Accumulation of hydroxyproline-rich glycoprotein mRNAs in response to fungal elicitor and infection

Exploring the extensin gene family: an updated genome-wide survey in plants and algae

Extensins (EXTs), a class of hydroxyproline-rich glycoprotein with multiple Ser-Pro3-5 motifs, are known to play roles in cell wall reinforcement and environmental responses. EXTs with repetitive Tyr-X-Tyr (YXY) motifs for crosslinking are referred as crosslinking EXTs. Our comprehensive study spanned 194 algal and plant species, categorizing EXTs into seven subfamilies: classical extensins (EXT I and II), arabinogalactan-protein extensins (AGP-EXTs), proline-rich extensin-like receptor kinases (PERKs), leucine-rich repeat extensins (LRX I and II), formin homology (FH) domain-containing extensins (FH-EXTs), proline-rich, arabinogalactan proteins, conserved cysteines (PAC) domain-containing extensins (PAC I and II), and eight-cysteine motif (8CM)-containing extensins (8CM-EXTs). In the examined dataset, EXTs were detected ubiquitously in plants but infrequently in algae, except for one Coccomyxa and four Chlamydomonadales species. No crosslinking EXTs were found in Poales or certain Zingiberales species. Notably, the previously uncharacterized EXT II, PAC II, and liverwort-specific 8CM-EXTs were found to be crosslinking EXTs. EXT II, featuring repetitive YY motifs instead of the conventional YXY motif, was exclusively identified in Solanaceae. Furthermore, tandem genes encoding distinctive 8CM-EXTs specifically expressed in the germinating spores of Marchantia polymorpha. This updated classification of EXT types allows us to propose a plausible evolutionary history of EXT genes during the course of plant evolution.

Portuguese Common Bean Natural Variation Helps to Clarify the Genetic Architecture of the Legume's Nutritional Composition and Protein Quality

Common bean is a nutritious food legume widely appreciated by consumers worldwide. It is a staple food in Latin America, and a component of the Mediterranean diet, being an affordable source of protein with high potential as a gourmet food. Breeding for nutritional quality, including both macro and micronutrients, and meeting organoleptic consumers' preferences is a difficult task which is facilitated by uncovering the genetic basis of related traits. This study explored the diversity of 106 Portuguese common bean accessions, under two contrasting environments, to gain insight into the genetic basis of nutritional composition (ash, carbohydrates, fat, fiber, moisture, protein, and resistant starch contents) and protein quality (amino acid contents and trypsin inhibitor activity) traits through a genome-wide association study. Single-nucleotide polymorphism-trait associations were tested using linear mixed models accounting for the accessions' genetic relatedness. Mapping resolution to the gene level was achieved in 56% of the cases, with 102 candidate genes proposed for 136 genomic regions associated with trait variation. Only one marker-trait association was stable across environments, highlighting the associations' environment-specific nature and the importance of genotype × environment interaction for crops' local adaptation and quality. This study provides novel information to better understand the molecular mechanisms regulating the nutritional quality in common bean and promising molecular tools to aid future breeding efforts to answer consumers' concerns.

Extensins: Self-Assembly, Crosslinking, and the Role of Peroxidases

The extensin (EXT) network is elaborated by the covalent intermolecular crosslinking of EXT glycoprotein monomers, and its proper assembly is important for numerous aspects of basic wall architecture and cellular defense. In this review, we discuss new advances in the secretion of EXT monomers and the molecular drivers of EXT network self-assembly. Many of the functions of EXTs are conferred through covalent crosslinking into the wall, so we also discuss the different types of known intermolecular crosslinks, the enzymes that are involved, as well as the potential for additional crosslinks that are yet to be identified. EXTs also function in wall architecture independent of crosslinking status, and therefore, we explore the role of non-crosslinking EXTs. As EXT crosslinking is upregulated in response to wounding and pathogen infection, we discuss a potential regulatory mechanism to control covalent crosslinking and its relationship to the subcellular localization of the crosslinking enzymes.

What we know about plant arginases?

Nitrogen is one of the essential element required for plant growth and development. In plants, most of the nitrogen is stored in arginine. Hence, metabolism of arginine to urea by arginase and its further hydrolysis to ammonia by urease is involved in nitrogen recycling to meet the metabolic demands of growing plants. In this respect, plant arginases differ from that of animals. Animals excrete urea while plants recycle the urea. However, the studies on the biochemical and biophysical characteristics of plant arginase are limited when compared to animal arginase(s). In this review, the structural and biochemical characteristics of various plant arginases are discussed. Moreover, the significance of arginase in nitrogen recycling is explained and recent literature on function and activation of plant arginases in response to various environmental (biotic and abiotic) insults is also presented.

Genome-wide identification and expression analysis of extensin genes in tomato

Extensins (EXTs) are major protein components in plant cell walls that play crucial roles in higher plants. The function of EXTs has been reported in several plants but is limited in tomato, especially in fruit ripening. In this study, we identified 83 EXTs in tomato, and divided them into seven groups. The gene intron-exon structure and protein-motif composition of SlEXTs were similar within each group but different among groups. SlEXT genes showed different expression patterns in roots, leaves, flowers and fruits, and some SlEXT gene expressions in flowers could be regulated by treatments of auxin, gibberellic acid and jasmonic acid. In particular, SlSEXT8 had higher and increased expression during tomato fruit ripening, and its expression could be induced by ethylene, suggesting SlSEXT8 may be involved in tomato fruit softening. The result provides insights into the function of EXTs, and will facilitate to further study EXT roles in tomato fruit ripening.

The Expression of Potato Expansin A3 (StEXPA3) and Extensin4 (StEXT4) Genes with Distribution of StEXPAs and HRGPs-Extensin Changes as an Effect of Cell Wall Rebuilding in Two Types of PVYNTN-Solanum tuberosum Interactions

The plant cell wall acts not only as a physical barrier, but also as a complex and dynamic structure that actively changes under different biotic and abiotic stress conditions. The question is, how are the different cell wall compounds modified during different interactions with exogenous stimuli such as pathogens? Plants exposed to viral pathogens respond to unfavorable conditions on multiple levels. One challenge that plants face under viral stress is the number of processes required for differential cell wall remodeling. The key players in these conditions are the cell wall genes and proteins, which can be regulated in specific ways during the interactions and have direct influences on the rebuilding of the cell wall structure. The cell wall modifications occurring in plants during viral infection remain poorly described. Therefore, this study focuses on cell wall dynamics as an effect of incompatible interactions between the potato virus Y (PVY^NTN) and resistant potatoes (hypersensitive plant), as well as compatible (susceptible plant) interactions. Our analysis describes, for the first time, the expression of the potato expansin A3 (StEXPA3) and potato extensin 4 (StEXT4) genes in PVY^NTN-susceptible and -resistant potato plant interactions. The results indicated a statistically significant induction of the StEXPA3 gene during a susceptible response. By contrast, we demonstrated the predominantly gradual activation of the StEXT4 gene during the hypersensitive response to PVY^NTN inoculation. Moreover, the in situ distributions of expansins (StEXPAs), which are essential cell wall-associated proteins, and the hydroxyproline-rich glycoprotein (HRGP) extensin were investigated in two types of interactions. Furthermore, cell wall loosening was accompanied by an increase in StEXPA deposition in a PVY^NTN-susceptible potato, whereas the HRGP content dynamically increased during the hypersensitive response, when the cell wall was reinforced. Ultrastructural localization and quantification revealed that the HRGP extensin was preferably located in the apoplast, but deposition in the symplast was also observed in resistant plants. Interestingly, during the hypersensitive response, StEXPA proteins were mainly located in the symplast area, in contrast to the susceptible potato where StEXPA proteins were mainly observed in the cell wall. These findings revealed that changes in the intracellular distribution and abundance of StEXPAs and HRGPs can be differentially regulated, depending on different types of PVY^NTN–potato plant interactions, and confirmed the involvement of apoplast and symplast activation as a defense response mechanism.

