Differential expression of peroxidase isogenes during the early stages of infection of the tropical forage legume Stylosanthes humilis by Colletotrichum gloeosporioides

A successful defense of the narrow-leafed lupin against anthracnose involves quick and orchestrated reprogramming of oxidation-reduction, photosynthesis and pathogenesis-related genes

Narrow-leafed lupin (NLL, Lupinus angustifolius L.) is a legume plant cultivated for grain production and soil improvement. Worldwide expansion of NLL as a crop attracted various pathogenic fungi, including Colletotrichum lupini causing a devastating disease, anthracnose. Two alleles conferring improved resistance, Lanr1 and AnMan, were exploited in NLL breeding, however, underlying molecular mechanisms remained unknown. In this study, European NLL germplasm was screened with Lanr1 and AnMan markers. Inoculation tests in controlled environment confirmed effectiveness of both resistance donors. Representative resistant and susceptible lines were subjected to differential gene expression profiling. Resistance to anthracnose was associated with overrepresentation of "GO:0006952 defense response", "GO:0055114 oxidation-reduction process" and "GO:0015979 photosynthesis" gene ontology terms. Moreover, the Lanr1 (83A:476) line revealed massive transcriptomic reprogramming quickly after inoculation, whereas other lines showed such a response delayed by about 42 h. Defense response was associated with upregulation of TIR-NBS, CC-NBS-LRR and NBS-LRR genes, pathogenesis-related 10 proteins, lipid transfer proteins, glucan endo-1,3-beta-glucosidases, glycine-rich cell wall proteins and genes from reactive oxygen species pathway. Early response of 83A:476, including orchestrated downregulation of photosynthesis-related genes, coincided with the successful defense during fungus biotrophic growth phase, indicating effector-triggered immunity. Mandelup response was delayed and resembled general horizontal resistance.

Clavibacter michiganensis ssp. michiganensis: bacterial canker of tomato, molecular interactions and disease management

Bacterial canker disease is considered to be one of the most destructive diseases of tomato (Solanum lycopersicum), and is caused by the seed-borne Gram-positive bacterium Clavibacter michiganensis ssp. michiganensis (Cmm). This vascular pathogen generally invades and proliferates in the xylem through natural openings or wounds, causing wilt and canker symptoms. The incidence of symptomless latent infections and the invasion of tomato seeds by Cmm are widespread. Pathogenicity is mediated by virulence factors and transcriptional regulators encoded by the chromosome and two natural plasmids. The virulence factors include serine proteases, cell wall-degrading enzymes (cellulases, xylanases, pectinases) and others. Mutational analyses of these genes and gene expression profiling (via quantitative reverse transcription-polymerase chain reaction, transcriptomics and proteomics) have begun to shed light on their roles in colonization and virulence, whereas the expression of tomato genes in response to Cmm infection suggests plant factors involved in the defence response. These findings may aid in the generation of target-specific bactericides or new resistant varieties of tomato. Meanwhile, various chemical and biological controls have been researched to control Cmm. This review presents a detailed investigation regarding the pathogen Cmm, bacterial canker infection, molecular interactions between Cmm and tomato, and current perspectives on improved disease management.

Biochemical evaluation of resistance responses of potato to different isolates of Alternaria solani

The resistance phenotypes of nine potato cultivars to five isolates of Alternaria solani, causal agent of early blight, were studied after inoculation and growth under greenhouse conditions. We identified potato cultivars with both susceptible and resistant phenotypes as well as A. solani isolates with varying degrees of aggressiveness. Two potato cultivars and two pathogen isolates were selected for biochemical analysis of phenol production and peroxidase activity after inoculation. Phenol compounds were evaluated 2, 4, 6, and 8 days after inoculation, while peroxidase activities were monitored daily for 10 days. Native polyacrylamide electrophoresis was used to identify one protein with peroxidase activity in extracts taken 6 days after inoculation. Significantly higher peroxidase activity as well as total phenol content in potato was correlated with resistance in the Iranian potato cultivar Diamond. Variability of responses within the same cultivar to different isolates of A. solani suggests genotypic diversity between isolates that results in phenotypic diversity for pathogen aggressiveness.

