A molecular evolutionary concept connecting nonhost resistance, pathogen host range, and pathogen speciation

Any given pathogenic microbial species typically colonizes a limited number of plant species. Plant species outside of this host range mount nonhost disease resistance to attempted colonization by the, in this case, non-adapted pathogen. The underlying mechanism of nonhost immunity and host immunity involves the same non-self detection systems, the combined action of nucleotide-binding and leucine-rich repeat (NB-LRR) proteins and pattern recognition receptors (PRRs). Here we hypothesize that the relative contribution of NB-LRR- and PRR-triggered immunity to nonhost resistance changes as a function of phylogenetic divergence time between host and nonhost. Similarly, changes in pathogen host range, e.g. host range expansions, appear to be driven by variation in pathogen effector repertoires, in turn leading to reproductive isolation and subsequent pathogen speciation.     

Loss of susceptibility as a novel breeding strategy for durable and broad-spectrum resistance

Recent studies on plant immunity have suggested that a pathogen should suppress induced plant defense in order to infect a plant species, which otherwise would have been a nonhost to the pathogen. For this purpose, pathogens exploit effector molecules to interfere with different layers of plant defense responses. In this review, we summarize the latest findings on plant factors that are activated by pathogen effectors to suppress plant immunity. By looking from a different point of view into host and nonhost resistance, we propose a novel breeding strategy: disabling plant disease susceptibility genes (S-genes) to achieve durable and broad-spectrum resistance.     

Arabidopsis Heterotrimeric G-Proteins Play a Critical Role in Host and Nonhost Resistance against Pseudomonas syringae Pathogens

Heterotrimeric G-proteins have been proposed to be involved in many aspects of plant disease resistance but their precise role in mediating nonhost disease resistance is not well understood. We evaluated the roles of specific subunits of heterotrimeric G-proteins using knock-out mutants of Arabidopsis Gα, Gβ and Gγ subunits in response to host and nonhost Pseudomonas pathogens. Plants lacking functional Gα, Gβ and Gγ1Gγ2 proteins displayed enhanced bacterial growth and disease susceptibility in response to host and nonhost pathogens. Mutations of single Gγ subunits Gγ1, Gγ2 and Gγ3 did not alter bacterial disease resistance. Some specificity of subunit usage was observed when comparing host pathogen versus nonhost pathogen. Overexpression of both Gα and Gβ led to reduced bacterial multiplication of nonhost pathogen P. syringae pv. tabaci whereas overexpression of Gβ, but not of Gα, resulted in reduced bacterial growth of host pathogen P. syringae pv. maculicola, compared to wild-type Col-0. Moreover, the regulation of stomatal aperture by bacterial pathogens was altered in Gα and Gβ mutants but not in any of the single or double Gγ mutants. Taken together, these data substantiate the critical role of heterotrimeric G-proteins in plant innate immunity and stomatal modulation in response to P. syringae.     

Phytosterols play a key role in plant innate immunity against bacterial pathogens by regulating nutrient efflux into the apoplast

Bacterial pathogens colonize a host plant by growing between the cells by utilizing the nutrients present in apoplastic space. While successful pathogens manipulate the plant cell membrane to retrieve more nutrients from the cell, the counteracting plant defense mechanism against nonhost pathogens to restrict the nutrient efflux into the apoplast is not clear. To identify the genes involved in nonhost resistance against bacterial pathogens, we developed a virus-induced gene-silencing-based fast-forward genetics screen in Nicotiana benthamiana. Silencing of N. benthamiana SQUALENE SYNTHASE, a key gene in phytosterol biosynthesis, not only compromised nonhost resistance to few pathovars of Pseudomonas syringae and Xanthomonas campestris, but also enhanced the growth of the host pathogen P. syringae pv tabaci by increasing nutrient efflux into the apoplast. An Arabidopsis (Arabidopsis thaliana) sterol methyltransferase mutant (sterol methyltransferase2) involved in sterol biosynthesis also compromised plant innate immunity against bacterial pathogens. The Arabidopsis cytochrome P450 CYP710A1, which encodes C22-sterol desaturase that converts β-sitosterol to stigmasterol, was dramatically induced upon inoculation with nonhost pathogens. An Arabidopsis Atcyp710A1 null mutant compromised both nonhost and basal resistance while overexpressors of AtCYP710A1 enhanced resistance to host pathogens. Our data implicate the involvement of sterols in plant innate immunity against bacterial infections by regulating nutrient efflux into the apoplast.     

