Showing papers on "Plant disease resistance published in 2002"

Tobacco Rar1, EDS1 and NPR1/NIM1 like genes are required for N‐mediated resistance to tobacco mosaic virus

Abstract: The tobacco N gene confers resistance to tobacco mosaic virus (TMV) and encodes a Toll-interleukin-1 receptor/nucleotide binding site/leucine-rich repeat (TIR-NBS-LRR) class protein. We have developed and used a tobacco rattle virus (TRV) based virus induced gene silencing (VIGS) system to investigate the role of tobacco candidate genes in the N-mediated signalling pathway. To accomplish this we generated transgenic Nicotiana benthamiana containing the tobacco N gene. The transgenic lines exhibit hypersensitive response (HR) to TMV and restrict virus spread to the inoculated site. This demonstrates that the tobacco N gene can confer resistance to TMV in heterologous N. benthamiana. We have used this line to study the role of tobacco Rar1-, EDS1-, and NPR1/NIM1- like genes in N-mediated resistance to TMV using a TRV based VIGS approach. Our VIGS analysis suggests that these genes are required for N function. EDS1-like gene requirement for the N function suggests that EDS1 could be a common component of bacterial, fungal and viral resistance signalling mediated by the TIR-NBS-LRR class of resistance proteins. Requirement of Rar1- like gene for N-mediated resistance to TMV and some powdery mildew resistance genes in barley provide the first example of converging points in the disease resistance signalling pathways mediated by TIR-NBS-LRR and CC-NBS-LRR proteins. The TRV based VIGS approach as described here to study N-mediated resistance signalling will be useful for the analysis of not only disease resistance signalling pathways but also of other signalling pathways in genetically intractable plant systems.

The RAR1 interactor SGT1, an essential component of R gene-triggered disease resistance

Abstract: Plant disease resistance (R) genes trigger innate immune responses upon pathogen attack. RAR1 is an early convergence point in a signaling pathway engaged by multiple R genes. Here, we show that RAR1 interacts with plant orthologs of the yeast protein SGT1, an essential regulator in the cell cycle. Silencing the barley gene Sgt1 reveals its role in R gene-triggered, Rar1-dependent disease resistance. SGT1 associates with SKP1 and CUL1, subunits of the SCF (Skp1-Cullin-F-box) ubiquitin ligase complex. Furthermore, the RAR1-SGT1 complex also interacts with two COP9 signalosome components. The interactions among RAR1, SGT1, SCF, and signalosome subunits indicate a link between disease resistance and ubiquitination.

EDS5, an Essential Component of Salicylic Acid–Dependent Signaling for Disease Resistance in Arabidopsis, Is a Member of the MATE Transporter Family

Abstract: The eds5 mutant of Arabidopsis (earlier named sid1) was shown previously to accumulate very little salicylic acid and PR-1 transcript after pathogen inoculation and to be hypersusceptible to pathogens. We have isolated EDS5 by positional cloning and show that it encodes a protein with a predicted series of nine to 11 membrane-spanning domains and a coil domain at the N terminus. EDS5 is homologous with members of the MATE (multidrug and toxin extrusion) transporter family. EDS5 expression is very low in unstressed plants and strongly induced by pathogens and UV-C light. The transcript starts to accumulate 2 hr after inoculation of Arabidopsis with an avirulent strain of Pseudomonas syringae or UV-C light exposure, and it stays induced for ∼2 days. EDS5 also is expressed after treatments with salicylic acid, indicating a possible positive feedback regulation. EDS5 expression after infection by certain pathogens as well as after UV-C light exposure depends on the pathogen response proteins EDS1, PAD4, and NDR1, indicating that the signal transduction pathways after UV-C light exposure and pathogen inoculation share common elements.

