Three aspartic acid residues of polygalacturonase-inhibiting protein (PGIP) from Phaseolus vulgaris are critical for inhibition of Fusarium phyllophilum PG

Cucumber PGIP2 is involved in resistance to gray mold disease

Polygalacturonase inhibitor protein (PGIP) restricts fungal growth and colonization and functions in plant immunity. Gray mold in cucumber is a common fungal disease caused by Botrytis cinerea, and is widespread and difficult to control in cucumber (Cucumis sativus L.) production. In this study, Cucumis sativus polygalacturonase-inhibiting protein 2 (CsPGIP2) was found to be upregulated in response to gray mold in cucumber. CsPGIP2 was detected in the endoplasmic reticulum, cell membrane, and cell wall after transient transformation of protoplasts and tobacco. A possible interaction between Botrytis cinerea polygalacturonase 3 (BcPG3) and CsPGIP2 was supported by protein interaction prediction and BiFC analysis. Transgenic Arabidopsis plants expressing CsPGIP2 were constructed and exhibited smaller areas of gray mold infection compared to wild type (WT) plants after simultaneous inoculation. Evans blue dye (EBD) confirmed greater damage for WT plants, with more intense dyeing than for the transgenic Arabidopsis. Interestingly, compared to WT, transgenic Arabidopsis exhibited higher superoxide dismutase (AtSOD1) expression, antioxidant enzyme activities, lignin content, net photosynthetic rate (Pn), and photochemical activity. Our results suggest that CsPGIP2 stimulates a variety of plant defense mechanisms to enhance transgenic Arabidopsis resistance against gray mold disease.

Exploring the potential of engineering polygalacturonase-inhibiting protein as an ecological, friendly, and nontoxic pest control agent

In plants, polygalacturonase-inhibiting proteins (PGIPs) play critical roles for resistance to fungal disease by inhibiting the pectin-depolymerizing activity of endopolygalacturonases (PGs), one type of enzyme secreted by pathogens that compromises plant cell walls and leaves the plant susceptible to disease. Here, the interactions between PGIPs from Phaseolus vulgaris (PvPGIP1 and PvPGIP2) and PGs from Aspergillus niger (AnPG2), Botrytis cinerea (BcPG1 and BcPG2), and Fusarium moniliforme (FmPG3) were reconstituted through a yeast two hybrid (Y2H) system to investigate the inhibition efficiency of various PvPGIP1 and 2 truncations and mutants. We found that tPvPGIP2_5-8, which contains LRR5 to LRR8 and is only one-third the size of the full length peptide, exhibits the same level of interactions with AnPG and BcPGs as the full length PvPGIP2 via Y2H. The inhibitory activities of tPvPGIP2_5-8 on the growth of A. niger and B. cinerea were then examined and confirmed on pectin agar. On pectin assays, application of both full length PvPGIP2 and tPvPGIP2_5-8 clearly slows down the growth of A. niger and B. cinerea. Investigation on the sequence-function relationships of PGIP utilizing a combination of site directed mutagenesis and a variety of peptide truncations suggests that LRR5 could have the most essential structural feature for the inhibitory activities, and may be a possible target for the future engineering of PGIP with enhanced activity. This study highlights the potential of plant-derived PGIPs as a candidate for future in planta evaluation as a pest control agent.

Amino acid substitutions in a polygalacturonase inhibiting protein (OsPGIP2) increases sheath blight resistance in rice

BACKGROUND

An economic strategy to control plant disease is to improve plant defense to pathogens by deploying resistance genes. Plant polygalacturonase inhibiting proteins (PGIPs) have a vital role in plant defense against phytopathogenic fungi by inhibiting fungal polygalacturonase (PG) activity. We previously reported that rice PGIP1 (OsPGIP1) inhibits PG activity in Rhizoctonia solani, the causal agent of rice sheath blight (SB), and is involved in regulating resistance to SB.

RESULT

Here, we report that OsPGIP2, the protein ortholog of OsPGIP1, does not possess PGIP activity; however, a few amino acid substitutions in a derivative of OsPGIP2, of which we provide support for L233F being the causative mutation, appear to impart OsPGIP2 with PG inhibition capability. Furthermore, the overexpression of mutated OsPGIP2L233F in rice significantly increased the resistance of transgenic lines and decreased SB disease rating scores. OsPGIP2L233F transgenic lines displayed an increased ability to reduce the tissue degradation caused by R. solani PGs as compared to control plants. Rice plants overexpressing OsPGIP2L233F showed no difference in agronomic traits and grain yield as compared to controls, thus demonstrating its potential use in rice breeding programs.

