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PubMed2002

The genomics parade of defense responses: to infinity and beyond.

Genomic-scale methods, such as cDNA microarrays, cDNA-AFLP analyses and proteomics are revolutionizing the study of plant-pathogen interactions, and are revealing a complex web of signaling cascades involved in plant defense responses. Recent studies have shown that responses to pathogens and environmental stresses are linked, suggesting that genes previously identified as stress-responsive may also play an active role in plant defense. As a result of proteomic analysis, proteins involved in early defense signaling are coming to light.

Gene Expression Regulation, Plant↗
PubMed2002

An evolutionarily conserved mediator of plant disease resistance gene function is required for normal Arabidopsis development.

Plants recognize many pathogens through the action of a diverse family of proteins called disease resistance (R) genes. The Arabidopsis R gene RPM1 encodes resistance to specific Pseudomonas syringae strains. We describe an RPM1-interacting protein that is an ortholog of TIP49a, previously shown to interact with the TATA binding protein (TBP) complex and to modulate c-myc- and beta-catenin-mediated signaling in animals. Reduction of Arabidopsis TIP49a (AtTIP49a) mRNA levels results in measurable increases of two R-dependent responses without constitutively activating defense responses, suggesting that AtTIP49a can act as a negative regulator of at least some R functions. Further, AtTIP49a is essential for both sporophyte and female gametophyte viability. Thus, regulators of R function overlap with essential modulators of plant development.

Arabidopsis↗
PubMed1993

Identification of a disease resistance locus in Arabidopsis that is functionally homologous to the RPG1 locus of soybean.

A new disease resistance locus in Arabidopsis, RPS3, was identified using a previously cloned avirulence gene from a non-Arabidopsis pathogen. The avrB avirulence gene from the soybean pathogen Pseudomonas syringae pv. glycinea was transferred into a P. syringae pv. tomato strain that is virulent on Arabidopsis, and conversion to avirulence was assayed on Arabidopsis plants. The avrB gene had avirulence activity on most, but not all, Arabidopsis ecotypes. Of 53 ecotypes examined, 45 were resistant to a P. syringae pv. tomato strain carrying avrB, and eight were susceptible. The inheritance of this resistance was examined using crosses between the resistant ecotype Col-0 and the susceptible ecotype Bla-2. In F2 plants from this cross, the ratio of resistant:susceptible plants was approximately 3:1, indicating that resistance to P. syringae expressing avrB is determined by a single dominant locus in ecotype Col-0, which we have designated RPS3. Using RFLP analysis, RPS3 was mapped to chromosome 3, adjacent to markers M583 and G4523, and < or = 1 cM from another disease resistance locus, RPM1. In soybean, resistance to P. syringae strains that carry avrB is controlled by the locus RPG1. Thus, RPG1 and RPS3 both confer avrB-specific disease resistance, suggesting that these genes may be homologs.

Arabidopsis↗
PubMed1997

Use of Arabidopsis recombinant inbred lines reveals a monogenic and a novel digenic resistance mechanism to Xanthomonas campestris pv campestris.

Infiltration of the Arabidopsis thaliana accession Landsberg erecta (Ler) with Xanthomonas campestris pv campestris isolate 2D520 results in extensive necrosis and limited chlorosis within 5-6 days post-inoculation (d.p.i.), which can lead to systemic necrosis within 23 d.p.i. in contrast, the accession Columbia (Col) remains asymptomatic after infiltration. Although both accessions support bacterial growth, 5-28-fold more bacteria are present in Ler than in Col leaf tissue. Inheritance studies indicate that three independent, dominant or partially dominant, nuclear genes condition resistance to X. c. campestris 2D520. The major gene, termed RXC2, conditions monogenic resistance to X. c.; campestris and was mapped to a 5.5 cM interval of chromosome V. Segregation data indicate that the locus RXC3 in conjunction with RXC4 confers digenic resistance to X. c. campestris. The combined action of RXC3 and RXC4 is correlated with a suppression of in planta bacterial levels and a suppression of symptoms relative to Ler. The RXC3 + RXC4-mediated resistance is novel in that although the Col allele of RXC4 contributes positively to resistance, it is the Ler and not the Col allele of RXC3 that contributes positively to resistance. RXC3 was mapped to the bottom arm of chromosome V in a 2.7 cM interval within the major recognition gene complex MRC-J, a cluster of genes involved in disease resistance. RXC4 was mapped to a 12 cM interval on chromosome II that also contains RXC1, a gene conferring tolerance to X. c. campestris.

Arabidopsis↗
PubMed2003

Expression profiling of the host response to bacterial infection: the transition from basal to induced defence responses in RPM1-mediated resistance.