The population genetics of plant pathogens and breeding strategies for durable resistance

Abstract: The durability of disease resistance is affected by the evolutionary potential of the pathogen population Pathogens with a high evolutionary potential are more likely to overcome genetic resistance than pathogens with a low evolutionary potential We will propose a set of guidelines to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure Under our model of pathogen evolution, the two most important parameters to consider are reproduction/mating system and gene/genotype flow Pathogens that pose the greatest risk of breaking down resistance genes are those that possess a mixed reproduction system, with at least one sexual cycle per growing season and asexual reproduction during the epidemic phase, and a high potential for gene flow The lowest risk pathogens are those with strict asexual reproduction and low potential for gene flow We will present examples of high- and low-risk pathogens Knowledge of the population genetic structure of the pathogen may offer insight into the best breeding strategy for durable resistance We will present broad guidelines suggesting a rational method for breeding durable resistance according to the population genetics of the pathogen

Overexpression of polyphenol oxidase in transgenic tomato plants results in enhanced bacterial disease resistance.

Abstract: Polyphenol oxidases (PPOs; EC 1.10.3.2 or EC 1.14.18.1) catalyzing the oxygen-dependent oxidation of phenols to quinones are ubiquitous among angiosperms and assumed to be involved in plant defense against pests and pathogens. In order to investigate the role of PPO in plant disease resistance, we made transgenic tomato (Lycopersiconesculentum Mill. cv. Money Maker) plants that overexpressed a potato (Solanumtuberosum L.) PPO cDNA under control of the cauliflower mosaic virus 35S promoter. The transgenic plants expressed up to 30-fold increases in PPO transcripts and 5- to 10-fold increases in PPO activity and immunodetectable PPO. As expected, these PPO-overexpressing transgenic plants oxidized the endogenous phenolic substrate pool at a higher rate than control plants. Three independent transgenic lines were selected to assess their interaction with the bacterial pathogen Pseudomonassyringae pv. tomato. The PPO-overexpressing tomato plants exhibited a great increase in resistance to P.syringae. Compared with control plants, these transgenic lines showed less severity of disease symptoms, with over 15-fold fewer lesions, and strong inhibition of bacterial growth, with over 100-fold reduction of bacterial population in the infected leaves. These results demonstrate the importance of PPO-mediated phenolic oxidation in restricting plant disease development.

Ubiquitin ligase-associated protein SGT1 is required for host and nonhost disease resistance in plants.

Abstract: Homologues of the yeast ubiquitin ligase-associated protein SGT1 are required for disease resistance in plants mediated by nucleotide-binding site/leucine-rich repeat (NBS-LRR) proteins. Here, by silencing SGT1 in Nicotiana benthamiana, we extend these findings and demonstrate that SGT1 has an unexpectedly general role in disease resistance. It is required for resistance responses mediated by NBS-LRR and other R proteins in which pathogen-derived elicitors are recognized either inside or outside the host plant cell. A requirement also exists for SGT1 in nonhost resistance in which all known members of a host species are resistant against every characterized isolate of a pathogen. Our findings show that silencing SGT1 affects diverse types of disease resistance in plants and support the idea that R protein-mediated and nonhost resistance may involve similar mechanisms.

Regulatory Role of SGT1 in Early R Gene-Mediated Plant Defenses

Abstract: Animal SGT1 is a component of Skp1-Cullin-F-box protein (SCF) ubiquitin ligases that target regulatory proteins for degradation. Mutations in one (SGT1b) of two highly homologous Arabidopsis SGT1 genes disable early plant defenses conferred by multiple resistance (R) genes. Loss of SGT1b function in resistance is not compensated for by SGT1a. R genes differ in their requirements for SGT1b and a second resistance signaling gene, RAR1, that was previously implicated as an SGT1 interactor. Moreover, SGT1b and RAR1 contribute additively to RPP5-mediated pathogen recognition. These data imply both operationally distinct and cooperative functions of SGT1 and RAR1 in plant disease resistance.

A tomato cysteine protease required for Cf-2-dependent disease resistance and suppression of autonecrosis.

Abstract: Little is known of how plant disease resistance (R) proteins recognize pathogens and activate plant defenses. Rcr3 is specifically required for the function of Cf-2, a Lycopersicon pimpinellifolium gene bred into cultivated tomato (Lycopersicon esculentum) for resistance to Cladosporium fulvum. Rcr3 encodes a secreted papain-like cysteine endoprotease. Genetic analysis shows Rcr3 is allelic to the L. pimpinellifolium Ne gene, which suppresses the Cf-2-dependent autonecrosis conditioned by its L. esculentum allele, ne (necrosis). Rcr3 alleles from these two species encode proteins that differ by only seven amino acids. Possible roles of Rcr3 in Cf-2-dependent defense and autonecrosis are discussed.