A symbiotic plant peroxidase involved in bacterial invasion of the tropical legume Sesbania rostrata

Aquatic nodulation on the tropical legume Sesbania rostrata occurs at lateral root bases via intercellular crack-entry invasion. A gene was identified (Srprx1) that is transiently up-regulated during the nodulation process and codes for a functional class III plant peroxidase. The expression strictly depended on bacterial nodulation factors (NFs) and could be modulated by hydrogen peroxide, a downstream signal for crack-entry invasion. Expression was not induced after wounding or pathogen attack, indicating that the peroxidase is a symbiosis-specific isoform. In situ hybridization showed Srprx1 transcripts around bacterial infection pockets and infection threads until they reached the central tissue of the nodule. A root nodule extensin (SrRNE1) colocalized with Srprx1 both in time and space and had the same NF requirement, suggesting a function in a similar process. Finally, in mixed inoculation nodules that were invaded by NF-deficient bacteria and differed in infection thread progression, infection-associated peroxidase transcripts were not observed. Lack of Srprx1 gene expression could be one of the causes for the aberrant structure of the infection threads.

Ookinete-induced midgut peroxidases detonate the time bomb in anopheline mosquitoes

Previous analysis of the temporal-spatial relationship between ookinete migration and the cellular localization of genes mediating midgut immune defense responses suggested that, in order to survive, parasites must complete invasion before toxic chemicals ("a bomb") are generated by the invaded cell. Recent studies indicate that ookinete invasion induces tyrosine nitration as a two-step reaction, in which NOS induction is followed by a localized increase in peroxidase activity. Peroxidases utilize nitrite and hydrogen peroxide as substrates, and detonate the time bomb by generating reactive nitrogen intermediates, such as nitrogen dioxide, which mediate nitration. There is evidence that peroxidases also mediate antimicrobial responses to bacteria, fungi and parasites in a broad range of biological systems including humans and plants. Defense reactions that generate toxic chemicals are also potentially harmful to the host mounting the response and often results in apoptosis. The two-step nitration pathway is probably an ancient response, as it has also been described in vertebrate leukocytes and probably evolved as a mechanism to circumscribe the toxic products generated during defense responses involving protein nitration.

Isolation and characterization of a polymorphic stigma-specific class III peroxidase gene from Senecio squalidus L. (Asteraceae)

A novel stigma-specific class III peroxidase gene, SSP (Stigma-Specific Peroxidase), has been isolated from the self-incompatible daisy Senecio squalidus L. (Asteraceae). Expression of SSP in flower buds is developmentally regulated, with maximal levels of expression coinciding with anthesis, when stigmas are most receptive to pollen and when self-incompatibility is fully developed. In situ hybridization revealed SSP expression to be localized exclusively to the specialized secretory epidermal cells (papillae) of the stigma, which receive and discriminate pollen. SSP is therefore the first tissue-specific and cell-specific peroxidase gene identified in a plant. SSP belongs to a distinct clade of class III plant peroxidases that possess two introns, instead of the more normal situation of three conserved introns. The deduced amino acid sequence of SSP revealed a 27 amino acid signal peptide, suggesting that the SSP protein is secreted to the cell wall of the stigmatic papillae. In-gel peroxidase activity assays showed that SSP has relatively low peroxidase activity compared to other, as yet uncharacterized, peroxidases present in stigmatic extracts. Six SSP alleles have been cloned from different lines of S. squalidus carrying a range of self-incompatibility (S)-alleles but there was no consistent association between the presence of a particular SSP allele and S-genotype indicating that SSP is not the female determinant of SSI in S. squalidus. Nevertheless, the precise expression of SSP in stigmatic papillae suggests that it may have a more general function in pollen-stigma interactions, or alternatively in protection of stigmas from pathogen attack. Extensive database screens have identified homologues of SSP in other plant species, but available expression data for these genes indicates that none are flower-specific, suggesting that SSP represents a new functional type of class III peroxidase specific to the stigma. We discuss the possible function(s) of S. squalidus SSP in pollen-stigma interactions and in protection of stigmas from pathogen attack.

Ten Rice Peroxidases Redundantly Respond to Multiple Stresses Including Infection with Rice Blast Fungus

Class III plant peroxidases are believed to function in diverse physiological processes including disease resistance and wound response, but predicted low substrate specificities and the presence of 70 or more isoforms have made it difficult to define a specific physiological function(s) for each gene. To select pathogen-responsive POX genes, we analyzed the expression profiles of 22 rice POX genes after infection with rice blast fungus. The expression of 10 POX genes among the 22 genes was induced after fungal inoculation in both compatible and incompatible hosts. Seven of the 10 POX genes were expressed at higher levels in the incompatible host than in the compatible host 6-24 h after inoculation by which time no fungus-induced lesions have appeared. Organ-specific expression and stress-induced expression by wounding and treatment with probenazole, an agrichemical against blast fungus, jasmonic acid, salicylic acid and 1-aminocyclopropane-1-carboxylate, a precursor of ethylene, indicated that rice POXs have individual characteristics and can be classified into several types. A comparison of the amino acid sequences of POXs showed that multiple isoforms with a high sequence similarity respond to stress in different or similar ways. Such redundant responses of POX genes may guarantee POX activities that are necessary for self-defense in plant tissues against environmental stresses including pathogen infection.

