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[This corrects the article DOI: 10.1371/journal.pone.0212540.].

Invertases (INVs) and pectin methylesterases (PMEs) are essential enzymes coordinating carbohydrate metabolism, stress responses, and sugar signaling. INVs catalyzes the cleavage of sucrose into glucose and fructose, exerting a pivotal role in sucrose metabolism, cellulose biosynthesis, nitrogen uptake, reactive oxygen species scavenging as well as osmotic stress adaptation. PMEs exert a dynamic control of pectin methylesterification to manage cell adhesion, cell wall porosity, and elasticity, as well as perception and signaling of stresses. INV and PME activities can be regulated by specific proteinaceous inhibitors, named INV inhibitors (INVIs) and PME Inhibitors (PMEIs). Despite targeting different enzymes, INVIs and PMEIs belong to the same large protein family named “Plant Invertase/Pectin Methylesterase Inhibitor Superfamily.” INVIs and PMEIs, while showing a low aa sequence identity, they share several structural properties. The two inhibitors showed mainly alpha-helices in their secondary structure and both form a non-covalent 1:1 complex with their enzymatic counterpart. Some PMEI members are organized in a gene cluster with specific PMEs. Although the most important physiological information was obtained in Arabidopsis thaliana , there are now several characterized INVI/PMEIs in different plant species. This review provides an integrated and updated overview of this fascinating superfamily, from the specific activity of characterized isoforms to their specific functions in plant physiology. We also highlight INVI/PMEIs as biotechnological tools to control different aspects of plant growth and defense. Some isoforms are discussed in view of their potential applications to improve industrial processes. A review of the nomenclature of some isoforms is carried out to eliminate confusion about the identity and the names of some INVI/PMEI member. Open questions, shortcoming, and opportunities for future research are also presented.

Pectin is synthesized in a highly methylesterified form in the Golgi cisternae and partially de‐methylesterified in muro by pectin methylesterases (PMEs). Arabidopsis thaliana produces a local and strong induction of PME activity during the infection of the necrotrophic fungus Botrytis cinerea. AtPME17 is a putative A. thaliana PME highly induced in response to B. cinerea. Here, a fine tuning of AtPME17 expression by different defence hormones was identified. Our genetic evidence demonstrates that AtPME17 strongly contributes to the pathogen‐induced PME activity and resistance against B. cinerea by triggering jasmonic acid–ethylene‐dependent PDF1.2 expression. AtPME17 belongs to group 2 isoforms of PMEs characterized by a PME domain preceded by an N‐terminal PRO region. However, the biochemical evidence for AtPME17 as a functional PME is still lacking and the role played by its PRO region is not known. Using the Pichia pastoris expression system, we demonstrate that AtPME17 is a functional PME with activity favoured by an increase in pH. AtPME17 performs a blockwise pattern of pectin de‐methylesterification that favours the formation of egg‐box structures between homogalacturonans. Recombinant AtPME17 expression in Escherichia coli reveals that the PRO region acts as an intramolecular inhibitor of AtPME17 activity. AtPME17 contributes to resistance against Botrytis cinerea by triggering jasmonic acid–ethylene‐dependent PDF1.2 expression. AtPME17 activity is inhibited by its PRO region and produces a blockwise pectin de‐methylesterification.

Although atmospheric cold plasma is well known for nonthermal inactivation of microorganisms on surfaces, few studies examine its application to liquid food within a package. This study explores the decontamination efficiency of high voltage atmospheric cold plasma (HVACP) on Salmonella enterica serovar Typhimurium ( S . enterica ) in orange juice (OJ). Both direct and indirect HVACP treatments of 25-mL OJ induce greater than a 5-log reduction in S . enterica following 30 s of treatment with air and MA65 gas with no storage. For 50-mL OJ, 120 s of direct HVACP treatment followed by 24-h storage induced a 2.9-log reduction of S . enterica in air and a 4.7-log reduction in MA65 gas; 120 s of indirect HVACP treatment followed by 24-h storage resulted in a 2.2-log reduction in air and a 3.8-log reduction in MA65. No significant ( P  < 0.05) Brix or pH change occurred following 120-s HVACP treatment. Applying 120-s HVACP direct treatment reduced vitamin C by 22% in air (compared to 50% for heat pasteurization) and pectin methylesterase activity by 74% in air and 82% in MA65. These results demonstrate that HVACP can effectively inactivate Salmonella in OJ with minimal quality degradation.