Unmethyl-esterified homogalacturonan and extensins seal Arabidopsis graft union

BACKGROUND

Grafting is a technique widely used in horticulture. The processes involved in grafting are diverse, and the technique is commonly employed in studies focusing on the mechanisms that regulate cell differentiation or response of plants to abiotic stress. Information on the changes in the composition of the cell wall that occur during the grafting process is scarce. Therefore, this study was carried out for analyzing the composition of the cell wall using Arabidopsis hypocotyls as an example. During the study, the formation of a layer that covers the surface of the graft union was observed. So, this study also aimed to describe the histological and cellular changes that accompany autografting of Arabidopsis hypocotyls and to perform preliminary chemical and structural analyses of extracellular material that seals the graft union.

RESULTS

During grafting, polyphenolic and lipid compounds were detected, along with extracellular deposition of carbohydrate/protein material. The spatiotemporal changes observed in the structure of the extracellular material included the formation of a fibrillar network, polymerization of the fibrillar network into a membranous layer, and the presence of bead-like structures on the surface of cells in established graft union. These bead-like structures appeared either "closed" or "open". Only three cell wall epitopes, namely: LM19 (un/low-methyl-esterified homogalacturonan), JIM11, and JIM20 (extensins), were detected abundantly on the cut surfaces that made the adhesion plane, as well as in the structure that covered the graft union and in the bead-like structures, during the subsequent stages of regeneration.

CONCLUSIONS

To the best of our knowledge, this is the first report on the composition and structure of the extracellular material that gets deposited on the surface of graft union during Arabidopsis grafting. The results showed that unmethyl-esterified homogalacturonan and extensins are together involved in the adhesion of scion and stock, as well as taking part in sealing the graft union. The extracellular material is of importance not only due to the potential pectin-extensin interaction but also due to its origin. The findings presented here implicate a need for studies with biochemical approach for a detailed analysis of the composition and structure of the extracellular material.

Cell wall extensins in root-microbe interactions and root secretions

Extensins are cell wall glycoproteins, belonging to the hydroxyproline-rich glycoprotein (HRGP) family, which are involved in many biological functions, including plant growth and defence. Several reviews have described the involvement of HRGPs in plant immunity but little focus has been given specifically to cell wall extensins. Yet, a large set of recently published data indicates that extensins play an important role in plant protection, especially in root-microbe interactions. Here, we summarise the current knowledge on this topic and discuss the importance of extensins in root defence. We first provide an overview of the distribution of extensin epitopes recognised by different monoclonal antibodies among plants and discuss the relevance of some of these epitopes as markers of the root defence response. We also highlight the implication of extensins in different types of plant interactions elicited by either pathogenic or beneficial micro-organisms. We then present and discuss the specific importance of extensins in root secretions, as these glycoproteins are not only found in the cell walls but are also released into the root mucilage. Finally, we propose a model to illustrate the impact of cell wall extensin on root secretions.

Functional characterization of unique enzymes in Xanthomonas euvesicatoria related to degradation of arabinofurano-oligosaccharides on hydroxyproline-rich glycoproteins

In this study, we clarified the functions of three uncharacterized enzymes, XCV2724, XCV2728, and XCV2729, in Xanthomonas euvesicatoria, the causal agent of bacterial spot of tomato and pepper. The genes corresponding to the three enzymes are homologs of hypBA1, hypBA2, and hypAA from Bifidobacterium longum and are unique to Xanthomonas spp. among plant pathogenic bacteria. Functional characterization of the recombinant enzymes expressed using microbial systems revealed that they degrade the arabinofurano-oligosaccharides present on hydroxyproline (Hyp)-rich glycoproteins (HRGPs) such as extensin and solanaceous lectins in plant cell walls. These enzymes work coordinately to degrade the oligosaccharides. First, XeHypAA (XCV2728), belonging to the glycoside hydrolase (GH) 43 family, releases L-arabinose from L-arabinofuranose (Araf)-α1,3-Araf-ß1,2-Araf-ß1,2-Araf-ß-Hyp (Ara4-Hyp), cleaving its α1,3 bond; second, XeHypBA2 (XCV2729), belonging to the GH121 family, releases the disaccharide Araf-ß1,2-Araf from Araf-ß1,2-Araf-ß1,2-Araf-ß-Hyp (Ara3-Hyp); finally, XeHypBA1 (XCV2724), belonging to GH family 127, releases L-arabinose from Araf-ß-Hyp (Ara-Hyp). In summary, the main oligosaccharide structure of Ara4-Hyp on the HRGPs is degraded to Ara3-Hyp, then to Ara-Hyp, and finally to Ara monosaccharides by the action of these three enzymes. HRGPs containing oligosaccharide substrates have been reported to contribute to plant defense, and interestingly, the promoter region of the operon (xehypBA2 and xehypAA) contains the plant-inducible promoter box for binding the regulator protein HrpX involved in pathogenicity. We then analyzed the expression level of the operon gene in hrp-inducing medium and in plants and constructed gene-deletion mutants. However, although the operon was evidently upregulated by HrpX, three single-gene deletion mutants (ΔxehypBA1, ΔxehypBA2, ΔxehypAA) and even a triple-gene deletion mutant (ΔxehypBA1-BA2-AA) remained pathogenic, and had no effect on nonhost resistance, either, indicating that these three enzymes are not involved in either pathogenicity or nonhost resistance reactions. This is the first report of enzymes in plant pathogenic bacteria that catalyze the degradation of Hyp-linked-L-arabinofuranosides in plant cell walls.

Genome-wide association mapping for resistance to leaf rust, stripe rust and tan spot in wheat reveals potential candidate genes

KEY MESSAGE

Genome-wide association mapping in conjunction with population sequencing map and Ensembl plants was used to identify markers/candidate genes linked to leaf rust, stripe rust and tan spot resistance in wheat. Leaf rust (LR), stripe rust (YR) and tan spot (TS) are some of the important foliar diseases in wheat (Triticum aestivum L.). To identify candidate resistance genes for these diseases in CIMMYT's (International Maize and Wheat Improvement Center) International bread wheat screening nurseries, we used genome-wide association studies (GWAS) in conjunction with information from the population sequencing map and Ensembl plants. Wheat entries were genotyped using genotyping-by-sequencing and phenotyped in replicated trials. Using a mixed linear model, we observed that seedling resistance to LR was associated with 12 markers on chromosomes 1DS, 2AS, 2BL, 3B, 4AL, 6AS and 6AL, and seedling resistance to TS was associated with 14 markers on chromosomes 1AS, 2AL, 2BL, 3AS, 3AL, 3B, 6AS and 6AL. Seedling and adult plant resistance (APR) to YR were associated with several markers at the distal end of chromosome 2AS. In addition, YR APR was also associated with markers on chromosomes 2DL, 3B and 7DS. The potential candidate genes for these diseases included several resistance genes, receptor-like serine/threonine-protein kinases and defense-related enzymes. However, extensive LD in wheat that decays at about 5 × 107 bps, poses a huge challenge for delineating candidate gene intervals and candidates should be further mapped, functionally characterized and validated. We also explored a segment on chromosome 2AS associated with multiple disease resistance and identified seventeen disease resistance linked genes. We conclude that identifying candidate genes linked to significant markers in GWAS is feasible in wheat, thus creating opportunities for accelerating molecular breeding.

Biochemical and functional characterization of an atypical plant l-arginase from Cilantro (Coriandrum sativam L.)

Arginase is one of the key enzymes responsible for maintaining the essential levels of nitrogen among plants, but biochemical and functional characterization of arginase among plants is limited. While screening for stable plant arginase, we found cilantro possessing an abundant and stable arginase. We purified arginase to apparent homogeneity (3300-fold purification) with a specific activity of 81,728 nmoles of urea formed/mg of protein/min and its eight-tryptic fragments had amino acid sequences identical to Arabidopsis thaliana arginase. Cilantro arginase exhibited absolute requirement for Mn²⁺ (0.5 mM-1 mM). Unlike other known plant arginases, cilantro arginase did not hydrolyse d-arginine and other arginine analogues. While for sulfhydryl reagents the enzyme was sensitive, l-NOHA, an arginase inhibitor showed only moderate inhibition - a property distinct from tomato arginase. We also found arginine derived amino acids and polyamines can regulate cilantro arginase in vitro. In addition, we also noticed an increase in cilantro arginase activity to both biotic and abiotic stress. We conclude that, cilantro may be used as a model plant to study plant arginases and to delineate arginase role, beyond its classical role in nitrogen recycling and polyamine biosynthesis.

