Abstract: Hydrogen peroxide (H2O2) has been implicated in many stress conditions. Control of H2O2 levels is complex and dissection of mechanisms generating and relieving H2O2 stress is difficult, particularly in intact plants. Here the role of the mycorrhizal inoculation in chili plants challenged with Phytophthora capsici was investigated to study the effect on hypersensitive response. In the treatment without mycorrhiza (treatment T3) and with mycorrhiza (considered treatment T4) visible disorders were detected two days after inoculation with P. capsici, but in the next days T3 plants rapidly developed 25% more necrotic lesions on the leaves than T4 plants. Leaf necrosis correlated with H2O2 accumulation and the greater damage observed in T3 plants coincided with larger accumulation of H2O2 after 12 h of inoculation accompanied with an increase in POX (peroxidase) and SOD (superoxide dismutase) activity. T4-infected and mycorrhizal plants exhibited an earlier accumulation of H2O2 starting 6 h after inoculation with lower levels compared to T3 plants. Correlated with observed damage, POX and SOD activity measured in T4 plants indirectly suggest a smaller accumulation of ROS (reactive oxygen species) leading to a decrease in the wounds observed and slightly diminishing the advance of the pathogen. According to these findings, we conclude that mycorrhizal colonization contributes significantly in maintaining the redox balance during oxidative stress, but the exact mechanism is still uncertain.
Abstract: Changes in peroxide levels, the activity of antioxidant enzymes and phenolic compounds were studied in Capsicum annuum L. var. annuum plants inoculated with Phytophthora capsici P1UDC (Galician field isolate). The extent of oxidative stress during the interaction Capsicum–Phytophthora was evidenced, at the final period of interaction, by the at least 87% increase in peroxide levels. Catalase activity showed the biggest changes in the stems by 114% increase, ascorbate peroxidase fell during the interaction, peroxidase levels remained constant from the first hours to the fifth day, and only showed an increment by 54 and 90% in stems and leaves, respectively, during the final period of interaction as phenolic compounds (free and bound fraction). Collectively, these results provide evidence that the interaction Capsicum–Phytophthora is accompanied by a substantial increase in oxidative stress, probably as a direct consequence of a progressive decline in the enzymatic systems responsible for catabolism of active oxygen species.
Abstract: Nine isolates of Phtophthora nicotianae were isolated from infected pepper plants. Their pathogenicity was studied in Capsicum annuum in comparison with P. nicotianae isolates from tomato and tobacco. The pathogenicity test showed that pepper isolates of P. nicotianae are adapted to their host. Banding patterns obtained by RAPD analysis with six oligonucleotide primers revealed polymorphism that grouped the isolates independently of the plant host. The polygenic dendrogram showed that pepper isolates were more similar to tomato isolates than to tobacco isolates. The RAPD bands of 1300 and 1500 bp, detected with primers OPD-01 and OPD-10, respectively, appeared specific to the most pathogenic pepper isolates. The OPK-08-1950 seems specific to the isolates of P. nicotianae from tomato. These results suggest that host specified might occur in P. nicotianae and that may be due to interspecific hybridization events resulting in novel pathogenic behavior.
Abstract: To find out more about the interaction between potato and Phytophthora infestans at the molecular level, we screened for genes induced early after infection using mRNA differential display. Among the twenty cDNA clones recovered in the screen, two were found to represent plant genes whose transcript levels increased during infection of intact plants. These two genes differed strikingly in their response to wounding. Stprx2, a putative peroxidase, responded slowly and transiently to wounding, and its expression pattern was similar to that of gst1, a well-described pathogen-induced gene of potato. The second gene, StNAC, was induced rapidly and strongly after wounding but not systemically. Transcript levels reached a maximum after around 1 h and returned to basal levels after ca. 24 h. StNAC has strong similarity to the ATAF subfamily of NAC domain proteins, a large family of putative transcriptional activators. Arabidopsis ATAF1 and ATAF2 were also shown to be induced by wounding. This implies that the ATAF genes are not merely structurally similar but also share a conserved role in stress responses.
Abstract: A variety of reactions to inoculation with Phytophthora cinnamomi ranging from high susceptibility to moderate resistance were found in 20 ecotypes of Arabidopsis thaliana. P. cinnamomi zoospores successfully colonised both root and leaf tissue of Arabidopsis and sporulation in the form of chlamydospores and sporangia occurred in leaves and roots of each ecotype but the number varied considerably between ecotypes. In the more susceptible ecotypes, colonisation was characterised by rapid intercellular growth and sporulation of the pathogen from 48 h post inoculation. In less susceptible ecotypes, P. cinnamomi was limited to a defined region within tissues. In response to P. cinnamomi infection, several ecotypes expressed active defence responses in both root and leaf tissue. Callose formation was closely associated with lesion restriction as was the production of the reactive oxygen species, hydrogen peroxide. The oxidative burst was not limited to the site of pathogen ingress but also occurred in distant, uninfected tissues. We have characterised an Arabidopsis–P. cinnamomi system that will be useful for further studies of active resistance mechanisms.
