-linking glycans. This review summarizes the major stages in cytokinesis, endomembrane components involved in cell plate assembly and its transition to a new cell wall. An animation that can be widely used for educational purposes further summarizes the process.

Degradation of de-esterified pctin/homogalacturonan by the polygalacturonase GhNSP is necessary for pollen exine formation and male fertility in cotton

The pollen wall exine provides a protective layer for the male gametophyte and is largely composed of sporopollenin, which comprises fatty acid derivatives and phenolics. However, the biochemical nature of the external exine is poorly understood. Here, we show that the male sterile line 1355A of cotton mutated in NO SPINE POLLEN (GhNSP) leads to defective exine formation. The GhNSP locus was identified through map-based cloning and confirmed by genetic analysis (co-segregation test and allele prediction using the CRISPR/Cas9 system). In situ hybridization showed that GhNSP is highly expressed in tapetum. GhNSP encodes a polygalacturonase protein homologous to AtQRT3, which suggests a function for polygalacturonase in pollen exine formation. These results indicate that GhNSP is functionally different from AtQRT3, the latter has the function of microspore separation. Biochemical analysis showed that the percentage of de-esterified pectin was significantly increased in the 1355A anthers at developmental stage 8. Furthermore, immunofluorescence studies using antibodies to the de-esterified and esterified homogalacturonan (JIM5 and JIM7) showed that the Ghnsp mutant exhibits abundant of de-esterified homogalacturonan in the tapetum and exine, coupled with defective exine formation. The characterization of GhNSP provides new understanding of the role of polygalacturonase and de-esterified homogalacturonan in pollen exine formation.

Structural Basis of the Function of Yariv Reagent-An Important Tool to Study Arabinogalactan Proteins

Arabinogalactan proteins are very abundant, heavily glycosylated plant cell wall proteins. They are intensively studied because of their crucial role in plant development as well as their function in plant defence. Research of these biomacromolecules is complicated by the lack of tools for their analysis and characterisation due to their extreme heterogeneity. One of the few available tools for detection, isolation, characterisation, and functional studies of arabinogalactan proteins is Yariv reagents. Yariv reagent is a synthetic aromatic glycoconjugate originally prepared as an antigen for immunization. Later, it was found that this compound can precipitate arabinogalactan proteins, namely, their ß-D-(1→3)-galactan structures. Even though this compound has been intensively used for decades, the structural basis of arabinogalactan protein precipitation by Yariv is not known. Multiple biophysical studies have been published, but none of them attempted to elucidate the three-dimensional structure of the Yariv-galactan complex. Here we use a series of molecular dynamics simulations of systems containing one or multiple molecules of ß-D-galactosyl Yariv reagent with or without oligo ß-D-(1→3)-galactan to predict the structure of the complex. According to our model of Yariv-galactan complexes, Yariv reagent forms stacked oligomers stabilized by π-π and CH/π interactions. These oligomers may contain irregularities. Galactan structures crosslink these Yariv oligomers. The results were compared with studies in literature.

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.

Arabinogalactan Proteins in Plant Roots - An Update on Possible Functions

Responsiveness to environmental conditions and developmental plasticity of root systems are crucial determinants of plant fitness. These processes are interconnected at a cellular level with cell wall properties and cell surface signaling, which involve arabinogalactan proteins (AGPs) as essential components. AGPs are cell-wall localized glycoproteins, often GPI-anchored, which participate in root functions at many levels. They are involved in cell expansion and differentiation, regulation of root growth, interactions with other organisms, and environmental response. Due to the complexity of cell wall functional and regulatory networks, and despite the large amount of experimental data, the exact molecular mechanisms of AGP-action are still largely unknown. This dynamically evolving field of root biology is summarized in the present review.

An Improved Protocol for the Synthesis and Purification of Yariv Reagents

Yariv reagents are glycoconjugate tris-azo dyes widely used in plant biology. These reagents are synthesized by diazo coupling between phloroglucinol and a para-diazophenyl glycoside. Despite their synthetic accessibility, well-defined protocols for obtaining pure Yariv reagents, and their complete compound characterization data, have not been reported. We report here optimized protocols used to synthesize, purify, and characterize a panel of six Yariv reagents and suggest approaches that could be valuable for the purification and characterization of other glycoconjugates as well.