Changes in transcription in leaves of Arabidopsis thaliana were characterised following challenge with strains of Pseudomonas syringae pv. tomato DC3000 allowing differentiation of basal resistance (hrpA mutants), gene-specific resistance (RPM1-specified interactions) and susceptibility (wild-type pathogen). In planta avirulence gene induction, changes in host [Ca2+]cyt and leaf collapse were used to delineate the transition from infection to induced resistance. The plant responds rapidly, dynamically and discriminately to infection by phytopathogenic bacteria. Within the first 2 h host transcriptional changes are common to all challenges indicating that Type III effector function does not contribute to early events in host transcriptome re-programming. The timing of induction for specific transcripts was reproducible, hierarchical and modulated at least in part through EDS1 function. R gene-specific transcripts were not observed until 3 h after inoculation. Intriguingly, the R gene-specific response proteins are expected to localise to diverse cellular addresses indicative of a global impact on cellular homeostasis. The altered transcriptional response rapidly manifests into initial symptoms of leaf collapse within 2 h, although establishment of the full macroscopic HR occurs significantly later.

Arabidopsis↗
PubMed2003

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

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.

Arabidopsis↗
PubMed2004

Molecular genetics of disease resistance in cereals.

AIMS: This Botanical Briefing attempts to summarize what is currently known about the molecular bases of disease resistance in cereal species and suggests future research directions. SCOPE: An increasing number of resistance (R) genes have been isolated from rice, maize, wheat and barley that encode both structurally related and unique proteins. This R protein diversity may be attributable to the different modus operandi employed by pathogen species in some cases, but it is also a consequence of multiple defence strategies being employed against phytopathogens. Mutational analysis of barley has identified additional genes required for activation of an R gene-mediated defence response upon pathogen infection. In some instances very closely related barley R proteins require different proteins for defence activation, demonstrating that, within a single plant species, multiple resistance signalling pathways and different resistance strategies have evolved to confer protection against a single pathogen species. Despite the apparent diversity of cereal resistance mechanisms, some of the additional molecules required for R protein function are conserved amongst cereal and dicotyledonous species and even other eukaryotic species. Thus the derivation of functional homologues and interacting partner proteins from other species is contributing to the understanding of resistance signalling in cereals. The potential and limit of utilizing the rice genome sequence for further R gene isolation from cereal species is also considered, as are the new biotechnological possibilities for disease control arising from R gene isolation. CONCLUSIONS: Molecular analyses in cereals have further highlighted the complexity of plant-pathogen co-evolution and have shown that numerous active and passive defence strategies are employed by plants against phytopathogens. Many advances in understanding the molecular basis of disease resistance in cereals have focused on monogenic resistance traits. Future research targets are likely to include less experimentally tractable, durable polygenic resistances and nonhost resistance mechanisms.

Breeding↗
PubMed2005

Involvement of the small GTPase Rac in the defense responses of tobacco to pathogens.

During the hypersensitive response (HR), plants accumulate reactive oxygen species (ROS) that are likely generated at least in part by an NADPH oxidase similar to that found in mammalian neutrophils. An essential regulator of mammalian NADPH oxidase is the small GTP-binding protein Rac. To investigate whether Rac also regulates the pathogen-induced oxidative burst in plants, a dominant negative form of the rice OsRac1 gene was overexpressed in tobacco carrying the N resistance gene. Following infection with Tobacco mosaic virus (TMV), DN-OsRacl plants developed smaller lesions than wild-type plants, accumulated lower levels of lipid peroxidation products, and failed to activate expression of antioxidant genes. These results, combined with the demonstration that superoxide and hydrogen peroxide levels were reduced in DN-OsRacl tobacco developing a synchronous HR triggered by transient expression of the TMV p50 helicase domain or the Pto and AvrPto proteins, suggest that ROS production is impaired. The dominant negative effect of DN-OsRacl could be rescued by transiently overexpressing the wild-type OsRac1 protein. TMV-induced salicylic acid accumulation also was compromised in DN-OsRacl tobacco. Interestingly, while systemic acquired resistance to TMV was not impaired, nonhost resistance to Pseudomonas syringae pv. maculicola ES4326 was suppressed. Thus, the effect DN-OsRac1 expression exerts on the resistance signaling pathway appears to vary depending on the identity of the inoculated pathogen.

Gene Expression Regulation, Plant↗
PubMedDate not supplied

Coat protein-mediated resistance in transgenic tobacco expressing the tobacco mosaic virus coat protein from tissue-specific promoters.

