Degree of pectin methyl esterification in endosperm cell walls is involved in embryo bending in Arabidopsis thaliana

Concluding Embryogenesis After Diaspora: Seed Germination in Illicium Parviflorum

Albuminous seeds, dispersed with a minimally developed embryo surrounded by nutrient storage tissue, are pervasive across extinct and extant early diverging angiosperm lineages. Typically, seed ontogenic studies have focused on the time between fertilization and seed release, but in albuminous seeds, embryogenesis is incomplete at the time of seed dispersal. Here, I studied the morphological and nutritional relationships between the embryo and the endosperm after seed dispersal in Illicium parviflorum (Austrobaileyales). Seeds of I. parviflorum germinate over a period of three months. Different stages during the germination process were anatomically evaluated using a combination of histochemistry and immunocytochemistry. At dispersal, the seeds of Illicium contain a tiny achlorophyllous embryo with minimal histological differentiation, surrounded by copious amounts of lipo-protein globules stored in the endosperm within cell walls rich in un-esterified pectins. Six weeks later, the embryo expanded and differentiated the vascular tissues before the emergence of the radicle through the seed coat, as the stored lipids and proteins coalesced within cells. Six weeks later, the cotyledons contained starch and complex lipids intracellularly, and accumulated low-esterified pectins in their cell walls. The proteolipid-rich albuminous seeds of Illicium exemplify how woody angiosperms of the Austrobaileyales, Amborellales, and many magnoliids release seeds with high-energy storage compounds that are reprocessed by embryos that complete development during germination. Seedlings of these lineages thrive in the understory of tropical environments, which match with the predicted habitats where angiosperms evolved.

Contrasting Cd accumulation of Arabidopsis halleri populations: a role for (1→4)-β-galactan in pectin

Cadmium (Cd) accumulation is highly variable among Arabidopsis halleri populations. To identify cell wall (CW) components that contribute to the contrasting Cd accumulation between PL22-H (Cd-hyperaccumulator) and I16-E (Cd-excluder), Cd absorption capacity of CW polysaccharides, CW mono- and poly- saccharides contents and CW glycan profiles were compared between these two populations. PL22-H pectin contained 3-fold higher Cd concentration than I16-E pectin in roots, and (1→4)-β-galactan pectic epitope showed the biggest difference between PL22-H and I16-E. CW-related differentially expressed genes (DEGs) between PL22-H and I16-E were identified and corresponding A. thaliana mutants were phenotyped for Cd tolerance and accumulation. A higher Cd translocation was observed in GALACTAN SYNTHASE1 A. thaliana knockout and overexpressor mutants, which both showed a lengthening of the RG-I sidechains after Cd treatment, contrary to the wild-type. Overall, our results support an indirect role for (1→4)-β-galactan in Cd translocation, possibly by a joint effect of regulating the length of RG-I sidechains, the pectin structure and interactions between polysaccharides in the CW. The characterization of other CW-related DEGs between I16-E and PL22-H selected allowed to identify a possible role in Zn translocation for BIIDXI and LEUNIG-HOMOLOG genes, which are both involved in pectin modification.

BIIDXI, a DUF642 Cell Wall Protein That Regulates Pectin Methyl Esterase Activity, Is Involved in Thermotolerance Processes in Arabidopsis thaliana

Plant cell wall remodeling is an important process during plant responses to heat stress. Pectins, a group of cell wall polysaccharides with a great diversity of complex chemical structures, are also involved in heat stress responses. Enzymatic activity of the pectin methyl esterases, which remove methyl groups from pectins in the cell wall, is regulated by DUF642 proteins, as described in different plants, including Arabidopsis thaliana and Oryza sativa. Our results demonstrated that heat stress altered the expression of the DUF642 gene, BIIDXI. There was an important decrease in BIIDXI expression during the first hour of HS, followed by an increase at 24 h. bdx-1 seedlings had less tolerance to heat stress but presented a normal heat stress response; HSFA2 and HSP22 expressions were highly increased, as they were in WT seedlings. Thermopriming triggered changes in pectin methyl esterase activity in WT seedlings, while no increases in PME activity were detected in bdx-1 seedlings at the same conditions. Taken together, our results suggest that BIIDXI is involved in thermotolerance via PME activation.