The barley stem rust-resistance gene Rpg1 is a novel disease-resistance gene with homology to receptor kinases

Abstract: Stem rust caused by Puccinia graminis f. sp. tritici was among the most devastating diseases of barley in the northern Great Plains of the U.S. and Canada before the deployment of the stem rust-resistance gene Rpg1 in 1942. Since then, Rpg1 has provided durable protection against stem rust losses in widely grown barley cultivars (cvs.). Extensive efforts to clone Rpg1 by synteny with rice provided excellent flanking markers but failed to yield the gene because it does not seem to exist in rice. Here we report the map-based cloning and characterization of Rpg1. A high-resolution genetic map constructed with 8,518 gametes and a 330-kb bacterial artificial chromosome contig physical map positioned the gene between two crossovers approximately 0.21 centimorgan and 110 kb apart. The region including Rpg1 was searched for potential candidate genes by sequencing low-copy probes. Two receptor kinase-like genes were identified. The candidate gene alleles were sequenced from resistant and susceptible cvs. Only one of the candidate genes showed a pattern of apparently functional gene structure in the resistant cvs. and defective gene structure in the susceptible cvs. identifying it as the Rpg1 gene. Rpg1 encodes a receptor kinase-like protein with two tandem protein kinase domains, a novel structure for a plant disease-resistance gene. Thus, it may represent a new class of plant resistance genes.

Potentiation of Developmentally Regulated Plant Defense Response by AtWRKY18, a Pathogen-Induced Arabidopsis Transcription Factor

Abstract: AtWRKY18 is a pathogen- and salicylic acid-induced Arabidopsis transcription factor containing the plant-specific WRKY zinc finger DNA-binding motif. In the present study, we have transformed Arabidopsis plants with AtWRKY18 under control of the cauliflower mosaic virus 35S promoter. Surprisingly, transgenic plants expressing high levels of AtWRKY18 were stunted in growth. When expressed at moderate levels, AtWRKY18 potentiated developmentally regulated defense responses in transgenic plants without causing substantial negative effects on plant growth. As they grew from seedling to mature stages, transgenic AtWRKY18 plant showed marked increase in the expression of pathogenesis-related genes and resistance to the bacterial pathogen Pseudomonas syringae, whereas wild-type plants exhibited little enhancement in these defense responses. Potentiation of developmentally regulated defense responses by AtWRKY18 was not associated with enhanced biosynthesis of salicylic acid but required the disease resistance regulatory protein NPR1/NIM1. Thus, AtWRKY18 can positively modulate defense-related gene expression and disease resistance. To study the regulated expression of AtWRKY18, we have identified a cluster of WRKY binding sites in the promoter of the gene and demonstrated that they acted as negative regulatory elements for the inducible expression of AtWRKY18. These negative cis-acting elements may prevent overexpression of AtWRKY18 during the activation of plant defense responses that could be detrimental to plant growth as inferred from the transgenic plants ectopically expressing the transgene.

Tomato Transcription Factors Pti4, Pti5, and Pti6 Activate Defense Responses When Expressed in Arabidopsis

Abstract: The Pti4, Pti5, and Pti6 proteins from tomato were identified based on their interaction with the product of the Pto disease resistance gene, a Ser-Thr protein kinase. They belong to the ethylene-response factor (ERF) family of plantunique transcription factors and bind specifically to the GCC-box cis element present in the promoters of many pathogenesis-related ( PR ) genes. Here, we show that these tomato ERFs are localized to the nucleus and function in vivo as transcription activators that regulate the expression of GCC box‐containing PR genes. Expression of Pti4 , Pti5 , or Pti 6 in Arabidopsis activated the expression of the salicylic acid‐regulated genes PR1 and PR2 . Expression of jasmonic acid‐ and ethylene-regulated genes, such as PR3 , PR4 , PDF1.2 , and Thi2.1 , was affected differently by each of the three tomato ERFs, with Arabidopsis -Pti4 plants having very high levels of PDF1.2 transcripts. Exogenous application of salicylic acid to Arabidopsis- Pti4 plants suppressed the increased expression of PDF1.2 but further stimulated PR1 expression. Arabidopsis plants expressing Pti4 displayed increased resistance to the fungal pathogen Erysiphe orontii and increased tolerance to the bacterial pathogen Pseudomonas syringae pv tomato . These results indicate that Pti4, Pti5, and Pti6 activate the expression of a wide array of PR genes and play important and distinct roles in plant defense.

