Showing papers on "Plant disease resistance published in 2003"

SNARE-protein-mediated disease resistance at the plant cell wall

Abstract: Failure of pathogenic fungi to breach the plant cell wall constitutes a major component of immunity of non-host plant species--species outside the pathogen host range--and accounts for a proportion of aborted infection attempts on 'susceptible' host plants (basal resistance). Neither form of penetration resistance is understood at the molecular level. We developed a screen for penetration (pen) mutants of Arabidopsis, which are disabled in non-host penetration resistance against barley powdery mildew, Blumeria graminis f. sp. hordei, and we isolated the PEN1 gene. We also isolated barley ROR2 (ref. 2), which is required for basal penetration resistance against B. g. hordei. The genes encode functionally homologous syntaxins, demonstrating a mechanistic link between non-host resistance and basal penetration resistance in monocotyledons and dicotyledons. We show that resistance in barley requires a SNAP-25 (synaptosome-associated protein, molecular mass 25 kDa) homologue capable of forming a binary SNAP receptor (SNARE) complex with ROR2. Genetic control of vesicle behaviour at penetration sites, and plasma membrane location of PEN1/ROR2, is consistent with a proposed involvement of SNARE-complex-mediated exocytosis and/or homotypic vesicle fusion events in resistance. Functions associated with SNARE-dependent penetration resistance are dispensable for immunity mediated by race-specific resistance (R) genes, highlighting fundamental differences between these two resistance forms.

Disease Resistance and Abiotic Stress Tolerance in Rice Are Inversely Modulated by an Abscisic Acid–Inducible Mitogen-Activated Protein Kinase

Abstract: Mitogen-activated protein kinase (MAPK) cascades play an important role in mediating stress responses in eukaryotic organisms. However, little is known about the role of MAPKs in modulating the interaction of defense pathways activated by biotic and abiotic factors. In this study, we have isolated and functionally characterized a stress-responsive MAPK gene (OsMAPK5) from rice. OsMAPK5 is a single-copy gene but can generate at least two differentially spliced transcripts. The OsMAPK5 gene, its protein, and kinase activity were inducible by abscisic acid as well as various biotic (pathogen infection) and abiotic (wounding, drought, salt, and cold) stresses. To determine its biological function, we generated and analyzed transgenic rice plants with overexpression (using the 35S promoter of Cauliflower mosaic virus) or suppression (using double-stranded RNA interference [dsRNAi]) of OsMAPK5. Interestingly, suppression of OsMAPK5 expression and its kinase activity resulted in the constitutive expression of pathogenesis-related (PR) genes such as PR1 and PR10 in the dsRNAi transgenic plants and significantly enhanced resistance to fungal (Magnaporthe grisea) and bacterial (Burkholderia glumae) pathogens. However, these same dsRNAi lines had significant reductions in drought, salt, and cold tolerance. By contrast, overexpression lines exhibited increased OsMAPK5 kinase activity and increased tolerance to drought, salt, and cold stresses. These results strongly suggest that OsMAPK5 can positively regulate drought, salt, and cold tolerance and negatively modulate PR gene expression and broad-spectrum disease resistance.

High throughput virus‐induced gene silencing implicates heat shock protein 90 in plant disease resistance

Abstract: Virus-induced gene silencing was used to assess the function of random Nicotiana benthamiana cDNAs in disease resistance. Out of 4992 cDNAs tested from a normalized library, there were 79 that suppressed a hypersensitive response (HR) associated with Pto-mediated resistance against Pseudomonas syringae. However, only six of these clones blocked the Pto-mediated suppression of P.syringae growth. The three clones giving the strongest loss of Pto resistance had inserts corresponding to HSP90 and also caused loss of Rx-mediated resistance against potato virus X and N-mediated tobacco mosaic virus resistance. The role of HSP90 as a cofactor of disease resistance is associated with stabilization of Rx protein levels and could be accounted for in part by SGT1 and other cofactors of disease resistance acting as co-chaperones. This approach illustrates the potential benefits and limitations of RNA silencing in forward screens of gene function in plants.

Brassinosteroid functions in a broad range of disease resistance in tobacco and rice.