A comparative analysis of nonhost resistance across the two Triticeae crop species wheat and barley

BACKGROUND

Nonhost resistance (NHR) protects plants against a vast number of non-adapted pathogens which implicates a potential exploitation as source for novel disease resistance strategies. Aiming at a fundamental understanding of NHR a global analysis of transcriptome reprogramming in the economically important Triticeae cereals wheat and barley, comparing host and nonhost interactions in three major fungal pathosystems responsible for powdery mildew (Blumeria graminis ff. ssp.), cereal blast (Magnaporthe sp.) and leaf rust (Puccinia sp.) diseases, was performed.

RESULTS

In each pathosystem a significant transcriptome reprogramming by adapted- or non-adapted pathogen isolates was observed, with considerable overlap between Blumeria, Magnaporthe and Puccinia. Small subsets of these general pathogen-regulated genes were identified as differentially regulated between host and corresponding nonhost interactions, indicating a fine-tuning of the general pathogen response during the course of co-evolution. Additionally, the host- or nonhost-related responses were rather specific for each pair of adapted and non-adapted isolates, indicating that the nonhost resistance-related responses were to a great extent pathosystem-specific. This pathosystem-specific reprogramming may reflect different resistance mechanisms operating against non-adapted pathogens with different lifestyles, or equally, different co-option of the hosts by the adapted isolates to create an optimal environment for infection. To compare the transcriptional reprogramming between wheat and barley, putative orthologues were identified. Within the wheat and barley general pathogen-regulated genes, temporal expression profiles of orthologues looked similar, indicating conserved general responses in Triticeae against fungal attack. However, the comparison of orthologues differentially expressed between host and nonhost interactions revealed fewer commonalities between wheat and barley, but rather suggested different host or nonhost responses in the two cereal species.

CONCLUSIONS

Taken together, our results suggest independent co-evolutionary forces acting on host pathosystems mirrored by barley- or wheat-specific nonhost responses. As a result of evolutionary processes, at least for the pathosystems investigated, NHR appears to rely on rather specific plant responses.

Small RNA transcriptomes of mangroves evolve adaptively in extreme environments

MicroRNAs (miRNAs) and endogenous small interfering RNAs (siRNAs) are key players in plant stress responses. Here, we present the sRNA transcriptomes of mangroves Bruguiera gymnorrhiza and Kandelia candel. Comparative computational analyses and target predictions revealed that mangroves exhibit distinct sRNA regulatory networks that differ from those of glycophytes. A total of 32 known and three novel miRNA families were identified. Conserved and mangrove-specific miRNA targets were predicted; the latter were widely involved in stress responses. The known miRNAs showed differential expression between the mangroves and glycophytes, reminiscent of the adaptive stress-responsive changes in Arabidopsis. B. gymnorrhiza possessed highly abundant but less conserved TAS3 trans-acting siRNAs (tasiRNAs) in addition to tasiR-ARFs, with expanded potential targets. Our results indicate that the evolutionary alteration of sRNA expression levels and the rewiring of sRNA-regulatory networks are important mechanisms underlying stress adaptation. We also identified sRNAs that are involved in salt and/or drought tolerance and nutrient homeostasis as possible contributors to mangrove success in stressful environments.

Arabinosylation Plays a Crucial Role in Extensin Cross-linking In Vitro

Extensins (EXTs) are hydroxyproline-rich glycoproteins (HRGPs) that are structural components of the plant primary cell wall. They are basic proteins and are highly glycosylated with carbohydrate accounting for >50% of their dry weight. Carbohydrate occurs as monogalactosyl serine and arabinosyl hydroxyproline, with arabinosides ranging in size from ~1 to 4 or 5 residues. Proposed functions of EXT arabinosylation include stabilizing the polyproline II helix structure and facilitating EXT cross-linking. Here, the involvement of arabinosylation in EXT cross-linking was investigated by assaying the initial cross-linking rate and degree of cross-linking of partially or fully de-arabinosylated EXTs using an in vitro cross-linking assay followed by gel permeation chromatography. Our results indicate that EXT arabinosylation is required for EXT cross-linking in vitro and the fourth arabinosyl residue in the tetraarabinoside chain, which is uniquely α-linked, may determine the initial cross-linking rate. Our results also confirm the conserved structure of the oligoarabinosides across species, indicating an evolutionary significance for EXT arabinosylation.

Identification of the Abundant Hydroxyproline-Rich Glycoproteins in the Root Walls of Wild-Type Arabidopsis, an ext3 Mutant Line, and Its Phenotypic Revertant

Extensins are members of the cell wall hydroxyproline-rich glycoprotein (HRGP) superfamily that form covalently cross-linked networks in primary cell walls. A knockout mutation in EXT3 (AT1G21310), the gene coding EXTENSIN 3 (EXT3) in Arabidopsis Landsberg erecta resulted in a lethal phenotype, although about 20% of the knockout plants have an apparently normal phenotype (ANP). In this study the root cell wall HRGP components of wild-type, ANP and the ext3 mutant seedlings were characterized by peptide fractionation of trypsin digested anhydrous hydrogen fluoride deglycosylated wall residues and by sequencing using LC-MS/MS. Several HRGPs, including EXT3, were identified in the wild-type root walls but not in walls of the ANP and lethal mutant. Indeed the ANP walls and walls of mutants displaying the lethal phenotype possessed HRGPs, but the profiles suggest that changes in the amount and perhaps type may account for the corresponding phenotypes.

NtProRP1, a novel proline-rich protein, is an osmotic stress-responsive factor and specifically functions in pollen tube growth and early embryogenesis in Nicotiana tabacum

Proline-rich proteins (PRPs) are known to play important roles in sexual plant reproduction. Most of the known proteins in the family were found in styles or pollen and modulate pollen tube growth. Here, we identified a novel member of the gene family, NtProRP1, which is preferentially expressed in tobacco pollen grains, pollen tubes and zygotes. NtProRP1 could be secreted into the extracellular space including the cell wall, and the predicted N-terminal signal peptide is crucial for its secretion. In NtProRP1-RNAi plants, pollen germination and pollen tube growth were significantly slower and showed zigzag or swell morphology in vitro. Early embryogenesis also exhibited aberrant development, indicative of its critical role in both pollen tube growth and early embryogenesis. Further investigation revealed that NtProRP1 plays a crucial role in osmotic stress response during pollen tube growth and is likely regulated by Tsi, a stress-responsive gene, suggesting that the regulatory mechanism is also involved in the stress response during sexual plant reproduction. These data provide evidence that NtProRP1 functions as a downstream factor of Tsi1 in the stress response and converges the stress signal into the modulation of pollen tube growth and early embryogenesis.

Wood chemistry analysis and expression profiling of a poplar clone expressing a tyrosine-rich peptide

Our study has identified pathways and gene candidates that may be associated with the greater flexibility and digestibility of the poplar cell walls. With the goal of facilitating lignin removal during the utilization of woody biomass as a biofuel feedstock, we previously transformed a hybrid poplar clone with a partial cDNA sequence encoding a tyrosine- and hydroxyproline-rich glycoprotein from parsley. A number of the transgenic lines released more polysaccharides following protease digestion and were more flexible than wild-type plants, but otherwise normal in phenotype. Here, we report that overexpression of the tyrosine-rich peptide encoding sequence in these transgenic poplar plants did not significantly alter total lignin quantity or quality (S/G lignin ratio), five- and six-carbon sugar contents, growth rate, or susceptibility to a major poplar fungal pathogen, Septoria musiva. Whole-genome microarray analysis revealed a total of 411 differentially expressed transcripts in transgenic lines, all with decreased transcript abundance relative to wild-type plants. Their corresponding genes were overrepresented in functional categories such as secondary metabolism, amino acid metabolism, and energy metabolism. Transcript abundance was decreased primarily for five types of genes encoding proteins involved in cell-wall organization and in lignin biosynthesis. The expression of a subset of 19 of the differentially regulated genes by qRT-PCR validated the microarray results. Our study has identified pathways and gene candidates that may be the underlying cause for the enhanced flexibility and digestibility of the stems of poplar plants expressing the TYR transgene.