PMR6, a Pectate Lyase–Like Gene Required for Powdery Mildew Susceptibility in Arabidopsis

Abstract: The plant genes required for the growth and reproduction of plant pathogens are largely unknown. In an effort to identify these genes, we isolated Arabidopsis mutants that do not support the normal growth of the powdery mildew pathogen Erysiphe cichoracearum . Here, we report on the cloning and characterization of one of these genes, PMR6 . PMR6 encodes a pectate lyase–like protein with a novel C-terminal domain. Consistent with its predicted gene function, mutations in PMR6 alter the composition of the plant cell wall, as shown by Fourier transform infrared spectroscopy. pmr6 -mediated resistance requires neither salicylic acid nor the ability to perceive jasmonic acid or ethylene, indicating that the resistance mechanism does not require the activation of well-described defense pathways. Thus, pmr6 resistance represents a novel form of disease resistance based on the loss of a gene required during a compatible interaction rather than the activation of known host defense pathways.

Durability of resistance against fungal, bacterial and viral pathogens; present situation

Abstract: In evolutionary sense no resistance lasts forever. The durability of a resistance can be seen as a quantitative trait; resistances may range from not durable at all (ephemeral, or transient) to highly durable. Ephemeral resistance occurs against fungi and bacteria with a narrow host range, specialists. It is characterised by a hypersensitive reaction (HR), major gene inheritance and many resistance genes, which often occur in multiple allelic series and/or complex loci. These resistance genes (alleles) interact in a gene-for-gene way with a virulence genes (alleles) in the pathogen to give an incompatible reaction. The pathogen neutralises the effect of the resistance gene by a loss mutation in the corresponding avirulence allele. The incompatible reaction is not elicited any more and the pathogenicity is restored. The pathogens can afford the loss of many avirulences without loss of fitness. Durable resistance against specialised fungi and bacteria is often quantitative and based upon the additive effects of some to several genes, the resulting resistance being of another nature than the hypersensitive reaction. This quantitative resistance is present to nearly all pathogens at low to fair levels in most commercial cultivars. Durable resistance of a monogenic nature occurs too and is usually of a non-HR type. Resistance against fungi and bacteria with a wide host range, generalists, is usually quantitative and durable. Resistances against viruses are often fairly durable, even if these are based on monogenic, race-specific, HR resistances. The level of specialisation does not seem to be associated with the durability of resistance.

Epigenetic variation in Arabidopsis disease resistance

Abstract: Plant pathogen resistance is mediated by a large repertoire of resistance (R) genes, which are often clustered in the genome and show a high degree of genetic variation. Here, we show that an Arabidopsis thaliana R-gene cluster is also subject to epigenetic variation. We describe a heritable but metastable epigenetic variant bal that overexpresses the R-like gene At4g16890 from a gene cluster on Chromosome 4. The bal variant and Arabidopsis transgenics overexpressing the At4g16890 gene are dwarfed and constitutively activate the salicylic acid (SA)-dependent defense response pathway. Overexpression of a related R-like gene also occurs in the ssi1 (suppressor of SA insensitivity 1) background, suggesting that ssi1 is mechanistically related to bal.

A R2R3-MYB gene, AtMYB30, acts as a positive regulator of the hypersensitive cell death program in plants in response to pathogen attack.

Abstract: Hypersensitive response (HR) is a programmed cell death that is commonly associated with disease resistance in plants. Among the different HR-related early induced genes, the AtMYB30 gene is specifically, rapidly, and transiently expressed during incompatible interactions between Arabidopsis and bacterial pathogens. Its expression was also shown to be deregulated in Arabidopsis mutants affected in the control of cell death initiation. Here, we demonstrate that overexpression in Arabidopsis and tobacco of AtMYB30 (i) accelerates and intensifies the appearance of the HR in response to different avirulent bacterial pathogens, (ii) causes HR-like responses to virulent strains, and (iii) increases resistance against different bacterial pathogens, and a virulent biotrophic fungal pathogen, Cercospora nicotianae. In antisense AtMYB30 Arabidopsis lines, HR cell death is strongly decreased or suppressed in response to avirulent bacterial strains, resistance against different bacterial pathogens decreased, and the expression of HR- and defense-related genes was altered. Taken together, these results strongly suggest that AtMYB30 is a positive regulator of hypersensitive cell death.

