Purified tomato polygalacturonase activity during thermal and high-pressure treatment

Biochemical characterization of Arabidopsis clade F polygalacturonase shows a substrate preference toward oligogalacturonic acids

Polygalacturonases (PGs) hydrolyze α-1,4-linked d-galacturonic acid (GalUA) in polygalacturonic acid. Previously, PG activity in pea seedlings was found in the Golgi apparatus, where pectin biosynthesis occurs. However, the corresponding genes encoding Golgi-localized PG proteins have never been identified in the higher plants. In this study, we cloned the 5 Arabidopsis genes encoding putative membrane-bound PGs from clade F PGs (AtPGFs) as the first step for the discovery of the Golgi-localized PGs. Five AtPGF proteins (AtPGF3, AtPGF6, AtPGF10, AtPGF14 and AtPGF16) were heterologously produced in Schizosaccharomyces pombe. Among these, only the AtPGF10 protein showed in vitro exo-type PG activity toward fluorogenic pyridylaminated-oligogalacturonic acids (PA-OGAs) as a substrate. The optimum PG activity was observed at pH 5.5 and 60°C. The recombinant AtPGF10 protein showed the maximum PG activities toward PA-OGA with 10 degrees of polymerization. The apparent K_m values for the PA-OGAs with 7, 11 and 14 degrees of polymerization were 8.0, 22, and 5.9 μM, respectively. This is the first report of the identification and enzymatic characterization of AtPGF10 as PG carrying putative membrane-bound domain.

Production and molecular characterization of tailored citrus pectin-derived compounds

In this study, tailored-made citrus pectin-derived compounds were produced through controlled enzymatic and/or chemical modifications of commercial citrus pectin with different degrees of methylesterification (DM) and similar average molecular weight (M_W). In the first treatment, degradation of the citrus pectin (CP) materials by endo-polygalacturonase (EPG) yielded pectins with average M_w's (between 2 and 60 kDa). Separation and identification of the oligosaccharide fraction present in these samples, revealed the presence of non-methylesterified galacturonic acid oligomers with degree of polymerization (DP) 1-5. In the second treatment, exploiting the combined effect of EPG and pectin lyase, compounds with M_W between 2 and 21 kDa, containing methylesterified and non-methylesterified polygalacturonans (DP 1-6), were generated. Finally, CP was sequentially modified by chemical saponification and the action of EPG. A sample of DM 11% and M_W 2.7 kDa, containing POS (DP 1-5), was produced. Diverse pectin-derived compounds were successfully generated for further studies exploring their functionality.

The exogenous application of AtPGLR, an endo-polygalacturonase, triggers pollen tube burst and repair

Plant cell wall remodeling plays a key role in the control of cell elongation and differentiation. In particular, fine-tuning of the degree of methylesterification of pectins was previously reported to control developmental processes as diverse as pollen germination, pollen tube elongation, emergence of primordia or elongation of dark-grown hypocotyls. However, how pectin degradation can modulate plant development has remained elusive. Here we report the characterization of a polygalacturonase (PG), AtPGLR, the gene for which is highly expressed at the onset of lateral root emergence in Arabidopsis. Due to gene compensation mechanisms, mutant approaches failed to determine the involvement of AtPGLR in plant growth. To overcome this issue, AtPGLR has been expressed heterologously in the yeast Pichia pastoris and biochemically characterized. We showed that AtPGLR is an endo-PG that preferentially releases non-methylesterified oligogalacturonides with a short degree of polymerization (< 8) at acidic pH. The application of the purified recombinant protein on Amaryllis pollen tubes, an excellent model for studying cell wall remodeling at acidic pH, induced abnormal pollen tubes or cytoplasmic leakage in the subapical dome of the pollen tube tip, where non-methylesterified pectin epitopes are detected. Those leaks could either be repaired by new β-glucan deposits (mostly callose) in the cell wall or promoted dramatic burst of the pollen tube. Our work presents the full biochemical characterization of an Arabidopsis PG and highlights the importance of pectin integrity in pollen tube elongation.

Hyperbaric Storage at Room Temperature for Fruit Juice Preservation

Hyperbaric storage is an innovative preservation method that consists of storing food under pressure, either at room or at low temperature, for time periods of days, weeks, or months. Recent scientific literature shows that hyperbaric storage at room temperature (HS-RT) could be an efficient method for fruit juice preservation. Depending on the level applied, pressure can inhibit and even inactivate the endogenous microflora of the fresh juice, while properly preserving other organoleptic and quality indicators. Even though the method has not yet been implemented in the food industry, its industrial viability has been evaluated from different points of view (product quality, consumer acceptation, vessel design, economic, or environmental, among others). The results reveal that HS-RT is effective in extending the shelf-life of both acidic and low-acidic fruit juices. Moreover, the energetic costs and the carbon footprint of HS-RT are considerably lower than those of refrigeration, therefore, HS-RT could be a reliable and environmentally friendly alternative to conventional cold storage. However, before industrial implementation, much more research is needed to clarify the effects of the storage conditions on the agents that cause fruit juice deterioration.