Abstract: Summary Brassinolide (BL), considered to be the most important brassinosteroid (BR) and playing pivotal roles in the hormonal regulation of plant growth and development, was found to induce disease resistance in plants. To study the potentialities of BL activity on stress responding systems, we analyzed its ability to induce disease resistance in tobacco and rice plants. Wild-type tobacco treated with BL exhibited enhanced resistance to the viral pathogen tobacco mosaic virus (TMV), the bacterial pathogen Pseudomonas syringae pv. tabaci (Pst), and the fungal pathogen Oidium sp. The measurement of salicylic acid (SA) in wild-type plants treated with BL and the pathogen infection assays using NahG transgenic plants indicate that BL-induced resistance does not require SA biosynthesis. BL treatment did not induce either acidic or basic pathogenesis-related (PR) gene expression, suggesting that BL-induced resistance is distinct from systemic acquired resistance (SAR) and wound-inducible disease resistance. Analysis using brassinazole 2001, a specific inhibitor for BR biosynthesis, and the measurement of BRs in TMV-infected tobacco leaves indicate that steroid hormone-mediated disease resistance (BDR) plays part in defense response in tobacco. Simultaneous activation of SAR and BDR by SAR inducers and BL, respectively, exhibited additive protective effects against TMV and Pst, indicating that there is no cross-talk between SAR- and BDR-signaling pathway downstream of BL. In addition to the enhanced resistance to a broad range of diseases in tobacco, BL induced resistance in rice to rice blast and bacterial blight diseases caused by Magnaporthe grisea and Xanthomonas oryzae pv. oryzae, respectively. Our data suggest that BDR functions in the innate immunity system of higher plants including dicotyledonous and monocotyledonous species.

Map-based isolation of the leaf rust disease resistance gene Lr10 from the hexaploid wheat (Triticum aestivum L.) genome

Abstract: More than 50 leaf rust resistance (Lr) genes against the fungal pathogen Puccinia triticina have been identified in the wheat gene pool, and a large number of them have been extensively used in breeding. Of the 50 Lr genes, all are known only from their phenotype and/or map position except for Lr21, which was cloned recently. For many years, the problems of molecular work in the large (1.6 × 1010 bp), highly repetitive (80%), and hexaploid bread wheat (Triticum aestivum L.) genome have hampered map-based cloning. Here, we report the isolation of the Lr gene Lr10 from hexaploid wheat by using a combination of subgenome map-based cloning and haplotype studies in the genus Triticum. Lr10 is a single-copy gene on chromosome 1AS. It encodes a CC-NBS-LRR type of protein with an N-terminal domain, which is under diversifying selection. When overexpressed in transgenic wheat plants, Lr10 confers enhanced resistance to leaf rust. Lr10 has similarities to RPM1 in Arabidopsis thaliana and to resistance gene analogs in rice and barley, but is not closely related to other wheat Lr genes based on Southern analysis. We conclude that map-based cloning of genes of agronomic importance in hexaploid wheat is now feasible, opening perspectives for molecular bread wheat improvement trough transgenic strategies and diagnostic allele detection.

A Gain-of-Function Mutation in a Plant Disease Resistance Gene Leads to Constitutive Activation of Downstream Signal Transduction Pathways in suppressor of npr1-1, constitutive 1

Abstract: Plants have evolved sophisticated defense mechanisms against pathogen infections, during which resistance (R) genes play central roles in recognizing pathogens and initiating defense cascades. Most of the cloned R genes share two common domains: the central domain, which encodes a nucleotide binding adaptor shared by APAF-1, certain R proteins, and CED-4 (NB-ARC), plus a C-terminal region that encodes Leu-rich repeats (LRR). In Arabidopsis, a dominant mutant, suppressor of npr1-1, constitutive 1 (snc1), was identified previously that constitutively expresses pathogenesis-related (PR) genes and resistance against both Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2. The snc1 mutation was mapped to the RPP4 cluster. In snc1, one of the TIR-NB-LRR–type R genes contains a point mutation that results in a single amino acid change from Glu to Lys in the region between NB-ARC and LRR. Deletions of this R gene in snc1 reverted the plants to wild-type morphology and completely abolished constitutive PR gene expression and disease resistance. The constitutive activation of the defense responses was not the result of the overexpression of the R gene, because its expression level was not altered in snc1. Our data suggest that the point mutation in snc1 renders the R gene constitutively active without interaction with pathogens. To analyze signal transduction pathways downstream of snc1, epistasis analyses between snc1 and pad4-1 or eds5-3 were performed. Although the resistance signaling in snc1 was fully dependent on PAD4, it was only partially affected by blocking salicylic acid (SA) synthesis, suggesting that snc1 activates both SA-dependent and SA-independent resistance pathways.

An ancient R gene from the wild potato species Solanum bulbocastanum confers broad-spectrum resistance to Phytophthora infestans in cultivated potato and tomato.

