Polygalacturonase-inhibiting protein is a structural component of plant cell wall

Ripening and rot: How ripening processes influence disease susceptibility in fleshy fruits

Fleshy fruits become more susceptible to pathogen infection when they ripen; for example, changes in cell wall properties related to softening make it easier for pathogens to infect fruits. The need for high-quality fruit has driven extensive research on improving pathogen resistance in important fruit crops such as tomato (Solanum lycopersicum). In this review, we summarize current progress in understanding how changes in fruit properties during ripening affect infection by pathogens. These changes affect physical barriers that limit pathogen entry, such as the fruit epidermis and its cuticle, along with other defenses that limit pathogen growth, such as preformed and induced defense compounds. The plant immune system also protects ripening fruit by recognizing pathogens and initiating defense responses involving reactive oxygen species production, mitogen-activated protein kinase signaling cascades, and jasmonic acid, salicylic acid, ethylene, and abscisic acid signaling. These phytohormones regulate an intricate web of transcription factors (TFs) that activate resistance mechanisms, including the expression of pathogenesis-related genes. In tomato, ripening regulators, such as RIPENING INHIBITOR and NON_RIPENING, not only regulate ripening but also influence fruit defenses against pathogens. Moreover, members of the ETHYLENE RESPONSE FACTOR (ERF) family play pivotal and distinct roles in ripening and defense, with different members being regulated by different phytohormones. We also discuss the interaction of ripening-related and defense-related TFs with the Mediator transcription complex. As the ripening processes in climacteric and non-climacteric fruits share many similarities, these processes have broad applications across fruiting crops. Further research on the individual contributions of ERFs and other TFs will inform efforts to diminish disease susceptibility in ripe fruit, satisfy the growing demand for high-quality fruit and decrease food waste and related economic losses.

Molecular dynamics simulation of the interaction of a raspberry polygalacturonase (RiPG) with a PG inhibiting protein (RiPGIP) isolated from ripening raspberry (Rubus idaeus cv. Heritage) fruit as a model to understand proteins interaction during fruit softening

Polygalacturonase (PG) is an important hydrolytic enzyme involved in pectin disassembly and the subsequent textural changes during fruit ripening. Although the interaction of fungal PGs with other proteins has been documented, the interaction of plant PGs with other plant proteins has not yet been studied. In this study, the molecular mechanisms involved in raspberry fruit ripening, particularly the polygalacturonase (RiPG) interaction with polygalacturonase inhibiting protein (RiPGIP) and substrate, were investigated with a structural approach. The 3D model of RiPG2 and RiPGIP3 was built using a comparative modeling strategy and validated using molecular dynamics (MD) simulations. The RiPG2 model structure comprises 11 complete coils of right-handed parallel β-helix architecture, with an average of 27 amino acid residues per turn. The structural model of the RiPGIP3 displays a typical structure of LRR protein, with the right-handed superhelical fold with an extended parallel β-sheet. The conformational interaction between the RiPG2 protein and RiPGIP3 showed that RiPGIP3 could bind to the enzyme and thereby leave the active site cleft accessible to the substrate. All this evidence indicates that RiPG2 enzyme could interact with RiPGIP3 protein. It can be a helpful model for evaluating protein-protein interaction as a potential regulator mechanism of hydrolase activity during pectin disassembly in fruit ripening.

Laccaria bicolor pectin methylesterases are involved in ectomycorrhiza development with Populus tremula × Populus tremuloides

The development of ectomycorrhizal (ECM) symbioses between soil fungi and tree roots requires modification of root cell walls. The pectin-mediated adhesion between adjacent root cells loosens to accommodate fungal hyphae in the Hartig net, facilitating nutrient exchange between partners. We investigated the role of fungal pectin modifying enzymes in Laccaria bicolor for ECM formation with Populus tremula × Populus tremuloides. We combine transcriptomics of cell-wall-related enzymes in both partners during ECM formation, immunolocalisation of pectin (Homogalacturonan, HG) epitopes in different methylesterification states, pectin methylesterase (PME) activity assays and functional analyses of transgenic L. bicolor to uncover pectin modification mechanisms and the requirement of fungal pectin methylesterases (LbPMEs) for ECM formation. Immunolocalisation identified remodelling of pectin towards de-esterified HG during ECM formation, which was accompanied by increased LbPME1 expression and PME activity. Overexpression or RNAi of the ECM-induced LbPME1 in transgenic L. bicolor lines led to reduced ECM formation. Hartig Nets formed with LbPME1 RNAi lines were shallower, whereas those formed with LbPME1 overexpressors were deeper. This suggests that LbPME1 plays a role in ECM formation potentially through HG de-esterification, which initiates loosening of adjacent root cells to facilitate Hartig net formation.