AtNSE1 and AtNSE3 are required for embryo pattern formation and maintenance of cell viability during Arabidopsis embryogenesis

Embryogenesis is an essential process during seed development in higher plants. It has previously been shown that mutation of the Arabidopsis non-SMC element genes AtNSE1 or AtNSE3 leads to early embryo abortion, and their proteins can interact with each other directly. However, the crucial regions of these proteins in this interaction and how the proteins are cytologically involved in Arabidopsis embryo development are unknown. In this study, we found that the C-terminal including the Ring-like motif of AtNSE1 can interact with the N-terminal of AtNSE3, and only the Ring-like motif is essential for binding with three α motifs of AtNSE2 (homologous to AtMMS21). Using genetic assays and by analysing molecular markers of cell fate decisions (STM, WOX5, and WOX8) in mutant nse1 and nse3 embryos, we found that AtNSE1 and AtNSE3 work non-redundantly in early embryo development, and that differentiation of the apical meristem and the hypophysis fails in the mutants, which have disrupted auxin transportation and responses. However, the upper cells of the suspensor in the mutants seem to have proper embryo cell identity. Cytological examination showed that cell death occurred from the early embryo stage, and that vacuolar programmed cell death and necrosis in the nse1 and nse3 mutant embryos led to ovule abortion. Thus, AtNSE1 and AtNSE3 are essential for maintaining cell viability and growth during early embryogenesis. Our results improve our understanding of the functions of SMC5/6 complex in early embryogenesis in Arabidopsis.

Gene expression and localization of arabinogalactan proteins during the development of anther, ovule, and embryo in rice

Arabinogalactan proteins (AGPs) are hyperglycosylated members of the hydroxyproline-rich glycoprotein (HRGP) superfamily and are widely distributed throughout the plant kingdom. In Oryza sativa (rice), the gene expression and biological function of AGPs only have received minimal research attention. Here, we used qRT-PCR to detect the expression patterns of OsAGP genes in various organs, and found that six genes were preferentially expressed in panicles, three genes were specifically expressed in anthers, and one gene in the stigma. Furthermore, using four specific monoclonal antibodies (JIM8, JIM13, LM2, MAC207), we observed the distribution of AGPs in rice anthers, ovules, and embryos. In anthers, the strong fluorescence signals of AGPs were present in tapetum cells, pollen mother cells, and mature pollens, suggesting that AGPs might be related to the development of anther and pollen. In ovules, signals of AGPs were specifically distributed in the three micropylar megaspores of the tetrad, and with intense signals in the egg cell and synergid cells in the mature embryo sac. This suggests that AGPs may be involved in megaspore determination and double fertilization. In embryos, the immunological signals of AGPs appeared in peripheral and inner cells at the early stage, and in the scutellum, plumule, and radicle at the late stage, indicating that AGPs may be associated with organ differentiation and maturation of embryos. In this study, we revealed that AGPs were widely distributed in rice anthers, ovules, and embryos, which lays a foundation for the functional investigation of AGPs in various processes of sexual reproduction.

Integration of mRNA and miRNA Analysis Reveals the Molecular Mechanism Underlying Salt and Alkali Stress Tolerance in Tobacco

Salinity is one of the most severe forms of abiotic stress and affects crop yields worldwide. Plants respond to salinity stress via a sophisticated mechanism at the physiological, transcriptional and metabolic levels. However, the molecular regulatory networks involved in salt and alkali tolerance have not yet been elucidated. We developed an RNA-seq technique to perform mRNA and small RNA (sRNA) sequencing of plants under salt (NaCl) and alkali (NaHCO3) stress in tobacco. Overall, 8064 differentially expressed genes (DEGs) and 33 differentially expressed microRNAs (DE miRNAs) were identified in response to salt and alkali stress. A total of 1578 overlapping DEGs, which exhibit the same expression patterns and are involved in ion channel, aquaporin (AQP) and antioxidant activities, were identified. Furthermore, genes involved in several biological processes, such as "photosynthesis" and "starch and sucrose metabolism," were specifically enriched under NaHCO3 treatment. We also identified 15 and 22 miRNAs that were differentially expressed in response to NaCl and NaHCO3, respectively. Analysis of inverse correlations between miRNAs and target mRNAs revealed 26 mRNA-miRNA interactions under NaCl treatment and 139 mRNA-miRNA interactions under NaHCO3 treatment. This study provides new insights into the molecular mechanisms underlying the response of tobacco to salinity stress.