Coat protein-mediated resistance (CP-MR) was studied in transgenic Nicotiana tabacum 'Xanthi nn' and 'Xanthi NN' that express chimeric tobacco mosaic virus (TMV) coat protein (CP) gene constructs using two different tissue-specific promoters. The Phaseolus vulgaris pal2 promoter leads to gene expression in the upper leaf epidermis and the xylem, while the rolC promoter from Agrobacterium rhizogenes leads to gene expression in pholem and leaf hair tip cells. Tissue-specific gene expression was verified using the gusA(uidA) reporter gene, while accumulation of TMV CP was verified by Western blot analysis. Transgenic Xanthi nn plants harboring the pal2-CP gene construct were partially resistant to TMV infection. On Xanthi NN plants that expressed the pal2-CP gene construct, fewer necrotic lesions were formed after TMV inoculation compared to nontransformed control plants. The level of resistance, however, was substantially less than in plant lines that expressed TMV CP from the cauliflower mosaic virus 35S promoter. By contrast, expression of the rolC-CP construct did not confer resistance in either Xanthi nn or Xanthi NN. The results provide further evidence that CP-MR to systemic TMV infection in tobacco is probably due to inhibition of infection rather than to effects on long-distance spread through the phloem.

Capsid↗
PubMed2002

Chemically induced virus resistance in Arabidopsis thaliana is independent of pathogenesis-related protein expression and the NPR1 gene.

Salicylic acid (SA) treatment triggers inhibition of replication or movement of several positive-sense RNA plant viruses in tobacco. This resistance can also be stimulated by nonlethal concentrations of cyanide and antimycin A (AA) without triggering induction of pathogenesis-related PR-1 protein genes. In two ecotypes of Arabidopsis thaliana (Columbia and Nössen), SA-induced resistance to a tobamovirus, Turnip vein clearing virus (TVCV), was also induced by nonlethal concentrations of cyanide and AA without concomitant induction of PR-1 gene expression. Furthermore, chemically induced resistance to TVCV, as well as the induction of the plant mitochondrial alternative oxidase (a potential target for the chemicals), was independent of NPR1, a gene that plays a key role downstream of SA in the induction of PR proteins. The chemically induced resistance to TVCV appeared to be due to inhibition of replication at the site of inoculation. Taken together, these results show that in Arabidopsis, as in tobacco, resistance to viruses can be induced via a distinct branch of the defensive signal transduction pathway. This suggests that the existence of this virus-specific branch may be widespread among plants.

Arabidopsis↗
PubMed2002

Functional and comparative bioinformatic analysis of expressed genes from wheat spikes infected with Fusarium graminearum.

Fusarium head blight, caused by the fungus Fusarium graminearum, is a major disease on wheat (Triticum aestivum L.). Expressed sequence tags (ESTs) were used to identify genes expressed during the wheat-F. graminearum interaction. We generated 4,838 ESTs from a cDNA library prepared from spikes of the partially resistant cultivar Sumai 3 infected with F. graminearum. These ESTs were composed of 2,831 singlet (single-copy transcripts) and 715 contigs (multiple-copy transcripts) for a total of 3,546 non-redundant sequences. Four sets of nonredundant sequences were identified. One set contains numerous, common biotic and abiotic stress-related genes. Many of these stress-related genes were represented by multiple ESTs, indicating that they are abundantly expressed. A second set comprised 16 nonredundant sequences from F. graminearum that may be required for pathogenicity. A subset of these fungal genes encodes proteins associated with plant cell wall degradation. A third set of 326 nonredundant sequences had no DNA or amino acid sequence similarity to almost 1 million plant and over 7 million animal sequences in dbEST (as of 22 June 2001). Thus, these 326 nonredundant sequences have only been found in our F. graminearum-infected 'Sumai 3' cDNA library. A fourth set of 29 nonredundant sequences was found in our F. graminearum-infected wheat and another plant-pathogen interaction cDNA library. Some of these sequences encode proteins that may act in establishing various plant-fungal interactions.

Cell Wall↗
PubMed2004

Temporal expression of PR-1 and enhanced mature plant resistance to virus infection is controlled by a single dominant gene in a new Nicotiana hybrid.

A new variety of Nicotiana edwardsonii, designated N. edwardsonii cv. Columbia, expresses pathogenesis-related (PR) proteins in a temporal manner 45 to 49 days postplanting and also exhibits enhanced resistance to Tobacco mosaic virus, Tobacco necrosis virus, and Tomato bushy stunt virus. In contrast, PR proteins were not expressed in the original N. edwardsonii variety at comparable ages but were induced after onset of a hypersensitive response to viral infection. The temporal induction of PR proteins in 'Columbia' was correlated with increases in salicylic acid and glycosylated salicylic acid. Earlier studies noted that some Nicotiana hybrids derived from interspecific crosses constitutively express PR proteins, but the genetic basis of this phenomenon had not been investigated, likely because many interspecific Nicotiana crosses are sterile. However, the close genetic relationship between N. edwardsonii and 'Columbia' indicated that a hybrid between these two plants might be fertile, and this proved to be true. Genetic crosses between 'Columbia' and N. edwardsonii demonstrated that a single, dominant gene conditioned temporal expression of PR proteins and enhanced resistance. This gene was designated TPR1 (for temporal expression of PR proteins).

