Polysaccharide Structures in the Outer Mucilage of Arabidopsis Seeds Visualized by AFM

Top five unanswered questions in plant cell surface research

Plant cell wall researchers were asked their view on what the major unanswered questions are in their field. This article summarises the feedback that was received from them in five questions. In this issue you can find equivalent syntheses for researchers working on bacterial, unicellular parasite and fungal systems.

Structure and growth of plant cell walls

Plant cells build nanofibrillar walls that are central to plant growth, morphogenesis and mechanics. Starting from simple sugars, three groups of polysaccharides, namely, cellulose, hemicelluloses and pectins, with very different physical properties are assembled by the cell to make a strong yet extensible wall. This Review describes the physics of wall growth and its regulation by cellular processes such as cellulose production by cellulose synthase, modulation of wall pH by plasma membrane H+-ATPase, wall loosening by expansin and signalling by plant hormones such as auxin and brassinosteroid. In addition, this Review discusses the nuanced roles, properties and interactions of cellulose, matrix polysaccharides and cell wall proteins and describes how wall stress and wall loosening cooperatively result in cell wall growth.

Multi-level chemical characterization and anti-inflammatory activity evaluation of the polysaccharides from Prunella vulgaris

Prunella vulgaris (PV) is a medicine and food homologous plant, but its quality evaluation seldom relies on the polysaccharides (PVPs). In this work, we established the multi-level fingerprinting and in vitro anti-inflammatory evaluation approaches to characterize and compare the polysaccharides of P. vulgaris collected from the major production regions in China. PVPs prepared from 22 batches of samples gave the content variation of 5.76-24.524 mg/g, but displayed high similarity in the molecular weight distribution. Hydrolyzed oligosaccharides with degrees of polymerization 2-14 were characterized with different numbers of pentose and hexose by HILIC-MS. The tested 22 batches of oligosaccharides exhibited visible differences in peak abundance, which failed to corelate to their production regions. All the PVPs contained Gal, Xyl, and Ara, as the main monosaccharides. Eleven batches among the tested PVPs showed the significant inhibitory effects on NO production on LPS-induced RAW264.7 cells at 10 μg/mL, but the exerted efficacy did not exhibit correlation with the production regions. Conclusively, we, for the first time, investigated the chemical features of PVPs at three levels, and assessed the chemical and anti-inflammatory variations among the different regions of P. vulgaris samples.

Molecular dynamics simulations and X-ray scattering show the κ-carrageenan disorder-to-order transition to be the formation of double-helices

Recent molecular dynamics simulations, verified experimentally by solution-state x-ray scattering experiments, have found that κ-carrageenan chains contain helical secondary structure, akin to that found in the solid-state, even in aqueous solution. Furthermore, upon the addition of ions to single chains the simulations found no evidence that any conformational transitions take place. These findings challenge the long-held assumption that the so-called disorder-to-order transition in carrageenan systems involves a uni-molecular 'coil-to-helix transition'. Herein, the results of further molecular dynamics simulations undertaken using pairs of κ-carrageenan chains in 0.1 M NaI solutions are reported, and are validated experimentally using state-of-the-art solution-state WAXS experiments. From initially separated chains double-helices are shown to form, leading the authors to propose 'two single helices-to-stabilized double-helix' as a description of the molecular events taking place during the disorder-to-order transition.

GoSAMTs are required for pectin methyl-esterification and mucilage release in seed coat epidermal cells

INTRODUCTION

GoSAMTs play a role in the methylation of polysaccharides synthesized by the Golgi. Pectin homogalacturonan (HG) methyl-esterification is essential for the proper function of this polysaccharide in cell walls. In order to better understand the role of GoSAMTs in HG biosynthesis, we analyzed mucilage methyl-esterification in gosamt mutants.

METHODS

To determine the function of GoSAMT1 and GoSAMT2 in HG methyl-esterification we utilized epidermal cells of seed coats, as these structures produce mucilage, which is a pectic matrix. We evaluated differences in seed surface morphology and quantified mucilage release. We measured methanol release, and used antibodies and confocal microscopy to analyze HG methyl-esterification in mucilage.