Transcriptome characteristics during cell wall formation of endosperm cellularization and embryo differentiation in Arabidopsis

Embryonic and endosperm development are important biological events during Arabidopsis seed development, and are controlled by dynamic changes in a range of gene expression. Nevertheless, the regulatory mechanisms of endosperm cellularization and embryo differentiation remain unclear. Here, we characterized the early embryo and endosperm development of the naa15 mutant that had abnormal embryo differentiation and incomplete endosperm cellularization compared to WT of Arabidopsis, and comparatively investigated the changes of gene expressions in WT seeds at 3, 4, and 5 days after pollination (3W, 4W, and 5W) and the white homozygous aborted naa15 seeds at 5, 6, and 7 DAP (5M, 6M, and 7M) from naa15-1/+ siliques using RNA sequencing and qPCR assays. The transcriptome analyses showed that there were 2040 and 3630 differentially expressed genes (DEGs) in 4W (at endosperm cellularization initiation stage and heart embryo stage) vs 3W (at syncytium stage and globular embryo stage), and 5W (at end of endosperm cellularization stage and torpedo embryo stage) vs 4W, respectively. The KEGG and GO analyses showed that lipid metabolic processes and transmembrane transport related to cell wall biogenesis, cell division and differentiation, the plant hormone signaling pathway, photosynthesis, and transcription regulator activity were evidently enriched in WT and naa15. The heatmap and qPCR analyses showed that auxin response genes (ARFs), auxin transport genes (PINs) cytokinin synthesis genes (LOGs), cytokinin dehydrogenase genes (CKXs), cytokinin receptor, transcription factors (MYB, bHLH, MADS-box, and ERF) were significantly downregulated in naa15 compared to WT. A series of cell wall genes annotated to xyloglucan endotransglycosylase/hydrolase, pectin methyl esterase, and pectin methyl esterase inhibitor were also identified in these DEGs. Moreover, using an immunofluorescent assay, the features of cell walls displayed that cellulose fluorescence signals in the embryo and endosperm of naa15 were significantly decreased, and the signals of low- and high- methyl esterification of pectin were also obviously decreased in the endosperm of naa15. In summary, we identified a large number of DEGs and investigated the features of cell walls during endosperm cellularization and embryonic differentiation, which provided important information on transcription and gene expression to reveal their regulatory mechanisms.

Dynamics of pectic homogalacturonan in cellular morphogenesis and adhesion, wall integrity sensing and plant development

Homogalacturonan (HG) is the most abundant pectin subtype in plant cell walls. Although it is a linear homopolymer, its modification states allow for complex molecular encoding. HG metabolism affects its structure, chemical properties, mobility and binding capacity, allowing it to interact dynamically with other polymers during wall assembly and remodelling and to facilitate anisotropic cell growth, cell adhesion and separation, and organ morphogenesis. HGs have also recently been found to function as signalling molecules that transmit information about wall integrity to the cell. Here we highlight recent advances in our understanding of the dual functions of HG as a dynamic structural component of the cell wall and an initiator of intrinsic and environmental signalling. We also predict how HG might interconnect the cell wall, plasma membrane and intracellular components with transcriptional networks to regulate plant growth and development.