Homogalacturonan-modifying enzymes: structure, expression, and roles in plants

Understanding the changes affecting the plant cell wall is a key element in addressing its functional role in plant growth and in the response to stress. Pectins, which are the main constituents of the primary cell wall in dicot species, play a central role in the control of cellular adhesion and thereby of the rheological properties of the wall. This is likely to be a major determinant of plant growth. How the discrete changes in pectin structure are mediated is thus a key issue in our understanding of plant development and plant responses to changes in the environment. In particular, understanding the remodelling of homogalacturonan (HG), the most abundant pectic polymer, by specific enzymes is a current challenge in addressing its fundamental role. HG, a polymer that can be methylesterified or acetylated, can be modified by HGMEs (HG-modifying enzymes) which all belong to large multigenic families in all species sequenced to date. In particular, both the degrees of substitution (methylesterification and/or acetylation) and polymerization can be controlled by specific enzymes such as pectin methylesterases (PMEs), pectin acetylesterases (PAEs), polygalacturonases (PGs), or pectate lyases-like (PLLs). Major advances in the biochemical and functional characterization of these enzymes have been made over the last 10 years. This review aims to provide a comprehensive, up to date summary of the recent data concerning the structure, regulation, and function of these fascinating enzymes in plant development and in response to biotic stresses.

Effect of Temperature and High Pressure on the Activity and Mode of Action of Fungal Pectin Methyl Esterase

Pectin was de-esterified with purified recombinant Aspergillus aculeatus pectin methyl esterase (PME) during isothermal-isobaric treatments. By measuring the release of methanol as a function of treatment time, the rate of enzymatic pectin conversion was determined. Elevated temperature and pressure were found to stimulate PME activity. The highest rate of PME-catalyzed pectin de-esterification was obtained when combining pressures in the range 200-300 MPa with temperatures in the range 50-55 degrees C. The mode of pectin de-esterification was investigated by characterizing the pectin reaction products by enzymatic fingerprinting. No significant effect of increasing pressure (300 MPa) and/or temperature (50 degrees C) on the mode of pectin conversion was detected.

Opportunities and Challenges in High Pressure Processing of Foods

Consumers increasingly demand convenience foods of the highest quality in terms of natural flavor and taste, and which are free from additives and preservatives. This demand has triggered the need for the development of a number of nonthermal approaches to food processing, of which high-pressure technology has proven to be very valuable. A number of recent publications have demonstrated novel and diverse uses of this technology. Its novel features, which include destruction of microorganisms at room temperature or lower, have made the technology commercially attractive. Enzymes and even spore forming bacteria can be inactivated by the application of pressure-thermal combinations, This review aims to identify the opportunities and challenges associated with this technology. In addition to discussing the effects of high pressure on food components, this review covers the combined effects of high pressure processing with: gamma irradiation, alternating current, ultrasound, and carbon dioxide or anti-microbial treatment. Further, the applications of this technology in various sectors - fruits and vegetables, dairy, and meat processing - have been dealt with extensively. The integration of high-pressure with other matured processing operations such as blanching, dehydration, osmotic dehydration, rehydration, frying, freezing / thawing and solid-liquid extraction has been shown to open up new processing options. The key challenges identified include: heat transfer problems and resulting non-uniformity in processing, obtaining reliable and reproducible data for process validation, lack of detailed knowledge about the interaction between high pressure, and a number of food constituents, packaging and statutory issues.

Mode of de-esterification of alkaline and acidic pectin methyl esterases at different pH conditions

Highly esterified citrus pectin was de-esterified at pH 4.5 and 8.0 by a fungal pectin methyl esterase (PME) that was shown to have an acidic isoelectric pH (pI) and an acidic pH optimum and by a plant PME that was characterized by an alkaline pI and an alkaline pH optimum. Interchain and intrachain de-esterification patterns were studied by digestion of the pectin products with endo-polygalacturonase and subsequent analysis using size exclusion and anion-exchange chromatography. No effect of pH was observed on the de-esterification mode of either of the two enzymes. Acidic, fungal PME converted pectin according to a multiple-chain mechanism, with a limited degree of multiple attack at the intrachain level, both at pH 4.5 and at pH 8.0. A multiple-attack mechanism, with a high degree of multiple attack, was more appropriate to describe the action mode of alkaline, plant PME, both at pH 4.5 and at pH 8.0.

Combined enzymatic and high-pressure processing affect cell wall polysaccharides in berries

The effect of high-pressure processing (HPP) on cell wall polysaccharides in berries was investigated. HPP decreased the degree of methyl esterification (DM), probably by activation of pectin methyl esterase (PME), and improved the extractability of pectins. When commercial enzyme mixtures were added to mashed berries, a synergistic effect was observed between treatment with commercial enzymes and HPP. Compared to treatment at atmospheric pressure, pectic polysaccharides were degraded to a larger extent when HPP was used. In contrast, hemicelluloses were hardly affected by the added enzymes when HPP was included, although they were degraded during similar treatment at atmospheric pressure. Additionally, the activity of rhamnose-releasing enzymes present in minor quantities might be enhanced after HPP, resulting in a decrease of rhamnose in the polymeric cell wall material. These results exploring the effect of HPP at representative conditions clearly point out the potential of HPP for polysaccharide modification.