Pectin is an important component of cell wall polysaccharides and is important for normal plant growth and development. As a major component of pectin in the primary cell wall, homogalacturonan (HG) is a long-chain macromolecular polysaccharide composed of repeated α-1,4-D-GalA sugar units. At the same time, HG is synthesized in the Golgi apparatus in the form of methyl esterification and acetylation. It is then secreted into the plasmodesmata, where it is usually demethylated by pectin methyl esterase (PME) and deacetylated by pectin acetylase (PAE). The synthesis and modification of HG are involved in polysaccharide metabolism in the cell wall, which affects the structure and function of the cell wall and plays an important role in plant growth and development. This paper mainly summarizes the recent research on the biosynthesis, modification and the roles of HG in plant cell wall.

Background The heavy metal cadmium (Cd) accumulates in the environment due to anthropogenic influences. It is unessential and harmful to all life forms. The plant cell wall forms a physical barrier against environmental stress and changes in the cell wall structure have been observed upon Cd exposure. In the current study, changes in the cell wall composition and structure of Medicago sativa stems were investigated after long-term exposure to Cd. Liquid chromatography coupled to mass spectrometry (LC-MS) for quantitative protein analysis was complemented with targeted gene expression analysis and combined with analyses of the cell wall composition. Results Several proteins determining for the cell wall structure changed in abundance. Structural changes mainly appeared in the composition of pectic polysaccharides and data indicate an increased presence of xylogalacturonan in response to Cd. Although a higher abundance and enzymatic activity of pectin methylesterase was detected, the total pectin methylation was not affected. Conclusions An increased abundance of xylogalacturonan might hinder Cd binding in the cell wall due to the methylation of its galacturonic acid backbone. Probably, the exclusion of Cd from the cell wall and apoplast limits the entry of the heavy metal into the symplast and is an important factor during tolerance acquisition.

Unilateral cross-incompatibility (UCI) is a unidirectional inter/intra-population reproductive barrier when both parents are self-compatible. Maize Gametophyte factor1 ( Ga1 ) is an intraspecific UCI system and has been utilized in breeding. However, the mechanism underlying maize UCI specificity has remained mysterious for decades. Here, we report the cloning of ZmGa1P , a pollen-expressed PECTIN METHYLESTERASE ( PME ) gene at the Ga1 locus that can confer the male function in the maize UCI system. Homozygous transgenic plants expressing ZmGa1P in a ga1 background can fertilize Ga1-S plants and can be fertilized by pollen of ga1 plants. ZmGa1P protein is predominantly localized to the apex of growing pollen tubes and may interact with another pollen-specific PME protein, ZmPME10-1, to maintain the state of pectin methylesterification required for pollen tube growth in Ga1-S silks. Our study discloses a PME-mediated UCI mechanism and provides a tool to manipulate hybrid breeding. Unilateral cross-incompatibility between certain varieties of maize prevents cross-fertilization and can facilitate hybrid breeding. Here the authors show that a PECTIN METHYLESTERASE gene is able to overcome this reproductive barrier and confer fertility when expressed in pollen of the male parent.

Background and aims The role of polysaccharide modification in metal accumulation in hyperaccumulators is still unknown. Our aim was to compare the differences in the role of root cell-wall polysaccharides in cadmium (Cd) accumulation between hyperaccumulating (HE) and non-hyperaccumulating ecotype (NHE) of Sedm alfredii. Methods Hydroponic experiments were performed to characterize root-to-shoot Cd translocation, cadmium species and polysaccharide modification in root cell-wall of S. alfredii using stable isotope tracing, X-ray absorption near edge structure and immunofluorescence localization techniques. Results Cd absorbed was more readily available for transport to the shoots by the HE roots than by the NHE roots, which is confirmed by a 6-fold higher 113Cd concentration in xylem sap. Root Cd efflux originated mainly from the cell walls. The concentration of cell-wall polysaccharides and activity of pectin methylesterase were higher in the NHE than in the HE in the absence of Cd, and even higher in the presence of Cd. More pectins were methylated in the HE than in the NHE, indicating more free pectic acid residues in the NHE. The cell-wall-bound Cd was retained more tightly in the NHE than in the HE. Conclusions Cadmium hyperaccumulation by HE of S. alfredii is associated with its enhanced Cd flux into the xylem, which is partly regulated by cell-wall polysaccharide modification in roots.

