Determination of pathogen-related enzyme action by mass spectrometry analysis of pectin breakdown products of plant cell walls

Transcriptomics Reveals the Effect of Short-Term Freezing on the Signal Transduction and Metabolism of Grapevine

Low temperature is an important factor limiting plant growth. Most cultivars of Vitis vinifera L. are sensitive to low temperatures and are at risk of freezing injury or even plant death during winter. In this study, we analyzed the transcriptome of branches of dormant cv. Cabernet Sauvignon exposed to several low-temperature conditions to identify differentially expressed genes and determine their function based on Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG)enrichment analyses. Our results indicated that exposure to subzero low temperatures resulted in damage to plant cell membranes and extravasation of intracellular electrolytes, and that this damage increased with decreasing temperature or increasing duration. The number of differential genes increased as the duration of stress increased, but most of the common differentially expressed genes reached their highest expression at 6 h of stress, indicating that 6 h may be a turning point for vines to tolerate extreme low temperatures. Several pathways play key roles in the response of Cabernet Sauvignon to low-temperature injury, namely: (1) the role of calcium/calmodulin-mediated signaling; (2) carbohydrate metabolism, including the hydrolysis of cell wall pectin and cellulose, decomposition of sucrose, synthesis of raffinose, and inhibition of glycolytic processes; (3) the synthesis of unsaturated fatty acids and metabolism of linolenic acid; and (4) the synthesis of secondary metabolites, especially flavonoids. In addition, pathogenesis-related protein may also play a role in plant cold resistance, but the mechanism is not yet clear. This study reveals possible pathways for the freezing response and leads to new insights into the molecular basis of the tolerance to low temperature in grapevine.

Botrytis cinerea infection accelerates ripening and cell wall disassembly to promote disease in tomato fruit

Postharvest fungal pathogens benefit from the increased host susceptibility that occurs during fruit ripening. In unripe fruit, pathogens often remain quiescent and unable to cause disease until ripening begins, emerging at this point into destructive necrotrophic lifestyles that quickly result in fruit decay. Here, we demonstrate that one such pathogen, Botrytis cinerea, actively induces ripening processes to facilitate infections and promote disease in tomato (Solanum lycopersicum). Assessments of ripening progression revealed that B. cinerea accelerated external coloration, ethylene production, and softening in unripe fruit, while mRNA sequencing of inoculated unripe fruit confirmed the corresponding upregulation of host genes involved in ripening processes, such as ethylene biosynthesis and cell wall degradation. Furthermore, an enzyme-linked immunosorbent assay (ELISA)-based glycomics technique used to assess fruit cell wall polysaccharides revealed remarkable similarities in the cell wall polysaccharide changes caused by both infections of unripe fruit and ripening of healthy fruit, particularly in the increased accessibility of pectic polysaccharides. Virulence and additional ripening assessment experiments with B. cinerea knockout mutants showed that induction of ripening depends on the ability to infect the host and break down pectin. The B. cinerea double knockout Δbc polygalacturonase1 Δbc polygalacturonase2 lacking two critical pectin degrading enzymes was incapable of emerging from quiescence even long after the fruit had ripened at its own pace, suggesting that the failure to accelerate ripening severely inhibits fungal survival on unripe fruit. These findings demonstrate that active induction of ripening in unripe tomato fruit is an important infection strategy for B. cinerea.

Structural characteristics of sulfated polysaccharides from Sargassum horneri and immune-enhancing activity of polysaccharides combined with lactic acid bacteria

Sargassum horneri (SH), a marine brown alga, is known to contain a variety of bioactive ingredients and previous studies reported sulfated polysaccharides in SH as a potential candidate for a functional ingredient. However, immune-enhancing activity combined with Lactobacillus plantarum (LAB) is not yet studied. In the present study, we attempted to characterize sulfated polysaccharides (SHCPs) in SH by MALDI-TOF/TOF mass spectrometry and evaluate their immune-enhancing effect on macrophage cells. The main residue of SHCPs in SH is 2-sulfated 1,4-linked L-fucose and this epitope combined with LAB shows immune enhancement properties through cytokine production at the cellular level and increases the population of lymphocytes and myelomonocytes in the adult zebrafish kidney. These results indicate that SHCPs, along with LAB, have potent immune-enhancing activity and may be utilized as a potential immunomodulatory ingredient.

