Pectin: cell biology and prospects for functional analysis

Pectin is a major component of primary cell walls of all land plants and encompasses a range of galacturonic acid-rich polysaccharides. Three major pectic polysaccharides (homogalacturonan, rhamnogalacturonan-I and rhamnogalacturonan-II) are thought to occur in all primary cell walls. This review surveys what is known about the structure and function of these pectin domains. The high degree of structural complexity and heterogeneity of the pectic matrix is produced both during biosynthesis in the endomembrane system and as a result of the action of an array of wall-based pectin-modifying enzymes. Recent developments in analytical techniques and in the generation of anti-pectin probes have begun to place the structural complexity of pectin in cell biological and developmental contexts. The in muro de-methyl-esterification of homogalacturonan by pectin methyl esterases is emerging as a key process for the local modulation of matrix properties. Rhamnogalacturonan-I comprises a highly diverse population of spatially and developmentally regulated polymers, whereas rhamnogalacturonan-II appears to be a highly conserved and stable pectic domain. Current knowledge of biosynthetic enzymes, plant and microbial pectinases and the interactions of pectin with other cell wall components and the impact of molecular genetic approaches are reviewed in terms of the functional analysis of pectic polysaccharides in plant growth and development.     

The structure, function, and biosynthesis of plant cell wall pectic polysaccharides

Plant cell walls consist of carbohydrate, protein, and aromatic compounds and are essential to the proper growth and development of plants. The carbohydrate components make up ∼90% of the primary wall, and are critical to wall function. There is a diversity of polysaccharides that make up the wall and that are classified as one of three types: cellulose, hemicellulose, or pectin. The pectins, which are most abundant in the plant primary cell walls and the middle lamellae, are a class of molecules defined by the presence of galacturonic acid. The pectic polysaccharides include the galacturonans (homogalacturonan, substituted galacturonans, and RG-II) and rhamnogalacturonan-I. Galacturonans have a backbone that consists of α-1,4-linked galacturonic acid. The identification of glycosyltransferases involved in pectin synthesis is essential to the study of cell wall function in plant growth and development and for maximizing the value and use of plant polysaccharides in industry and human health. A detailed synopsis of the existing literature on pectin structure, function, and biosynthesis is presented.     

植物果胶的生物合成与功能

果胶作为植物细胞壁多糖之一,其结构和功能非常复杂。果胶主要由同型半乳糖醛酸聚糖(HG)、鼠李半乳糖醛酸聚糖I (RGI)和鼠李半乳糖醛酸聚糖II (RGII)组成。果胶类成分在维持细胞壁结构的完整性以及细胞间黏附和信号转导等方面发挥重要作用。研究果胶类成分的结构、分布和功能,对理解细胞壁高级结构的构建和功能具有重要意义...     

Pectin structure and biosynthesis

Pectin is structurally and functionally the most complex polysaccharide in plant cell walls. Pectin has functions in plant growth, morphology, development, and plant defense and also serves as a gelling and stabilizing polymer in diverse food and specialty products and has positive effects on human health and multiple biomedical uses. Pectin is a family of galacturonic acid-rich polysaccharides including homogalacturonan, rhamnogalacturonan I, and the substituted galacturonans rhamnogalacturonan II (RG-II) and xylogalacturonan (XGA). Pectin biosynthesis is estimated to require at least 67 transferases including glycosyl-, methyl-, and acetyltransferases. New developments in understanding pectin structure, function, and biosynthesis indicate that these polysaccharides have roles in both primary and secondary cell walls. Manipulation of pectin synthesis is expected to impact diverse plant agronomical properties including plant biomass characteristics important for biofuel production.     

Enzymatic Deconstruction of Backbone Structures of the Ramified Regions in Pectins

The pectic enzymes are a diverse group of enzymes that collectively degrade pectin, a mixture of highly heterogeneous and branched polysaccharides rich in d-galacturonic acids forming a major component of the primary cell wall of plants. This review covers key enzymes that function to deconstruct the “ramified region” of pectin. The enzymes include glycoside hydrolases and polysaccharide lyases that degrade complex pectic domains consisting of rhamnogalacturonans, xylogalacturonans, and other heterogeneous polymers. The chemical nature of the pectic substrates for the enzymes is presented. The biochemical properties of the enzymes, the mechanisms of enzyme actions, and related structures and functions, are described. Applications of these enzymes in fruit juice processing and in the production of bioactive compounds, as well as their technological relevance to the deconstruction of cell wall structures for biomass conversion are discussed.     