Transcriptome comparison of susceptible and resistant wheat in response to powdery mildew infection

Powdery mildew (Pm) caused by the infection of Blumeria graminis f. sp. tritici (Bgt) is a worldwide crop disease resulting in significant loss of wheat yield. To profile the genes and pathways responding to the Bgt infection, here, using Affymetrix wheat microarrays, we compared the leaf transcriptomes before and after Bgt inoculation in two wheat genotypes, a Pm-susceptible cultivar Jingdong 8 (S) and its near-isogenic line (R) carrying a single Pm resistant gene Pm30. Our analysis showed that the original gene expression status in the S and R genotypes of wheat was almost identical before Bgt inoculation, since only 60 genes exhibited differential expression by P = 0.01 cutoff. However, 12 h after Bgt inoculation, 3014 and 2800 genes in the S and R genotype, respectively, responded to infection. A wide range of pathways were involved, including cell wall fortification, flavonoid biosynthesis and metabolic processes. Furthermore, for the first time, we show that sense-antisense pair genes might be participants in wheat-powdery mildew interaction. In addition, the results of qRT-PCR analysis on several candidate genes were consistent with the microarray data in their expression patterns. In summary, this study reveals leaf transcriptome changes before and after powdery mildew infection in wheat near-isogenic lines, suggesting that powdery mildew resistance is a highly complex systematic response involving a large amount of gene regulation.

Overexpression of arginase in Arabidopsis thaliana influences defence responses against Botrytis cinerea

Arabidopsis possesses two arginase-encoding genes, ARGAH1 and ARGAH2, catalysing the catabolism of arginine into ornithine and urea. Arginine and ornithine are both precursors for polyamine biosynthetic pathways. We observed an accumulation of ARGAH2 mRNA in Arabidopsis upon inoculation with the necrotrophic pathogen Botrytis cinerea. Transgenic lines displaying either overexpression of ARGAH2 or simultaneous silencing of both Arabidopsis arginase-encoding genes were created and their resistance to B. cinerea infection evaluated. Overexpression of arginase resulted in changes in amino acid accumulation, while polyamine levels remained largely unaffected. Silencing lines were affected in both amino acid and putrescine accumulation. Arabidopsis plants overexpressing the arginase gene were less susceptible to B. cinerea, whereas silencing lines remained as susceptible as the wild type. We discuss how arginase might interact with plant defence mechanisms. These results provide new insights into amino acid metabolic changes under stress.

Transcriptomics and systems biology analysis in identification of specific pathways involved in cacao resistance and susceptibility to witches' broom disease

This study reports on expression analysis associated with molecular systems biology of cacao-Moniliophthora perniciosa interaction. Gene expression data were obtained for two cacao genotypes (TSH1188, resistant; Catongo, susceptible) challenged or not with the fungus M. perniciosa and collected at three time points through disease. Using expression analysis, we identified 154 and 227 genes that are differentially expressed in TSH1188 and Catongo, respectively. The expression of some of these genes was confirmed by RT-qPCR. Physical protein-protein interaction (PPPI) networks of Arabidopsis thaliana orthologous proteins corresponding to resistant and susceptible interactions were obtained followed by cluster and gene ontology analyses. The integrated analysis of gene expression and systems biology allowed designing a general scheme of major mechanisms associated with witches' broom disease resistance/susceptibility. In this sense, the TSH1188 cultivar shows strong production of ROS and elicitors at the beginning of the interaction with M. perniciosa followed by resistance signal propagation and ROS detoxification. On the other hand, the Catongo genotype displays defense mechanisms that include the synthesis of some defense molecules but without success in regards to elimination of the fungus. This phase is followed by the activation of protein metabolism which is achieved with the production of proteasome associated with autophagy as a precursor mechanism of PCD. This work also identifies candidate genes for further functional studies and for genetic mapping and marker assisted selection.

Influence of over-expression of cytosolic aspartate aminotransferase on amino acid metabolism and defence responses against Botrytis cinerea infection in Arabidopsis thaliana

Arabidopsis possesses several genes encoding aspartate aminotransferase, which catalyzes the bidirectional conversion of aspartate into glutamate. These amino acids together with asparagine and glutamine play an important role in N storage and distribution. In addition, they act as precursors for other amino acids. The gene encoding cytosolic aspartate aminotransferase, Asp2, was found to be induced upon infection with the necrotrophic pathogen Botrytis cinerea in Arabidopsis. Asp2 over-expression lines and a T-DNA insertion mutant were used to study the role of aspartate aminotransferase in Arabidopsis defence responses. Over-expression of Asp2 led to changes in aspartate content and aspartate-derived amino acids. The Asp2 knockout mutant was also slightly affected in its amino acid composition. Under standard growth conditions, the Asp2 transgenic lines did not show morphological changes in comparison with the wild-type. However, transgenic lines with the highest Asp2 expression displayed more spreading lesions when infected with B. cinerea. We discuss how this gene involved in amino acid metabolism might interact with plant defence responses.

Immunohistochemical analysis of cell wall hydroxyproline-rich glycoproteins in the roots of resistant and susceptible wax gourd cultivars in response to Fusarium oxysporum f. sp. Benincasae infection and fusaric acid treatment

Hydroxyproline-rich glycoproteins (HRGPs) play a defensive role in host-pathogen interactions. However, specific roles of individual HRGPs in plant defense against pathogen are poorly understood. Changes in extracellular distribution and abundance of individual cell wall HRGPs were investigated on root sections of two wax gourd (Benincasa hispida Cogn.) cultivars (Fusarium wilt resistant and susceptible, respectively), which were analyzed by immunolabelling with 20 monoclonal antibodies recognizing different epitopes of extensins and arabinogalactan proteins (AGPs) after being inoculated with Fusarium oxysporum f. sp. Benincasae or treated with fusaric acid (FA). These analyses revealed the following: (1) The levels of JIM11 and JIM20 interacting extensins were higher in the resistant cultivar. Either treatment caused a dramatic decrease in signal in both cultivars, but some new signal appeared in the rhizodermis. (2) The AGPs or rhamnogalacturonan containing CCRCM7-epitope were enhanced in the resistant cultivar, but not in the susceptible one by either treatment. (3) Either treatment caused a slight increase in the levels of the AGPs recognized by LM2 and JIM16, but there were no differences between two cultivars. (4) The MAC204 signal nearly disappeared after FA treatment, but this was not the case with pathogen attack. (5) The LM14 signal slightly decreased after both treatments in both cultivars, but a less decrease was observed with the resistant cultivar. These results indicate that the CCRCM7 epitope likely contributed to the resistance of wax gourd to this pathogen, and JIM11 and JIM20 interacting extensins as well as LM2, LM14, MAC204 and JIM16 interacting AGPs were involved in the host-pathogen interaction.

Prunus domestica pathogenesis-related protein-5 activates the defense response pathway and enhances the resistance to fungal infection

Pathogenesis-related protein-5 (PR-5) has been implicated in plant disease resistance and its antifungal activity has been demonstrated in some fruit species. However, their roles, especially their interactions with the other defense responses in plant cells, are still not fully understood. In this study, we have cloned and characterized a new PR-5 cDNA named PdPR5-1 from the European plum (Prunus domestica). Expression of PdPR5-1 was studied in different cultivars varying in resistance to the brown rot disease caused by the necrotrophic fungus Monilinia fructicola. In addition transgenic Arabidopsis, ectopically expressing PdPR5-1 was used to study its role in other plant defense responses after fungal infection. We show that the resistant cultivars exhibited much higher levels of transcripts than the susceptible cultivars during fruit ripening. However, significant rise in the transcript levels after infection with M. fructicola was observed in the susceptible cultivars too. Transgenic Arabidopsis plants exhibited more resistance to Alternaria brassicicola. Further, there was a significant increase in the transcripts of genes involved in the phenylpropanoid biosynthesis pathway such as phenylalanine ammonia-lyase (PAL) and phytoalexin (camalexin) pathway leading to an increase in camalexin content after fungal infection. Our results show that PdPR5-1 gene, in addition to its anti-fungal properties, has a possible role in activating other defense pathways, including phytoalexin production.