New Players in the Interaction Between Beetle Polygalacturonases and Plant Polygalacturonase-Inhibiting Proteins: Insights From Proteomics and Gene Expression Analyses

Plants possess various defense strategies to counter attacks from microorganisms or herbivores. For example, plants reduce the cell-wall-macerating activity of pathogen- or insect-derived polygalacturonases (PGs) by expressing PG-inhibiting proteins (PGIPs). PGs and PGIPs belong to multi-gene families believed to have been shaped by an evolutionary arms race. The mustard leaf beetle Phaedon cochleariae expresses both active PGs and catalytically inactive PG pseudoenzymes. Previous studies demonstrated that (i) PGIPs target beetle PGs and (ii) the role of PG pseudoenzymes remains elusive, despite having been linked to the pectin degradation pathway. For further insight into the interaction between plant PGIPs and beetle PG family members, we combined affinity purification with proteomics and gene expression analyses, and identified novel inhibitors of beetle PGs from Chinese cabbage (Brassica rapa ssp. pekinensis). A beetle PG pseudoenzyme was not targeted by PGIPs, but instead interacted with PGIP-like proteins. Phylogenetic analysis revealed that PGIP-like proteins clustered apart from "classical" PGIPs but together with proteins, which have been involved in developmental processes. Our results indicate that PGIP-like proteins represent not only interesting novel PG inhibitor candidates in addition to "classical" PGIPs, but also fascinating new players in the arms race between herbivorous beetles and plant defenses.

Direct evidence for a new mode of plant defense against insects via a novel polygalacturonase-inhibiting protein expression strategy

Plant cell wall-associated polygalacturonase-inhibiting proteins (PGIPs) are widely distributed in the plant kingdom. They play a crucial role in plant defense against phytopathogens by inhibiting microbial polygalacturonases (PGs). PGs hydrolyze the cell wall polysaccharide pectin and are among the first enzymes to be secreted during plant infection. Recent studies demonstrated that herbivorous insects express their own PG multi-gene families, raising the question whether PGIPs also inhibit insect PGs and protect plants from herbivores. Preliminary evidence suggested that PGIPs may negatively influence larval growth of the leaf beetle Phaedon cochleariae (Coleoptera: Chrysomelidae) and identified BrPGIP3 from Chinese cabbage (Brassica rapa ssp. pekinensis) as a candidate. PGIPs are predominantly studied in planta because their heterologous expression in microbial systems is problematic and instability and aggregation of recombinant PGIPs has complicated in vitro inhibition assays. To minimize aggregate formation, we heterologously expressed BrPGIP3 fused to a glycosylphosphatidylinositol (GPI) membrane anchor, immobilizing it on the extracellular surface of insect cells. We demonstrated that BrPGIP3_GPI inhibited several P. cochleariae PGs in vitro, providing the first direct evidence of an interaction between a plant PGIP and an animal PG. Thus, plant PGIPs not only confer resistance against phytopathogens, but may also aid in defense against herbivorous beetles.

Identification and characterization of the first pectin methylesterase gene discovered in the root lesion nematode Pratylenchus penetrans

Similar to other plant-parasitic nematodes, root lesion nematodes possess an array of enzymes that are involved in the degradation of the plant cell wall. Here we report the identification of a gene encoding a cell wall-degrading enzyme, pectin methylesterase PME (EC 3.1.1.11), in the root lesion nematode Pratylenchus penetrans. Both genomic and coding sequences of the gene were cloned for this species, that included the presence of four introns which eliminated a possible contamination from bacteria. Expression of the Pp-pme gene was localized in the esophageal glands of P. penetrans as determined by in situ hybridization. Temporal expression of Pp-pme in planta was validated at early time points of infection. The possible function and activity of the gene were assessed by transient expression of Pp-pme in plants of Nicotiana benthamiana plants via a Potato virus X-based vector. To our knowledge, this is the first report on identification and characterization of a PME gene within the phylum Nematoda.

