Race-cultivar Interactions Research Articles

Dual transcriptional characterization of spinach and Peronospora effusa during resistant and susceptible race-cultivar interactions

BackgroundSpinach downy mildew, caused by the obligate oomycete pathogen, Peronospora effusa remains a major concern for spinach production. Disease control is predominantly based on development of resistant spinach cultivars. However, new races and novel isolates of the pathogen continue to emerge and overcome cultivar resistance. Currently there are 20 known races of P. effusa. Here we characterized the transcriptomes of spinach, Spinacia oleracea, and P. effusa during disease progression using the spinach cultivar Viroflay, the near isogenic lines NIL1 and NIL3, and P. effusa races, R13 and R19, at 24 h post inoculation and 6 days post inoculation. A total of 54 samples were collected and subjected to sequencing and transcriptomic analysis.ResultsDifferentially expressed gene (DEG) analysis in resistant spinach interactions of R13-NIL1 and R19-NIL3 revealed spinach DEGs from protein kinase-like and P-loop containing families, which have roles in plant defense. The homologous plant defense genes included but were not limited to, receptor-like protein kinases (Spiol0281C06495, Spiol06Chr21559 and Spiol06Chr24027), a BAK1 homolog (Spiol0223C05961), genes with leucine rich repeat motifs (Spiol04Chr08771, Spiol04Chr01972, Spiol05Chr26812, Spiol04Chr11049, Spiol0084S08137, Spiol03Chr20299) and ABC-transporters (Spiol02Chr28975, Spiol06Chr22112, Spiol06Chr03998 and Spiol04Chr09723). Additionally, analysis of the expression of eight homologous to previously reported downy mildew resistance genes revealed that some are differentially expressed during resistant reactions but not during susceptible reactions. Examination of P. effusa gene expression during infection of susceptible cultivars identified expressed genes present in R19 or R13 including predicted RxLR and Crinkler effector genes that may be responsible for race-specific virulence on NIL1 or NIL3 spinach hosts, respectively.ConclusionsThese findings deliver foundational insight to gene expression in both spinach and P. effusa during susceptible and resistant interactions and provide a library of candidate genes for further exploration and functional analysis. Such resources will be beneficial to spinach breeding efforts for disease resistance in addition to better understanding the virulence mechanisms of this obligate pathogen.

The type III (Hrp) secretion pathway of plant pathogenic bacteria: trafficking harpins, Avr proteins, and death.

The ability of plant pathogenic bacteria to deliver deathtriggering proteins to the interior of plant cells was revealed in a rapid succession of papers in 1996 that transformed our concepts of bacterial plant pathogenicity. The breakthrough came with the convergence of work on Hrp systems and Avr proteins, an understanding of which requires an introduction to the most prevalent bacterial pathogens of plants, the cardinal importance of the Hrp pathway, and the paradoxical phenotype associated with avr genes. Plant pathogenic bacteria in the genera Erwinia, Pseudomonas, Xanthomonas, and Ralstonia cause diverse, and sometimes devastating, diseases in many different plants, but they all share three characteristics: they colonize the intercellular spaces of plants, they are capable of killing plant cells, and they possess hrp genes. Many of these pathogens are host specific. In host plants, they produce various symptoms after several days of multiplication, whereas in nonhost plants, they trigger the hypersensitive response (HR), a rapid, defense-associated, programmed death of plant cells at the site of invasion (21, 43). With inoculum levels typically encountered in natural environments, the HR produces individual dead plant cells that are scattered within successfully defended healthy tissue (71). However, experimental infiltration of high inoculum levels (.10 bacterial cells/ml) results in macroscopically observable death of the entire infiltrated tissue, usually within 24 h (42). Pioneer screens for random transposon mutants with impaired plant interactions yielded a prevalent class that was designated Hrp, that is, deficient in both HR elicitation in nonhost plant species and pathogenicity (and parasitic growth) in host species (49, 56). This complete loss of pathogenic behavior results from mutation of any one of several hrp genes, which largely encode components of a type III protein secretion system (73). Because the capacity to elicit the HR is a convenient marker for the capacity to be pathogenic and these two abilities have a common genetic basis, the “simple” problem of HR elicitation is being studied as an entry to the larger problem of pathogenesis. A key part of the HR puzzle is that HR elicitation and the resulting limitation in host range can occur if the pathogen possesses any one of many possible avr (avirulence) genes that interact with corresponding R (resistance) genes in the host plant. Such “gene-for-gene” interactions result in recognition of the bacterium and the triggering of plant defenses. For example, Pseudomonas syringae pv. glycinea is one of over 40 P. syringae pathovars differing largely in host range among plant species and is subdivided into races on the basis of their interactions with genetically distinct cultivars of its host, soybean. Those race-cultivar interactions involving matching bacterial avr and plant R genes result in the HR and avirulence, i.e.; failure of the bacterium to produce disease. The R genes encode components of a parasite surveillance system and are crossed into crops from wild relatives by plant breeders for disease control. avr genes are identified and cloned on the basis of the avirulence they confer on virulent races in appropriate test plants (39, 69). In most cases, it is not clear why plant pathogens carry avr genes that betray them to host defenses but new insights into this question are discussed below. Both hrp and avr genes were originally defined on the basis of the phenotypes they confer on bacteria interacting with plants. Molecular studies have revealed a functional relationship between the products of these two classes of genes and an underlying similarity with a key virulence system of several animal pathogens. Yersinia, Salmonella, and Shigella spp. transfer virulence effector proteins directly into animal cells via the type III pathway (16, 17, 62, 67, 84). Similarly, plant pathogens use the Hrp type III pathway to transfer Avr effector proteins to the interior of plant cells. The genetic dissection of type III secretion systems is just beginning, and little is known of the mechanisms of protein translocation. In this review, we will describe (i) the recently completed inventory of genes directing type III secretion in plant pathogens and new insights into type III secretion mechanisms gained from research with Hrp systems, (ii) two classes of proteins (harpins and pilins) that are secreted by the Hrp type III pathway when plant pathogens are grown in media that mimic plant intercellular fluids, (iii) evidence that Avr proteins are delivered by the Hrp pathway directly to the interior of plant cells, and (iv) a resulting new paradigm for bacterial plant pathogenicity. The focus will be on quite recent work, and readers are referred to other reviews for a classic introduction to the HR phenomenon (43), earlier investigations of the Hrp system (11), avr genes (20, 46), and a wider perspective on bacterial virulence systems and plant responses (2).