A large family of class III plant peroxidases

Class III plant peroxidase (POX), a plant-specific oxidoreductase, is one of the many types of peroxidases that are widely distributed in animals, plants and microorganisms. POXs exist as isoenzymes in individual plant species, and each isoenzyme has variable amino acid sequences and shows diverse expression profiles, suggesting their involvement in various physiological processes. Indeed, studies have provided evidence that POXs participate in lignification, suberization, auxin catabolism, wound healing and defense against pathogen infection. Little, however, is known about the signal transduction for inducing expression of the pox genes. Recent studies have provided information on the regulatory mechanisms of wound- and pathogen-induced expression of some pox genes. These studies suggest that pox genes are induced via different signal transduction pathways from those of other known defense-related genes.

Expression of the Shpx2 peroxidase gene of Stylosanthes humilis in transgenic tobacco leads to enhanced resistance to Phytophthora parasitica pv. nicotianae and Cercospora nicotianae

Abstract Previous research indicated that the constitutive expression of a pathogen-inducible peroxidase gene (Shpx6a) from Stylosanthes humilis in transgenic plants resulted in enhanced resistance to fungal pathogens ( Kazan, K., Goulter, K.C., Way, H.M. and Manners, J.M. (1998) Expression of a pathogenesis-related peroxidase of Stylosanthes humilis in transgenic tobacco and canola and its effect on disease development. Plant Sci. 136, 207-217). We have now investigated another pathogen-inducible peroxidase gene of S. humilis, termed Shpx2, which is highly divergent from Shpx6a. Constitutive expression of the Shpx2 cDNA was obtained in tobacco using the 35S CaMV promoter, and up to a 12-fold increase in total peroxidase activity was observed in the leaves of transgenic plants compared to nontransgenic controls. Disease development was evaluated after inoculations in T(1) and T(2) transgenic lines expressing Shpx2. Lesion expansion was significantly (P < 0.05) reduced by up to 25% and 50% on leaves and stems, respectively, of transgenic plants expressing high levels of peroxidase compared to nontransgenic controls, following inoculation with Phytophthora parasitica pv. nicotianae, the cause of black shank disease. In addition, plant survival and recovery were greatly enhanced in transgenic plants following stem inoculations of plants with this plant pathogen. A significant (55%, P < 0.05) reduction in lesion number was observed in transgenic plants with high levels of peroxidase activity following inoculation with the fungus Cercospora nicotianae, the cause of frog-eye disease. No significant differences in disease development were observed between the lines expressing Shpx2 and controls following inoculation with the bacterium Pseudomonas syringae pv. tabaci, the cause of wildfire disease. These results provide evidence for a role for this peroxidase gene in plant defence, and suggest that diverse peroxidase genes may be employed as components of strategies aimed at the engineering of disease resistance.

An HR-induced tobacco peroxidase gene is responsive to spermine, but not to salicylate, methyl jasmonate, and ethephon

In Tobacco mosaic virus (TMV)-infected tobacco plants carrying the N resistance gene, a hypersensitive reaction or response (HR) occurs to enclose the virus in the infected tissue. Although a contribution of peroxidases to the resistance has been proposed, no evidence has been presented that tobacco peroxidase genes respond to HR. Here, we describe the HR-induced expression of a tobacco peroxidase gene (tpoxC1) whose induction kinetics were slightly different from those of acidic and basic tobacco pathogenesis-related (PR) protein genes. Interestingly, tpoxC1 was insensitive to the inducers of PR genes such as salicylic acid, methyl jasmonate, and ethephon. Spermine activated tpoxC1 gene expression at a low level and both acidic and basic PR gene expression at a considerably higher level. These results indicate that the induced expression of tpoxC1 is regulated differently from that of classical tobacco PR genes in the N gene-mediated self-defense system in tobacco plants.

On becoming a parasite: evaluating the role of wall oxidases in parasitic plant development

BACKGROUND

The temporal and spatial control of the transition from vegetative to parasitic growth is critical to any parasite, but is essential to the sessile parasitic plants. It has been proposed that this transition in Striga spp. is controlled simply by an exuded oxidase that converts host cell-surface phenols into benzoquinones which act as developmental signals that mediate the transition. An understanding of this mechanism may identify the critical molecular events that made possible the evolution of parasitism in plants.