Chris Lamb: a visionary leader in plant science

Christopher John Lamb (1950-2009) made major contributions to the field of plant defense gene activation, particularly through his studies on signal transduction mechanisms. Between 1994 and 2004, he published a series of seminal papers that outlined the involvement of hydrogen peroxide, nitric oxide, lipid transfer proteins, and aspartic proteases as critical components of local and/or systemic resistance during plant-microbe interactions. Prior to this, he had been one of the first to establish the fact that induced defense responses resulted from transcriptional activation of sets of coordinately regulated genes. Chris obtained his B.S and PhD degrees in biochemistry from the University of Cambridge, United Kingdom, moving to the Botany School at the University of Oxford as a postdoctoral fellow in 1975 and to the Biochemistry Department in Oxford as a Departmental Demonstrator in 1978. He was appointed founding director of the Plant Biology Laboratory at the Salk Institute for Biological Studies in La Jolla, California in 1982, and occupied the last ten years of his life as Director of the John Innes Center, Norwich, United Kingdom. In spite of spending most of his career as a director at two of the world's most prestigious institutes, formal recognition of his achievements came late in life, with election to the Royal Society of London in 2008 and endowment of the honor of Commander of the British Empire (CBE) for his contributions to British plant science by Queen Elizabeth II in 2009. Sadly, Chris did not live to attend the official ceremony at which he would receive his CBE.

Changes in growth and cell wall extensibility of maize silks following pollination

In response to pollination maize silks undergo an accelerated process of senescence which involves an inhibition of elongation. To gain insight into the mechanism underlying this growth response, the relationships among silk elongation kinetics, cell wall biophysical properties, pollen tube growth, and expansin protein abundance were investigated. The inhibition of silk elongation became apparent beyond 12 h after pollination. Pollinated walls were less responsive in assays of extension induced by pollen beta-expansin. Expansin protein abundance and endogenous expansin activity were not considerably reduced after pollination. Silk wall plastic compliance was significantly reduced 6 h post-pollination and beyond, suggesting that the wall undergoes structural modifications leading to its rigidification in response to pollination. The reduction in the plastic compliance occurred locally and progressively, shortly after pollen tubes traversed through a region of silk. Though numerous pollen grains germinated and initiated pollen tubes at the silk tip, the density of pollen tubes gradually declined along the length of the silk and only 1-2 reached the ovary even 24 h after pollination. These results support the notion that pollination-induced cell wall rigidification plays multiple roles in maize reproduction, including inhibition of silk growth and prevention of polyspermy.

Plant defense genes are regulated by ethylene

One of the earliest detectable events during plant-pathogen interaction is a rapid increase in ethylene biosynthesis. This gaseous plant stress hormone may be a signal for plants to activate defense mechanisms against invading pathogens such as bacteria, fungi, and viruses. The effect of ethylene on four plant genes involved in three separate plant defense response pathways was examined; these included (i and ii) genes that encode L-phenylalanine ammonia-lyase (EC 4.3.1.5) and 4-coumarate:CoA ligase [4-coumarate:CoA ligase (AMP-forming), EC 6.2.1.12], enzymes of the phenylpropanoid pathway, (iii) the gene encoding chalcone synthase, an enzyme of the flavonoid glycoside pathway, and (iv) the genes encoding hydroxyproline-rich glycoprotein, a major protein component(s) of plant cell walls. Blot hybridization analysis of mRNA from ethylene-treated carrot roots reveals marked increases in the levels of phenylalanine ammonia-lyase mRNA, 4-coumarate CoA ligase mRNA, chalcone synthase mRNA, and certain hydroxyproline-rich glycoprotein transcripts. The effect of ethylene on hydroxyproline-rich glycoprotein mRNA accumulation was different from that of wounding. Ethylene induces two hydroxyproline-rich glycoprotein mRNAs (1.8 and 4.0 kilobases), whereas wounding of carrot root leads to accumulation of an additional hydroxyproline-rich mRNA (1.5 kilobases). These results indicate that at least two distinct signals, ethylene and a wound signal, can affect the expression of plant defense-response genes.

Specific localization of a plant cell wall glycine-rich protein in protoxylem cells of the vascular system

An antibody against glycine-rich protein 1.8 of bean (Phaseolus vulgaris L.) was used for immunogold/silver localization of the protein in different organs of the plant. In hypocotyls, ovaries, and seed coats, the protein was found specifically in xylem cells of the vascular tissue. In hypocotyls, only protoxylem cells were labeled with the antibody, which indicates a remarkable cell-type specificity for accumulation of this cell wall protein. In mature hypocotyls, the protein was restricted to the same subset of xylem cells but was no longer detected on tissue prints, where a positive antibody reaction depends on the transfer of soluble material from plant tissue to the nitrocellulose filter. This indicates that the glycine-rich protein is insolubilized in the cell wall during development. In longitudinal sections of tracheary elements of young hypocotyls and seed coats, the antibody stained a pattern very similar to that of the lignified secondary thickenings of the cell wall, which suggests a close functional relationship between glycine-rich protein and lignin deposition during cell wall biogenesis in protoxylem cells.

Characterization of cDNA for nodulin-75 of soybean: A gene product involved in early stages of root nodule development

Establishment of a nitrogen-fixing root nodule is accompanied by a developmentally regulated expression of nodulin genes, only some of which, the so-called early nodulin genes, are expressed in stages preceding actual nitrogen fixation. We have isolated soybean cDNA clones representing early nodulin genes and have studied clone pENOD2 in detail. The cDNA insert of this clone hybridizes to nodule-specific RNA of 1200 nucleotides in length. The RNA that was hybrid-selected by the cloned ENOD2 DNA was in vitro translated to produce two nodulins with an apparent M(r) of 75,000, the N-75 nodulins. These two nodulins differ slightly in charge and one does not contain methionine. The amino acid sequence deduced from the DNA sequence shows that proline accounts for 45% of the 240 residues in these nodulins and the sequence contains at least 20 repeating heptapeptide units. The amino acid composition of none of the (hydroxy)proline-rich (glyco)proteins described in plants resembles the composition of the N-75 nodulins, especially with respect to the high glutamic acid and the low serine content. This suggests that the N-75 nodulins belong to a hitherto unidentified class of presumably structural proteins. The genes encoding the N-75 nodulins were found to be expressed in nodule-like structures devoid of intracellular bacteria and infection threads, indicating that these nodulins do not function in the infection process but more likely function in nodule morphogenesis.

Identification of quantitative trait loci for resistance to shoot fly in sorghum [Sorghum bicolor (L.) Moench]

The shoot fly is one of the most destructive insect pests of sorghum at the seedling stage. Deployment of cultivars with improved shoot fly resistance would be facilitated by the use of molecular markers linked to QTL. The objective of this study was to dissect the genetic basis of resistance into QTL, using replicated phenotypic data sets obtained from four test environments, and a 162 microsatellite marker-based linkage map constructed using 168 RILs of the cross 296B (susceptible) x IS18551 (resistant). Considering five component traits and four environments, a total of 29 QTL were detected by multiple QTL mapping (MQM) viz., four each for leaf glossiness and seedling vigor, seven for oviposition, six for deadhearts, two for adaxial trichome density and six for abaxial trichome density. The LOD and R (2) (%) values of QTL ranged from 2.6 to 15.0 and 5.0 to 33%, respectively. For most of the QTL, IS18551 contributed resistance alleles; however, at six QTL, alleles from 296B also contributed to resistance. QTL of the related component traits were co-localized, suggesting pleiotropy or tight linkage of genes. The new morphological marker Trit for trichome type was associated with the major QTL for component traits of resistance. Interestingly, QTL identified in this study correspond to QTL/genes for insect resistance at the syntenic maize genomic regions, suggesting the conservation of insect resistance loci between these crops. For majority of the QTL, possible candidate genes lie within or very near the ascribed confidence intervals in sorghum. Finally, the QTL identified in the study should provide a foundation for marker-assisted selection (MAS) programs for improving shoot fly resistance in sorghum.

