Research progress of pectin methylesterase and its inhibitors

As an important pectin enzyme, pectin methylesterase (PME) can hydrolyze methyl esters, release methanol and reduce esterification. It plays an essential role in regulating pollen tube development, root extension, and fruit ripening. Pectin methylesterase inhibitors (PMEI) can specifically bind PME and inhibit its activity, which jointly determine the esterification degree of pectin. PMEI has important application prospects in plant pest control, fruit and vegetable processing fields. In this paper, the gene families, crystal structures, molecular recognition, as well as applications in plants and industry all are reviewed for the PME and PMEI systems, and finally the semi-rational design of PMEI is discussed and prospected.

Integrative analysis of pectin methylesterase (PME) and PME inhibitors in tomato (Solanum lycopersicum): Identification, tissue-specific expression, and biochemical characterization

Although previous studies have demonstrated that the degree of demethylesterification of pectin polysaccharides is modulated during tomato fruit ripening, its involvement in vegetative organ development has been seldom investigated. As a first step in understanding the importance of pectin modification during vegetative stages, we used chemical, biochemical, and molecular approaches to analyze PMEs and PMEIs in tomato plants. We found that tomato cell walls isolated from vegetative tissues as well as the fruit contain substantial quantities of pectin, and different degrees of methylesterification were evident in different tissues. Our chemical study was further substantiated by immunolocalization analysis, which showed that selective removal of pectin-bound methyl groups is required for proper organ development and growth. In the tomato genome, there exists 79 PMEs and 48 PMEIs with temporally and spatially regulated expression. As a case study, we showed that two tomato PMEIs (SolycPMEI13 and SolycPMEI14) exhibited PMEI activities. This is the first report regarding the genome-wide identification and expression profiling of PME/PMEIs in tomato and the first chemical evidence of the differential degrees of pectin methylesterification in vegetative and reproductive tissues. Taken together, our findings provide an important tool to unravel the molecular and physiological functions of tomato PME and PMEI in further study.

Rice pectin methylesterase inhibitor28 (OsPMEI28) encodes a functional PMEI and its overexpression results in a dwarf phenotype through increased pectin methylesterification levels

Pectin methylesterases (PMEs, EC 3.1.1.11) belonging to carbohydrate esterase family 8 cleave the ester bond between a galacturonic acid and an methyl group and the resulting change in methylesterification level plays an important role during the growth and development of plants. Optimal pectin methylesterification status in each cell type is determined by the balance between PME activity and post-translational PME inhibition by PME inhibitors (PMEIs). Rice contains 49 PMEIs and none of them are functionally characterized. Genomic sequence analysis led to the identification of rice PMEI28 (OsPMEI28). Recombinant OsPMEI28 exhibited inhibitory activity against commercial PME protein with the highest activities detected at pH 8.5. Overexpression of OsPMEI28 in rice resulted in an increased level of cell wall bound methylester groups and differential changes in the composition of cell wall neutral monosaccharides and lignin content in culm tissues. Consequently, transgenic plants overexpressing OsPMEI28 exhibited dwarf phenotypes and reduced culm diameter. Our data indicate that OsPMEI28 functions as a critical structural modulator by regulating the degree of pectin methylesterification and that an impaired status of pectin methylesterification affects physiochemical properties of the cell wall components and causes abnormal cell extensibility in rice culm tissues.

Molecular and biochemical characterization of rice pectin methylesterase inhibitors (OsPMEIs)

Cell wall modifications such as partial degradation and depolymerization by cell wall hydrolases are normal cellular processes and are required for the functionalities of different cell types. Pectin, one of the major cell wall polysaccharides, is predominantly found in primary cell walls and middle lamellae and is subjected to in muro modification, primarily by cell wall-localized pectin methylesterases (PMEs). Molecular biochemical studies have demonstrated that enzymatic activities of PMEs are governed by multiple pectin methylesterase inhibitors (PMEIs), which consequently control the pectin methylesterification status. Although a few studies in Arabidopsis have shown the importance of this PMEI-mediated regulation in the biophysical properties of cell walls, little is known about the molecular physiological functions of rice PMEIs. We found 49 members of the PMEI family in the rice genome. Analysis of their transcript levels by quantitative real-time PCR and meta expression analysis showed that they are regulated spatially and temporally, as well as in response to diverse stresses. Quantification of cell wall-bound methylesters indicated that the degree of pectin methylesterification is developmentally regulated; in particular, higher PMEI activities were detected in cell wall proteins prepared from young leaves. Furthermore, an activity assay demonstrated that two recombinant OsPMEI proteins (OsPMEI8 and 12) were able to inhibit the enzymatic activity of a commercial PME protein. Subcellular localization indicated that OsPMEI8 is targeted to the middle lamella and OsPMEI12 is localized in the plasma membrane and nucleus. Taken together, our findings provide the first molecular and biochemical evidence for functional characterization of PMEIs in rice growth and development.

Molecular and biochemical characterization of rice pectin methylesterase inhibitors (OsPMEIs)

Cell wall modifications such as partial degradation and depolymerization by cell wall hydrolases are normal cellular processes and are required for the functionalities of different cell types. Pectin, one of the major cell wall polysaccharides, is predominantly found in primary cell walls and middle lamellae and is subjected to in muro modification, primarily by cell wall-localized pectin methylesterases (PMEs). Molecular biochemical studies have demonstrated that enzymatic activities of PMEs are governed by multiple pectin methylesterase inhibitors (PMEIs), which consequently control the pectin methylesterification status. Although a few studies in Arabidopsis have shown the importance of this PMEI-mediated regulation in the biophysical properties of cell walls, little is known about the molecular physiological functions of rice PMEIs. We found 49 members of the PMEI family in the rice genome. Analysis of their transcript levels by quantitative real-time PCR and meta expression analysis showed that they are regulated spatially and temporally, as well as in response to diverse stresses. Quantification of cell wall-bound methylesters indicated that the degree of pectin methylesterification is developmentally regulated; in particular, higher PMEI activities were detected in cell wall proteins prepared from young leaves. Furthermore, an activity assay demonstrated that two recombinant OsPMEI proteins (OsPMEI8 and 12) were able to inhibit the enzymatic activity of a commercial PME protein. Subcellular localization indicated that OsPMEI8 is targeted to the middle lamella and OsPMEI12 is localized in the plasma membrane and nucleus. Taken together, our findings provide the first molecular and biochemical evidence for functional characterization of PMEIs in rice growth and development.

A lower content of de-methylesterified homogalacturonan improves enzymatic cell separation and isolation of mesophyll protoplasts in Arabidopsis

Cell adhesion occurs primarily at the level of middle lamella which is mainly composed by pectin polysaccharides. These can be degraded by cell wall degrading enzymes (CWDEs) during developmental processes to allow a controlled separation of plant cells. Extensive cell wall degradation by CWDEs with consequent cell separation is performed when protoplasts are isolated from plant tissues by using mixtures of CWDEs. We have evaluated whether modification of pectin affects cell separation and protoplast isolation. Arabidopsis plants overexpressing the pectin methylesterase inhibitors AtPMEI-1 or AtPMEI-2, and Arabidopsis pme3 plants, mutated in the gene encoding pectin methylesterase 3, showed an increased efficiency of isolation of viable mesophyll protoplasts as compared with Wild Type Columbia-0 plants. The release of protoplasts was correlated with the reduced level of long stretches of de-methylesterified homogalacturonan (HGA) present in these plants. Response to elicitation, cell wall regeneration and efficiency of transfection in protoplasts from transgenic plants was comparable to those of wild type protoplasts.