Pattern-triggered immunity restricts host colonization by endophytic fusaria,but does not affect endophyte-mediated resistance

Fusarium oxysporum (Fo) is best known as a host-specific vascular pathogen causing major crop losses. Most Fo strains, however, are root endophytes potentially conferring endophyte-mediated resistance (EMR). EMR is a mechanistically poorly understood root-specific induced resistance response induced by endophytic or nonhost pathogenic Fo strains. Like other types of induced immunity, such as systemic acquired resistance or induced systemic resistance, EMR has been proposed to rely on the activation of the pattern-triggered immunity (PTI) system of the plant. PTI is activated upon recognition of conserved microbe-associated molecular patterns (MAMPs) of invading microbes. Here, we investigated the role of PTI in controlling host colonization by Fo endophytes and their ability to induce EMR to the tomato pathogen Fo f. sp. lycopersici (Fol). Transgenic tomato and Arabidopsis plants expressing the Fo effector gene Avr2 are hypersusceptible to bacterial and fungal infection. Here we show that these plants are PTI-compromised and are nonresponsive to bacterial- (flg22) and fungal- (chitosan) MAMPs. We challenged the PTI-compromised tomato mutants with the EMR-conferring Fo endophyte Fo47, the nonhost pathogen Fom (a melon pathogen), and with Fol. Compared to wild-type plants, Avr2-tomato plants became hypercolonized by Fo47 and Fom. Surprisingly, however, EMR towards Fol, induced by either Fo47 or Fom, was unaffected in these plants. These data show that EMR-based disease resistance is independent from the conventional defence pathways triggered by PTI, but that PTI is involved in restricting host colonization by nonpathogenic Fo isolates.     

Nonhost resistance of barley is successfully manifested against Magnaporthe grisea and a closely related Pennisetum-infecting lineage but is overcome by Magnaporthe oryzae

Magnaporthe oryzae is a major pathogen of rice (Oryza sativa L.) but is also able to infect other grasses, including barley (Hordeum vulgare L.). Here, we report a study using Magnaporthe isolates collected from other host plant species to evaluate their capacity to infect barley. A nonhost type of resistance was detected in barley against isolates derived from genera Pennisetum (fontaingrass) or Digitaria (crabgrass), but no resistance occurred in response to isolates from rice, genus Eleusine (goosegrass), wheat (Triticum aestivum L.), or maize (Zea mays L.), respectively. Restriction of pathogen growth in the nonhost interaction was investigated microscopically and compared with compatible interactions. Real-time polymerase chain reaction was used to quantify fungal biomass in both types of interaction. The phylogenetic relationship among the Magnaporthe isolates used in this study was investigated by inferring gene trees for fragments of three genes, actin, calmodulin, and beta-tubulin. Based on phylogenetic analysis, we could distinguish different species that were strictly correlated with the ability of the isolates to infect barley. We demonstrated that investigating specific host interaction phenotypes for a range of pathogen isolates can accurately highlight genetic diversity within a pathogen population.     

Durable resistance to wheat stem rust needed

The recent outbreak of a new wheat stem rust race capable of parasitizing many commercial wheat cultivars highlights the need for durable disease resistance in crop plants. More advanced breeding approaches using quantitative disease resistance genes and resistance gene pyramids are being used to combat wheat stem rust and other diseases, though widespread adoption of these breeding methodologies is needed to maintain resistance efficacy. Advances in understanding the molecular basis of plant disease resistance at both host and nonhost levels offers further possibilities for stem rust resistance using biotechnological approaches. However, truly durable resistance to wheat stem rust and other phytopathogens seems an unlikely prospect in the face of continually evolving pathogen populations.     