Extensin over-expression in Arabidopsis limits pathogen invasiveness

SUMMARY The function of the cell wall protein extensin has been the subject of much speculation since it was first isolated over 40 years ago. In order to investigate the role of extensins in plant defence, we used the gain-of-function strategy to generate transgenic Arabidopsis plants over-expressing the EXT1 extensin gene. These were infected with the virulent bacterial pathogen Pseudomonas syringae DC3000 and symptom development was monitored. Lesions on the transgenics were on average five-fold smaller than those on the wild-type, did not increase in area over the time period of infection, accumulated a small bacterial load and showed very little chlorosis outside the lesion boundary. By contrast, lesions on the wild-type were large, spread to over 50% of the leaf area, continued to increase in size over the time course of the infection, accumulated a bacterial load 100-fold higher than that found in the transgenics, and showed a large chlorotic area outside the lesion boundary. SEM of lesions showed no evidence of bacteria at the lesion boundary in the extensin-over-expressing transgenics, whereas bacteria were always seen at the lesion boundary on the wild-type. Analysis of transgenics carrying an EXT1-GUS promoter-reporter fusion showed expression of GUS in a ring around the boundary of the lesion. Basal defences and signal transduction pathways involved in plant defence were not perturbed in the transgenics, as shown by the analysis of the expression of PR1 and PDF1.2 genes. These results show that extensin over-expression limits pathogen invasiveness.

Osmotin and thaumatin from grape: a putative general defense mechanism against pathogenic fungi

ABSTRACT Little information is available concerning the expression of pathogenesis-related (PR) proteins in grapevine (Vitis vinifera) and their effect properties on the major fungal pathogens of grape. A systematic study was performed on the effect of total or individual grape proteins on mycelial growth, spore germination, and germ tube growth of Uncinula necator, Phomopsis viticola, and Botrytis cinerea. Two proteins, identified as PR proteins by immunological methods and by N-terminal sequencing as osmotin and thaumatin-like protein, exhibited strong antifungal activities in vitro, blocking the growth of Phomopsis viticola and Botrytis cinerea mycelia. In addition, they inhibited spore germination and germ tube growth of U. necator, Phomopsis viticola, and Botrytis cinerea. The presence of both proteins displayed a synergistic effect. The expression of osmotin and thaumatin-like protein was induced in grapevine leaves and berries infected with U. necator and Phomopsis viticola. Thaumatin previously was thought to occur exclusively in berries. Immunoblot analyses revealed the accumulation of the two PR proteins in infected leaves and berries, supporting a role in vivo in increasing the resistance of grapevine to fungal attack.

Characterization of Plasmopara-resistance in grapevine using in vitro plants

Although the exact mechanisms by which grapevine cells operate to reduce disease incidence caused by the downy mildew fungus Plasmopara viticola are not fully elucidated, our cytological results obtained from infected in vitro-plants confirm that enhanced disease resistance is associated with an expression of distinct reactions in a chronological order. An increased production of reactive oxygen species (superoxide radicals, 4-6 hours post infection, hpi) was followed by a hypersensitive response (6-8 hpi), an increased activity of peroxidase in cells flanking the infection area and in the vascular tissue (10-12 hpi) and an increased production, accumulation or conversion of phenolic compounds (12-15 hpi). These mechanisms seem also to be present in susceptible varieties as shown after an inoculation with non-host oomycetic pathogens on the basis of peroxidase activity, but they do not become activated after P. viticola infection. The investigation of the peroxidase activity in leaves at several time points after an infection with P. viticola indicated that there is a strong correlation between the POX activity in leaves of in vitro-plants and the resistance of grapevine plants to P. viticola in the field.

Target genes for OBP3, a Dof transcription factor, include novel basic helix-loop-helix domain proteins inducible by salicylic acid

Overexpression of a salicylic-acid (SA)-inducible Arabidopsis DNA binding with one finger (Dof) transcription factor, called OBF-binding protein 3 (OBP3; AtDof3.6), has previously been shown to result in growth defects. In this study, suppressive subtraction hybridization (SSH) was used to isolate genes induced in an OBP3-overexpression line and several putative clones, called OBP3-responsive genes (ORGs), were isolated. The link with the induced expression levels of these genes and OBP3 overexpression was confirmed by analysing additional OBP3-overexpression lines. ORG1 through ORG4 are novel genes, while ORG5 is an extensin gene, AtExt1. While ORG4 has no similarity with other proteins in the database, ORG1 has weak similarity in different regions of the predicted protein with CDC2 and fibrillin. ORG2 and ORG3 share 80% overall identity in their deduced amino acid sequences and contain a basic helix-loop-helix DNA-binding domain, suggesting that ORG2 and ORG3 may be transcription factors. The expression of the ORG1, ORG2 and ORG3 genes was co-regulated under all conditions examined including upregulation by SA and downregulation by jasmonic acid (JA). Fifteen OBP3-silenced lines were generated to further explore the function of OBP3. Although there were no visible phenotypic changes in any of these lines, the expression of ORG1, ORG2 and ORG3 was reduced. Among the ORG genes, ORG1, ORG2 and ORG3 contained the highest number of potential Dof-binding sites in the promoter region, and their expression was significantly increased within 3 h after induction of OBP3 expression using an inducible promoter system, and closely reflected the expression levels of the exogenous OBP3 protein. The results from the gain-of-function and loss-of-function experiments suggest that the ORG1, ORG2 and ORG3 genes are direct target genes of OBP3.

[Stimulation of plant natural defenses]

Some defense mechanisms of plants are of the passive type while others are induced after perception of the pathogenic microorganism (very specific gene-for-gene recognition) or of microbial components (non specific elicitors). These recognition events trigger an array of plant signals and a cascade of signalling pathways which activate a battery of metabolic alterations responsible for the observed induced resistance. These include the stimulated production of low molecular weight molecules with antibiotic activity, cell wall reinforcement by deposition and cross-linking of various macromolecules, and accumulation of a wide range of PR ('pathogenesis-related') proteins that exhibit direct and/or indirect antimicrobial activities. The present studies aim to characterize natural elicitors or design chemical messengers capable of triggering an array of plant defense responses. Treatments of plants with elicitors could be an alternative strategy of crop protection with a more satisfactory preservation of the environment.

The molecular basis of plant cell wall extension

In all terrestrial and aquatic plant species the primary cell wall is a dynamic structure, adjusted to fulfil a diversity of functions. However a universal property is its considerable mechanical and tensile strength, whilst being flexible enough to accommodate turgor and allow for cell elongation. The wall is a composite material consisting of a framework of cellulose microfibrils embedded in a matrix of non-cellulosic polysaccharides, interlaced with structural proteins and pectic polymers. The assembly and modification of these polymers within the growing cell wall has, until recently, been poorly understood. Advances in cytological and genetic techniques have thrown light on these processes and have led to the discovery of a number of wall-modifying enzymes which, either directly or indirectly, play a role in the molecular basis of cell wall expansion.

A leucine-rich repeat region is conserved in pollen extensin-like (Pex) proteins in monocots and dicots

We previously isolated a pollen-specific gene encoding a pollen tube wall-associated glycoprotein with a globular domain and an extensin domain from maize (mPex1). To evaluate which protein domains might be important for function, we isolated a second monocot gene (mPex2) and a dicot gene (tPex). Each gene encodes a signal sequence, an N-terminal globular domain comprised of a variable region, a leucine-rich repeat (LRR) with an adjacent cysteine-rich region, a transition region and an extensin-like C-terminal domain. The LRRs of the maize and tomato Pex proteins are highly conserved. Although the extensin domains in the maize and tomato proteins vary in length and in amino acid sequence, they are likely to be structurally conserved. Additional putative Pex gene sequences were identified by either GenBank search (Arabidopsis) or PCR (sorghum and potato): all encode conserved LRRs. The presence of a conserved LRR in the known and potential Pex proteins strongly suggests that this motif is involved in the binding of a specific ligand during pollen tube growth. Gene expression studies using RNA and protein blotting as well as promoter-reporter gene fusions in transient and stable transformation indicate that the tomato Pex gene is pollen-specific.