Cloning, expression analysis and recombinant expression of a gene encoding a polygalacturonase-inhibiting protein from tobacco, Nicotiana tabacum

Polygalacturonase inhibiting proteins (PGIPs) are major defensive proteins produced by plant cell walls that play a crucial role in pathogen resistance by reducing polygalacturonase (PG) activity. In the present study, a novel PGIP gene was isolated from tobacco (Nicotiana tabacum), hereafter referred as NtPGIP. A full-length NtPGIP cDNA of 1,412 bp with a 186 bp 5'-untranslated region (UTR), and 209 bp 3'-UTR was cloned from tobacco, NtPGIP is predicted to encode a protein of 338 amino acids. The NtPGIP sequence from genomic DNA showed no introns and sequence alignments of NtPGIP's deduced amino acid sequence showed high homology with known PGIPs from other plant species. Moreover, the putative NtPGIP protein was closely clustered with several Solanaceae PGIPs. Further, the expression profile of NtPGIP was examined in tobacco leaves following stimulation with the oomycete Phytophthora nicotianae and other stressors, including salicylic acid (SA), abscisic acid (ABA), salt, and cold treatment. The results showed that all of the treatments up-regulated the expression of NtPGIP at different times. To understand the biochemical activity of NtPGIP gene, a full-length NtPGIP cDNA sequence was subcloned into a pET28a vector and transformed into E. coli BL21 (DE3). Recombinant proteins were successfully induced by 1.0 nmol/L IPTG and the purified proteins effectively inhibited Phytophthora capsici PG activity. The results of this study suggest that NtPGIP may be a new candidate gene with properties that could be exploited in plant breeding.

How the rice weevil breaks down the pectin network: Enzymatic synergism and sub-functionalization

Pectin is the most complex polysaccharide in nature and highly abundant in plant cell walls and middle lamellae, where it functions in plant growth and development. Phytopathogens utilize plant pectin as an energy source through enzyme-mediated degradation. These pectolytic enzymes include polygalacturonases (PGs) of the GH28 family and pectin methylesterases (PMEs) of the CE8 family. Recently, PGs were also identified in herbivorous insects of the distantly related plant bug, stick insect and Phytophaga beetle lineages. Unlike all other insects, weevils possess PMEs in addition to PGs. To investigate pectin digestion in insects and the role of PMEs in weevils, all PME and PG family members of the rice weevil Sitophilus oryzae were heterologously expressed and functionally characterized. Enzymatically active and inactive PG and PME family members were identified. The loss of activity can be explained by a lack of substrate binding correlating with substitutions of functionally important amino acid residues. We found subfunctionalization in both enzyme families, supported by expression pattern and substrate specificities as well as evidence for synergistic pectin breakdown. Our data suggest that the rice weevil might be able to use pectin as an energy source, and illustrates the potential of both PG and PME enzyme families to functionally diversify after horizontal gene transfer.

Analysis of Quality-Related Parameters in Mature Kernels of Polygalacturonase Inhibiting Protein (PGIP) Transgenic Bread Wheat Infected with Fusarium graminearum

Fusarium head blight, caused by the fungus Fusarium graminearum, has a detrimental effect on both productivity and qualitative properties of wheat. To evaluate its impact on wheat flour, we compared its effect on quality-related parameters between a transgenic bread wheat line expressing a bean polygalacturonase inhibiting protein (PGIP) and its control line. We have compared metabolic proteins, the amounts of gluten proteins and their relative ratios, starch content, yield, extent of pathogen contamination, and deoxynivalenol (DON) accumulation. These comparisons showed that Fusarium significantly decreases the amount of starch in infected control plants, but not in infected PGIP plants. The flour of PGIP plants contained also a lower amount of pathogen biomass and DON accumulation. Conversely, both gluten and metabolic proteins were not significantly influenced either by the transgene or by fungal infection. These results indicate that the transgenic PGIP expression reduces the level of infection, without changing significantly the wheat seed proteome and other quality-related parameters.