Signalling in Rhizobacteria-Induced Systemic Resistance in Arabidopsis thaliana

Abstract: To protect themselves from disease, plants have evolved sophisticated defence mechanisms in which the signal molecules salicylic acid, jasmonic acid and ethylene often play crucial roles. Elucidation of signalling pathways controlling disease resistance is a major objective in research on plant-pathogen interactions. The capacity of a plant to develop a broad spectrum, systemic acquired resistance (SAR) after primary infection with a necrotizing pathogen is well-known and its signal transduction pathway extensively studied. Plants of which the roots have been colonized by specific strains of non-pathogenic fluorescent Pseudomonas spp. develop a phenotypically similar form of protection that is called rhizobacteria-mediated induced systemic resistance (ISR). In contrast to pathogen-induced SAR, which is regulated by salicylic acid, rhizobacteria-mediated ISR is controlled by a signalling pathway in which jasmonic acid and ethylene play key roles. In the past eight years, the model plant species Arabidopsis thaliana was explored to study the molecular basis of rhizobacteria-mediated ISR. Here we review current knowledge of the signal transduction steps involved in the ISR pathway that leads from recognition of the rhizobacteria in the roots to systemic expression of broad-spectrum disease resistance in aboveground foliar tissues.

RAR1 and NDR1 contribute quantitatively to disease resistance in Arabidopsis, and their relative contributions are dependent on the R gene assayed.

Abstract: Plant disease resistance (R) genes mediate specific pathogen recognition, leading to a successful immune response. Downstream responses include ion fluxes, an oxidative burst, transcriptional reprogramming, and, in many cases, hypersensitive cell death at the infection site. We used a transgenic Arabidopsis line carrying the bacterial avirulence gene avrRpm1 under the control of a steroid-inducible promoter to select for mutations in genes required for RPM1-mediated recognition and signal transduction. We identified an allelic series of eight mutants that also were allelic to the previously identified pbs2 mutation. Positional cloning revealed this gene to be AtRAR1, the Arabidopsis ortholog of barley RAR1, a known mediator of R function. AtRAR1 is required for both full hypersensitive cell death and complete disease resistance mediated by many, but not all, tested R genes. Double mutant analysis of Atrar1 in combination with the R signal intermediate ndr1 suggests that AtRAR1 and NDR1 can operate in both linear and parallel signaling events, depending on the R gene function triggered. In Atrar1 null plants, the levels of RPM1-myc are reduced severely, suggesting that AtRAR1 may regulate R protein stability or accumulation.

Wheat Stripe Rust Epidemics and Races of Puccinia striiformis f. sp. tritici in the United States in 2000.

Abstract: Wheat stripe rust, caused by Puccinia striiformis f. sp. tritici, is most destructive in the western United States and has become increasingly important in the south-central states. The disease has been monitored by collaborators through field surveys and in disease nurseries throughout the United States. In the year 2000, stripe rust occurred in more than 20 states throughout the country, which was the most widespread occurrence in recorded history. Although fungicide applications in many states reduced yield losses, the disease caused multimillion dollar losses in the United States, especially in Arkansas and California. One of the prevalent cultivars, RSI 5, had a yield loss of about 50% in the Sacramento-San Joaquin Delta region of California. In the Pacific Northwest, wheat losses due to stripe rust were minimal because cultivars with durable resistance were widely grown and the weather in May 2000 was not favorable for the disease. To identify races of the pathogen, stripe rust collections from 20 states across the United States were analyzed on 20 wheat differential cultivars, including Clement (Yr9, YrCle), Compair (Yr8, Yr19), and the Yr8 and Yr9 near-isogenic lines. In 2000, 21 previously identified races and 21 new races were identified. Of the 21 new races, 8 were pathotypes with combinations of virulences previously known to exist in the United States, and 13 had virulences to one or more of the lines Yr8, Yr9, Clement, or Compair. This is the first report of virulence to Yr8 and Yr9 in the United States. Most of the new races were also virulent on Express. Races that are virulent on Express have been identified in California since 1998. The races virulent on Yr8, Yr9, and Express were widely distributed in California and states east of the Rocky Mountains in 2000. The epidemic in 2000 demonstrates that increased efforts to breed for stripe rust resistance are needed in California, the south-central states, and some other states in the Great Plains. Diversification of resistance genes and use of durable resistance should prevent large-scale and severe epidemics.