RESULTS

We observed morphological differences on the seed surface and delayed, uneven mucilage release in gosamt1-1gosamt2-1 double mutants. We also found changes in the distal wall length indicating abnormal cell wall breakage in this double mutant. Using methanol release and immunolabeling, we confirmed that GoSAMT1 and GoSAMT2 are involved in HG methyl-esterification in mucilage. However, we did not find evidence of decreasing HG in the gosamt mutants. Confocal microscopy analyses detected different patterns in the adherent mucilage and a greater number of low-methyl-esterified domains near the seed coat surface, which correlates with a greater number of "egg-box" structures in this region. We also detected a shift in the partitioning between the Rhamnogalacturonan-I soluble and adherent layers of the double mutant, which correlated with increased amounts of arabinose and arabinogalactan-protein in the adherent mucilage.

DISCUSSION

The results show that the HG synthesized in gosamt mutant plants is less methyl esterified, resulting in more egg-box structures, which stiffen the cell walls in epidermal cells and change the rheological properties of the seed surface. The increased amounts of arabinose and arabinogalactan-protein in adherent mucilage, also suggests that compensation mechanisms were triggered in the gosamt mutants.

Building an extensible cell wall

This article recounts, from my perspective of four decades in this field, evolving paradigms of primary cell wall structure and the mechanism of surface enlargement of growing cell walls. Updates of the structures, physical interactions, and roles of cellulose, xyloglucan, and pectins are presented. This leads to an example of how a conceptual depiction of wall structure can be translated into an explicit quantitative model based on molecular dynamics methods. Comparison of the model's mechanical behavior with experimental results provides insights into the molecular basis of complex mechanical behaviors of primary cell wall and uncovers the dominant role of cellulose-cellulose interactions in forming a strong yet extensible network.

Cell wall-localized BETA-XYLOSIDASE4 contributes to immunity of Arabidopsis against Botrytis cinerea

Plant cell walls constitute physical barriers that restrict access of microbial pathogens to the contents of plant cells. The primary cell wall of multicellular plants predominantly consists of cellulose, hemicellulose, and pectin, and its composition can change upon stress. BETA-XYLOSIDASE4 (BXL4) belongs to a seven-member gene family in Arabidopsis (Arabidopsis thaliana), one of which encodes a protein (BXL1) involved in cell wall remodeling. We assayed the influence of BXL4 on plant immunity and investigated the subcellular localization and enzymatic activity of BXL4, making use of mutant and overexpression lines. BXL4 localized to the apoplast and was induced upon infection with the necrotrophic fungal pathogen Botrytis cinerea in a jasmonoyl isoleucine-dependent manner. The bxl4 mutants showed a reduced resistance to B. cinerea, while resistance was increased in conditional overexpression lines. Ectopic expression of BXL4 in Arabidopsis seed coat epidermal cells rescued a bxl1 mutant phenotype, suggesting that, like BXL1, BXL4 has both xylosidase and arabinosidase activity. We conclude that BXL4 is a xylosidase/arabinosidase that is secreted to the apoplast and its expression is upregulated under pathogen attack, contributing to immunity against B. cinerea, possibly by removal of arabinose and xylose side-chains of polysaccharides in the primary cell wall.

Rhamnogalacturonan-I forms mucilage: behind its simplicity, a cutting-edge organization

Zhang Y, Yin Q, Qin W, Gao H, Du J, Chen J, Li H, Zhou G, Wu H, Wu A-M. 2022. The Class II KNOX family members KNAT3 and KNAT7 redundantly participate in Arabidopsis seed coat mucilage biosynthesis. Journal of Experimental Botany 73, 3477–3495.