Background The migration of cadmium (Cd) from contaminated soil to rice is a cause for concern. However, the molecular mechanism underlying the response of rice roots to various Cd stresses remains to be clarified from the viewpoint of the co-expression network at a system-wide scale. Results We employed a comparative RNAseq-based approach to identify early Cd-responsive differentially expressed genes (DEGs) in rice ‘Nipponbare’ seedling roots after 1 h of high-Cd treatment. A multiplicity of the identified 1772 DEGs were implicated in hormone signaling and transcriptional regulation, particularly NACs and WRKYs were all upregulated under Cd stress. All of the 6 Cd-upregulated ABC transporters were pleiotropic drug resistance proteins (PDRs), whereas all of the 6 ZRT/IRT-like proteins (ZIPs) were consistently downregulated by Cd treatment. To further confirm our results of this early transcriptomic response to Cd exposure, we then conducted weighted gene co-expression network analysis (WGCNA) to re-analyze our RNAseq data in combination with other 11 previously published RNAseq datasets for rice roots exposed to diverse concentrations of Cd for extended treatment periods. This integrative approach identified 271 transcripts as universal Cd-regulated DEGs that are key components of the Cd treatment coupled co-expression module. A global view of the 164 transcripts with annotated functions in pathway networks revealed several Cd-upregulated key functional genes, including transporter ABCG36/OsPDR9, 12-oxo-phytodienoic acid reductases (OPRs) for JA synthesis, and ZIM domain proteins JAZs in JA signaling, as well as OsWRKY10 , NAC, and ZFP transcription factors. More importantly, 104 of these, including ABCG36/OsPDR9 , OsNAC3 , as well as several orthologs in group metalloendoproteinase, plastocyanin-like domain containing proteins and pectin methylesterase inhibitor, may respond specifically to various Cd pressures, after subtracting the 60 general stress-responsive genes reported to be commonly upregulated following multiple stresses. Conclusion An integrative approach was implemented to identify DEGs and co-expression network modules in response to various Cd pressures, and 104 of the 164 annotatable universal Cd-responsive DEGs may specifically respond to various Cd pressures. These results provide insight into the universal molecular mechanisms beneath the Cd response in rice roots, and suggest many promising targets for improving the rice acclimation process against Cd toxicity.

Background Panax notoginseng (Burk) F.H. Chen is an essential plant in the family of Araliaceae. Its seeds are classified as a type of morphophysiological dormancy (MPD), and are characterized by recalcitrance during the after-ripening process. However, it is not clear about the molecular mechanism on the after-ripening in recalcitrant seeds. Results In this study, exogenous supply of gibberellic acid (GA 3 ) with different concentrations shortened after-ripening process and promoted the germination of P. notoginseng seeds. Among the identified plant hormone metabolites, exogenous GA 3 results in an increased level of endogenous hormone GA 3 through permeation. A total of 2971 and 9827 differentially expressed genes (DEGs) were identified in response to 50 mg L −1 GA 3 (LG) and 500 mg L −1 GA 3 (HG) treatment, respectively, and the plant hormone signal and related metabolic pathways regulated by GA 3 was significantly enriched. Weighted gene co-expression network analysis (WGCNA) revealed that GA 3 treatment enhances GA biosynthesis and accumulation, while inhibiting the gene expression related to ABA signal transduction. This effect was associated with higher expression of crucial seed embryo development and cell wall loosening genes, Leafy Contyledon1 ( LEC1 ), Late Embryogenesis Abundant ( LEA ), expansins ( EXP ) and Pectinesterase ( PME ). Conclusions Exogenous GA 3 application promotes germination and shorts the after-ripening process of P. notoginseng seeds by increasing GA 3 contents through permeation. Furthermore, the altered ratio of GA and ABA contributes to the development of the embryo, breaks the mechanical constraints of the seed coat and promotes the protrusion of the radicle in recalcitrant P. notoginseng seeds. These findings improve our knowledge of the contribution of GA to regulating the dormancy of MPD seeds during the after-ripening process, and provide new theoretical guidance for the application of recalcitrant seeds in agricultural production and storage.