CONCLUSIONS

In summary, our results provide a new target gene for breeding SB resistance through genome-editing or natural allele mining.

Amino acid substitutions in a polygalacturonase inhibiting protein (OsPGIP2) increases sheath blight resistance in rice

BACKGROUND

An economic strategy to control plant disease is to improve plant defense to pathogens by deploying resistance genes. Plant polygalacturonase inhibiting proteins (PGIPs) have a vital role in plant defense against phytopathogenic fungi by inhibiting fungal polygalacturonase (PG) activity. We previously reported that rice PGIP1 (OsPGIP1) inhibits PG activity in Rhizoctonia solani, the causal agent of rice sheath blight (SB), and is involved in regulating resistance to SB.

RESULT

Here, we report that OsPGIP2, the protein ortholog of OsPGIP1, does not possess PGIP activity; however, a few amino acid substitutions in a derivative of OsPGIP2, of which we provide support for L233F being the causative mutation, appear to impart OsPGIP2 with PG inhibition capability. Furthermore, the overexpression of mutated OsPGIP2^L233F in rice significantly increased the resistance of transgenic lines and decreased SB disease rating scores. OsPGIP2^L233F transgenic lines displayed an increased ability to reduce the tissue degradation caused by R. solani PGs as compared to control plants. Rice plants overexpressing OsPGIP2^L233F showed no difference in agronomic traits and grain yield as compared to controls, thus demonstrating its potential use in rice breeding programs.

CONCLUSIONS

In summary, our results provide a new target gene for breeding SB resistance through genome-editing or natural allele mining.

Identification of I-7 expands the repertoire of genes for resistance to Fusarium wilt in tomato to three resistance gene classes

The tomato I-3 and I-7 genes confer resistance to Fusarium oxysporum f. sp. lycopersici (Fol) race 3 and were introgressed into the cultivated tomato, Solanum lycopersicum, from the wild relative Solanum pennellii. I-3 has been identified previously on chromosome 7 and encodes an S-receptor-like kinase, but little is known about I-7. Molecular markers have been developed for the marker-assisted breeding of I-3, but none are available for I-7. We used an RNA-seq and single nucleotide polymorphism (SNP) analysis approach to map I-7 to a small introgression of S. pennellii DNA (c. 210 kb) on chromosome 8, and identified I-7 as a gene encoding a leucine-rich repeat receptor-like protein (LRR-RLP), thereby expanding the repertoire of resistance protein classes conferring resistance to Fol. Using an eds1 mutant of tomato, we showed that I-7, like many other LRR-RLPs conferring pathogen resistance in tomato, is EDS1 (Enhanced Disease Susceptibility 1) dependent. Using transgenic tomato plants carrying only the I-7 gene for Fol resistance, we found that I-7 also confers resistance to Fol races 1 and 2. Given that Fol race 1 carries Avr1, resistance to Fol race 1 indicates that I-7-mediated resistance, unlike I-2- or I-3-mediated resistance, is not suppressed by Avr1. This suggests that Avr1 is not a general suppressor of Fol resistance in tomato, leading us to hypothesize that Avr1 may be acting against an EDS1-independent pathway for resistance activation. The identification of I-7 has allowed us to develop molecular markers for marker-assisted breeding of both genes currently known to confer Fol race 3 resistance (I-3 and I-7). Given that I-7-mediated resistance is not suppressed by Avr1, I-7 may be a useful addition to I-3 in the tomato breeder's toolbox.