ERF4 and MYB52 transcription factors play antagonistic roles in regulating homogalacturonan de-methylesterification in Arabidopsis seed coat mucilage

Homogalacturonan (HG), a component of pectin, is synthesized in the Golgi apparatus in its fully methylesterified form. It is then secreted into the apoplast where it is typically de-methylesterified by pectin methylesterases (PME). Secretion and de-esterification are critical for normal pectin function, yet the underlying transcriptional regulation mechanisms remain largely unknown. Here, we uncovered a mechanism that fine-tunes the degree of HG de-methylesterification (DM) in the mucilage that surrounds Arabidopsis thaliana seeds. We demonstrate that the APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) transcription factor (TF) ERF4 is a transcriptional repressor that positively regulates HG DM. ERF4 expression is confined to epidermal cells in the early stages of seed coat development. The adhesiveness of the erf4 mutant mucilage was decreased as a result of an increased DM caused by a decrease in PME activity. Molecular and genetic analyses revealed that ERF4 positively regulates HG DM by suppressing the expression of three PME INHIBITOR genes (PMEIs) and SUBTILISIN-LIKE SERINE PROTEASE 1.7 (SBT1.7). ERF4 shares common targets with the TF MYB52, which also regulates pectin DM. Nevertheless, the erf4-2 myb52 double mutant seeds have a wild-type mucilage phenotype. We provide evidence that ERF4 and MYB52 regulate downstream gene expression in an opposite manner by antagonizing each other's DNA-binding ability through a physical interaction. Together, our findings reveal that pectin DM in the seed coat is fine-tuned by an ERF4-MYB52 transcriptional complex.

Transport of UDP-rhamnose by URGT2, URGT4, and URGT6 modulates rhamnogalacturonan-I length

Rhamnogalacturonan-I biosynthesis occurs in the lumen of the Golgi apparatus, a compartment where UDP-Rhamnose and UDP-Galacturonic Acid are the main substrates for synthesis of the backbone polymer of pectin. Recent studies showed that UDP-Rha is transported from the cytosol into the Golgi apparatus by a family of six UDP-rhamnose/UDP-galactose transporters (URGT1-6). In this study, analysis of adherent and soluble mucilage (SM) of Arabidopsis thaliana seeds revealed distinct roles of URGT2, URGT4, and URGT6 in mucilage biosynthesis. Characterization of SM polymer size showed shorter chains in the urgt2 urgt4 and urgt2 urgt4 urgt6 mutants, suggesting that URGT2 and URGT4 are mainly involved in Rhamnogalacturonan-I (RG-I) elongation. Meanwhile, mutants in urgt6 exhibited changes only in adherent mucilage (AM). Surprisingly, the estimated number of RG-I polymer chains present in urgt2 urgt4 and urgt2 urgt4 urgt6 mutants was higher than in wild-type. Interestingly, the increased number of shorter RG-I chains was accompanied by an increased amount of xylan. In the urgt mutants, expression analysis of other genes involved in mucilage biosynthesis showed some compensation. Studies of mutants of transcription factors regulating mucilage formation indicated that URGT2, URGT4, and URGT6 are likely part of a gene network controlled by these regulators and involved in RG-I synthesis. These results suggest that URGT2, URGT4, and URGT6 play different roles in the biosynthesis of mucilage, and the lack of all three affects the production of shorter RG-I polymers and longer xylan domains.

The primary, secondary, and structures of higher levels of pectin polysaccharides

Pectin is a heteropolysaccharide abundant in the cell wall of plants and is obtained mainly from fruit (citrus and apple), thus its properties are particularly prone to changes occurring during ripening process. Properties of pectin depend on the string-like structure (conformation, stiffness) of the molecules that determines their mutual interaction and with the surrounding environment. Therefore, in this review the primary, secondary, and structures of higher levels of pectin chains are discussed in relation to external factors including crosslinking mechanisms. The review shows that the primary structure of pectin is relatively well known, however, we still know little about the conformation and properties of the more realistic systems of higher orders involving side chains, functional groups, and complexes of pectin domains. In particular, there is lack of knowledge on the influence of postharvest changes and extraction method on the primary and secondary structure of pectin that would affect conformation in a given environment and assembly to higher structural levels. Exploring the above-mentioned issues will allow to improve our understanding of pectin functionality and will help to tailor new functionalities for the food industry based on natural but often biologically variable source. The review also demonstrates that atomic force microscopy is a very convenient and adequate tool for the evaluation of pectin conformation since it allows for the relatively straightforward stretching of the pectin molecule in order to measure the force-extension curve which is directly related to its stiffness or flexibility.