Genetic mapping of a dominant gene conferring resistance to cassava mosaic disease

Abstract: Cassava mosaic disease (CMD) is the most-important disease of cassava (Manihot esculenta) in Africa, and is a potential threat to Latin American (LA) cassava production. Although this viral disease is still unknown in LA, its vector – the whitefly – has recently been found. The disease is best controlled through host-plant resistance, which was first found in third backcross derivatives of an interspecific cross between cassava and Manihot glaziovii, and is thought to be polygenic. Recently, high levels of resistance were also found in several Nigerian cassava landraces. Classical genetic analysis and molecular genetic-mapping of the landraces showed that a major dominant gene confers this resistance. Bulk segregant analysis (BSA) was used to quickly identify a simple sequence repeat (SSR) marker linked to the CMD-resistance gene. The marker, SSRY28, is located on linkage group R of the male-parent-derived molecular genetic map. The gene, designated as CMD2, is flanked by the SSR and RFLP marker GY1 at 9 and 8 cM, respectively. To our knowledge, this is the first report of qualitative virus resistance in cassava, and of molecular markers that tag CMD resistance in cassava. We discuss the use of markers linked to CMD2 for marker-assisted breeding of CMD resistance in Latin America and for increasing the cost-effectiveness of resistance breeding in Africa.

An EDS1 orthologue is required for N‐mediated resistance against tobacco mosaic virus

Abstract: In Arabidopsis, EDS1 is essential for disease resistance conferred by a structural subset of resistance (R) proteins containing a nucleotide-binding site, leucine-rich-repeats and amino-terminal similarity to animal Toll and Interleukin-1 (so-called TIR-NBS-LRR proteins). EDS1 is not required by NBS-LRR proteins that possess an amino-terminal coiled-coil motif (CC-NBS-LRR proteins). Using virus-induced gene silencing (VIGS) of a Nicotiana benthaminana EDS1 orthologue, we investigated the role of EDS1 in resistance specified by structurally distinct R genes in transgenic N. benthamiana. Resistance against tobacco mosaic virus mediated by tobacco N, a TIR-NBS-LRR protein, was EDS1-dependent. Two other R proteins, Pto (a protein kinase), and Rx (a CC-NBS-LRR protein) recognizing, respectively, a bacterial and viral pathogen did not require EDS1. These data, together with the finding that expression of N. benthamiana and Arabidopsis EDS1 mRNAs are similarly regulated, lead us to conclude that recruitment of EDS1 by TIR-NBS-LRR proteins is evolutionarily conserved between dicotyledenous plant species in resistance against bacterial, oomycete and viral pathogens. We further demonstrate that VIGS is a useful approach to dissect resistance signaling pathways in a genetically intractable plant species.

Genetic variation for disease resistance and tolerance among Arabidopsis thaliana accessions

Abstract: Pathogens can be an important selective agent in plant evolution because they can severely reduce plant fitness and growth. However, the role of pathogen selection on plant evolution depends on the extent of genetic variation for resistance traits and their covariance with host fitness. Although it is usually assumed that resistance traits will covary with plant fitness, this assumption has not been tested rigorously in plant-pathogen interactions. Many plant species are tolerant to herbivores, decoupling the relationship between resistance and fitness. Tolerance to pathogens can reduce selection for resistance and alter the effect of pathogens on plant evolution. In this study, we measured three components of Arabidopsis thaliana resistance (pathogen growth, disease symptoms, and host fitness) to the bacteria Pseudomonas syringae and investigated their covariation to determine the relative importance of resistance and tolerance. We observed extensive quantitative variation in the severity of disease symptoms, the bacterial population size, and the effect of infection on host fitness among 19 accessions of A. thaliana infected with P. syringae. The severity of disease symptoms was strongly and positively correlated with bacterial population size. Although the average fitness of infected plants was smaller than noninfected plants, we found no correlation between the bacterial growth or symptoms expressed by different accessions of A. thaliana and their relative fitness after infection. These results indicate that the accessions studied vary in tolerance to P. syringae, reducing the strength of selection on resistance traits, and that symptoms and bacterial growth are not good predictors of host fitness.