Abstract: Late blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating disease for potato cultivation. Here, we describe the positional cloning of the Rpi-blb1 gene from the wild potato species Solanum bulbocastanum known for its high levels of resistance to late blight. The Rpi-blb1 locus, which confers full resistance to complex isolates of P. infestans and for which race specificity has not yet been demonstrated, was mapped in an intraspecific S. bulbocastanum population on chromosome 8, 0.3 cM from marker CT88. Molecular analysis of a bacterial artificial chromosome (BAC) clone spanning the Rpi-blb1 locus identified a cluster of four candidate resistance gene analogues of the coiled coil, nucleotide-binding site, leucine-rich repeat (CC-NBS-LRR) class of plant resistance (R) genes. One of these candidate genes, designated the Rpi-blb1 gene, was able to complement the susceptible phenotype in a S. tuberosum and tomato background, demonstrating the potential of interspecific transfer of broad-spectrum late blight resistance to cultivated Solanaceae from sexually incompatible host species. Paired comparisons of synonymous and non-synonymous nucleotide substitutions between different regions of Rpi-blb1 paralogues revealed high levels of synonymous divergence, also in the LRR region. Although amino acid diversity between Rpi-blb1 homologues is centred on the putative solvent exposed residues of the LRRs, the majority of nucleotide differences in this region have not resulted in an amino acid change, suggesting conservation of function. These data suggest that Rpi-blb1 is relatively old and may be subject to balancing selection.

HLM1, an Essential Signaling Component in the Hypersensitive Response, Is a Member of the Cyclic Nucleotide–Gated Channel Ion Channel Family

Abstract: The hypersensitive response (HR) in plants is a programmed cell death that is commonly associated with disease resistance. A novel mutation in Arabidopsis, hlm1, which causes aberrant regulation of cell death, manifested by a lesion-mimic phenotype and an altered HR, segregated as a single recessive allele. Broad-spectrum defense mechanisms remained functional or were constitutive in the mutant plants, which also exhibited increased resistance to a virulent strain of Pseudomonas syringae pv tomato. In response to avirulent strains of the same pathogen, the hlm1 mutant showed differential abilities to restrict bacterial growth, depending on the avirulence gene expressed by the pathogen. The HLM1 gene encodes a cyclic nucleotide–gated channel, CNGC4. Preliminary study of the HLM1/CNGC4 gene pro-duct in Xenopus oocytes (inside-out patch-clamp technique) showed that CNGC4 is permeable to both K+ and Na+ and is activated by both cGMP and cAMP. HLM1 gene expression is induced in response to pathogen infection and some pathogen-related signals. Thus, HLM1 might constitute a common downstream component of the signaling pathways leading to HR/resistance.

Two MAPK cascades, NPR1, and TGA transcription factors play a role in Pto-mediated disease resistance in tomato

Abstract: The tomato Pto kinase confers resistance to the causative agent of bacterial speck disease, Pseudomonas syringae pv. tomato, by recognizing the pathogen effector proteins AvrPto or AvrPtoB. Pto-mediated resistance requires multiple signal transduction pathways and has been shown to activate many defense responses including an oxidative burst, rapid changes in the expression of over 400 genes, and localized cell death. We have tested the role in Pto-mediated resistance in tomato of a set of 21 genes from other species known to be involved in defense-related signaling. Expression of each gene was suppressed by virus-induced gene silencing (VIGS) and the effect on disease symptoms and bacterial growth during the tomato-Pseudomonas incompatible interaction was determined. We found that Pto-mediated resistance was compromised by silencing of genes encoding two mitogen-activated protein (MAP) kinase kinases, MEK1 and MEK2, two MAP kinases, NTF6 and wound-induced protein kinase (WIPK), a key regulator of systemic acquired resistance (SAR), NPR1, and two transcription factors, TGA1a and TGA2.2. A lesser impact on Pto-mediated resistance was observed in plants silenced for RAR1 and COI1. The identification of nine genes that play a role in resistance to bacterial speck disease both advances our knowledge of Pto signal transduction and demonstrates the conservation of many defense signaling components among diverse plant species.

Greenhouse and field testing of transgenic wheat plants stably expressing genes for thaumatin‐like protein, chitinase and glucanase against Fusarium graminearum

Abstract: Genes encoding pathogenesis-related (PR-) proteins isolated from a cDNA library of Fusarium graminearum-infected wheat spikes of scab-resistant cultivar 'Sumai-3' were transformed into susceptible spring wheat, 'Bobwhite' using a biolistic transformation protocol, with the goal of enhancing levels of resistance against scab. Twenty-four putative transgenic lines expressing either a single PR-protein gene or combinations thereof were regenerated. Transgene expression in a majority of these lines (20) was completely silenced in the T(1) or T(2) generations. Four transgenic wheat lines showed stable inheritance and expression of either a single transgene or transgene combinations up to four generations. One line co-expressing a chitinase and beta-1,3-glucanase gene combination, when bioassayed against scab showed a delay in the spread of the infection (type II resistance) under greenhouse conditions. This line and a second transgenic line expressing a rice thaumatin-like protein gene (tlp) which had moderate resistance to scab in previous greenhouse trials, along with susceptible and resistance checks were evaluated for resistance to scab under field conditions. None of the transgenic lines had resistance to scab in the field under conditions of strong pathogen, suggesting these plants lacked effective resistance to initial infection (type I resistance) under these conditions. As far as is known, this is the first report of field evaluation of transgenic wheat expressing genes for PR-proteins against disease resistance.