Developmental and pathogen-induced accumulation of transcripts of polygalacturonase-inhibiting protein in Phaseolus vulgaris L.

Expression of the polygalacturonase-inhibiting-protein gene ( pgip) during development of French bean (Phaseolus vulgaris L. cv. Red Kidney) was analysed in various organs, including leaves, stems and roots of etiolated and light-grown seedlings, light-grown adult plants, as well as flowers, pods, and seeds. In light-grown seedlings and plants, pgip transcripts were detected in all tissues examined, with higher levels found in the transition zone between the elongating and mature region of young hypocotyls, and in the basal region of the stem of adult plants. In etiolated hypocotyls, levels of pgip transcripts in the elongating region were five- to six fold higher than in light-grown hypocotyls. High levels of transcripts were also observed in pods. Accumulation of pgip mRNA was also followed in both compatible and incompatible race-cultivar interactions between Colletotrichum lindemuthianum races α and γ, respectively, and P. vulgaris by Northern blot analysis and in situ hybridisation. In situ experiments showed, in incompatible interactions, a rapid, intense and transient accumulation of pgip mRNA in epidermal cells proximal to the site of infection, and, less intense, within the cortical parenchyma underneath. In compatible interactions, no substantial accumulation of pgip mRNA was observed in hypocotyls; a very weak accumulation was observed in leaves during lesion formation.

The accumulation of PGIP is correlated with the hypersensitive response in racecultivar interactions

The accumulation of PGIP is correlated with the hypersensitive response in racecultivar interactions

Accumulation of the pathogenesis-related tomato leaf protein P14 as an early indicator of incompatibility in the interaction between Cladosporium fulvum (Syn. Fulvia fulva) and tomato

The interaction between Cladosporium fulvum and tomato was used as a model gene-for-gene relationship to study the accumulation of the pathogenesis-related protein P14 in time-course experiments. P14 is present in limited amounts in healthy controls. In 20 different race-cultivar interactions which were tested for the accumulation of P14 (eight incompatible and 12 compatible interactions) it was found that P14 accumulated 4–6 days earlier in incpmpatible interactions than in compatible ones. Although the differential accumulation of P14 was quantitative rather than qualitative in nature, it can be used as an early indicator of incompatibility in the interaction between C. fulvum and tomato. The accumulation of P14 appeared to be correlated with the hypersensitive response or some other form of stress. In the compatible interaction a significant accumulation of P14 occurred at the switch from biotrophy to necrotrophy. Abiotic induction by substances such as ethephon, acrylic acid and HgCl2 leads to accumulation of P14, but at lower levels and more slowly than induction by C. fulvum. Abiotic induction was always local in contrast to induction by C. fulvum. which could be systemic over short distances. A significant amount of P14 accumulated in the intercellular spaces of tomato indicating that P14 is at least partly secreted into the apoplast after synthesis.