RESULTS

PoxA and PoxB are identified as the only apoplastic phenol oxidases in Striga asiatica seedlings, and the genes encoding them have been cloned and sequenced. These peroxidase enzymes are capable of oxidizing the 60 known inducing phenols into a small set of benzoquinones, and it is these quinones that induce parasitic development. Analysis of the reaction requirements and comparisons to host enzymes, however, lead us to argue that PoxA and PoxB are not necessary for host recognition.

CONCLUSIONS

A new model is proposed where constitutive production of an activated oxygen species (in the case of Striga, H2O2) mediates host recognition. This strategy would allow a parasite to exploit abundant host enzymes to produce the diffusible recognition signals by converting a standard host defense into a parasitic offense.

Differential induction of a peroxidase gene family during infection of rice by Xanthomonas oryzae pv. oryzae

Induction of peroxidase has been correlated with resistant interactions between rice and Xanthomonas oryzae pv. oryzae. To assist in analysis of the role of rice peroxidases in plant defense against the bacterial pathogen, three peroxidase genes, POX22.3, POX8.1, and POX5.1, were identified from a rice cDNA library that was constructed from leaves of plants undergoing a resistant reaction. These genes were highly similar in nucleic acid and amino acid sequences and belonged to a gene family. The three genes showed differential expression in infiltrated rice leaves during pathogen interactions and mechanical stress. Only two peroxidase genes, POX8.1 and POX22.3, were predominantly expressed during resistant interactions. These two genes also were expressed during susceptible interactions, but induction was delayed compared with resistant interactions. POXgX9, a fourth peroxidase gene that was isolated from a genomic library, is adjacent to POX22.3 in the rice genome and has greater than 90% similarity in nucleotide and amino acid sequence identity to POX22.3. Interestingly, POXgX9 was expressed only in the roots of rice plants. While POX22.3 was expressed in both leaves and roots, POX8.1 and POX5.1 were not detected in roots but were induced in leaves by mechanical wounding at different times after treatment. POX22.3, POX8.1, and POX5.1 were estimated to be present in single copies in rice haploid genome. These results indicate that different members of the rice peroxidase gene family are distinctly regulated in response to various environmental cues.

Expression of a highly basic peroxidase gene in NaCl-adapted tomato cell suspensions

A tomato peroxidase gene, TPX2, that is only weakly expressed in the roots of young tomato seedlings is highly expressed in tomato suspension cells adapted to high external NaCl concentration. The protein encoded by this gene, with an isolectric point value of approximately 9.6, is found in the culture medium of the growing cells. Our data suggest that the expression of TPX2 in the salt-adapted cells is not the result of the elicitation imposed by the in vitro culture or the presence of high NaCl concentration in the medium.

A peroxidase gene promoter induced by phytopathogens and methyl jasmonate in transgenic plants

The expression of two closely related peroxidase isogenes, Shpx6a and Shpx6b, of the legume Stylosanthes humilis was studied using isogene-specific reverse transcriptase PCR techniques. Results indicated that transcripts of both genes were rapidly induced following inoculation with the fungal pathogen Colletotrichum gloeosporioides, wounding and treatment with the defense regulator methyl jasmonate (MeJA). In contrast treatment of leaves of S. humilis with abscisic acid (ABA) and salicylic acid (SA) did not induce transcripts of either isogene. A genomic clone containing the Shpx6b gene was isolated and 594 bp of 5' sequence upstream of the translation start was fused in frame to the coding region of the uidA reporter gene and introduced into tobacco. Expression from the Shpx6b promoter in transgenic plants was determined by histochemical staining and quantitative assays of beta-glucuronidase (GUS). In transgenic tobacco, GUS expression was detected in cotyledons, vascular cells of young leaves, anthers, pollen, and the stigma and style. Wounding of the tobacco plants produced very localized GUS staining. Much more extensive staining for GUS was observed following inoculation of tobacco leaves with conidia of the fungal pathogen Cercospora nicotianae and the inoculation of wound sites with mycelium of the Oomycete pathogen Phytophthora parasitica var. nicotianae. Treatment of mature leaves with methyl jasmonate induced GUS activity while treatment with ABA, SA, and H2O2 had no effect. A similar strong induction of GUS activity was measured in young transgenic seedlings germinated on MeJA while some, but much weaker, induction of GUS activity was observed in seedlings treated with SA. The sequence of the promoter contained motifs homologous to putative cis elements in other plant genes responsive to MeJA. The Shpx6b gene is the first plant peroxidase gene shown to be induced by both microbial pathogens and MeJA and its promoter will be useful for investigations of signaling processes during fungal infection and for the expression of foreign gene products at infection sites.