Fusarium Head Blight of Wheat and Barley

Abstract: Fusarium head blight of wheat and barley , Fusarium head blight of wheat and barley , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Microsatellite markers for genes lr34/yr18 and other quantitative trait Loci for leaf rust and stripe rust resistance in bread wheat.

Abstract: Leaf rust and stripe rust, caused by Puccinia triticina and P. striiformis, respectively, are important diseases of wheat in many countries. In this study we sought to identify molecular markers for adult plant resistance genes that could aid in incorporating such durable resistance into wheat. We used a doubled haploid population from a Japanese cv. Fukuho-komugi x Israeli wheat Oligoculm cross that had segregated for resistance to leaf rust and stripe rust in field trials. Joint and/or single-year analyses by composite interval mapping identified two quantitative trait loci (QTL) that reduced leaf rust severity and up to 11 and 7 QTLs that might have influenced stripe rust severity and infection type, respectively. Four common QTLs reduced stripe rust severity and infection type. Except for a QTL on chromosome 7DS, no common QTL for leaf rust and stripe rust was detected. QTL-7DS derived from 'Fukuho-komugi' had the largest effect on both leaf rust and stripe rust severities, possibly due to linked resistance genes Lr34/Yr18. The microsatellite locus Xgwm295.1, located almost at the peak of the likelihood ratio contours for both leaf and stripe rust severity, was closest to Lr34/Yr18. QTLs located on 1BL for leaf rust severity and 3BS for stripe rust infection type were derived from 'Oligoculm' and considered to be due to genes Lr46 and Yr30, respectively. Most of the remaining QTLs for stripe rust severity or infection type had smaller effects. Our results indicate there is significant diversity for genes that have minor effects on stripe rust resistance, and that successful detection of these QTLs by molecular markers should be helpful both for characterizing wheat genotypes effectively and combining such resistance genes.

Molecular Marker Mapping of Leaf Rust Resistance Gene Lr46 and Its Association with Stripe Rust Resistance Gene Yr29 in Wheat

Abstract: Leaf and stripe rusts, caused by Puccinia triticina and P. striiformis, respectively, are globally important fungal diseases of wheat that cause significant annual yield losses. A gene that confers slow rusting resistance to leaf rust, designated as Lr46, has recently been located on wheat chromosome 1B. The objectives of our study were to establish the precise genomic location of gene Lr46 using molecular approaches and to determine if there was an association of this locus with adult plant resistance to stripe rust. A population of 146 F(5) and F(6) lines produced from the cross of susceptible 'Avocet S' with resistant 'Pavon 76' was developed and classified for leaf rust and stripe rust severity for three seasons. Using patterns of segregation for the two diseases, we estimated that at least two genes with additive effects conferred resistance to leaf rust and three to four genes conferred resistance to stripe rust. Bulked segregant analysis and linkage mapping using amplified fragment length polymorphisms with the 'Avocet' x 'Pavon 76' population, F(3) progeny lines of a single chromosome recombinant line population from the cross 'Lalbahadur' x 'Lalbahadur (Pavon 1B)', and the International Triticeae Mapping Initiative population established the genomic location of Lr46 at the distal end of the long arm of wheat chromosome 1B. A gene that is closely linked to Lr46 and confers moderate levels of adult plant resistance to stripe rust is identified and designated as Yr29.

Mapping of quantitative trait loci for field resistance to Fusarium head blight in an European winter wheat

