Nonhost resistance genes and race-specific resistance

Identification and utilization of a sow thistle powdery mildew as a poorly adapted pathogen to dissect post-invasion non-host resistance mechanisms in Arabidopsis

To better dissect non-host resistance against haustorium-forming powdery mildew pathogens, a sow thistle powdery mildew isolate designated Golovinomyces cichoracearum UMSG1 that has largely overcome penetration resistance but is invariably stopped by post-invasion non-host resistance of Arabidopsis thaliana was identified. The post-invasion non-host resistance is mainly manifested as the formation of a callosic encasement of the haustorial complex (EHC) and hypersensitive response (HR), which appears to be controlled by both salicylic acid (SA)-dependent and SA-independent defence pathways, as supported by the susceptibility of the pad4/sid2 double mutant to the pathogen. While the broad-spectrum resistance protein RPW8.2 enhances post-penetration resistance against G. cichoracearum UCSC1, a well-adapted powdery mildew pathogen, RPW8.2, is dispensable for post-penetration resistance against G. cichoracearum UMSG1, and its specific targeting to the extrahaustorial membrane is physically blocked by the EHC, resulting in HR cell death. Taken together, the present work suggests an evolutionary scenario for the Arabidopsis-powdery mildew interaction: EHC formation is a conserved subcellular defence evolved in plants against haustorial invasion; well-adapted powdery mildew has evolved the ability to suppress EHC formation for parasitic growth and reproduction; RPW8.2 has evolved to enhance EHC formation, thereby conferring haustorium-targeted, broad-spectrum resistance at the post-invasion stage.

A Promoter from Pea Gene DRR206 Is Suitable to Regulate an Elicitor-Coding Gene and Develop Disease Resistance

ABSTRACT Plant nonhost disease resistance is characterized by the induction of multiple defense genes. The pea DRR206 gene is induced following inoculation with pathogens and treatment with abiotic agents, and moderately induced by wounding. A deletion series of DRR206 promoter segments was fused with the beta-glucuronidase (GUS) reporter gene and transiently transferred to tobacco, potato, and pea. GUS activity revealed that two upstream regions of the DRR206 promoter were particularly important for activation in the three plant species. Putative cis regulatory elements within the DRR206 promoter included a wound/pathogen- inducible box (W/P-box) and a WRKY box (W-box). Gel shift assays with nuclear extracts from treated and untreated tissue with the W/P-box revealed both similar and unique protein-DNA complexes from pea, potato, and tobacco. Tobacco was stably transformed with gene constructs of the DRR206 promoter fused with a DNase elicitor gene from Fusarium solani f. sp. phaseoli, FsphDNase. Pathogenicity tests indicated that the FsphDNase elicitor conferred resistance against Pseudomonas syringae pv. tabaci and Alternaria alternata in tobacco. Transgenic potatoes showed some sensitivity to the FsphDNase gene providing less protection against Phytophthora infestans. Thus, the elicitor-coding gene, FsphDNase, is capable of generating resistance in a heterologous plant system (tobacco) when fused with defined regions of the pea DRR206 promoter.

Host-parasite interactions: elicitation of defense responses in plants with chitosan

The plant's defense response against pathogens can be elicited by numerous external signals. Plant pathogens known to be incompatible on a given plant species can elicit strong disease resistance responses, whereas an adapted compatible pathogen generates a weaker response and thus can more readily infect the plant tissue. The plant's response can be manipulated genetically by the transfer of "R" genes (single dominant genes for race-specific disease resistance) or by treatment with elicitors such as chitosan. Both of these manipulations can result in the rapid activation of a subset of genes called PR (pathogenesis-related) genes, generally regarded as the genes that functionally develop disease resistance. There appear to be multiple modes by which chitosan can increase PR gene function, including activating cell surface or membrane receptors and internal effects on the plant's DNA conformation that in turn influence gene transcription. A novel strategy for controlling PR gene expression proposes to transform plants with a chitosan-inducible gene promoter linked in line with a single signal gene capable of rapid, intense induction of an entire set of PR genes, thereby enabling the control of disease resistance by external chitosan applications.

Analysis of host-induced response in the rice blast fungus Magnaporthe grisea using two-dimensional polyacrylamide gel electrophoresis

Two-dimensional (2-D) polyacrylamide gel electrophoresis of proteins was used to study the response of the rice blast fungus to extracts prepared from resistant and susceptible rice cultivars. A protein of molecular mass 31 kDa was induced by a susceptible host extract, while the fungus exposed to extract from the resistant cultivar and the untreated samples did not show the presence of this protein. Levels of this 31 kDa protein increased 30-fold, 72 h after treatment with plant extracts, with the concomitant appearance of at least sixteen other novel proteins. Fungus treated with extracts of resistant host or the untreated samples did not show any of these proteins while the proteins specific to different growth stages appeared as expected. Analysis of the extracellular samples showed induction of a 17 kDa protein after 72 h in the culture treated with susceptible host extract. Since the resistant host extract does not cause induction of any protein it is likely that the proteins induced in response to the susceptible host are expressed during the disease process and/or its establishment. Our study demonstrates usefulness of 2-D analysis in understanding host-pathogen interactions.