Potentially durable resistance mechanisms in plants to specialised fungal pathogens

Abstract: Summary Specialised plant pathogens are in many ways adapted to exploit their host plants. Infective propagules should reach the appropriate plant tissue, gain access to the tissue and negate or suppress various kinds of constitutive and inducible resistance mechanisms. The resistance type most frequently deployed in plant breeding is the racespecific resistance, where a hypersensitive response of plant tissue is elicited by an avirulence factor produced by the pathogen. The great disadvantage of this type of resistance is, that it is often ephemeral. Detailed screening of germplasm may result in the discovery of alternative defence mechanisms not associated with hypersensitivity, that may be durable. Avoidance mechanisms may reduce the chance of infection. Upright plant habit has been reported to decrease spore deposition in cereals. Crop architecture may also affect aspects such as humidity and aeration in the crop, and hence the chances for successful infection by pathogen propagules. Other examples of avoidance are leaf surface properties that interfere with leaf wettability, germ tube orientation and finding of stomata to enter the leaf. Stomata in some accessions of Hordeum chilense are excessively covered by cuticular wax that prevent rust fungal germ tubes from perceiving the stomata, resulting in failure of penetration of the pathogen into the leaf. There is evidence that in compatible host species, biotrophic pathogenic fungi induce basic compatibility by suppressing defence mechanisms. Failure of this induction results in abortion of the infection attempt. Several cases of apparently durable resistance are discussed that are based on failure of haustorium formation, and are not associated with hypersensitivity. They may represent cases where the pathogen has problems in establishing basic compatibility.

Expression of the antiapoptotic baculovirus p35 gene in tomato blocks programmed cell death and provides broad-spectrum resistance to disease

Abstract: The sphinganine analog mycotoxin, AAL-toxin, induces a death process in plant and animal cells that shows apoptotic morphology. In nature, the AAL-toxin is the primary determinant of the Alternaria stem canker disease of tomato, thus linking apoptosis to this disease caused by Alternaria alternata f. sp. lycopersici. The product of the baculovirus p35 gene is a specific inhibitor of a class of cysteine proteases termed caspases, and naturally functions in infected insects. Transgenic tomato plants bearing the p35 gene were protected against AAL-toxin-induced death and pathogen infection. Resistance to the toxin and pathogen co-segregated with the expression of the p35 gene through the T3 generation, as did resistance to A. alternata, Colletotrichum coccodes, and Pseudomonas syringae pv. tomato. The p35 gene, stably transformed into tomato roots by Agrobacterium rhizogenes, protected roots against a 30-fold greater concentration of AAL-toxin than control roots tolerated. Transgenic expression of a p35 binding site mutant (DQMD to DRIL), inactive against animal caspases-3, did not protect against AAL-toxin. These results indicate that plants possess a protease with substrate-site specificity that is functionally equivalent to certain animal caspases. A biological conclusion is that diverse plant pathogens co-opt apoptosis during infection, and that transgenic modification of pathways regulating programmed cell death in plants is a potential strategy for engineering broad-spectrum disease resistance in plants.

A small GTP-binding host protein is required for entry of powdery mildew fungus into epidermal cells of barley.