In silico analysis of the polygalacturonase inhibiting protein 1 from apple, Malus domestica

Background

The Malus domestica polygalacturonase inhibiting protein 1 (MdPGIP1) gene, encoding the M. domestica polygalacturonase inhibiting protein 1 (MdPGIP1), was isolated from the Granny Smith apple cultivar (GenBank accession no. DQ185063). The gene was used to transform tobacco and potato for enhanced resistance against fungal diseases.

Findings

Analysis of the MdPGIP1 nucleotide sequence revealed that the gene comprises 993 nucleotides that encode a 330 amino acid polypeptide. In silico characterization of the MdPGIP1 polypeptide revealed domains typical of PGIP proteins, which include a 24 amino acid putative signal peptide, a potential cleavage site [Alanine-Leucine-Serine (ALS)] for the signal peptide, a 238 amino acid leucine-rich repeat (LRR) domain, a 46 amino acid N-terminal domain and a 22 amino acid C-terminal domain. The hydropathic evaluation of MdPGIP1 indicated a repetitive hydrophobic motif in the LRR domain and a hydrophilic surface area consistent with a globular protein. The typical consensus glycosylation sequence of Asn-X-Ser/Thr was identified in MdPGIP1, indicating potential N-linked glycosylation of MdPGIP1. The molecular mass of non-glycosylated MdPGIP1 was calculated as 36.615 kDa and the theoretical isoelectric point as 6.98. Furthermore, the secondary and tertiary structure of MdPGIP1 was modelled, and revealed that MdPGIP1 is a curved and elongated molecule that contains sheet B1, sheet B2 and 3₁₀-helices on its LRR domain.

Conclusion

The overall properties of the MdPGIP1 protein is similar to that of the prototypical Phaseolus vulgaris PGIP 2 (PvPGIP2), and the detected differences supported its use in biotechnological applications as an inhibitor of targeted fungal polygalacturonases (PGs).

An update on polygalacturonase-inhibiting protein (PGIP), a leucine-rich repeat protein that protects crop plants against pathogens

Polygalacturonase inhibiting proteins (PGIPs) are cell wall proteins that inhibit the pectin-depolymerizing activity of polygalacturonases secreted by microbial pathogens and insects. These ubiquitous inhibitors have a leucine-rich repeat structure that is strongly conserved in monocot and dicot plants. Previous reviews have summarized the importance of PGIP in plant defense and the structural basis of PG-PGIP interaction; here we update the current knowledge about PGIPs with the recent findings on the composition and evolution of pgip gene families, with a special emphasis on legume and cereal crops. We also update the information about the inhibition properties of single pgip gene products against microbial PGs and the results, including field tests, showing the capacity of PGIP to protect crop plants against fungal, oomycetes and bacterial pathogens.

Plant Polygalacturonases Involved in Cell Elongation and Separation-The Same but Different?

Plant cells are surrounded by the primary cell wall, a rigid framework that needs to be modified in order to allow cell growth. Recent data suggest that in addition to the cellulose-hemicellulose network, the pectin matrix plays a critical role in determining the elasticity of the primary cell wall. Polygalacturonases are key homogalacturonan-hydrolyzing enzymes that function in a wide range of developmental processes. In this review, we present recent progress in understanding the role of polygalacturonases during cell elongation and separation. In discussing the specificities and possible redundancies of polygalacturonases, we focus particularly on newly discovered Arabidopsis mutants that have measurable loss-of-function phenotypes. However, data from other species are included when necessary.

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.