Isolation and Characterization of an Elicitor of Necrosis Isolated from Intercellular Fluids of Compatible Interactions of Cladosporium fulvum (Syn. Fulvia fulva) and Tomato

Intercellular fluids of compatible race-cultivar interactions of Cladosporium fulvum and tomato contain specific elicitors of necrosis. These elicitors which are of fungal origin induce chlorosis and necrosis in resistant but not in susceptible plants. With the tomato cultivar Sonatine (carrying resistance gene Cf9, resistant to the fungal races 0, 4, 5, 2, 2.4, and 2.4.5 but susceptible to race 2.4.5.9) as the test plant for assaying necrosis-inducing activity, we isolated and partially characterized an elicitor of necrosis on this cultivar. The elicitor bound to CM-Sephadex but not to DEAE-Sephadex; it was stable to heat (10 minutes at 100 degrees C), HCl (0.01 normal), NaOH (0.01 normal), and NaIO(4) (0.02 molar), sensitive to pronase and protease (from Bacillus polymyxa) but not to other proteases such as alpha-chymotrypsin and trypsin. After electrophoresis of partially purified elicitor preparations under low pH conditions, the necrosis-inducing activity was association with a peptide with an apparent molecular weight of 5500. Races 0, 4, 5, 2.4, and 2.4.5 but not race 2.4.5.9 produced this elicitor in high yields. The elicitor is probably a product of avirulence gene A9 which is present in all races except in race 2.4.5.9 and induces necrosis in cultivars carrying resistance gene Cf9.

Origin of specific elicitors of chlorosis and necrosis occurring in intercellular fluids of compatible interactions of Cladosporium fulvum (syn. Fulvia fulva) and tomato

Specific elicitors of chlorosis and necrosis were obtained from intercellular fluids of compatible race-cultivar interactions of Cladosporium fulvum and tomato. The specificity of the elicitors was not determined by the gene(s) for resistance present in the susceptible cultivar from which the intercellular fluids were obtained but by the race of the invading fungus, i.e. by the virulence genes present in the race. The fungus is most probably the producer of the specific elicitors. The role of the host plant in the production of the specific elicitors is discussed.

Some observations on hyphal growth and host responses in leaf-sheath of rice cultivars inoculated with Pyricularia fungi.

Hyphal growth and host responses were investigated in rice leaf-sheaths inoculated with Pyricularia oryzae and other Pyricularia spp. When the sheaths of the nine rice cultivars, having different genes of resistance to P. oryzae, were inoculated with the incompatible race of rice blast fungi, there were large differences in hyphal growth among the cultivars. In the cvs. Kanto 51 and Toride 1, the growth of most intercellular hyphae of the incompatible races was very poor in the values of both the average and highest indices; all invading hyphae were located in the first invaded cells. In the cvs. Ishikari-shiroke and Yashiro-mochi, however, most invading hyphae of the incompatible races developed well into the sheath cells; fungal behaviors and morphological changes of host cells showed almost same tendency as those in the compatible race-cultivar interactions. When non-detached sheaths of those cultivars were inoculated with the incompatible races of P. oryzae, blast lesions similar to susceptible lesion were formed on the surface of leaf-sheath 10 days after inoculation. On the other hand, in case of non-host interactions with non-pathogenic Pyricularia spp., hyphal growth was very poor in all combinations.

Evidence for the occurrence of race and cultivar-specific elicitors of necrosis in intercellular fluids of compatible interactions of Cladosporium fulvum and tomato

Race and cultivar-specific elicitors were isolated from intercellular fluids of compatible race-cultivar interactions of Cladosporium fulvum and tomato. The elicitors specifically induced chlorosis and necrosis in resistant but not in susceptible plants.Elicitors obtained from Cladosporium fulvum grown on synthetic media or on intercellular fluids of healthy tomato cultivars were neither race nor cultivar-specific, since they induced necrosis in all cultivars irrespective of the resistance genes the cultivars contained.The intercellular fluids of compatible race-cultivar interactions do not appear to contain specific suppressors of necrosis, or if they do their activity is masked by the specific elicitors.