Abstract: Small GTP-binding proteins such as those from the RAC family are cytosolic signal transduction proteins that often are involved in processing of extracellular stimuli. Plant RAC proteins are implicated in regulation of plant cell architecture, secondary wall formation, meristem signaling, and defense against pathogens. We isolated a RacB homolog from barley (Hordeum vulgare) to study its role in resistance to the barley powdery mildew fungus (Blumeria graminis f.sp. hordei). RacB was constitutively expressed in the barley epidermis and its expression level was not strongly influenced by inoculation with B. graminis. However, after biolistic bombardment of barley leaf segments with RacB-double-stranded RNA, sequencespecific RNA interference with RacB function inhibited fungal haustorium establishment in a cell-autonomous and genotype-specific manner. Mutants compromised in function of the Mlo wild-type gene and the Ror1 gene (genotype mlo5 ror1) that are moderately susceptible to B. graminis showed no alteration in powdery mildew resistance upon RacB-specific RNA interference. Thus, the phenotype, induced by RacB-specific RNA interference, was apparently dependent on the same processes as mlo5-mediated broad resistance, which is suppressed by ror1. We conclude that an RAC small GTP-binding protein is required for successful fungal haustorium establishment and that this function may be linked to MLO-associated functions.

Chromosomal location of genes for resistance to powdery mildew in common wheat (Triticum aestivum L. em Thell.) 7. Gene Pm29 in line Pova

Abstract: Chromosomal localization and linkage mapping of a powdery mildewresistance gene were conducted in the resistant wheat line Pova, derivedfrom a Triticum aestivum cv. Poros-Aegilops ovata-alien additionline. Monosomic analysis revealed that a major dominant gene was locatedon chromosome 7D. This gene possessed a distinct disease response patternagainst a differential set of Blumeria graminis tritici isolates andsegregated independently from resistance gene Pm19 also located onwheat chromosome 7D. Molecular genetic analysis showed that theresistance gene in Pova was specifically located on the long arm ofchromosome 7D closely linked to one RFLP and three AFLP markers. It isproposed that the new gene be designated Pm29.

Identification of resistance gene analogs linked to a powdery mildew resistance locus in grapevine.

Abstract: Oligonucleotide primers, designed to conserved regions of nucleotide binding site (NBS) motifs within previously cloned pathogen resistance genes, were used to amplify resistance gene analogs (RGAs) from grapevine. Twenty eight unique grapevine RGA sequences were identified and subdivided into 22 groups on the basis of nucleic acid sequence-identity of approximately 70% or greater. Representatives from each group were used in a bulked segregant analysis strategy to screen for restriction fragment length polymorphisms linked to the powdery mildew resistance locus, Run1, introgressed into Vitis vinifera L. from the wild grape species Muscadinia rotundifolia. Three RGA markers were found to be tightly linked to the Run1 locus. Of these markers, two (GLP1-12 and MHD145) cosegregated with the resistance phenotype in 167 progeny tested, whereas the third marker (MHD98) was mapped to a position 2.4 cM from the Run1 locus. The results demonstrate the usefulness of RGA sequences, when used in combination with bulked segregant analysis, to rapidly generate markers tightly linked to resistance loci in crop species.

Plant Biotechnology: The Genetic Manipulation of Plants

Abstract: Introduction 1. Plant genomes - the organisation and expression of plant genes 2. Plant tissue culture 3. Techniques for plant transformation 4. Binary vectors for plant transformation 5. The genetic manipulation of herbicide resistance 6. The genetic manipulation of pest resistance 7. Plant disease resistance 8. Reducing the effects of viral diseases 9. Strategies for stress tolerance 10. The improvement of crop yield and quality 11. Molecular farming / "pharming" 12. Future prospects for GM crops

An ankyrin repeat-containing protein plays a role in both disease resistance and antioxidation metabolism.

Abstract: Summary The Arabidopsis ankyrin repeat-containing protein AKR2 was identified as a GF14λ-interacting protein in a yeast two-hybrid screening (GF14λ is a 14-3-3 protein). Reduced expression of AKR2 by using the antisense technique results in small necrotic areas in leaves accompanied by higher production of H2O2, similar to the hypersensitive response to pathogen infection in plant disease resistance. Transcripts of genes encoding pathogen-induced protein PR-1 (pathogen-related protein 1) and stress-responsive protein GST6 (glutathione S-transferase 6) are increased in antisense plants. The resistance to a bacterial pathogen infection was also increased by at least 10-fold in antisense plants. AKR2 also interacts with another GF14λ-interacting protein, the ascorbate peroxidase 3 that scavenges H2O2 in plant cells. These data suggest that AKR2 may be a negative regulator of PR-1 expression, and is probably involved in the regulation of antioxidation metabolism that is shared by both disease resistance and stress responses.