Abstract: Fusarium head blight (FHB) caused by Fusarium culmorum is an economically important disease of wheat that may cause serious yield and quality losses under favorable climate conditions The development of disease-resistant cultivars is the most effective control strategy Worldwide, there is heavy reliance on the resistance pool originating from Asian wheats, but excellent field resistance has also been observed among European winter wheats The objective of this study was to map and characterize quantitative traits loci (QTL) of resistance to FHB among European winter wheats A population of 194 recombinant inbred lines (RILs) was genotyped from a cross between two winter wheats Renan (resistant)/Recital (susceptible) with microsatellites, AFLP and RFLP markers RILs were assessed under field conditions For 3 years in one location Nine QTLs were detected, and together they explained 30–45% of the variance, depending on the year Three of the QTLs were stable over the 3 years One stable QTL, QFhsinra2b, was mapped to chromosome 2B and two QTLs QFhsinra5a2 and QFhsinra5a3, to chromosome 5A; each of these QTLs explained 69–186% of the variance Other QTLs were identified on chromosome 2A, 3A, 3B, 5D, and 6D, but these had a smaller effect on FHB resistance One of the two QTLs on chromosome 5A was linked to gene B1 controlling the presence of awns Overlapping QTLs for FHB resistance were those for plant height or/and flowering time Our results confirm that wheat chromosomes 2A, 3A, 3B, and 5A carry FHB resistance genes, and new resistance factors were identified on chromosome arms 2BS and 5AL Markers flanking these QTLs should be useful tools for combining the resistance to FHB of Asian and European wheats to increase the resistance level of cultivars

Recognition Specificity and RAR1/SGT1 Dependence in Barley Mla Disease Resistance Genes to the Powdery Mildew Fungus

Abstract: A large number of resistance specificities to the powdery mildew fungus Blumeria graminis f. sp. hordei map to the barley Mla locus. This complex locus harbors multiple members of three distantly related gene families that encode proteins that contain an N-terminal coiled-coil (CC) structure, a central nucleotide binding (NB) site, a Leu-rich repeat (LRR) region, and a C-terminal non-LRR (CT) region. We identified Mla12, which encodes a CC-NB-LRR-CT protein that shares 89 and 92% identical residues with the known proteins MLA1 and MLA6. Slow Mla12-triggered resistance was altered dramatically to a rapid response by overexpression of Mla12. A series of reciprocal domains swaps between MLA1 and MLA6 identified in each protein recognition domain for cognate powdery mildew fungus avirulence genes (AvrMla1 and AvrMla6). These domains were within different but overlapping LRR regions and the CT part. Unexpectedly, MLA chimeras that confer AvrMla6 recognition exhibited markedly different dependence on Rar1, a gene required for the function of some but not all Mla resistance specificities. Furthermore, uncoupling of MLA6-specific function from RAR1 also uncoupled the response from SGT1, a protein known to associate physically with RAR1. Our findings suggest that differences in the degree of RAR1 dependence of different MLA immunity responses are determined by intrinsic properties of MLA variants and place RAR1/SGT1 activity downstream of and/or coincident with the action of resistance protein–containing recognition complexes.

ERECTA, an LRR receptor-like kinase protein controlling development pleiotropically affects resistance to bacterial wilt

Abstract: Bacterial wilt, one of the most devastating bacterial diseases of plants worldwide, is caused by Ralstonia solanacearum and affects many important crop species. We show that several strains isolated from solanaceous crops in Europe are pathogenic in different accessions of Arabidopsis thaliana. One of these strains, 14.25, causes wilting symptoms in A. thaliana accession Landsberg erecta (Ler) and no apparent symptoms in accession Columbia (Col-0). Disease development and bacterial multiplication in the susceptible Ler accession depend on functional hypersensitive response and pathogenicity (hrp) genes, key elements for bacterial pathogenicity. Genetic analysis using Ler x Col-0 recombinant inbred lines showed that resistance is governed by at least three loci: QRS1 (Quantitative Resistance to R. solanacearum) and QRS2 on chromosome 2, and QRS3 on chromosome 5. These loci explain about 90% of the resistance carried by the Col-0 accession. The ERECTA gene, which encodes a leucine-rich repeat receptor-like kinase (LRR-RLK) and affects development of aerial organs, is dimorphic in our population and lies close to QRS1. Susceptible Ler plants transformed with a wild-type ERECTA gene, and the LER line showed increased disease resistance to R. solanacearum as indicated by reduced wilt symptoms and impaired bacterial growth, suggesting unexpected cross-talk between resistance and developmental pathways.

