2020 ). Important types of edible coatings are those made of pectin, or pectic polysaccharides, which are primarily found in the cell walls of plants and in citrus fruits ( Sun et al. 2021b ). Pectin is composed mostly of molecules of galacturonic acid
Potato (Solanum tuberosum L.) periderm forms a barrier at the surface of the tuber that protects it from infection and dehydration. Immature periderm is susceptible to excoriation (skinning injury), which results in costly storage loses and market quality defects. The periderm consists of three different cell types: phellem (skin), phellogen (cork cambium), and phelloderm (parenchyma-like cells). The phellogen serves as a lateral meristem for the periderm and is characterized by thin radial walls that are labile to fracture while the periderm is immature and the phellogen is actively dividing, thus rendering the tuber susceptible to excoriation. As the periderm matures the phellogen becomes inactive, its cell walls thicken and become resistant to fracture, and thus the tuber becomes resistant to excoriation. Little is known about the changes in cell wall polymers that are associated with tuber periderm maturation and the concurrent development of resistance to excoriation. Various changes in pectins (galacturonans and rhamnogalacturonans) and extensin may be involved in this maturational process. The objectives of this research were to compare immunolabeling of homogalacturonan (HG) epitopes to labeling of rhamnogalacturonan I (RG-I) and extensin epitopes to better understand the depositional patterns of these polymers in periderm cell walls and their involvement in tuber periderm maturation. Immunolabeling with the monoclonal antibodies JIM5 and JIM7 (recognizing a broad range of esterified HG) confirmed that HG epitopes are lacking in phellogen walls of immature periderm, but increased greatly upon maturation of the periderm. Labeling of a (1,4)-β-galactan epitope found in RG-I and recognized by the monoclonal antibody LM5 was abundant in phelloderm cell walls, but sparse in most phellem cell walls. LM5 labeling was very sparse in the walls of meristematically active phellogen cells of immature periderm, but increased dramatically upon periderm maturation. Deposition of a (1,5)-α-l-arabinan epitope found in RG-I and recognized by LM6 was abundant in phelloderm and phellogen cell walls, but was sparse in phellem cell walls. LM6 labeling of phellogen walls did not change upon periderm maturation, indicating that different RG-1 epitopes are regulated independently during maturation of the periderm. Labeling with the monoclonal antibody LM1 for an extensin epitope implied that extensin is lacking in phellem cell walls, but is abundant in phelloderm cell walls. Phellogen cell walls did not label with LM1 in immature periderm, but were abundantly labeled with LM1 in mature periderm. These immunolabeling studies identify pectin and extensin depositions as likely biochemical processes involved in the thickening and related strengthening of phellogen walls upon inactivation of the phellogen layer as a lateral meristem and maturation of the periderm in potato tuber. These results provide unique and new insight into the identities of some of the biological processes that may be targeted in the development of new technologies to enhance resistance to tuber skinning injury for improved harvest, handling and storage properties.
Abbreviations. ASP, alkaline-soluble pectin; CSP, chelate-soluble pectin; HPSEC, high performance size exclusion chromatography; IV, intrinsic viscosity; M, molecular weight; MF, melting flesh; NMF, nonmelting flesh, R g , radius of gyration; WAF
postharvest quality. Calcium has many roles in the plant. A high proportion of Ca in plant cells is found in the cell wall/middle lamella region ( Marschner, 1995 ). Here it is bound to carboxyl groups of polygalacturonic acids (pectin), where it links
Abbreviations: ASP, alkaline-soluble pectin; CSP, Chelator-soluble pectin; F, firm; FF, fairly firm; HF, hemicellulosic fraction; IG, immature green; MG, mature green; OR overripe; SR, soft ripe. 1 Current address: Department of Pomology, 1045
Abstract
Pectin methylesterase (PME, EC 3.1.1.11) was added to locular gel and pericarp of ripening tomato (Lycopersicon esculentum Mill.) and cell wall alterations were examined. Treatment of mature tomato locular gel with purified PME resulted in release of protoplasts. No protoplasts could be detected from immature locular gel or pericarp at any ripening stage examined under similar conditions. Net solubilization of pectins occurred with PME treatment in both tissues. Pectins solubilized with buffer or PME were of high molecular weight. Maximum protoplast release and pectin solubilization occurred at pH 5.0. Increased pectin solubilization in locular gel and pericarp is a result of polygalacturonase action on PME-induced deesterified pectin, the preferred substrate. Release of protoplasts from PME-treated mature locular gel suggests that maturation in this tissue involves alterations in cell wall structure that do not occur in pericarp.
Acid hydrolysis-generated pectic oligomers have been shown to affect ripening of tomato fruit by inducing both acceleration of reddening and increased ethylene biosynthesis (Campbell & Labavitch, 1991 Plant Physiol 97:706-713). In the present work, homogeneous size classes of these oligomers were demonstrated to have different impacts on ethylene production of tomato fruit pericarp discs. Endogenous oligomeric material of the same size classes was isolated from ripening tomato tissues and also tested for biological activity. They promoted some aspects of ripening as shown by increased ACC and ethylene production, which suggests that pectic oligomers are potential regulators of the ripening process in tomatoes. A metabolic origin for these oligomers is suggested by the fact that they are produced by in vitro polygalacturonase I treatment of polygalacturonic acid or tomato pectin.
presence of tannins ( Gardner, 1975 ). Nonesterified pectin. Ruthenium red, Alcian blue 8GX, and toluidine blue O were used to identify nonesterified pectin and other acidic polysaccharides. Sections were flooded with a 0.05% aqueous solution of
Our previous research has indicated that the pit membrane regulates deep supercooling of xylem parenchyma in woody plants. This area of the cell wall is composed of three layers that may be rich in pectins. Since pectins may define the porosity of the cell wall they may also regulate deep supercooling. The present study examined pectin distribution in ray cells using monoclonal antibodies, that recognize un-esterified (JIM5) and methyl-esterified (JIM7) epitopes of pectin, in conjuction with immunogold electron microscopy. Antibodies were obtained courtesy of J. Paul Knox, John Innes Inst., U.K. Dormant and non-dormant tissues of Prunus persica, Cornus florida and Salix babylonica were utilized. Labelling with JIM7 revealed that methyl-esterified pectins were abundant and evenly distributed within the primary cell wall and amorphous layer. Labelling with JIM5 revealed that un-esterified pectins were located specifically within the pit membrane, in the outer region of the primary cell wall. No differences were observed between species, however, preliminary data indicated that JIM5 labelling was greater in dormant than in non-dormant tissues.
Softening to a normal melting flesh texture in peaches involves a combined participation between polymers located in the middle lamella and primary cell wall. Pectins located in the primary cell wall polysaccharide matrix which cosolubilize when hemicellulose is extracted with KOH have received less attention than the chelator or sodium carbonate soluble pectin likely to be associated with the middle lamella. We conducted a series of extractions for cell walls prepared from softening peach fruit (47, 30, and 15 N firmness) using 0.5 m imidazole, sodium carbonate and a graded series of KOH. Hemicellulose-associated pectin was a substantial proportion of most KOH extracts (30 to 50 mole percent) and fractionated on size exclusion chromatography as a high apparent molecular weight peak which became more prominent as fruit softened and could be separated from two lower apparent molecular weight peaks by anion exchange chromatography. The nature of a hemicellulose-pectin interaction in peach was apparently by physical entrapment, versus covalent cross-linking. Softening related changes in hemicellulose-associated pectin will be addressed.