Pectin De-methylesterification and AGP Increase Promote Cell Wall Remodeling and Are Required During Somatic Embryogenesis of Quercus suber

Somatic embryogenesis is a reliable system for in vitro plant regeneration, with biotechnological applications in trees, but the regulating mechanisms are largely unknown. Changes in cell wall mechanics controlled by methylesterification of pectins, mediated by pectin methylesterases (PMEs) and pectin methyl esterase inhibitors (PMEIs) underlie many developmental processes. Arabinogalactan proteins (AGPs) are highly glycosylated proteins located at the surface of plasma membranes, in cell walls, and in extracellular secretions, with key roles in a range of different processes. In this study, we have investigated changes in two cell wall components, pectins and AGPs, during somatic embryogenesis in Quercus suber, a forest tree of high economic and ecologic value. At early embryogenesis stages, cells of proembryogenic masses showed high levels of esterified pectins and expression of QsPME and QsPMEI genes encoding a PME and a putative PMEI, respectively. At advanced stages, differentiating cells of heart, torpedo and cotyledonary embryos exhibited walls rich in de-esterified pectins, while QsPME gene expression and PME activity progressively increased. AGPs were detected in cell walls of proembryogenic masses and somatic embryos. QsLys-rich-AGP18, QsLys-rich-AGP17, and QsAGP16L1 gene expression increased with embryogenesis progression, as did the level of total AGPs, detected by dot blot with β-glucosyl Yariv reagent. Immuno dot blot, immunofluorescence assays and confocal analysis using monoclonal antibodies to high- (JIM7, LM20) and low- (JIM5, LM19) methylesterified pectins, and to certain AGP epitopes (LM6, LM2) showed changes in the amount and distribution pattern of esterified/de-esterified pectins and AGP epitopes, that were associated with proliferation and differentiation and correlated with expression of the PME and AGP genes analyzed. Pharmacological treatments with catechin, an inhibitor of PME activity, and Yariv reagent, which blocks AGPs, impaired the progression of embryogenesis, with pectin de-esterification and an increase in AGP levels being necessary for embryo development. Findings indicate a role for pectins and AGPs during somatic embryogenesis of cork oak, promoting the cell wall remodeling during the process. They also provide new insights into the regulating mechanisms of somatic embryogenesis in woody species, for which information is still scarce, opening up new possibilities to improve in vitro embryo production in tree breeding.

Post-Golgi Trafficking and Transport of Cell Wall Components

The cell wall, a complex macromolecular composite structure surrounding and protecting plant cells, is essential for development, signal transduction, and disease resistance. This structure is also integral to cell expansion, as its tensile resistance is the primary balancing mechanism against internal turgor pressure. Throughout these processes, the biosynthesis, transport, deposition, and assembly of cell wall polymers are tightly regulated. The plant endomembrane system facilitates transport of polysaccharides, polysaccharide biosynthetic and modifying enzymes and glycoproteins through vesicle trafficking pathways. Although a number of enzymes involved in cell wall biosynthesis have been identified, comparatively little is known about the transport of cell wall polysaccharides and glycoproteins by the endomembrane system. This review summarizes our current understanding of trafficking of cell wall components during cell growth and cell division. Emerging technologies, such as vesicle glycomics, are also discussed as promising avenues to gain insights into the trafficking of structural polysaccharides to the apoplast.

Arabinogalactan Proteins Are Involved in Salt-Adaptation and Vesicle Trafficking in Tobacco by-2 Cell Cultures

Arabinogalactan proteins (AGPs) are a highly diverse family of glycoproteins that are commonly found in most plant species. However, little is known about the physiological and molecular mechanisms of their function. AGPs are involved in different biological processes such as cell differentiation, cell expansion, tissue development and somatic embryogenesis. AGPs are also involved in abiotic stress response such as salinity modulating cell wall expansion. In this study, we describe how salt-adaptation in tobacco BY-2 cell cultures induces important changes in arabinogalactan proteins distribution and contents. Using the immuno-dot blot technique with different anti-AGP antibodies (JIM13, JIM15, and others), we observed that AGPs were highly accumulated in the culture medium of salt-adapted tobacco cells, probably due to the action of phospholipases. We located these AGP epitopes using immunogold labeling in the cytoplasm associated to the endoplasmic reticulum, the golgi apparatus, and vesicles, plasma membrane and tonoplast. Our results show that salt-adaptation induced a significant reduction of the cytoplasm, plasma membrane and tonoplast content of these epitopes. Yariv reagent was added to the control and salt-adapted tobacco cell cultures, leading to cell death induction in control cells but not in salt-adapted cells. Ultrastructural and immunogold labeling revealed that cell death induced by Yariv reagent in control cells was due to the interaction of Yariv reagent with the AGPs linked to the plasma membranes. Finally, we propose a new function of AGPs as a possible sodium carrier through the mechanism of vesicle trafficking from the apoplast to the vacuoles in salt-adapted tobacco BY-2 cells. This mechanism may contribute to sodium homeostasis during salt-adaptation to high saline concentrations.