Enhanced Transcription of the Arabidopsis Disease Resistance Genes RPW8.1 and RPW8.2 via a Salicylic Acid–Dependent Amplification Circuit Is Required for Hypersensitive Cell Death

Abstract: The Arabidopsis disease resistance (R) genes RPW8.1 and RPW8.2 couple the recognition of powdery mildew pathogens of this plant with the subsequent induction of a localized necrosis, or hypersensitive response (HR). The HR restricts the spread of the infection and renders the plant resistant. One-third of Arabidopsis plants transformed with a genomic fragment containing RPW8.1 and RPW8.2 developed spontaneous HR-like lesions (SHL) in the absence of pathogens. We demonstrate that SHL occurs in transgenic lines that contain multiple copies of the transgene and express RPW8.1 and RPW8.2 at high levels. SHL is associated with salicylic acid (SA) accumulation, and at the site of the lesion, there is increased expression of RPW8.1, increased production of H2O2, and increased expression of pathogenesis-related genes. These lesions are physiologically similar to the pathogen-induced HR mediated by RPW8.1 and RPW8.2. Significantly, environmental conditions that suppress SHL suppress the transcription of RPW8.1 and RPW8.2 and also suppress resistance to powdery mildews, even in transgenic lines containing RPW8.1 and RPW8.2 that normally do not express SHL. Furthermore, treatment with SA increases the transcription of RPW8.1 and RPW8.2, induces SHL, and enhances resistance to powdery mildews. We conclude that HR requires the transcription of RPW8.1 and RPW8.2, which is regulated independently of the pathogen by SA-dependent feedback amplification.

Natural selection for polymorphism in the disease resistance gene Rps2 of Arabidopsis thaliana.

Abstract: Pathogen resistance is an ecologically important phenotype increasingly well understood at the molecular genetic level. In this article, we examine levels of avrRpt2-dependent resistance and Rps2 locus DNA sequence variability in a worldwide sample of 27 accessions of Arabidopsis thaliana. The rooted parsimony tree of Rps2 sequences drawn from a diverse set of ecotypes includes a deep bifurcation separating major resistance and susceptibility clades of alleles. We find evidence for selection maintaining these alleles and identify the N-terminal part of the leucine-rich repeat region as a probable target of selection. Additional protein variants are found within the two major clades and correlate well with measurable differences among ecotypes in resistance to the avirulence gene avrRpt2 of the pathogen Pseudomonas syringae. Long-lived polymorphisms have been observed for other resistance genes of A. thaliana; the Rps2 data suggest that the long-term maintenance of phenotypic variation in resistance genes may be a general phenomenon and are consistent with diversifying selection acting in concert with selection to maintain variation.

Identification and Validation of Markers Linked to Broad-Spectrum Stem Rust Resistance Gene Sr2 in Wheat (Triticum aestivum L.)

Abstract: Stem rust resistance in wheat (Triticum aestivum L.) conferred by the Sr2 gene has remained effective against Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn. worldwide for more than 50 yr. Sr2 resistance is associated with variable levels of disease symptons; it shows recessive inheritance and is expressed primarily during the adult-plant stage. A genetic and physical interval for the Sr2 gene was determined on the short arm of chromosome 3B of wheat. Flanking microsatellite markers were identified and one tightly linked marker (gwm533) was shown to be associated with presence of Sr2. This marker was validated in a wide range of germplasm and can now be implemented in marker-assisted breeding to facilitate selection for this durable, broad-spectrum but difficult to score rust resistance gene.

MSI-99, a magainin analogue, imparts enhanced disease resistance in transgenic tobacco and banana.

Abstract: Magainin is one of the earliest reported antimicrobial peptides isolated from skin secretions of the African clawed frog Xenopus laevis. A synthetic substitution analogue of magainin, MSI-99, is employed in this study to impart disease resistance in transgenic tobacco ( Nicotiana tabacum L.) and banana [( Musa spp. cv. Rasthali (AAB)]. This peptide inhibited the growth and spore germination of Fusarium oxysporum f.sp. cubense at 16 µg/ml. MSI-99 has been subcloned into plant expression vectors pMSI164 and pMSI168, targeting the peptide into the cytoplasm and extracellular spaces, respectively. Tobacco plants transformed with pMSI168 showed enhanced resistance against Sclerotinia sclerotiorum, Alternaria alternata and Botrytis cinerea. Transgenic banana pants were obtained for both pMSI164 and pMSI168 transformations and showed resistance to F. oxysporum f.sp. cubense and Mycosphaerella musicola. The transgenic nature of the transformants and expression of this peptide was confirmed through polymerase chain reaction (PCR) and reverse transcription (RT)–PCR. The results suggest that MSI-99 can be useful in imparting enhanced disease resistance in transgenic plants.