Arabidopsis nonhost resistance gene PSS1 confers immunity against an oomycete and a fungal pathogen but not a bacterial pathogen that cause diseases in soybean

ABSTRACT: BACKGROUND: Nonhost resistance (NHR) provides immunity to all members of a plant species against all isolates of a microorganism that is pathogenic to other plant species. Three Arabidopsis thaliana PEN (penetration deficient) genes, PEN1, 2 and 3 have been shown to provide NHR against the barley pathogen Blumeria graminis f. sp. hordei at the prehaustorial level. Arabidopsis pen1-1 mutant lacking the PEN1 gene is penetrated by the hemibiotrophic oomycete pathogen Phytophthora sojae, the causal organism of the root and stem rot disease in soybean. We investigated if there is any novel nonhost resistance mechanism in Arabidopsis against the soybean pathogen, P. sojae. RESULTS: The P. sojae susceptible (pss) 1 mutant was identified by screening a mutant population created in the Arabidopsis pen1-1 mutant that lacks penetration resistance against the non adapted barley biotrophic fungal pathogen, Blumeria graminis f. sp. hordei. Segregation data suggested that PEN1 is not epistatic to PSS1. Responses of pss1 and pen1-1 to P. sojae invasion were distinct and suggest that PSS1 may act at both pre- and post-haustorial levels, while PEN1 acts at the pre-haustorial level against this soybean pathogen. Therefore, PSS1 encodes a new form of nonhost resistance. The pss1 mutant is also infected by the necrotrophic fungal pathogen, Fusarium virguliforme, which causes sudden death syndrome in soybean. Thus, a common NHR mechanism is operative in Arabidopsis against both hemibiotrophic oomycetes and necrotrophic fungal pathogens that are pathogenic to soybean. However, PSS1 does not play any role in immunity against the bacterial pathogen, Pseudomonas syringae pv. glycinea, that causes bacterial blight in soybean. We mapped PSS1 to a region very close to the southern telomere of chromosome 3 that carries no known disease resistance genes. CONCLUSIONS: The study revealed that Arabidopsis PSS1 is a novel nonhost resistance gene that confers a new form of nonhost resistance against both a hemibiotrophic oomycete pathogen, P. sojae and a necrotrophic fungal pathogen, F. virguliforme that cause diseases in soybean. However, this gene does not play any role in the immunity of Arabidopsis to the bacterial pathogen, P. syringae pv. glycinea, which causes bacterial blight in soybean. Identification and further characterization of the PSS1 gene would provide further insights into a new form of nonhost resistance in Arabidopsis, which could be utilized in improving resistance of soybean to two serious pathogens.     

Nonhost and basal resistance: how to explain specificity?

Nonhost resistance to plant pathogens can be constitutive or induced by microbes. Successful pathogens suppress microbe-induced plant defences by delivering appropriate effectors, which are apparently not sufficiently effective on nonhost plant species, as can be concluded from the strong host specificity of many biotroph plant pathogens. Such effectors act on particular plant targets, such as promoters or motifs in expressed sequences. Despite much progress in the elucidation of the molecular aspects of nonhost resistance to plant pathogens, very little is known about the genes that determine whether effectors can or cannot suppress the basal defence. In hosts they can, in nonhosts they cannot. The targets determining the host status of plants can be identified in inheritance studies. Recent reports have indicated that nonhost resistance is inherited polygenically, and exhibits strong similarity and association with the basal resistance of plants to adapted pathogens.     