The Arabidopsis thaliana LysM-containing Receptor-Like Kinase 2 is required for elicitor-induced resistance to pathogens

In Arabidopsis thaliana, perception of chitin from fungal cell walls is mediated by three LysM-containing Receptor-Like Kinases (LYKs): CERK1, which is absolutely required for chitin perception, and LYK4 and LYK5, which act redundantly. The role in plant innate immunity of a fourth LYK protein, LYK2, is currently not known. Here we show that CERK1, LYK2 and LYK5 are dispensable for basal susceptibility to B. cinerea but are necessary for chitin-induced resistance to this pathogen. LYK2 is dispensable for chitin perception and early signalling events, though it contributes to callose deposition induced by this elicitor. Notably, LYK2 is also necessary for enhanced resistance to B. cinerea and Pseudomonas syringae induced by flagellin and for elicitor-induced priming of defence gene expression during fungal infection. Consistently, overexpression of LYK2 enhances resistance to B. cinerea and P. syringae and results in increased expression of defence-related genes during fungal infection. LYK2 appears to be required to establish a primed state in plants exposed to biotic elicitors, ensuring a robust resistance to subsequent pathogen infections.

High-CO2 Treatment Prolongs the Postharvest Shelf Life of Strawberry Fruits by Reducing Decay and Cell Wall Degradation

Improved methods are needed to extend the shelf life of strawberry fruits. The objective of this study was to determine the postharvest physiological mechanism of high-CO₂ treatment in strawberries. Harvested strawberries were stored at 10 °C after 3 h of exposure to a treatment with 30% CO₂ or air. Pectin and gene expression levels related to cell wall degradation were measured to assess the high-CO₂ effects on the cell wall and lipid metabolism. Strawberries subjected to high-CO₂ treatment presented higher pectin content and firmness and lower decay than those of control fruits. Genes encoding cell wall-degrading enzymes (pectin methylesterase, polygalacturonase, and pectate lyase) were downregulated after high-CO₂ treatment. High-CO₂ induced the expression of oligogalacturonides, thereby conferring defense against Botrytis cinerea in strawberry fruits, and lowering the decay incidence at seven days after its inoculation. Our findings suggest that high-CO₂ treatment can maintain strawberry quality by reducing decay and cell wall degradation.

Physiological effects of zero-valent iron nanoparticles in rhizosphere on edible crop, Medicago sativa (Alfalfa), grown in soil

We investigated the effects of nanoscale zero-valent iron (nZVI) that has been widely used for groundwater remediation on a terrestrial crop, Medicago sativa (Alfalfa), and comprehensively addressed its development and growth in soil culture. Root lengths, chlorophyll, carbohydrate and lignin contents were compared, and no physiological phytotoxicity was observed in the plants. In the roots, using an omics-based analytical, we found evidence of OH radical-induced cell wall loosening from exposure to nZVI, resulting in increased root lengths that were approximately 1.5 times greater than those of the control. Moreover, germination index (GI) was employed to physiologically evaluate the impact of nZVI on germination and root length. In regard to chlorophyll concentration, nZVI-treated alfalfa exhibited a higher value in 20-day-old seedlings, whereas the carbohydrate and lignin contents were slightly decreased in nZVI-treated alfalfa. Additionally, evidence for translocation of nZVI into plant tissues was also found. Vibrating sample magnetometry on shoots revealed the translocation of nZVI from the root to shoot. In this study, using an edible crop as a representative model, the potential impact of reactive engineered nanomaterials that can be exposed to the ecosystem on plant is discussed.

Oligogalacturonide production upon Arabidopsis thaliana-Botrytis cinerea interaction

Despite an ever-increasing interest for the use of pectin-derived oligogalacturonides (OGs) as biological control agents in agriculture, very little information exists-mainly for technical reasons-on the nature and activity of the OGs that accumulate during pathogen infection. Here we developed a sensitive OG profiling method, which revealed unsuspected features of the OGs generated during infection of Arabidopsis thaliana with the fungus Botrytis cinerea Indeed, in contrast to previous reports, most OGs were acetyl- and methylesterified, and 80% of them were produced by fungal pectin lyases, not by polygalacturonases. Polygalacturonase products did not accumulate as larger size OGs but were converted into oxidized GalA dimers. Finally, the comparison of the OGs and transcriptomes of leaves infected with B. cinerea mutants with reduced pectinolytic activity but with decreased or increased virulence, respectively, identified candidate OG elicitors. In conclusion, OG analysis provides insights into the enzymatic arms race between plant and pathogen and facilitates the identification of defense elicitors.