Controlled expression of pectic enzymes in Arabidopsis thaliana enhances biomass conversion without adverse effects on growth

Lignocellulosic biomass from agriculture wastes is a potential source of biofuel, but its use is currently limited by the recalcitrance of the plant cell wall to enzymatic digestion. Modification of the wall structural components can be a viable strategy to overcome this bottleneck. We have previously shown that the expression of a fungal polygalacturonase (pga2 from Aspergillus niger) in Arabidopsis and tobacco plants reduces the levels of de-esterified homogalacturonan in the cell wall and significantly increases saccharification efficiency. However, plants expressing pga2 show stunted growth and reduced biomass production, likely as a consequence of an extensive loss of pectin integrity during the whole plant life cycle. We report here that the expression in Arabidopsis of another pectic enzyme, the pectate lyase 1 (PL1) of Pectobacterium carotovorum, under the control of a chemically inducible promoter, results, after induction of the transgene, in a saccharification efficiency similar to that of plants expressing pga2. However, lines with high levels of transgene induction show reduced growth even in the absence of the inducer. To overcome the problem of plant fitness, we have generated Arabidopsis plants that express pga2 under the control of the promoter of SAG12, a gene expressed only during senescence. These plants expressed pga2 only at late stages of development, and their growth was comparable to that of WT plants. Notably, leaves and stems of transgenic plants were more easily digested by cellulase, compared to WT plants, only during senescence. Expression of cell wall-degrading enzymes at the end of the plant life cycle may be therefore a useful strategy to engineer crops unimpaired in biomass yield but improved for bioconversion.

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.

Experimental and bioinformatic characterization of a recombinant polygalacturonase-inhibitor protein from pearl millet and its interaction with fungal polygalacturonases

Polygalacturonases (PGs) are hydrolytic enzymes employed by several phytopathogens to weaken the plant cell wall by degrading homopolygalacturonan, a major constituent of pectin. Plants fight back by employing polygalacturonase-inhibitor proteins (PGIPs). The present study compared the inhibition potential of pearl millet PGIP (Pennisetum glaucum; PglPGIP1) with the known inhibition of Phaseolus vulgaris PGIP (PvPGIP2) against two PGs, the PG-II isoform from Aspergillus niger (AnPGII) and the PG-III isoform from Fusarium moniliforme (FmPGIII). The key rationale was to elucidate the relationship between the extent of sequence similarity of the PGIPs and the corresponding PG inhibition potential. First, a pearl millet pgip gene (Pglpgip1) was isolated and phylogenetically placed among monocot PGIPs alongside foxtail millet (Setaria italica). Upstream sequence analysis of Pglpgip1 identified important cis-elements responsive to light, plant stress hormones, and anoxic stress. PglPGIP1, heterologously produced in Escherichia coli, partially inhibited AnPGII non-competitively with a pH optimum between 4.0 and 4.5, and showed no inhibition against FmPGIII. Docking analysis showed that the concave surface of PglPGIP1 interacted strongly with the N-terminal region of AnPGII away from the active site, whereas it weakly interacted with the C-terminus of FmPGIII. Interestingly, PglPGIP1 and PvPGIP2 employed similar motif regions with few identical amino acids for interaction with AnPGII at non-substrate-binding sites; however, they engaged different regions of AnPGII. Computational mutagenesis predicted D126 (PglPGIP1)-K39 (AnPGII) to be the most significant binding contact in the PglPGIP1-AnPGII complex. Such protein-protein interaction studies are crucial in the future generation of designer host proteins for improved resistance against ever-evolving pathogen virulence factors.

The pgip family in soybean and three other legume species: evidence for a birth-and-death model of evolution

BACKGROUND

Polygalacturonase-inhibiting proteins (PGIPs) are leucine-rich repeat (LRR) plant cell wall glycoproteins involved in plant immunity. They are typically encoded by gene families with a small number of gene copies whose evolutionary origin has been poorly investigated. Here we report the complete characterization of the full complement of the pgip family in soybean (Glycine max [L.] Merr.) and the characterization of the genomic region surrounding the pgip family in four legume species.

RESULTS

BAC clone and genome sequence analyses showed that the soybean genome contains two pgip loci. Each locus is composed of three clustered genes that are induced following infection with the fungal pathogen Sclerotinia sclerotiorum (Lib.) de Bary, and remnant sequences of pgip genes. The analyzed homeologous soybean genomic regions (about 126 Kb) that include the pgip loci are strongly conserved and this conservation extends also to the genomes of the legume species Phaseolus vulgaris L., Medicago truncatula Gaertn. and Cicer arietinum L., each containing a single pgip locus. Maximum likelihood-based gene trees suggest that the genes within the pgip clusters have independently undergone tandem duplication in each species.