Response to Xanthomonas campestris pv. vesicatoria in tomato involves regulation of ethylene receptor gene expression

Although ethylene regulates a wide range of defense-related genes, its role in plant defense varies greatly among different plant-microbe interactions. We compared ethylene's role in plant response to virulent and avirulent strains of Xanthomonas campestris pv. vesicatoria in tomato (Lycopersicon esculentum Mill.). The ethylene-insensitive Never ripe (Nr) mutant displays increased tolerance to the virulent strain, while maintaining resistance to the avirulent strain. Expression of the ethylene receptor genes NR and LeETR4 was induced by infection with both virulent and avirulent strains; however, the induction of LeETR4 expression by the avirulent strain was blocked in the Nr mutant. To determine whether ethylene receptor levels affect symptom development, transgenic plants overexpressing a wild-type NR cDNA were infected with virulent X. campestris pv. vesicatoria. Like the Nr mutant, the NR overexpressors displayed greatly reduced necrosis in response to this pathogen. NR overexpression also reduced ethylene sensitivity in seedlings and mature plants, indicating that, like LeETR4, this receptor is a negative regulator of ethylene response. Therefore, pathogen-induced increases in ethylene receptors may limit the spread of necrosis by reducing ethylene sensitivity.

A defective seed coat pattern (Net) is correlated with the post-transcriptional abundance of soluble proline-rich cell wall proteins

The pigmented seed coats of several soybean (Glycine max (L.) Merr.) plant introductions and isolines have unusual defects that result in cracking of the mature seed coat exposing the endosperm and cotyledons. It has previously been shown that the T (tawny) locus that controls the color of trichomes on stems and leaves also has an effect on both the structure and pigmentation of the seed coat. Distribution of pigmentation on the seed coat is controlled by alleles of the I (inhibitor) locus. It was also found that total seed coat proteins were difficult to extract from pigmented seed coats with i T genotypes because they have procyanidins that exhibit tannin properties. We report that the inclusion of poly-L-proline in the extraction buffer out-competes proteins for binding to procyanidins. Once this problem was solved, we examined expression of the proline-rich cell wall proteins PRP1 and PRP2 in pigmented genotypes with the dominant T allele. We found that both homozygous i T and i t genotypes have reduced soluble PRP1 levels. The epistatic interaction of the double recessive genotype at both loci is necessary to produce the pigmented, defective seed coat phenotype characteristic of seed coats with the double recessive i and t alleles. This implies a novel effect of an enzyme in the flavonoid pathway on seed coat structure in addition to its effect on flavonoids, anthocyanidins, and proanthocyanidins. No soluble PRP1 polypeptides were detectable in pigmented seed coats (i T genotypes) of isolines that also display a net-like pattern of seed coat cracking, known as the Net defect. PRP2 was also absent in one of the these lines. However, both PRP1 and PRP2 cytoplasmic mRNAs were found in the Net-defective seed coats. Together with in vitro translation studies, these results suggest that the absence of soluble PRP polypeptides in the defective Net lines is post-translational and could be due to a more rapid or premature insolubilization of PRP polypeptides within the cell wall matrix.

Decreased cell wall digestibility in canola transformed with chimeric tyrosine decarboxylase genes from opium poppy

Tyrosine decarboxylase (TYDC) is a common plant enzyme involved in the biosynthesis of numerous secondary metabolites, including hydroxycinnamic acid amides. Although a definite function has not yet been determined, amides have been proposed to form a physical barrier against pathogens because they are usually found as integral cell wall components. Canola (Brassica napus) was independently transformed with chimeric genes (35S::TYDC1 and 35S::TYDC2) under the transcriptional control of the cauliflower mosaic virus 35S promoter, and encoding two TYDC isoforms from opium poppy (Papaver somniferum). All T0 plants displayed a suppressed level of wild-type TYDC activity, and transgene mRNAs were not detected. Silencing of 35S::TYDC1 was overcome in the T1 progeny of self-pollinated T0 plants, since high levels of TYDC1 mRNAs were detected, and TYDC activity increased up to 4-fold compared with wild-type levels. However, TYDC1 mRNA levels decreased in T2 plants and were not detected in the T3 progeny. TYDC activity also gradually declined in T2 and T3 plants to nearly wild-type levels. In contrast, silencing of 35S::TYDC2 was maintained through four consecutive generations. T1 plants with a 3- to 4-fold increase in wild-type TYDC activity showed a 30% decrease in cellular tyrosine pools and a 2-fold increase in cell wall-bound tyramine compared with wild-type plants. An increase in cell wall-bound aromatic compounds was also detected in these T1 plants by ultraviolet autofluorescence microscopy. The relative digestibility of cell walls measured by protoplast release efficiency was inversely related to the level of TYDC activity.

Induction of mRNA accumulation corresponding to a gene encoding a cell wall hydroxyproline-rich glycoprotein by fungal elicitors

The Hrgp (hydroxyproline-rich glycoprotein) gene codes in maize for one of the most abundant proteins of the cell wall. HRGPs may contribute to the structural support of the wall and they have also been involved in plant defense mechanisms. This second aspect has been tested for the Hrgp gene in maize where, in contrast with the situation in dicot species, the gene is encoded by a single-copy sequence. Hrgp mRNA accumulation is induced in maize suspension-cultured cells by elicitors, isolated either from maize pathogenic or non-pathogenic fungi. The induction of Hrgp mRNA accumulation by elicitor extracted from Fusarium moniliforme has been studied in detail. The level of induction depends on elicitor concentration and remains high until at least 24 h. Ethylene and protein phosphorylation appear to be involved in the transduction pathway of Hrgp gene activation by the F. moniliforme elicitor but not by 5 microM methyl jasmonate or 1 mM salycilic acid. Different compounds known to participate in plant stress responses such as ascorbic acid or reduced glutathione have also a positive effect on Hrgp mRNA accumulation.

A metalloprotease from Xanthomonas campestris that specifically degrades proline/hydroxyproline-rich glycoproteins of the plant extracellular matrix

Culture supernatants of Xanthomonas campestris pv. campestris contain an enzymic activity capable of degrading gp120, a proline-rich glycoprotein associated with the extracellular matrix of the vascular bundles in petioles of turnip (Brassica campestris). This activity did not reside in any of the three previously characterized proteases of X. campestris pv. campestris that were identified by their action against the model substrate beta-casein. The novel enzyme was purified by ion-exchange and size-exclusion high-performance liquid chromatography (HPLC). The enzyme, which has no activity against beta-casein, is active against some plant glycoproteins of the hydroxyproline-rich class such as extensin from potato and tomato and gpS-3, a glycoprotein induced in B. campestris petioles by wounding. Other hydroxyproline-rich glycoproteins, such as the solanaceous lectins, were not substrates however. Studies of the products released upon degradation of tomato extensin suggested that the degradative mechanism was proteolysis. Inhibitor studies suggested that the enzyme was a zinc-requiring metalloprotease. Extracellular matrix glycoproteins of the proline-rich and hydroxyproline-rich classes have been implicated in plant resistance to microbial attack, hence their degradation by X. campestris pv. campestris may have considerable significance for black rot pathogenesis.