Neofusicoccum parvum Colonization of the Grapevine Woody Stem Triggers Asynchronous Host Responses at the Site of Infection and in the Leaves

Grapevine trunk diseases cause important economic losses in vineyards worldwide. Neofusicoccum parvum, one of the most aggressive causal agents of the trunk disease Botryosphaeria dieback, colonizes cells and tissues of the grapevine wood, leading to the formation of an internal canker. Symptoms then extend to distal shoots, with wilting of leaves and bud mortality. Our aim was to characterize the transcriptional dynamics of grapevine genes in the woody stem and in the leaves during Neofusicoccum parvum colonization. Genome-wide transcriptional profiling at seven distinct time points (0, 3, and 24 hours; 2, 6, 8, and 12 weeks) showed that both stems and leaves undergo extensive transcriptomic reprogramming in response to infection of the stem. While most intense transcriptional responses were detected in the stems at 24 hours, strong responses were not detected in the leaves until the next sampling point at 2 weeks post-inoculation. Network co-expression analysis identified modules of co-expressed genes common to both organs and showed most of these genes were asynchronously modulated. The temporal shift between stem vs. leaf responses affected transcriptional modulation of genes involved in both signal perception and transduction, as well as downstream biological processes, including oxidative stress, cell wall rearrangement and cell death. Promoter analysis of the genes asynchronously modulated in stem and leaves during N. parvum colonization suggests that the temporal shift of transcriptional reprogramming between the two organs might be due to asynchronous co-regulation by common transcriptional regulators. Topology analysis of stem and leaf co-expression networks pointed to specific transcription factor-encoding genes, including WRKY and MYB, which may be associated with the observed transcriptional responses in the two organs.

Short oligogalacturonides induce pathogen resistance-associated gene expression in Arabidopsis thaliana

BACKGROUND

Oligogalacturonides (OGs) are important components of damage-associated molecular pattern (DAMP) signaling and influence growth regulation in plants. Recent studies have focused on the impact of long OGs (degree of polymerization (DP) from 10-15), demonstrating the induction of plant defense signaling resulting in enhanced defenses to necrotrophic pathogens. To clarify the role of trimers (trimeric OGs, DP3) in DAMP signaling and their impact on plant growth regulation, we performed a transcriptomic analysis through the RNA sequencing of Arabidopsis thaliana exposed to trimers.

RESULTS

The transcriptomic data from trimer-treated Arabidopsis seedlings indicate a clear activation of genes involved in defense signaling, phytohormone signaling and a down-regulation of genes involved in processes related to growth regulation and development. This is further accompanied with improved defenses against necrotrophic pathogens triggered by the trimer treatment, indicating that short OGs have a clear impact on plant responses, similar to those described for long OGs.

CONCLUSIONS

Our results demonstrate that trimers are indeed active elicitors of plant defenses. This is clearly indicated by the up-regulation of genes associated with plant defense signaling, accompanied with improved defenses against necrotrophic pathogens. Moreover, trimers simultaneously trigger a clear down-regulation of genes and gene sets associated with growth and development, leading to stunted seedling growth in Arabidopsis.