Arabinogalactan proteins have deep roots in eukaryotes: identification of genes and epitopes in brown algae and their role in Fucus serratus embryo development

Arabinogalactan proteins (AGPs) are highly glycosylated, hydroxyproline-rich proteins found at the cell surface of plants, where they play key roles in developmental processes. Brown algae are marine, multicellular, photosynthetic eukaryotes. They belong to the phylum Stramenopiles, which is unrelated to land plants and green algae (Chloroplastida). Brown algae share common evolutionary features with other multicellular organisms, including a carbohydrate-rich cell wall. They differ markedly from plants in their cell wall composition, and AGPs have not been reported in brown algae. Here we investigated the presence of chimeric AGP-like core proteins in this lineage. We report that the genome sequence of the brown algal model Ectocarpus siliculosus encodes AGP protein backbone motifs, in a gene context that differs considerably from what is known in land plants. We showed the occurrence of AGP glycan epitopes in a range of brown algal cell wall extracts. We demonstrated that these chimeric AGP-like core proteins are developmentally regulated in embryos of the order Fucales and showed that AGP loss of function seriously impairs the course of early embryogenesis. Our findings shine a new light on the role of AGPs in cell wall sensing and raise questions about the origin and evolution of AGPs in eukaryotes.

Live-cell imaging and optical manipulation of Arabidopsis early embryogenesis

Intercellular communications are essential for cell proliferation and differentiation during plant embryogenesis. However, analysis of intercellular communications in living material in real time is difficult owing to the restricted accessibility of the embryo within the flower. We established a live-embryo imaging system to visualize cell division and cell fate specification in Arabidopsis thaliana from zygote division in real time. We generated a cell-division lineage tree for early embryogenesis in Arabidopsis. Lineage analysis showed that both the direction and time course of cell division between sister cells differed along the apical-basal or radial axes. Using the Arabidopsis kpl mutant, in which single-fertilization events are frequent, we showed that endosperm development is not required for pattern formation during early embryogenesis. Optical manipulation demonstrated that damage to the embryo initial cell induces cell fate conversion of the suspensor cell to compensate for the disrupted embryo initial cell even after cell fate is specified.

Polysaccharide deposition during cytokinesis: Challenges and future perspectives

De novo formation of a new cell wall partitions the cytoplasm of the dividing cell during plant cytokinesis. The development of the cell plate, a transient sheet-like structure, requires the accumulation of vesicles directed by the phragmoplast to the cell plate assembly matrix. Fusion and fission of the accumulated vesicles are accompanied by the deposition of polysaccharides and cell wall structural proteins; together, they are leading to the stabilization of the formed structure which after insertion into the parental wall lead to the maturation of the nascent cross wall. Callose is the most abundant polysaccharide during cell plate formation and during maturation is gradually replaced by cellulose. Matrix polysaccharides such as hemicellulose, and pectins presumably are present throughout all developmental stages, being delivered to the cell plate by secretory vesicles. The availability of novel chemical probes such as endosidin 7, which inhibits callose formation at the cell plate, has proved useful for dissecting the temporal accumulation of vesicles at the cell plate and establishing the critical role of callose during cytokinesis. The use of emerging approaches such as chemical genomics combined with live cell imaging; novel techniques of polysaccharide detection including tagged polysaccharide substrates, newly characterized polysaccharide antibodies and vesicle proteomics can be used to develop a comprehensive model of cell plate development.

Identification, characterization, and transcription analysis of xylogen-like arabinogalactan proteins in rice (Oryza sativa L.)

BACKGROUND

Xylogen, a chimeric arabinogalactan protein containing a non-specific lipid transfer protein domain, can promote xylem cell differentiation. No comprehensive study has been carried out on the XYLP gene family in rice. As a first step in research on this gene family and as a useful strategy in general, a genome-wide analysis of the OsXYLP gene family is thus needed.