Gibberellin biosynthesis and the regulation of plant development

Gibberellins (GAs) form a large family of plant growth substances with distinct functions during the whole life cycle of higher plants. The rate of GA biosynthesis and catabolism determines how the GA hormone pool occurs in plants in a tissue and developmentally regulated manner. With the availability of genes coding for GA biosynthetic enzymes, our understanding has improved dramatically of how GA plant hormones regulate and integrate a wide range of growth and developmental processes. This review focuses on two plant systems, pumpkin and Arabidopsis, which have added significantly to our understanding of GA biosynthesis and its regulation. In addition, we present models for regulation of GA biosynthesis in transgenic plants, and discuss their suitability for altering plant growth and development.     

The plant secretory pathway seen through the lens of the cell wall

Secretion in plant cells is often studied by looking at well-characterised, evolutionarily conserved membrane proteins associated with particular endomembrane compartments. Studies using live cell microscopy and fluorescent proteins have illuminated the highly dynamic nature of trafficking, and electron microscopy studies have resolved the ultrastructure of many compartments. Biochemical and molecular analyses have further informed about the function of particular proteins and endomembrane compartments. In plants, there are over 40 cell types, each with highly specialised functions, and hence potential variations in cell biological processes and cell wall structure. As the primary function of secretion in plant cells is for the biosynthesis of cell wall polysaccharides and apoplastic transport complexes, it follows that utilising our knowledge of cell wall glycosyltransferases (GTs) and their polysaccharide products will inform us about secretion. Indeed, this knowledge has led to novel insights into the secretory pathway, including previously unseen post-TGN secretory compartments. Conversely, our knowledge of trafficking routes of secretion will inform us about polarised and localised deposition of cell walls and their constituent polysaccharides/glycoproteins. In this review, we look at what is known about cell wall biosynthesis and the secretory pathway and how the different approaches can be used in a complementary manner to study secretion and provide novel insights into these processes.     

Polypotency of the immunomodulatory effect of pectins

Pectins are the major component of plant cell walls, and they display diverse biological activities including immunomodulation. The pectin macromolecule contains fragments of linear and branched regions of polysaccharides such as homogalacturonan, rhamnogalacturonan-I, xylogalacturonan, and apiogalacturonan. These structural features determine the effect of pectins on the immune system. The backbones of pectic macromolecules have immunosuppressive activity. Pectins containing greater than 80% galacturonic acid residues were found to decrease macrophage activity and inhibit the delayed-type hypersensitivity reaction. Branched galacturonan fragments result in a biphasic immunomodulatory action. The branched region of pectins mediates both increased phagocytosis and antibody production. The fine structure of the galactan, arabinan, and apiogalacturonan side chains determines the stimulating interaction between pectin and immune cells. This review summarizes data regarding the relationship between the structure and immunomodulatory activity of pectins isolated from the plants of the European north of Russia and elucidates the concept of polypotency of pectins in native plant cell walls to both stimulate and suppress the immune response. The possible mechanisms of the immunostimulatory and anti-inflammatory effects of pectins are also discussed.     

The plant cell wall: Biosynthesis,construction, and functions

The plant cell wall is composed of multiple biopolymers, representing one of the most complex structural networks in nature. Hundreds of genes are involved in building such a natural masterpiece. However, the plant cell wall is the least understood cellular structure in plants. Due to great progress in plant functional genomics,manyachievementshavebeenmadein uncovering cell wall biosynthesis, assembly, and architecture, as well as cell wall regulation and signaling. Such information has significantly advanced our understanding of the roles of the cell wall in many biological and physiological processes and has enhanced our utilization of cell wall materials. The use of cutting-edge technologies such as single-molecule imaging,nuclear magnetic resonance spectroscopy, and atomic force microscopy has provided much insight into the plant cell wall as an intricate nanoscale network, opening up unprecedented possibilities for cell wall research. In this review,we summarize the major advances made in understanding the cell wall in this era of functional genomics, including the latest findings on the biosynthesis, construction, and functions of the cell wall.     

Identification and functional characterization of the distinct plant pectin esterases PAE8 and PAE9 and their deletion mutants