Ribosomal protein QM/RPL10 positively regulates defence and protein translation mechanisms during nonhost disease resistance

Ribosomes play an integral part in plant growth, development, and defence responses. We report here the role of ribosomal protein large (RPL) subunit QM/RPL10 in nonhost disease resistance. The RPL10-silenced Nicotiana benthamiana plants showed compromised disease resistance against nonhost pathogen Pseudomonas syringae pv. tomato T1. The RNA-sequencing analysis revealed that many genes involved in defence and protein translation mechanisms were differentially affected due to silencing of NbRPL10. Arabidopsis AtRPL10 RNAi and rpl10 mutant lines showed compromised nonhost disease resistance to P. syringae pv. tomato T1 and P. syringae pv. tabaci. Overexpression of AtRPL10A in Arabidopsis resulted in reduced susceptibility against host pathogen P. syringae pv. tomato DC3000. RPL10 interacts with the RNA recognition motif protein and ribosomal proteins RPL30, RPL23, and RPS30 in the yeast two-hybrid assay. Silencing or mutants of genes encoding these RPL10-interacting proteins in N. benthamiana or Arabidopsis, respectively, also showed compromised disease resistance to nonhost pathogens. These results suggest that QM/RPL10 positively regulates the defence and translation-associated genes during nonhost pathogen infection.     

Secondary metabolites from nonhost plants affect the motility and viability of phytopathogenic Aphanomyces cochlioides zoospores

The motile zoospores of the damping-off pathogen Aphanomyces cochlioides aggregate on host plants (e.g., sugar beet, spinach) guided by the host-specific plant signal cochliophilin A before infection. To assess the potential role of secondary metabolites in nonhost resistance, acetone extracts of 200 nonhost traditional medicinal plants from Chinese and Bangladeshi origins were tested for the motility behaviour of A. cochlioides zoospores using a particle bioassay method. Nearly one third of the tested plant extracts exhibited diverse deleterious activities such as repellent, stimulant, motility halting and lysis against A. cochlioides zoospores. Among these active plants, an extract of the Chinese medicinal plant Dalbergia odorifera displayed potent repellent activity toward zoospores. Chromatographic separation of D. odorifera constituents revealed that the repellent activity was regulated by the cumulative effect of three motility-affecting isoflavonoids, viz. (+/-)-medicarpin (repellent at 150 microg/ml), (-)-claussequinone (stimulant at 100 microg/ml) and formononetin (stimulant and attractant at 50 microg/ml). A mixture (1:1:1, w/w/w) of these three compounds exhibited only repellent activity toward zoospores at a concentration lower than 50 microg/ml. These results suggest that nonhost plants might possess potential bioactive secondary metabolites to ward off zoosporic phytopathogens.     

SGT1 positively regulates the process of plant cell death during both compatible and incompatible plant–pathogen interactions

SGT1 (suppressor of G2 allele of Skp1), an interactor of SCF (Skp1‐Cullin‐F‐box) ubiquitin ligase complexes that mediate protein degradation, plays an important role at both G1–S and G2–M cell cycle transitions in yeast, and is highly conserved throughout eukaryotes. Plant SGT1 is required for both resistance (R) gene‐mediated disease resistance and nonhost resistance to certain pathogens. Using virus‐induced gene silencing (VIGS) in Nicotiana benthamiana, we demonstrate that SGT1 positively regulates the process of cell death during both host and nonhost interactions with various pathovars of Pseudomonas syringae. Silencing of NbSGT1 in N. benthamiana plants delays the induction of hypersensitive response (HR)‐mediated cell death against nonhost pathogens and the development of disease‐associated cell death caused by the host pathogen P. syringae pv. tabaci. Our results further demonstrate that NbSGT1 is required for Erwinia carotovora‐ and Sclerotinia sclerotiorum‐induced disease‐associated cell death. Overexpression of NbSGT1 in N. benthamiana accelerates the development of HR during R gene‐mediated disease resistance and nonhost resistance. Our data also indicate that SGT1 is required for pathogen‐induced cell death, but is not always necessary for the restriction of bacterial multiplication in planta. Therefore, we conclude that SGT1 is an essential component affecting the process of cell death during both compatible and incompatible plant–pathogen interactions.     