Abstract: The sterol fraction of Glycine max (soybean) was found to contain a mixture of 13 major sterols which differed dramatically in composition between seeds and shoots. Typical C4-desmethyl Δ5-sterols, including sitosterol, predominate the sterol mixture of shoots, whereas C4-methyl sterol intermediates, cycloartenol and 24(28)-methylene cycloartanol, accumulate in seeds. The significance of modified sterol profile of shoot compared to seed was relevant to the physiology of Phytophthora sojae, a phytopathogen of soybean shown to be auxotrophic for sterol. Sterols native to the host plant containing a C4-methyl group, such as cycloartenol, were not utilized by the fungus. Alternatively, all Δ5-sterols added to the culture media of P. sojae supported normal growth and promoted viable oospore production. The results demonstrate the importance of sterols in plant–fungal interactions and offer the possibility of bioengineering the phytosterol pathway for resistance to phytopathogens which scavenge specific sterols of the host plant to complete the life cycle.
Abstract: The genus Phytophthora includes many highly destructive plant pathogens. In many Phytophthora species, pathogen dispersal and initiation of plant infection are achieved by motile, biflagellate zoospores that are chemotactically attracted to suitable infection sites. In order to study gene expression in zoospores, we have constructed a cDNA library using mRNA from zoospores of Phytophthora nicotianae. The library was arrayed and screened using probes derived from mycelium or zoospore mRNA. More than 400 clones representing genes preferentially expressed in zoospores were identified and sequenced from the 5prime end of the insert. The expressed sequence tags (ESTs) generated were found to represent 240 genes. The ESTs were compared to sequences in GenBank and in the Phytophthora Genome Consortium database, and classified according to putative function based on homology to known proteins. To further characterize the identified genes, a colony array was created on replicate nylon filters and screened with probes derived from four Phytophthora developmental stages including zoospores, germinating cysts, vegetative mycelium and sporulating hyphae, and from inoculated and uninoculated tobacco seedlings. Data from sequence analysis and colony array screening were compiled into a local database, and searched to identify genes that are preferentially expressed in zoospores for future functional analysis.
Abstract: The RGD tripeptide sequence, a cell adhesion motif present in several extracellular matrix proteins of mammalians, is involved in numerous plant processes. In plant-pathogen interactions, the RGD motif is believed to reduce plant defence responses by disrupting adhesions between the cell wall and plasma membrane. Photoaffinity cross-linking of [125I]-azido-RGD heptapeptide in the presence of purified plasma membrane vesicles of Arabidopsis thaliana led to label incorporation into a single protein with an apparent molecular mass of 80 kDa. Incorporation could be prevented by excess RGD peptides, but also by the IPI-O protein, an RGD-containing protein secreted by the oomycete plant pathogen Phytophthora infestans. Hydrophobic cluster analysis revealed that the RGD motif of IPI-O (positions 53–56) is readily accessible for interactions. Single amino acid mutations in the RGD motif in IPI-O (of Asp56 into Glu or Ala) resulted in the loss of protection of the 80-kDa protein from labelling. Thus, the interaction between the two proteins is mediated through RGD recognition and the 80-kDa RGD-binding protein has the characteristics of a receptor for IPI-O. The IPI-O protein also disrupted cell wall-plasma membrane adhesions in plasmolysed A. thaliana cells, whereas IPI-O proteins mutated in the RGD motif (D56A and D56E) did not.
Abstract: Infection with Likubin bacterium (LB) followed by Phytophthora parasitica increased the mortality of sour orange and pummelo seedlings, and enhanced the P. parasitica-induced root rot in all the four types of citrus tested. The LB-induced enhancement of root infection by P. parasitica was apparent within 1thinsph of exposure to zoospore suspension. The enhancement of P. parasitica-induced root rot was affected by the infection sequence. Inoculation of sour orange seedlings with LB before P. parasitica was more effective in increasing P. parasitica-induced root rot than LB and P. parasitica concomitantly or LB after P. parasitica. Grafting P. parasitica susceptible scions of ponkan (Citrus reticulata) onto P. parasitica-tolerant rootstocks of sour orange greatly increased the susceptibility of rootstocks to P. parasitica. Results also demonstrate the enhancement of LB-induced symptoms by P. parasitica in citrus plants.
Abstract: The plant fungal pathogen Fusarium oxysporum (Fox) is the causal agent of root rot or wilt diseases in several plant species, including crops such as tomato (Solanum lycopersicum), banana (Musa sapientum) and asparagus (Asparagus officinalis). Colonization of plants by Fox leads to the necrosis of the infected tissues, a subsequent collapse of vascular vessels and decay of the plant. Plant resistance to Fox appears to be monogenic or oligogenic depending on the host. Perception of Fox by plants follows the concept of elicitor-induced immune response, which in turn activates several plant defense signaling pathways. Here, we review the Fox-derived elicitors identified so far and the interaction among the different signaling pathways mediating plant resistance to Fox.
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