Main conclusion

PAE8 and PAE9 have pectin acetylesterase activity and together remove one-third of the cell wall acetate associated with pectin formation in Arabidopsis leaves. In pae8 and pae9 mutants, substantial amounts of acetate accumulate in cell walls. In addition, the inflorescence stem height is decreased. Pectic polysaccharides constitute a significant part of the primary cell walls in dicotyledonous angiosperms. This diverse group of polysaccharides has been implicated in several physiological processes including cell-to-cell adhesion and pathogenesis. Several pectic polysaccharides contain acetyl-moieties directly affecting their physical properties such as gelling capacity, an important trait for the food industry. In order to gain further insight into the biological role of pectin acetylation, a reverse genetics approach was used to investigate the function of genes that are members of the Pectin AcetylEsterase gene family (PAE) in Arabidopsis. Mutations in two members of the PAE family (PAE8 and PAE9) lead to cell walls with an approximately 20 % increase in acetate content. High-molecular-weight fractions enriched in pectic rhamnogalacturonan I (RGI) extracted from the mutants had increased acetate content. In addition, the pae8 mutant displayed increased acetate content also in low-molecular-weight pectic fractions. The pae8/pae9-2 double mutant exhibited an additive effect by increasing wall acetate content by up to 37 %, suggesting that the two genes are not redundant and act on acetyl-substituents of different pectic domains. The pae8 and pae8/pae9-2 mutants exhibit reduced inflorescence growth underscoring the role of pectic acetylation in plant development. When heterologously expressed and purified, both gene products were shown to release acetate from the corresponding mutant pectic fractions in vitro. PAEs play a significant role in modulating the acetylation state of pectic polymers in the wall, highlighting the importance of apoplastic metabolism for the plant cell and plant growth.     

The Arabidopsis root hair cell wall formation mutant lrx1 is suppressed by mutations in the RHM1 gene encoding a UDP-L-rhamnose synthase

Cell and cell wall growth are mutually dependent processes that must be tightly coordinated and controlled. LRR-extensin1 (LRX1) of Arabidopsis thaliana is a potential regulator of cell wall development, consisting of an N-terminal leucine-rich repeat domain and a C-terminal extensin-like domain typical for structural cell wall proteins. LRX1 is expressed in root hairs, and lrx1 mutant plants develop distorted root hairs that often swell, branch, or collapse. The aberrant cell wall structures found in lrx1 mutants point toward a function of LRX1 during the establishment of the extracellular matrix. To identify genes that are involved in an LRX1-dependent developmental pathway, a suppressor screen was performed on the lrx1 mutant, and two independent rol1 (for repressor of lrx1) alleles were isolated. ROL1 is allelic to Rhamnose Biosynthesis1, which codes for a protein involved in the biosynthesis of rhamnose, a major monosaccharide component of pectin. The rol1 mutations modify the pectic polysaccharide rhamnogalacturonan I and, for one allele, rhamnogalacturonan II. Furthermore, the rol1 mutations cause a change in the expression of a number of cell wall-related genes. Thus, the lrx1 mutant phenotype is likely to be suppressed by changes in pectic polysaccharides or other cell wall components.     

Polygalacturonase45 cleaves pectin and links cell proliferation and morphogenesis to leaf curvature in Arabidopsis thaliana

Regulating plant architecture is a major goal in current breeding programs. Previous studies have increased our understanding of the genetic regulation of plant architecture, but it is also essential to understand how organ morphology is controlled at the cellular level. In the cell wall, pectin modification and degradation are required for organ morphogenesis, and these processes involve a series of pectin-modifying enzymes. Polygalacturonases (PGs) are a major group of pectin-hydrolyzing enzymes that cleave pectin backbones and release oligogalacturonides (OGs). PG genes function in cell expansion and separation, and contribute to organ expansion, separation and dehiscence in plants. However, whether and how they influence other cellular processes and organ morphogenesis are poorly understood. Here, we characterized the functions of Arabidopsis PG45 (PG45) in organ morphogenesis using genetic, developmental, cell biological and biochemical analyses. A heterologously expressed portion of PG45 cleaves pectic homogalacturonan in vitro, indicating that PG45 is a bona fide PG. PG45 functions in leaf and flower structure, branch formation and organ growth. Undulation in pg45 knockout and PG45 overexpression leaves is accompanied by impaired adaxial–abaxial polarity, and loss of PG45 shortens the duration of cell proliferation in the adaxial epidermis of developing leaves. Abnormal leaf curvature is coupled with altered pectin metabolism and autogenous OG profiles in pg45 knockout and PG45 overexpression leaves. Together, these results highlight a previously underappreciated function for PGs in determining tissue polarity and regulating cell proliferation, and imply the existence of OG-based signaling pathways that modulate plant development.     