Arabidopsis NHO1 is required for general resistance against Pseudomonas bacteria

Nonhost interactions are prevalent between plants and specialized phytopathogens. Although it has great potential for providing crop plants with durable resistance, nonhost resistance is poorly understood. Here, we show that nonhost resistance is controlled, at least in part, by general resistance. Arabidopsis plants are resistant to the nonhost pathogen Pseudomonas syringae pv phaseolicola NPS3121 and completely arrest bacterial multiplication in the plant. Ten Arabidopsis mutants were isolated that were compromised in nonhost (nho) resistance to P. s. phaseolicola. Among these, nho1 is caused by a single recessive mutation that defines a novel gene. nho1 is defective in nonspecific resistance to Pseudomonas bacteria, because it also supported the growth of P. s. tabaci and P. fluorescens bacteria, both of which are nonpathogenic on Arabidopsis. In addition, the nho1 mutation also compromised resistance mediated by RPS2, RPS4, RPS5, and RPM1. Interestingly, the nho1 mutation had no effect on the growth of the virulent bacteria P. s. maculicola ES4326 and P. s. tomato DC3000, but it partially restored the in planta growth of the DC3000 hrpS(-) mutant bacteria. Thus, the virulent bacteria appear to evade or suppress NHO1-mediated resistance by means of an Hrp-dependent virulence mechanism.     

Effects of nonhost and host plants on insect herbivory covarying with plant size in the cruciferous plant <Emphasis Type="Italic">Turritis glabra</Emphasis>

Correlation between plant size and reproductive output may be modified by herbivory in accordance with host plant density and the presence of nonhost plants. To elucidate the effects of nonhost plant density and host plant density on the intensity of herbivory and reproductive output of the host plant in relation to plant size under natural conditions, we investigated the abundance of three lepidopteran insects, Plutella maculipennis, Anthocharis scolymus, and Pieris rapae the intensity of herbivory, and fruit set of their host plant, Turritis glabra (Cruciferae). To elucidate the effects of nonhost and host plant density, we selected four categories of plots under natural conditions: low density of nonhost and high density of host plants; low density of both nonhost and host plants; high density of both nonhost and host plants; and high density of nonhost and low density of host plants. The plant size indicated by stem diameter was a good predictor of the abundance of all herbivorous species. The effects of density of nonhost and host plants on the abundance of insects varied among species and stages of insects. As the abundance of insects affected the intensity of herbivory, herbivory was more apparent on larger host plants in plots with low density of both nonhost and host plants. Consequently, the correlation between plant size and the number of fruits disappeared in low plots with density of both nonhost and host plants. In this T. glabra– herbivorous insect system, the density of nonhost plants and host plants plays an important role in modifying the relationship between plants and herbivores under natural conditions. Received: July 19, 1999 / Accepted: June 15, 2000     

Mechanisms of powdery mildew resistance in the Vitaceae family

The cultivated grapevine, Vitis vinifera, is a member of the Vitaceae family, which comprises over 700 species in 14 genera. Vitis vinifera is highly susceptible to the powdery mildew pathogen Erysiphe necator. However, other species within the Vitaceae family have been reported to show resistance to this fungal pathogen, but little is known about the mechanistic basis of this resistance. Therefore, the frequency of successful E. necator penetration events, in addition to programmed cell death (PCD) responses, were investigated in a representative genotype from a range of different species within the Vitaceae family. The results revealed that penetration resistance and PCD-associated responses, or combinations of both, are employed by the different Vitaceae genera to limit E. necator infection. In order to further characterize the cellular processes involved in the observed penetration resistance, specific inhibitors of the actin cytoskeleton and secretory/endocytic vesicle trafficking function were employed. These inhibitors were demonstrated to successfully break the penetration resistance in V. vinifera against the nonadapted powdery mildew E. cichoracearum. However, the use of these inhibitors with the adapted powdery mildew E. necator unexpectedly revealed that, although secretory and endocytic vesicle trafficking pathways play a crucial role in nonhost penetration resistance, the adapted powdery mildew species may actually require these pathways to successfully penetrate the plant host.