RESULTS

In this study, we identified 21 XYLP genes from the rice genome and comprehensively analyzed their protein structures, phylogenetic relationships, chromosomal locations, and gene duplication status. Our results indicate that gene duplication has played major roles in the expansion of the OsXYLP gene family. We used expressed sequence tag, microarray, massively parallel signature sequencing, and quantitative real-time PCR data to analyze OsXYLP gene expression during various developmental stages and under abiotic stress conditions. We found that many OsXYLP genes are abundantly expressed in vascular tissues and seeds, with some genes regulated under hormonal or abiotic stresses. In addition, we identified knockout mutants of OsXYLP7 and OsXYLP16 and discovered that the mutant xylp7 has a defect in stem height.

CONCLUSIONS

We analyzed expression profiles of 21 XYLP genes and characterized the structures and evolutionary relationships of their proteins. Our results demonstrate that the rice XYLP gene family may play roles in plant vascular system development and hormone signaling. Among the 21 detected OsXYLPs, 19 are newly identified genes encoding arabinogalactan proteins. Our results provide comprehensive insights that will assist future research on the biological functions of the rice XYLP gene family.

Virus induced gene silencing of three putative prolyl 4-hydroxylases enhances plant growth in tomato (Solanum lycopersicum)

Proline hydroxylation is a major posttranslational modification of hydroxyproline-rich glycoproteins (HRGPs) that is catalyzed by prolyl 4-hydroxylases (P4Hs). HRGPs such as arabinogalactan proteins (AGPs) and extensios play significant roles on cell wall structure and function and their implication in cell division and expansion has been reported. We used tobacco rattle virus (TRV)-based virus induced gene silencing to investigate the role of three tomato P4Hs, out of ten present in the tomato genome, in growth and development. Eight-days old tomato seedlings were infected with the appropriate TRV vectors and plants were allowed to grow under standard conditions for 6 weeks. Lower P4H mRNA levels were associated with lower hydroxyproline content in root and shoot tissues indicating successful gene silencing. P4H-silenced plants had longer roots and shoots and larger leaves. The increased leaf area can be attributed to increased cell division as indicated by the higher leaf epidermal cell number in SlP4H1- and SlP4H9-silenced plants. In contrast, SlP4H7-silenced plants had larger leaves due to enhanced cell expansion. Western blot analysis revealed that silencing of SlP4H7 and SlP4H9 was associated with reduced levels of JIM8-bound AGP and JIM11-bound extensin epitopes, while silencing of SlP4H1 reduced only the levels of AGP proteins. Collectively these results show that P4Hs have significant and distinct roles in cell division and expansion of tomato leaves.

Arabinogalactan protein profiles and distribution patterns during microspore embryogenesis and pollen development in Brassica napus

Arabinogalactan proteins (AGPs), present in cell walls, plasma membranes and extracellular secretions, are massively glycosylated hydroxyproline-rich proteins that play a key role in several plant developmental processes. After stress treatment, microspores cultured in vitro can reprogramme and change their gametophytic developmental pathways towards embryogenesis, thereby producing embryos which can further give rise to haploid and double haploid plants, important biotechnological tools in plant breeding. Microspore embryogenesis constitutes a convenient system for studying the mechanisms underlying cell reprogramming and embryo formation. In this work, the dynamics of both AGP presence and distribution were studied during pollen development and microspore embryogenesis in Brassica napus, by employing a multidisciplinary approach using monoclonal antibodies for AGPs (LM2, LM6, JIM13, JIM14, MAC207) and analysing the expression pattern of the BnAGP Sta 39-4 gene. Results showed the developmental regulation and defined localization of the studied AGP epitopes during the two microspore developmental pathways, revealing different distribution patterns for AGPs with different antigenic reactivity. AGPs recognized by JIM13, JIM14 and MAC207 antibodies were related to pollen maturation, whereas AGPs labelled by LM2 and LM6 were associated with embryo development. Interestingly, the AGPs labelled by JIM13 and JIM14 were induced with the change of microspore fate. Increases in the expression of the Sta 39-4 gene, JIM13 and JIM14 epitopes found specifically in 2-4 cell stage embryo cell walls, suggested that AGPs are early molecular markers of microspore embryogenesis. Later, LM2 and LM6 antigens increased progressively with embryo development and localized on cell walls and cytoplasmic spots, suggesting an active production and secretion of AGPs during in vitro embryo formation. These results give new insights into the involvement of AGPs as potential regulating/signalling molecules in microspore reprogramming and embryogenesis.