Identification of a locus controlling Verticillium disease symptom response in Arabidopsis thaliana

Abstract: Verticillium dahliae Klebahn is a soil-borne fungal pathogen causing vascular diseases. The pathogen penetrates the host through the roots, spreads through the xylem, and systemically colonizes both resistant and susceptible genotypes. To elucidate the genetic and molecular bases of plant-Verticillium interactions, we have developed a pathosystem utilizing Arabidopsis thaliana and an isolate of V. dahliae pathogenic to both cruciferous and non-cruciferous crops. Relative tolerance (based on symptom severity) but no immunity was found in a survey of Arabidopsis ecotypes. Anthocyanin accumulation, stunting, and chlorosis were common symptoms. Specific responses of the more susceptible ecotype Columbia were induction of early flowering and dying. The more tolerant ecotype C-24 was characterized by pathogen-induced delay of transition to flowering and mild chlorosis symptoms. Genetic analysis indicated that a single dominant locus, Verticillium dahliae-tolerance (VET1), likely functioning also as a negative regulator of the transition to flowering, was able to convey increased tolerance. VET1 was mapped on chromosome IV. The differential symptom responses observed between ecotypes were not correlated with different rates of fungal tissue colonization or with differential transcript accumulation of PR-1 and PDF1.2 defense genes whose activation was not detected during the Arabidopsis-V. dahliae interaction. Impairment in salicylic acid (SA)- or jasmonic acid (JA)-dependent signaling did not cause hypersensitivity to the fungal infection, whereas ethylene insensitivity led to reduced chlorosis and ABA deficiency to reduced anthocyanin accumulation. The results of this study clearly indicated that the ability of V. dahliae to induce disease symptoms is also connected to the genetic control of development and life span in Arabidopsis.

Cloning and characterization of the durable tomato mosaic virus resistance gene Tm-2(2) from Lycopersicon esculentum.

Abstract: In tomato, infections by tomato mosaic virus are controlled by durable Tm-22 resistance. In order to gain insight into the processes underlying disease resistance and its durability, we cloned and analysed the Tm-22 resistance gene and the susceptible allele, tm-2. The Tm-22 gene was isolated by transposon tagging using a screen in which plants with a destroyed Tm-22 gene survive. The Tm-22 locus consists of a single gene that encodes an 861 amino acid polypeptide, which belongs to the CC-NBS-LRR class of resistance proteins. The putative tm-2 allele was cloned from susceptible tomato lines via PCR with primers based on the Tm-22 sequence. Interestingly, the tm-2 gene has an open reading frame that is comparable to the Tm-22 allele. Between the tm-2 and the Tm-22 polypeptide 38 amino acid differences are present of which 26 are located in the second half of the LRR-domain. Susceptible tomato plants, which were transformed with the Tm-22 gene, displayed resistance against ToMV infection. In addition, virus specificity, displayed by the Tm-22 resistance was conserved in these transgenic lines. To explain the durability of this resistance, it is proposed that the Tm-22-encoded resistance is aimed at the Achilles' heel of the virus.

Genetic analysis of loci associated with partial resistance to Sclerotinia sclerotiorum in rapeseed ( Brassica napus L.)

Abstract: Sclerotinia stem rot is the most devastating disease of rapeseed (Brassica napus L) in China Quantitative trait loci (QTLs) involved in resistance to Sclerotinia sclerotiorum were detected in a rapeseed population of 128-F2:3 families derived from a cross between the male sterility restorer line H5200 and a partial resistant line Ning RS-1 A total of 107 molecular markers including 72 RFLPs, 30 AFLPs, 3 SSRs and 2 RAPDs were employed to construct a genetic linkage map with 23 linkage groups covering 1,6257 cM with an average space of 152 cM Resistance was assessed empirically at two developmental stages: with a detached leaf inoculation at the seedling stage and in vivo stem inoculation at the mature plant stage The observed resistance was scored for each plant as leaf resistance at the seedling stage (LRS) and stem resistance at the mature plant stage (SRM) A total of 13 loci were identified by one-way ANOVA and six QTLs were detected with MapMaker-QTL We found that three of the six QTLs were associated with leaf resistance at the seedling stage and collectively accounted for 407% of the total phenotypic variation, each accounting for 232%, 166% and 136% respectively Three QTLs were found corresponding to the disease resistance at the mature plant stage, explaining 490% of the phenotypic variation Epistasis was observed for the resistance and the additive by additive interactions were the predominant type of epistasis It was concluded that both single-locus QTLs and epistatic interactions played important roles in Sclerotinia resistance in rapeseed

Targeted Activation Tagging of the Arabidopsis NBS-LRR gene, ADR1, Conveys Resistance to Virulent Pathogens

Abstract: A transgenic Arabidopsis line containing a chimeric PR-1::luciferase (LUC) reporter gene was subjected to mutagenesis with activation tags. Screening of lines via high-throughput LUC imaging identified a number of dominant Arabidopsis mutants that exhibited enhanced PR-1 gene expression. Here, we report the characterization of one of these mutants, designated activated disease resistance (adr) 1. This line showed constitutive expression of a number of key defense marker genes and accumulated salicylic acid but not ethylene or jasmonic acid. Furthermore, adr1 plants exhibited resistance against the biotrophic pathogens Peronospora parasitica and Erysiphe cichoracearum but not the necrotrophic fungus Botrytis cinerea. Analysis of a series of adr1 double mutants suggested that adr1-mediated resistance against P. parasitica was salicylic acid (SA)-dependent, while resistance against E. cichoracearum was both SA-dependent and partially NPR1-dependent. The ADR1 gene encoded a protein possessing a number of key features, including homology to subdomains of protein kinases, a nucleotide binding domain, and leucine-rich repeats. The controlled, transient expression of ADR1 conveyed striking disease resistance in the absence of yield penalty, highlighting the potential utility of this gene in crop protection.