Molecular mechanisms involved in bacterial speck disease resistance of tomato

An important recent advance in the field of plant–microbe interactions has been the cloning of genes that confer resistance to specific viruses, bacteria, fungi or nematodes. Disease resistance ( R ) genes encode proteins with predicted structural motifs consistent with them having roles in signal recognition and transduction. The future challenge is to understand how R gene products specifically perceive defence–eliciting signals from the pathogen and transduce those signals to pathways that lead to the activation of plant defence responses. In tomatoes, the Pto kinase (product of the Pto R gene) confers resistance to strains of the bacterial speck pathogen, Pseudomonas syringae pv. tomato , that carry the corresponding avirulence gene avrPto . Resistance to bacterial speck disease is initiated by a mechanism involving the physical interaction of the Pto kinase and the AvrPto protein. This recognition event initiates signalling events that lead to defence responses including an oxidative burst, the hypersensitive response and expression of pathogenesis–related genes. Pto–interacting (Pti) proteins have been identified that appear to act downstream of the Pto kinase and our current studies are directed at elucidating the roles of these components.

Control of expression of a gene encoding an extensin by phytochrome and a blue light receptor in spores of Adiantum capillus-veneris L

In the present study, using a newly developed fluorescent differential display technique, we have carried out large-scale screening for genes whose expression was regulated by phytochrome and antagonistically by a blue light receptor in the spores of the fern Adiantum capillus-veneris L. Spores after imbibition were briefly irradiated with red, red/blue or blue light and collected 8 h after the irradiation. Total RNA was isolated from each sample and used to make cDNA with an oligo-dT primer. The cDNA was then used as a template for PCR with the oligo-dT primer and 80 arbitrary primers. The resulting PCR products were analyzed by an automated fluorescent DNA sequencer. Among 8000 displayed bands, we identified 15 up-regulated and four down-regulated bands by red light, and this red light effect was irreversibly reversed by blue light. We cloned one of the up-regulated cDNA fragments and used it to screen a cDNA library prepared from the spores. The isolated insert is predicted to encode Ser-(Pro)n repeats and showed homology with cell wall-associated extensins. The expression of this cDNA was induced 8 h after a red light treatment and the red light induction was photoreversibly prevented by far-red light and photoirreversibly by blue light. The mRNA of this gene was detectable 4 h after red light irradiation and gradually increased in germinating spores.

Role of calcium in signal transduction during the hypersensitive response caused by basidiospore-derived infection of the cowpea rust fungus

The hypersensitive response (HR) of disease-resistant plant cells to fungal invasion is a rapid cell death that has some features in common with programmed cell death (apoptosis) in animals. We investigated the role of cytosolic free calcium ([Ca2+]i) in the HR of cowpea to the cowpea rust fungus. By using confocal laser scanning microscopy in conjunction with a calcium reporter dye, we found a slow, prolonged elevation of [Ca2+]i in epidermal cells of resistant but not susceptible plants as the fungus grew through the cell wall. [Ca2+]i levels declined to normal levels as the fungus entered and grew within the cell lumen. This elevation was related to the stage of fungal growth and not to the speed of initiation of subsequent cell death. Elevated [Ca2+]i levels also represent the first sign of the HR detectable in this cowpea-cowpea rust fungus system. The increase in [Ca2+]i was prevented by calcium channnel inhibitors. This effect was consistent with pharmacological tests in which these inhibitors delayed the HR. The data suggest that elevation of [Ca2+]i is involved in signal transduction leading to the HR during rust fungal infection.

Purification and characterization of a galactose-rich basic glycoprotein in tobacco

We found a galactose-rich basic glycoprotein (GBGP) in the cell walls of cultured tobacco (Nicotiana tabacum) cells. GBGP and extensin were isolated as the major components of basic, salt-extracted cell wall glycoproteins. GBGP and extensin were separated by gel filtration in 6 M guanidine hydrochloride as 49- and 90-kD peaks, respectively, and further purified with reverse-phase chromatography. The protein moiety of GBGP constitutes about one-half of the molecule (W/W) and contains lysine (16%), proline (12%), hydroxyproline (10%), tyrosine (4%), alanine (7%), leucine (6%), and cystine (1.4%). Galactose accounted for 72% of the sugar moiety, arabinose content was low (17%), and a significant amount of mannose (7%) was found. No immunological cross-reaction was detected between GBGP and extensin. The antibody against native GBGP with sugar chains reacted with other glycoproteins on the gel blots, whereas the antibodies against deglycosylated GBGP and native extensin were highly specific. Immunolocalization analysis in tobacco stems showed that GBGP is specific to parenchyma tissue and that extensin localizes in the epidermis. This tissue-specific and exclusive distribution suggests important functions of these basic glycoproteins.

Induction of extracellular matrix glycoproteins in Brassica petioles by wounding and in response to Xanthomonas campestris

A panel of monoclonal antibodies that recognize plant extracellular matrix glycoproteins previously implicated in plant-microbe interactions was used to study the effects of pathogen inoculation and wounding on glycoproteins in petioles of Brassica campestris. The panel of monoclonals comprised two sets: JIM11, JIM12, and JIM20 recognize epitopes carried on hydroxyproline-rich glycoproteins (HRGPs) (M. Smallwood, A. Beven, N. Donovan, S. J. Neitl, J. Peart, K. Roberts, and J. P. Knox, Plant J. 5:237-246, 1994); MAC204 and MAC265 recognize glycoproteins of the Rhizobium infection thread (K. A. VandenBosch, D. J. Bradley, S. Perotto, G. W. Butcher, and N. J. Brewin, EMBO J. 8:335-342, 1989). Wounding or inoculation of petioles with avirulent strains of pathovars of Xanthomonas campestris induced the synthesis of two new groups of antigens: gp160 ran as a smear on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with apparent molecular mass from 120 to 200 kDa and was recognized by JIM20 and MAC204; gpS remained in the stacking gel on SDS-PAGE and was recognized by JIM11, JIM20, and MAC204. The response to virulent strains of pathovars of X. campestris was either less pronounced or absent. gpS comprised several components that were resolved by cation-exchange chromatography. Some of these components were characterized as extensin-like HRGPs. The level of induction of the gpS group of antigens by virulent strains was not altered by mutation of a number of genes required for basic pathogenicity or by heat-killing the bacteria.

The Pto kinase conferring resistance to tomato bacterial speck disease interacts with proteins that bind a cis-element of pathogenesis-related genes

In tomato, the Pto kinase confers resistance to bacterial speck disease by recognizing the expression of a corresponding avirulence gene, avrPto, in the pathogen Pseudomonas syringae pv. tomato. Using the yeast two-hybrid system, we have identified three genes, Pti4, Pti5 and Pti6, that encode proteins that physically interact with the Pto kinase. Pti4/5/6 each encode a protein with characteristics that are typical of transcription factors and are similar to the tobacco ethylene-responsive element-binding proteins (EREBPs). Using a gel mobility-shift assay, we demonstrate that, similarly to EREBPs, Pti4/5/6 specifically recognize and bind to a DNA sequence that is present in the promoter region of a large number of genes encoding 'pathogenesis-related' (PR) proteins. Expression of several PR genes and a tobacco EREBP gene is specifically enhanced upon Pto-avrPto recognition in tobacco. These observations establish a direct connection between a disease resistance gene and the specific activation of plant defense genes.

Characterization of a tobacco extensin gene and regulation of its gene family in healthy plants and under various stress conditions

A genomic clone (Ext 1.4) encoding an extensin was isolated from a Nicotiana tabacum genomic library. The encoded polypeptide showed features characteristic of extensins such as Ser-(Pro)4 repeats and a high content in Tyr and Lys residues. The presence of one Tyr-Leu-Tyr-Lys motif suggests the possibility for one intramolecular isodityrosine cross-link whereas numerous Val-Tyr-Lys motifs may participate in intermolecular cross-links. This extensin appears to be close to an extensin already characterized in N. tabacum but very different from the Ext 1.2 extensin of N. sylvestris. The analysis of genomic DNA gel blots using probes spanning different parts of the gene showed that the Ext 1.4 gene belongs to a complex multigene family having one member originating from N. sylvestris and three members from N. tomentosiformis. The Ext 1.4 specific probe found a 1.4 kb mRNA in stems, roots, ovaries and germinating seeds of healthy plants. The Ext 1.4 gene family is strongly induced in actively dividing cell suspension cultures and after wounding of leaves or stems in conditions where root formation occurs. On the contrary, it is not induced in leaves in response to a hypersensitive reaction to a viral infection or after elicitor treatment.