Novel insights of ethylene role in strawberry cell wall metabolism

Due to its organoleptic and nutraceutical qualities, strawberry fruit (Fragaria x ananassa, Duch) is a worldwide important commodity. The role of ethylene in the regulation of strawberry cell wall metabolism was studied in fruit from Toyonoka cultivar harvested at white stage, when most changes associated with fruit ripening have begun. Fruit were treated with ethephon, an ethylene-releasing reagent, or with 1-methylcyclopropene (1-MCP), a competitive inhibitor of ethylene action, maintaining a set of non-treated fruit as controls for each condition. Ethephon treated-fruit showed higher contents of hemicelluloses, cellulose and neutral sugars regarding controls, while 1-MCP-treated fruit showed a lower amount of those fractions. On the other hand, ethephon-treated fruit presented a lower quantity of galacturonic acid from ionically and covalently bound pectins regarding controls, while 1-MCP-treated fruit showed higher contents of those components. We also explored the ethylene effect over the mRNA accumulation of genes related to pectins and hemicelluloses metabolism, and a relationship between gene expression patterns and cell wall polysaccharides contents was shown. Moreover, we detected that strawberry necrotrophic pathogens growth more easily on plates containing cell walls from ethephon-treated fruit regarding controls, while a lower growth rate was observed when cell walls from 1-MCP treated fruit were used as the only carbon source, suggesting an effect of ethylene on cell wall structure. Around 60% of strawberry cell wall is made up of pectins, which in turns is 70% made by homogalacturonans. Our findings support the idea of a central role for pectins on strawberry fruit softening and a participation of ethylene in the regulation of this process.

Effects of Dietary Zinc Pectin Oligosaccharides Chelate Supplementation on Growth Performance, Nutrient Digestibility and Tissue Zinc Concentrations of Broilers

The experiment was conducted to investigate the effects of zinc pectin oligosaccharides (Zn-POS) chelate on growth performance, nutrient digestibility, and tissue zinc concentrations of Arbor Acre broilers aged from 1 to 42 days. A total of 576 1-day-old broilers were randomly assigned into 4 groups with 9 replicates per group and 16 chicks per replicate. Chicks were fed either a basal diet (control) or basal diet supplemented with Zn-POS at 300 (Zn-POS-300), 600 (Zn-POS-600), or 900 mg/kg (Zn-POS-900), respectively, for 42 days. A 3-day metabolism trial was conducted during the last week of the experiment feeding. The average daily gain and the average daily feed intake of Zn-POS-600 were significantly higher (P < 0.05) than those of either the control, Zn-POS-300, or Zn-POS-900. Zn-POS-600 had the highest apparent digestibility of dry matter, crude protein, and metabolic energy among all groups. The control group had the lowest apparent digestibility of dry matter (P < 0.05), whereas the apparent digestibility of dry matter in Zn-POS-600 was higher (P < 0.05) than that of Zn-POS-300. The apparent digestibility of crude protein in Zn-POS-600 or Zn-POS-900 was higher (P < 0.05) compared to Zn-POS-300 or the control. The apparent digestibility of metabolic energy in Zn-POS-600 or Zn-POS-900 was higher (P < 0.05) than that of Zn-POS-300. Zn-POS-600 had the highest liver zinc concentrations (P < 0.05), while Zn-POS-900 had the highest pancreatic zinc concentrations (P < 0.05). Our data suggest that the supplementation of 600 mg/kg Zn-POS is optimal in improving the average daily gain and the average daily feed intake, utilization of dietary dry matter and crude protein, and increasing tissue zinc concentrations in liver and pancreas of broilers.

Oligogalacturonic acids promote tomato fruit ripening through the regulation of 1-aminocyclopropane-1-carboxylic acid synthesis at the transcriptional and post-translational levels

BACKGROUND

Oligogalacturonic acids (OGs) are oligomers of alpha-1,4-linked galacturonosyl residues that are released from cell walls by the hydrolysis of polygalacturonic acids upon fruit ripening and under abiotic/biotic stress. OGs may induce ethylene production and fruit ripening, however, the mechanism(s) behind these processes is unknown.

RESULTS

Tomato cultivar 'Ailsa Craig' (AC) and mutant Neverripe, ripening inhibitor, non-ripening, and colorless non-ripening fruits were treated with OGs at different stages. Only AC fruits at mature green stage 1 showed an advanced ripening phenomenon, although transient ethylene production was detected in all of the tomato fruits. Ethylene synthesis genes LeACS2 and LeACO1 were rapidly up-regulated, and the phosphorylated LeACS2 protein was detected after OGs treatment. Protein kinase/phosphatase inhibitors significantly affected the ripening process induced by the OGs. As a potential receptor of OGs, LeWAKL2 was also up-regulated in their presence.