The Plant Invertase/Pectin Methylesterase Inhibitor Superfamily

Invertases (INVs) and pectin methylesterases (PMEs) are essential enzymes coordinating carbohydrate metabolism, stress responses, and sugar signaling. INVs catalyzes the cleavage of sucrose into glucose and fructose, exerting a pivotal role in sucrose metabolism, cellulose biosynthesis, nitrogen uptake, reactive oxygen species scavenging as well as osmotic stress adaptation. PMEs exert a dynamic control of pectin methylesterification to manage cell adhesion, cell wall porosity, and elasticity, as well as perception and signaling of stresses. INV and PME activities can be regulated by specific proteinaceous inhibitors, named INV inhibitors (INVIs) and PME Inhibitors (PMEIs). Despite targeting different enzymes, INVIs and PMEIs belong to the same large protein family named "Plant Invertase/Pectin Methylesterase Inhibitor Superfamily." INVIs and PMEIs, while showing a low aa sequence identity, they share several structural properties. The two inhibitors showed mainly alpha-helices in their secondary structure and both form a non-covalent 1:1 complex with their enzymatic counterpart. Some PMEI members are organized in a gene cluster with specific PMEs. Although the most important physiological information was obtained in Arabidopsis thaliana, there are now several characterized INVI/PMEIs in different plant species. This review provides an integrated and updated overview of this fascinating superfamily, from the specific activity of characterized isoforms to their specific functions in plant physiology. We also highlight INVI/PMEIs as biotechnological tools to control different aspects of plant growth and defense. Some isoforms are discussed in view of their potential applications to improve industrial processes. A review of the nomenclature of some isoforms is carried out to eliminate confusion about the identity and the names of some INVI/PMEI member. Open questions, shortcoming, and opportunities for future research are also presented.

Overview of the Role of Cell Wall DUF642 Proteins in Plant Development

The DUF642 protein family is found exclusively in spermatophytes and is represented by 10 genes in Arabidopsis and in most of the 24 plant species analyzed to date. Even though the primary structure of DUF642 proteins is highly conserved in different spermatophyte species, studies of their expression patterns in Arabidopsis have shown that the spatial-temporal expression pattern for each gene is specific and consistent with the phenotypes of the mutant plants studied so far. Additionally, the regulation of DUF642 gene expression by hormones and environmental stimuli was specific for each gene, showing both up- and down-regulation depending of the analyzed tissue and the intensity or duration of the stimuli. These expression patterns suggest that the DUF642 genes are involved throughout the development and growth of plants. In general, changes in the expression patterns of DUF642 genes can be related to changes in pectin methyl esterase activity and/or to changes in the degree of methyl-esterified homogalacturonans during plant development in different cell types. Thus, the regulation of pectin methyl esterases mediated by DUF642 genes could contribute to the regulation of the cell wall properties during plant growth.

BIIDXI, a DUF642 cell wall protein, is involved in hypocotyl growth via auxin efflux

Auxin is involved in hypocotyl elongation in response to different environmental factors. BIIDXI is a cell wall DUF642 protein that participates in the regulation of the degree of pectin-methylesterification of the cell wall in different tissues, including hypocotyls. Under continuous light, bdx-1 seedlings presented longer hypocotyls than those of WT, while BIIDXI-overexpressed hypocotyls were auxin resistant. Auxin accumulation was observed in epidermal cells from bdx-1 hypocotyls, and the distribution pattern of PIN1 proteins differed. Moreover, the gravitropic response of bdx-1, a process that is highly dependent on auxin flux, was increased. In this study, we determined that BIIDXI is involved in hypocotyl elongation through the regulation of auxin flux.

Emerging Functions for Cell Wall Polysaccharides Accumulated during Eudicot Seed Development

The formation of seeds is a reproductive strategy in higher plants that enables the dispersal of offspring through time and space. Eudicot seeds comprise three main components, the embryo, the endosperm and the seed coat, where the coordinated development of each is important for the correct formation of the mature seed. In addition, the seed coat protects the quiescent progeny and can provide transport mechanisms. A key underlying process in the production of seed tissues is the formation of an extracellular matrix termed the cell wall, which is well known for its essential function in cytokinesis, directional growth and morphogenesis. The cell wall is composed of a macromolecular network of polymers where the major component is polysaccharides. The attributes of polysaccharides differ with their composition and charge, which enables dynamic remodeling of the mechanical and physical properties of the matrix by adjusting their production, modification or turnover. Accordingly, the importance of specific polysaccharides or modifications is increasingly being associated with specialized functions within seed tissues, often through the spatio-temporal accumulation or remodeling of particular polymers. Here, we review the evolution and accumulation of polysaccharides during eudicot seed development, what is known of their impact on wall architecture and the diverse roles associated with these in different seed tissues.