植物细胞壁同聚半乳糖醛酸的代谢与功能

果胶是细胞壁多糖的重要组成成分,对植物正常的生长发育十分重要。作为初生细胞壁中果胶的一种主要组成成分,同聚半乳糖醛酸（homogalacturonan,HG）是由α-D-半乳糖醛酸单体经α-（1,4）-糖苷键连接起来的一种长链大分子物质。HG的合成和降解参与了细胞壁中的多糖代谢,影响了细胞壁的结构和功能。同时,HG精确的去甲酯化以及HG所参与的细胞壁关联激酶（WAKs）和促分裂原活化蛋白激酶（MAPKs）相关的信号转导途径,在植物生长发育中也发挥着重要作用。该文主要从HG的合成、降解和循环利用以及HG的作用等方面对植物细胞壁中HG的研究进展进行了阐述。     

Comparative genomics of pectinacetylesterases: Insight on function and biology

Pectin acetylation influences the gelling ability of this important plant polysaccharide for the food industry. Plant apoplastic pectinacetylesterases (PAEs) play a key role in regulating the degree of pectin acetylation and modifying their expression thus represents one way to engineer plant polysaccharides for food applications. Identifying the major active enzymes within the PAE gene family will aid in our understanding of this biological phenomena as well as provide the tools for direct trait manipulation. Using comparative genomics we propose that there is a minimal set of 4 distinct PAEs in plants. Possible functional diversification of the PAE family in the grasses is also explored with the identification of 3 groups of PAE genes specific to grasses.     

果胶甲酯酶抑制蛋白(PMEIs)的功能性研究进展

细胞壁是一种复杂的动态网络结构,在植物生长发育、胁迫应答和免疫抗性过程中起着重要的调控和防御作用。果胶(pectin)是细胞初生壁结构中多糖的主要成分之一;其中,同型半乳糖醛酸聚糖(HG)是果胶多糖组分中含量最丰富的线性聚合物。HG的甲基酯化程度变化会导致其酶解形成凝胶,从而影响果胶结构的稳定性。果胶甲酯酶抑制蛋白(PMEIs)通过翻译后机制调控果胶甲酯酶(PMEs)活性,微调果胶多糖甲酯化修饰平衡后,维持细胞壁的完整性和生物力学特性。研究发现,PMEI-PME互作调控果胶甲酯化修饰的稳态是决定细胞黏附、细胞壁硬度和弹性以及器官形态发生的关键因素,同时也是细胞壁应对逆境、释放抗性信号和免疫防御的分子模式。主要对PMEIs在调节植物器官发育过程和应对不同胁迫因子发挥的抗逆功能及调控机制等最新研究进展作出综述。鉴于PMEIs在木本植物中的体内生理活性和调控机制仍有待探索,可为后续填补该领域的研究空白提供理论依据和策略参考。     

Pectic substances,pectic enzymes and haze formation in fruit juices

The pectic substances, located primarily in the middle lamella between cells in higher plant tissues, are complex polysaccharides. They include the negatively charged rhamnogalacturonans, and the neutral arabinogalactans I and II and l-arabinans. These polysaccharides add viscosity to juices but may also form hazes and precipitates and retard maximum recovery of juices from the fruit. The rhamnogalacturonans are degraded by the enzymes pectin methylesterase and polygalacturonase normally present in plant tissues and by these enzymes and pectate lyase in microbially derived commercial pectic enzymes added during processing. The presence of a?abinofuranosidase, which degrades l-arabinans, in commercial pectic enzyme preparations, can cause haze formation in juices such as apple and pear.     

Altered pectin composition in primary cell walls of korrigan, a dwarf mutant of Arabidopsis deficient in a membrane-bound endo-1,4-β-glucanase

Korrigan (kor) is a dwarf mutant of Arabidopsis thaliana (L.) Heynh. that is deficient in a membrane-bound endo-1,4-beta-glucanase. The effect of the mutation on the pectin network has been studied in kor by microscopical techniques associated with various probes specific for different classes of pectic polysaccharides. The localisation of native crystalline cellulose was also examined using the cellobiohydrolase I-gold probe. The investigations were focused on the external cell walls of the epidermis, a cell layer that, in a number of plant species, has been shown to be growth limiting. Anionic sites associated with pectic polymers were quantified using the cationic gold probe. Homogalacturonans were quantified using polyclonal anti-polygalacturonic acid/rhamnogalacturonan I antibodies recognising polygalacturonic acid, and monoclonal JIM7 and JIM5 antibodies recognising homogalacturonans with a high or low degree of methyl-esterification, respectively. Rhamnogalacturonans were quantified with two monoclonal antibodies, LM5, recognising beta-1,4 galactan side chains of rhamnogalacturonan I, and CCRCM2. Our results show a marked increase in homogalacturonan epitopes and a decrease in rhamnogalacturonan epitopes in kor compared to the wild type. A substantial decrease in cellobiohydrolase I-gold labelling was also observed in the mutant cell walls. These findings demonstrate that a deficiency in an endo-1,4-beta-glucanase, which is in principle not directly implicated in pectin metabolism, can induce important changes in pectin composition in the primary cell wall. The changes indicate the existence of feedback mechanisms controlling the synthesis and/or deposition of pectic polysaccharides in primary cell walls.