CONCLUSIONS

We demonstrated that OGs promoted tomato fruit ripening by inducing ethylene synthesis through the regulation of LeACS2 at transcriptional and post-translational levels.

Sensitive detection and measurement of oligogalacturonides in Arabidopsis

Oligogalacturonides (OGs) are pectin fragments derived from the partial hydrolysis of the plant cell wall pectin; they are elicitors of various defense responses. While their activity is well documented, the detection of OGs produced in planta is still a challenging task. A protocol has been developed for the extraction and analysis of OGs from small samples of Arabidopsis tissues by using fluorescent labeled OGs, which allowed to monitor the efficiency of extraction. An efficient recovery was obtained by using a combination of calcium chelating agents at acidic pH. Off-line coupling of high performance anion exchange chromatography with matrix assisted laser desorption ionization- time of flight-mass spectrometryor nanoESI-Orbitrap-MS/MS was used for the identification and characterization of oligosaccharides. The protocol was successfully applied to detect OGs by using low amounts (50 mg) of Arabidopsis leaves and very low amounts (30 mg) of senescent leaves. The protocol was also successfully used to detect OGs in Arabidopsis cell wall material digested with pectinases. The proposed extraction protocol followed by sensitive and high-resolution analysis methods allowed detection of OGs released from the cell wall in Arabidopsis tissues by using minimal sample material. The protocol may be useful to study OG-triggered plant immunity and cell wall remodeling during Arabidopsis growth and development.

Exposure of iron nanoparticles to Arabidopsis thaliana enhances root elongation by triggering cell wall loosening

In this study, we investigated the effect of nZVI on plant root elongation in Arabidopsis thaliana and showed, for the first time, that nZVI enhanced root elongation by inducing OH radical-induced cell wall loosening. Exposure of plants to 0.5 g/L nZVI enhanced root elongation by 150-200% over that in the control, and further mechanistic studies showed that this occurred via nZVI-mediated OH radical-induced cell wall loosening. The oxidation capacity of nZVI, leading to release of H2O2, allowed it to cause OH radical-induced cell wall loosening in roots. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometers (MALDI-TOFMS)-based analysis clearly revealed that pectin-polysaccharides in roots were degraded; they are one of the main matrix-polysaccharide-connecting and load-bearing polymers in cell walls. Rapid root elongation led to structural changes in root cell walls: reduction of cell wall thickness and a bias on the orientation of cellulose microfibrils. Additionally, the asymmetrical distribution of tensional strength resulted from the OH radical-induced cell wall loosening enhanced endocytosis. These findings emphasize that OH radical-induced cell wall loosening is important for mechanical regulation of the cell wall and provide new insights into the cellular responses of plants exposed to reactive metal nanoparticles.

Genome-wide transcriptional profiling of Botrytis cinerea genes targeting plant cell walls during infections of different hosts

Cell walls are barriers that impair colonization of host tissues, but also are important reservoirs of energy-rich sugars. Growing hyphae of necrotrophic fungal pathogens, such as Botrytis cinerea (Botrytis, henceforth), secrete enzymes that disassemble cell wall polysaccharides. In this work we describe the annotation of 275 putative secreted Carbohydrate-Active enZymes (CAZymes) identified in the Botrytis B05.10 genome. Using RNAseq we determined which Botrytis CAZymes were expressed during infections of lettuce leaves, ripe tomato fruit, and grape berries. On the three hosts, Botrytis expressed a common group of 229 potentially secreted CAZymes, including 28 pectin backbone-modifying enzymes, 21 hemicellulose-modifying proteins, 18 enzymes that might target pectin and hemicellulose side-branches, and 16 enzymes predicted to degrade cellulose. The diversity of the Botrytis CAZymes may be partly responsible for its wide host range. Thirty-six candidate CAZymes with secretion signals were found exclusively when Botrytis interacted with ripe tomato fruit and grape berries. Pectin polysaccharides are notably abundant in grape and tomato cell walls, but lettuce leaf walls have less pectin and are richer in hemicelluloses and cellulose. The results of this study not only suggest that Botrytis targets similar wall polysaccharide networks on fruit and leaves, but also that it may selectively attack host wall polysaccharide substrates depending on the host tissue.