Enhanced fungal resistance in transgenic cotton expressing an endochitinase gene from Trichoderma virens.

Abstract: Summary Mycoparasitic fungi are proving to be rich sources of antifungal genes that can be utilized to genetically engineer important crops for resistance against fungal pathogens. We have transformed cotton and tobacco plants with a cDNA clone encoding a 42 kDa endochitinase from the mycoparasitic fungus, Trichoderma virens . Plants from 82 independently transformed callus lines of cotton were regenerated and analysed for transgene expression. Several primary transformants were identified with endochitinase activities that were significantly higher than the control values. Transgene integration and expression was confirmed by Southern and Northern blot analyses, respectively. The transgenic endochitinase activities were examined in the leaves of transgenic tobacco as well as in the leaves, roots, hypocotyls and seeds of transgenic cotton. Transgenic plants with elevated endochitinase activities also showed the expected 42 kDa endochitinase band in fluorescence, gel-based assays performed with the leaf extracts in both species. Homozygous T 2 plants of the high endochitinase-expressing cotton lines were tested for disease resistance against a soil-borne pathogen, Rhizoctonia solani and a foliar pathogen, Alternaria alternata . Transgenic cotton plants showed significant resistance to both

Sensitization of defense responses and activation of programmed cell death by a pathogen-induced receptor-like protein kinase in Arabidopsis

Abstract: During the search for potential target genes of WRKY DNA-binding transcription factors, we have previously identified four pathogen-induced Arabidopsis genes (CRK5, CRK6, CRK10 and CRK11) encoding receptor-like protein kinases (RLKs) containing novel cysteine-rich repeats in their extracellular domains. In the present study, we transformed Arabidopsis plants with the RLK genes under control of the constitutive CaMV 35S promoter or a steroid-inducible Gal4 promoter. Expression of CRK5, but not the three other RLK genes, resulted in significant alterations in defense responses and leaf growth in transgenic plants. In transgenic plants harboring the 35S::CRK5 construct, significantly elevated and constitutive expression of CRK5 correlated with enhanced leaf growth and increased resistance to the bacterial pathogen Pseudomonas syringae. The enhanced disease resistance in the transgenic plants was associated with more rapidly induced expression of the PR1 gene after pathogen infection. In transgenic plants transformed with CRK5 under control of the steroid-inducible promoter, expression of the transgene was induced at relatively high levels after the steroid application and this induced expression of CRK5 triggered hypersensitive response-like cell death. Induced CRK5 expression also activated cell death in the npr1, ndr1 and eds1 mutants and in the transgenic nahG plants that fail to accumulate salicylic acid. Thus, the novel RLK is capable of activating multiple distinct defense responses depending on the manner and/or the levels of its over-expression in transgenic plants.

Identification and fine mapping of Pi33, the rice resistance gene corresponding to the Magnaporthe grisea avirulence gene ACE1

Abstract: Rice blast disease is a major constraint for rice breeding. Nevertheless, the genetic basis of resistance remains poorly understood for most rice varieties, and new resistance genes remain to be identified. We identified the resistance gene corresponding to the cloned avirulence gene ACE1 using pairs of isogenic strains of Magnaporthe grisea differing only by their ACE1 allele. This resistance gene was mapped on the short arm of rice chromosome 8 using progenies from the crosses IR64 (resistant) x Azucena (susceptible) and Azucena x Bala (resistant). The isogenic strains also permitted the detection of this resistance gene in several rice varieties, including the differential isogenic line C101LAC. Allelism tests permitted us to distinguish this gene from two other resistance genes [ Pi11 and Pi-29(t)] that are present on the short arm of chromosome 8. Segregation analysis in F(2) populations was in agreement with the existence of a single dominant gene, designated as Pi33. Finally, Pi33 was finely mapped between two molecular markers of the rice genetic map that are separated by a distance of 1.6 cM. Detection of Pi33 in different semi-dwarf indica varieties indicated that this gene could originate from either one or a few varieties.