Functional characterisation of wheat Pgip genes reveals their involvement in the local response to wounding

Polygalacturonase-inhibiting proteins (PGIPs) are cell wall leucine-rich repeat (LRR) proteins involved in plant defence. The hexaploid wheat (Triticum aestivum, genome AABBDD) genome contains one Pgip gene per genome. Tapgip1 (B genome) and Tapgip2 (D genome) are expressed in all tissues, whereas Tapgip3 (A genome) is inactive because of a long terminal repeat, Copia retrotransposon insertion within the coding region. To verify whether Tapgip1 and Tapgip2 encode active PGIPs and are involved in the wheat defence response, we expressed them transiently and analysed their expression under stress conditions. Neither TaPGIP1 nor TaPGIP2 showed inhibition activity in vitro against fungal polygalacturonases. Moreover, a wheat genotype (T. turgidum ssp. dicoccoides) lacking active homologues of Tapgip1 or Tapgip2 possesses PGIP activity. At transcript level, Tapgip1 and Tapgip2 were both up-regulated after fungal infection and strongly induced following wounding. This latter result has been confirmed in transgenic wheat plants expressing the β-glucuronidase (GUS) gene under control of the 5'-flanking region of Tdpgip1, a homologue of Tapgip1 with an identical sequence. Strong and transient GUS staining was mainly restricted to the damaged tissues and was not observed in adjacent tissues. Taken together, these results suggest that Tapgips and their homologues are involved in the wheat defence response by acting at the site of the lesion caused by pathogen infection.

Cell wall integrity: targeted post-synthetic modifications to reveal its role in plant growth and defense against pathogens

The plant cell wall, a dynamic network of polysaccharides and glycoproteins of significant compositional and structural complexity, functions in plant growth, development and stress responses. In recent years, the existence of plant cell wall integrity (CWI) maintenance mechanisms has been demonstrated, but little is known about the signaling pathways involved, or their components. Examination of key mutants has shed light on the relationships between cell wall remodeling and plant cell responses, indicating a central role for the regulatory network that monitors and controls cell wall performance and integrity. In this review, we present a short overview of cell wall composition and discuss post-synthetic cell wall modification as a valuable approach for studying CWI perception and signaling pathways.

Regulation of the grapevine polygalacturonase-inhibiting protein encoding gene: expression pattern, induction profile and promoter analysis

Regulation of defense in plants is a complex process mediated by various signaling pathways. Promoter analysis of defense-related genes is useful to understand these signaling pathways involved in regulation. To this end, the regulation of the polygalacturonase-inhibiting protein encoding gene from Vitis vinifera L. (Vvpgip1) was analyzed with regard to expression pattern and induction profile as well as the promoter in terms of putative regulatory elements present, core promoter size and the start of transcription. Expression of Vvpgip1 is tissue-specific and developmentally regulated. Vvpgip1 expression was induced in response to auxin, salicylic acid and sugar treatment, wounding and pathogen infection. The start of transcription was mapped to 17 bp upstream of the ATG and the core promoter was mapped to the 137 bp upstream of the ATG. Fructose- and Botrytis responsiveness were identified in the region between positions -3.1 and -1.5 kb. The analyses showed induction in water when the leaves were submersed and this response and the response to wounding mapped to the region between positions -1.1 and -0.1 kb. In silico analyses revealed putative cis-acting elements in these areas that correspond well to the induction stimuli tested.

Molecular cloning, functional analysis and localization of a novel gene encoding polygalacturonase-inhibiting protein in Chorispora bungeana

Polygalacturonase-inhibiting proteins (PGIPs) are plant defense proteins. To date, no spatial distribution of PGIPs and interaction between PGIPs and nitric oxide (NO) in plant were described. Here, we first reported the full-length cDNA sequence of PGIP of Chorispora bungeana (CbPGIP1). Notably, immunofluorescence localization showed that the CbPGIP was evenly distributed in leaves but it was mainly localized in epidermis and vascular bundle in stems and roots. Further studies indicated that CbPGIP had higher abundance in roots than in stems and leaves. Conversely, the bulk PGIP of C. bungeana showed a higher activity in leaves than in stems and roots. In addition, quantitative real-time polymerase chain reaction demonstrated that CbPGIP1 expression was induced by Stemphylium solani, salicylic acid (SA), 4, -4 degrees C and NO. This is a first report attempting to predict if NO can induce the PGIP expression. Taken together, these findings showed that the gene was spatially regulated and NO and SA might take part in CbPGIP1 expression induced by biotic and abiotic stresses. This study highlighted the potential importance of CbPGIP1 and NO in plant resistance.