CONCLUSIONS

The paleopolyploid soybean genome contains two pgip loci comprised in large and highly conserved duplicated regions, which are also conserved in bean, M. truncatula and C. arietinum. The genomic features of these legume pgip families suggest that the forces driving the evolution of pgip genes follow the birth-and-death model, similar to that proposed for the evolution of resistance (R) genes of NBS-LRR-type.

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.

The pepper extracellular xyloglucan-specific endo-β-1,4-glucanase inhibitor protein gene, CaXEGIP1, is required for plant cell death and defense responses

Plants produce various proteinaceous inhibitors to protect themselves against microbial pathogen attack. A xyloglucan-specific endo-β-1,4-glucanase inhibitor1 gene, CaXEGIP1, was isolated and functionally characterized in pepper (Capsicum annuum) plants. CaXEGIP1 was rapidly and strongly induced in pepper leaves infected with avirulent Xanthomonas campestris pv vesicatoria, and purified CaXEGIP1 protein significantly inhibited the hydrolytic activity of the glycoside hydrolase74 family xyloglucan-specific endo-β-1,4-glucanase from Clostridium thermocellum. Soluble-modified green fluorescent protein-tagged CaXEGIP1 proteins were mainly localized to the apoplast of onion (Allium cepa) epidermal cells. Agrobacterium tumefaciens-mediated overexpression of CaXEGIP1 triggered pathogen-independent, spontaneous cell death in pepper and Nicotiana benthamiana leaves. CaXEGIP1 silencing in pepper conferred enhanced susceptibility to virulent and avirulent X. campestris pv vesicatoria, accompanied by a compromised hypersensitive response and lowered expression of defense-related genes. Overexpression of dexamethasone:CaXEGIP1 in Arabidopsis (Arabidopsis thaliana) enhanced resistance to Hyaloperonospora arabidopsidis infection. Comparative histochemical and proteomic analyses revealed that CaXEGIP1 overexpression induced a spontaneous cell death response and also increased the expression of some defense-related proteins in transgenic Arabidopsis leaves. This response was also accompanied by cell wall thickening and darkening. Together, these results suggest that pathogen-inducible CaXEGIP1 positively regulates cell death-mediated defense responses in plants.

Spatial distribution of polygalacturonase-inhibiting proteins in Arabidopsis and their expression induced by Stemphylium solani infection

Disease-induced polygalacturonase-inhibiting proteins (PGIPs) are the major defense proteins which play an important role in resistance to infection of pathogens. To date, the AtPGIP expression in Arabidopsis induced by Stemphylium solani (S. solani) was not described. Here the distribution of AtPGIPs and their expression induced by S. solani infection in Arabidopsis was reported. Notably, immunofluorescence localization showed that the AtPGIPs were distributed in leaves, petioles, stems and roots of 5 week old Arabidopsis, but they were mainly localized in epidermis, vascular bundles and vascular cylinder. Further studies indicated that the transcription level of AtPGIP1 and AtPGIP2 was both up-regulated in response to infection with S. solani which caused hypersensitive cell death, but the transcription level of AtPGIP2 was less induced than AtPGIP1. Consistently, the bulk AtPGIPs of Arabidopsis showed a higher activity in leaves infected by S. solani. Taken together, our preliminary results showed that AtPGIPs were spatially distributed and AtPGIP expression might take part in resistance to infection of S. solani. This study might highlight the potential importance of AtPGIPs and plant disease resistance.

SPR and differential proteolysis/MS provide further insight into the interaction between PGIP2 and EPGs

By using surface plasmon resonance (SPR) technology, the kinetics of the interaction of various fungal endopolygalacturonases (EPGs) (13 EPGs) with Phaseolus vulgaris (bean) PGIP2 was carried out to determine whether or not there is any interaction between polygalacturonases-inhibiting protein (PGIP) and EPG. The effect of polygalacturonic acid (PGA) on these interactions was also evaluated. The results show that all EPGs evaluated bind to PGIP2, except for AnPGb and the strength of the interaction depends on the EPG/PGIP2 pairing. Further, the presence of PGA has a moderate to strong effect on the EPG/PGIP2 interaction and the strength of the effect is dependent on the exact EPG/PGIP2 pairing. The differences in affinity in the absence and presence of PGA, suggest a certain level of cooperativity. These results indicate a three-component complex similar to that observed for the heparin-ATIII-thrombin, the FGF-FGFR-heparin, or the hedgehog-interference hedgehog-heparan complexes. This data points to an architecture in which the inhibitor binds at a location distant from the substrate binding site. Furthermore, we applied differential proteolysis mass spectrometry (DPMS) to study the location of the binding site between EPG and PGIP2. DPMS studies indicate that PGIP2 does not bind AnPGII, AnPGa, and AnPGc directly over the active site but instead binds on the face opposite to the active site, creating an allosteric interaction.