Quiescent and necrotrophic lifestyle choice during postharvest disease development

Insidious fungal infections by postharvest pathogens remain quiescent during fruit growth until, at a particular phase during fruit ripening and senescence, the pathogens switch to the necrotrophic lifestyle and cause decay. During ripening, fruits undergo physiological processes, such as activation of ethylene biosynthesis, cuticular changes, and cell-wall loosening-changes that are accompanied by a decline of antifungal compounds, both those that are preformed and those that are inducible secondary metabolites. Pathogen infection of the unripe host fruit initiates defensive signal-transduction cascades, culminating in accumulation of antifungal proteins that limit fungal growth and development. In contrast, development of the same pathogens during fruit ripening and storage activates a substantially different signaling network, one that facilitates aggressive fungal colonization. This review focuses on responses induced by the quiescent pathogens of postharvest diseases in unripe host fruits. New genome-scale experimental approaches have begun to delineate the complex and multiple networks of host and pathogen responses activated to maintain or to facilitate the transition from the quiescent to the necrotrophic lifestyle.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Elaboration of extensin-pectin thin film model of primary plant cell wall

With the aim of mimicking the plant cell wall, a layer by layer approach was used to build a thin film consisting of successive adsorption of pectin and extensin. Elaboration of the thin film was monitored by surface plasmon resonance, quartz crystal microbalance, and ellipsometry. All data indicate that formation of the film was successful and that growth occurred according to a nonuniform growth. It is likely that diffusion of the polymers occurred within the multilayer structure and that the final structure is not constituted by layered individual pectin and extensin films. Polymer rearrangements were also supported by the atomic force microscopy images that show a smoother surface after extensin adsorption than after pectin deposition.

Ripening-regulated susceptibility of tomato fruit to Botrytis cinerea requires NOR but not RIN or ethylene

Fruit ripening is a developmental process that is associated with increased susceptibility to the necrotrophic pathogen Botrytis cinerea. Histochemical observations demonstrate that unripe tomato (Solanum lycopersicum) fruit activate pathogen defense responses, but these responses are attenuated in ripe fruit infected by B. cinerea. Tomato fruit ripening is regulated independently and cooperatively by ethylene and transcription factors, including NON-RIPENING (NOR) and RIPENING-INHIBITOR (RIN). Mutations in NOR or RIN or interference with ethylene perception prevent fruit from ripening and, thereby, would be expected to influence susceptibility. We show, however, that the susceptibility of ripe fruit is dependent on NOR but not on RIN and only partially on ethylene perception, leading to the conclusion that not all of the pathways and events that constitute ripening render fruit susceptible. Additionally, on unripe fruit, B. cinerea induces the expression of genes also expressed as uninfected fruit ripen. Among the ripening-associated genes induced by B. cinerea are LePG (for polygalacturonase) and LeExp1 (for expansin), which encode cell wall-modifying proteins and have been shown to facilitate susceptibility. LePG and LeExp1 are induced only in susceptible rin fruit and not in resistant nor fruit. Thus, to infect fruit, B. cinerea relies on some of the processes and events that occur during ripening, and the fungus induces these pathways in unripe fruit, suggesting that the pathogen itself can initiate the induction of susceptibility by exploiting endogenous developmental programs. These results demonstrate the developmental plasticity of plant responses to the fungus and indicate how known regulators of fruit ripening participate in regulating ripening-associated pathogen susceptibility.

Strangers in the matrix: plant cell walls and pathogen susceptibility

Early in infection, pathogens encounter the outer wall of plant cells. Because pathogen hydrolases targeting the plant cell wall are well-known components of virulence, it has been assumed that wall disassembly by the plant itself also contributes to susceptibility, and now this has been established experimentally. Understanding how plant morphological and developmental remodeling and pathogen cell wall targeted virulence influence infections provides new perspectives about plant-pathogen interactions. The plant cell wall can be an effective physical barrier to pathogens, but also it is a matrix where many proteins involved in pathogen perception are delivered. By breaching the wall, a pathogen potentially reveals itself to the plant and activates responses, setting off events that might halt or limit its advance.