Arabidopsis Proteins↗
PubMed2005

A stilbene synthase gene (SbSTS1) is involved in host and nonhost defense responses in sorghum.

A chalcone synthase (CHS)-like gene, SbCHS8, with high expressed sequence tag abundance in a pathogen-induced cDNA library, was identified previously in sorghum (Sorghum bicolor). Genomic Southern analysis revealed that SbCHS8 represents a single-copy gene. SbCHS8 expression was induced in sorghum mesocotyls following inoculation with Cochliobolus heterotrophus and Colletotrichum sublineolum, corresponding to nonhost and host defense responses, respectively. However, the induction was delayed by approximately 24 h when compared to the expression of at least one of the other SbCHS genes. In addition, SbCHS8 expression was not induced by light and did not occur in a tissue-specific manner. SbCHS8, together with SbCHS2, was overexpressed in transgenic Arabidopsis (Arabidopsis thaliana) tt4 (transparent testa) mutants defective in CHS activities. SbCHS2 rescued the ability of these mutants to accumulate flavonoids in seed coats and seedlings. In contrast, SbCHS8 failed to complement the mutation, suggesting that the encoded enzyme does not function as a CHS. To elucidate their biochemical functions, recombinant proteins were assayed with different phenylpropanoid-Coenzyme A esters. Flavanones and stilbenes were detected in the reaction products of SbCHS2 and SbCHS8, respectively. Taken together, our data demonstrated that SbCHS2 encodes a typical CHS that synthesizes naringenin chalcone, which is necessary for the formation of different flavonoid metabolites. On the other hand, SbCHS8, now retermed SbSTS1, encodes an enzyme with stilbene synthase activity, suggesting that sorghum accumulates stilbene-derived defense metabolites in addition to the well-characterized 3-deoxyanthocyanidin phytoalexins.

Acyltransferases↗
PubMed2001

The disease resistance signaling components EDS1 and PAD4 are essential regulators of the cell death pathway controlled by LSD1 in Arabidopsis.

Specific recognition of pathogens is mediated by plant disease resistance (R) genes and translated into a successful defense response. The extent of associated hypersensitive cell death varies from none to an area encompassing cells surrounding an infection site, depending on the R gene activated. We constructed double mutants in Arabidopsis between positive regulators of R function and a negative regulator of cell death, LSD1, to address whether genes required for normal R function also regulate the runaway cell death observed in lsd1 mutants. We report here that EDS1 and PAD4, two signaling genes that mediate some but not all R responses, also are required for runaway cell death in the lsd1 mutant. Importantly, this novel function of EDS1 and PAD4 is operative when runaway cell death in lsd1 is initiated through an R gene that does not require EDS1 or PAD4 for disease resistance. NDR1, another component of R signaling, also contributes to the control of plant cell death. The roles of EDS1 and PAD4 in regulating lsd1 runaway cell death are related to the interpretation of reactive oxygen intermediate-derived signals at infection sites. We further demonstrate that the fate of superoxide at infection sites is different from that observed at the leading margins of runaway cell death lesions in lsd1 mutants.

Arabidopsis↗
PubMed2005

Induced disease resistance and gene expression in cereals.

Disease resistance strategies reduce chemical input into the environment and are therefore powerful approaches to sustainable agriculture. Induced resistance (IR) has emerged as a potential alternative, or a complementary strategy, for crop protection. IR signifies the control of pathogens and pests by prior activation of plant defence pathways. A molecular understanding of IR in cereals, including the most important global crops wheat and rice, has been largely missing. Evidence indicating that central elements of IR pathways are conserved among Di- and Monocotyledoneae has only recently been presented, although their regulation and interaction with other plant pathways may be quite divergent. We present here a synopsis of current molecular knowledge of cereal IR mechanisms.

Edible Grain↗
PubMed1997

Signaling in plant-microbe interactions.

Analysis of viral and bacterial pathogenesis has revealed common themes in the ways in which plants and animals respond to pathogenic agents. Pathogenic bacteria use macromolecule delivery systems (types III and IV) to deliver microbial avirulence proteins and transfer DNA-protein complexes directly into plant cells. The molecular events that constitute critical steps of plant-pathogen interactions seem to involve ligand-receptor mechanisms for pathogen recognition and the induction of signal transduction pathways in the plant that lead to defense responses. Unraveling the molecular basis of disease resistance pathways has laid a foundation for the rational design of crop protection strategies.

Animals↗