Structural resolution of the complex between a fungal polygalacturonase and a plant polygalacturonase-inhibiting protein by small-angle X-ray scattering

We report here the low-resolution structure of the complex formed by the endo-polygalacturonase from Fusarium phyllophilum and one of the polygalacturonase-inhibiting protein from Phaseolus vulgaris after chemical cross-linking as determined by small-angle x-ray scattering analysis. The inhibitor engages its concave surface of the leucine-rich repeat domain with the enzyme. Both sides of the enzyme active site cleft interact with the inhibitor, accounting for the competitive mechanism of inhibition observed. The structure is in agreement with previous site-directed mutagenesis data and has been further validated with structure-guided mutations and subsequent assay of the inhibitory activity. The structure of the complex may help the design of inhibitors with improved or new recognition capabilities to be used for crop protection.

The bean polygalacturonase-inhibiting protein 2 (PvPGIP2) is highly conserved in common bean (Phaseolus vulgaris L.) germplasm and related species

Polygalacturonase-inhibiting proteins (PGIPs) are extracellular plant protein inhibitors of endo-polygalacturonases (PGs) that belong to the leucine-rich repeat (LRR) protein family. In bean, PGIP is encoded by a small gene family of four members among which Pvpgip2 encodes the most wide-spectrum and efficient inhibitor of fungal PGs. In order to evaluate the sequence polymorphism of Pvpgip2 and its functional significance, we have analyzed a number of wild and cultivated bean (P. vulgaris) accessions of Andean and Mesoamerican origin, and some genotypes from the related species P. coccineus, P. acutifolius, and P. lunatus. Our analyses indicate that the protein encoded by Pvpgip2 is highly conserved in the bean germplasm. The few detected polymorphic sites correspond to synonymous substitutions and only two wild genotypes contain a Pvpgip2 with a single non-synonymous replacement. Sequence comparison showed a slightly larger variation in the related bean species P. coccineus, P. acutifolius, and P. lunatus and confirmed the known phylogenetic relationships with P. vulgaris. The majority of the replacements were within the xxLxLxx region of the leucine rich repeat (LRR) domain and none of them affected residues contributing to structural features. The variant PGIPs were expressed in Nicotiana benthamiana using PVX as vector and their inhibitory activity compared to that of PvPPGIP2. All the variants were able to fully inhibit the four fungal PGs tested with minor differences. Taken together these results support the hypothesis that the overall sequence conservation of PGIP2 and minor variation at specific sites is necessary for high-affinity recognition of different fungal PGs.

Integration of evolutionary and desolvation energy analysis identifies functional sites in a plant immunity protein

Plant immune responses often depend on leucine-rich repeat receptors that recognize microbe-associated molecular patterns or pathogen-specific virulence proteins, either directly or indirectly. When the recognition is direct, a molecular arms race takes place where plant receptors continually and rapidly evolve in response to virulence factor evolution. A useful model system to study ligand-receptor coevolution dynamics at the protein level is represented by the interaction between pathogen-derived polygalacturonases (PGs) and plant polygalacturonase-inhibiting proteins (PGIPs). We have applied codon substitution models to PGIP sequences of different eudicotyledonous families to identify putative positively selected sites and then compared these sites with the propensity of protein surface residues to interact with protein partners, based on desolvation energy calculations. The 2 approaches remarkably correlated in pinpointing several residues in the concave face of the leucine-rich repeat domain. These residues were mutated into alanine and their effect on the recognition of several PGs was tested, leading to the identification of unique hotspots for the PGIP-PG interaction. The combined approach used in this work can be of general utility in cases where structural information about a pattern-recognition receptor or resistance-gene product is available.