Partial demethylation of oligogalacturonides by pectin methyl esterase 1 is required for eliciting defence responses in wild strawberry (Fragaria vesca)

In addition to the role of the cell wall as a physical barrier against pathogens, some of its constituents, such as pectin-derived oligogalacturonides (OGA), are essential components for elicitation of defence responses. To investigate how modifications of pectin alter defence responses, we expressed the fruit-specific Fragaria x ananassa pectin methyl esterase FaPE1 in the wild strawberry Fragaria vesca. Pectin from transgenic ripe fruits differed from the wild-type with regard to the degree and pattern of methyl esterification, as well as the average size of pectin polymers. Purified oligogalacturonides from the transgenic fruits showed a reduced degree of esterification compared to oligogalacturonides from wild-type fruits. This reduced esterification is necessary to elicit defence responses in strawberry. The transgenic F. vesca lines had constitutively activated pathogen defence responses, resulting in higher resistance to the necrotropic fungus Botrytis cinerea. Further studies in F. vesca and Nicotiana benthamiana leaves showed that the elicitation capacity of the oligogalacturonides is more specific than previously envisaged.

Resistance to Botrytis cinerea induced in Arabidopsis by elicitors is independent of salicylic acid, ethylene, or jasmonate signaling but requires PHYTOALEXIN DEFICIENT3

Oligogalacturonides (OGs) released from plant cell walls by pathogen polygalacturonases induce a variety of host defense responses. Here we show that in Arabidopsis (Arabidopsis thaliana), OGs increase resistance to the necrotrophic fungal pathogen Botrytis cinerea independently of jasmonate (JA)-, salicylic acid (SA)-, and ethylene (ET)-mediated signaling. Microarray analysis showed that about 50% of the genes regulated by OGs, including genes encoding enzymes involved in secondary metabolism, show a similar change of expression during B. cinerea infection. In particular, expression of PHYTOALEXIN DEFICIENT3 (PAD3) is strongly up-regulated by both OGs and infection independently of SA, JA, and ET. OG treatments do not enhance resistance to B. cinerea in the pad3 mutant or in underinducer after pathogen and stress1, a mutant with severely impaired PAD3 expression in response to OGs. Similarly to OGs, the bacterial flagellin peptide elicitor flg22 also enhanced resistance to B. cinerea in a PAD3-dependent manner, independently of SA, JA, and ET. This work suggests, therefore, that elicitors released from the cell wall during pathogen infection contribute to basal resistance against fungal pathogens through a signaling pathway also activated by pathogen-associated molecular pattern molecules.

Detection and identification of rhamnogalacturonan lyase activity in intercellular spaces of expanding cotton cotyledons

Rhamnogalacturonan lyase (RG lyase) activity has been detected and its relative activity measured in vivo during the expansion of cotton (Gossypium hirsutum L.) cotyledons. Rhamnogalacturonan (RG) oligomers labeled with a fluorescent tag were injected into the intercellular spaces of cotton cotyledons and, after incubation, the digested substrate was rinsed out. Enzyme digestion products were detected and identified by capillary zone electrophoresis. Rhamnogalacturonan lyase products were identified as such by co-migration with the digestion products of linear RG oligomers when the oligomers were treated with fungal RG lyase but not when treated with fungal RG hydrolase. In addition, reaction of plant RG lyase digestion products of RG oligomers with I(2)/KI, which selectively removes unsaturated galactopyranosyluronic acid (GaLap) residues formed at the non-reducing end of the oligomer, converted the plant digestion products into RG oligomers that co-migrated with fungal RG hydrolase products. The activity of the enzyme in the intercellular spaces of cotton cotyledons is very low and could be detected most easily when not >0.03 nmol of substrate was injected in a approximately 0.7-cm(2) area and incubated in vivo for 2-6 h. Rhamnogalacturonan lyase activity was the highest in rapidly expanding 3- to 4-day-old cotyledons and gradually decreased during the slow-down in expansion over the next 2-3 days. The RG lyase activity was also detected when the APTS (8-aminopyrene-1,3,6-trisulfonic acid, trisodium salt)-labeled substrates were introduced into intercellular spaces by infiltration instead of injection, indicating that the activity was not induced by wounding or released into the apoplast by cell damage. An exo-RG galacturonohydrolase activity was also found, but RG hydrolase and exo-RG rhamnohydrolase were not detected.