Morphological and molecular confirmation of Albugo resedae (Albuginales; Oomycota) as a distinct species from A. candida

The genus Albugo s.str. causes white blister rust on four families of the Brassicales, Brassicaceae, Capparaceae, Cleomaceae, and Resedaceae. Recent phylogenetic studies have revealed that several host specific lineages are present within Albugo on Brassicales, while it was also confirmed that Albugo candida has an exceptionally wide host range which extends from Brassicaceae to Cleomaceae and Capparaceae. The Albugo species infecting the Resedaceae was attributed in monographic studies as well as local floras to either A. resedae or, applying a broader species concept, to A. candida. In the present study, A. resedae specimens were morphologically and molecularly compared to the five Albugo species so far confirmed from Brassicales, A. candida, A. koreana, A. laibachii, A. lepidii, and A. voglmayrii. Both morphological differences of oospore ornamentation and phylogenetic analysis of cox2 mtDNA sequences provided evidence that A. resedae is distinct from A. candida and from the additional four species so far described from Brassicaceae. It thus seems possible that so far unknown factors restrict Albugo candida to Brassicaceae and its sister families, Cleomaceae and Capparaceae.     

Phylogeny of an Albugo sp. infecting Barbarea vulgaris in Denmark and its frequency of symptom development in natural populations of two evolutionary divergent plant types

The oomycete Albugo candida has long been considered a broad spectrum generalist pathogen, but recent studies suggest that it is diverged into several more specialized species in addition to the generalist Albugo candida sensu stricto. Whereas these species cause the disease white blister rust in many crucifer plants, asymptomatic endophytic infections may be important in the epidemiology of others. One of the plant species attacked by Albugo sp. is the wild crucifer Barbarea vulgaris ssp. arcuata, which is diverged into two phytochemically and genetically different types with different geographical distributions in Europe. These were previously shown to differ strongly in propensity to develop white rust upon controlled infections in the greenhouse. Here, we analyse the phylogenetic relatedness of this local Albugo sp. field isolate to other species and lines of Albugo spp., including others collected on B. vulgaris. We further ask whether the difference in incidence of white rust between the two types of B. vulgaris are also expressed in natural populations.     

A new species of <Emphasis Type="Italic">Pustula</Emphasis> (Oomycetes,Albuginales) is the causal agent of sunflower white rust

White blister rust of sunflower is an emerging disease that is among the most important diseases in this crop in South Africa and has recently spread to Europe. For the genus Albugo, it has been demonstrated that species are mostly at least host genus specific and that several previously overlooked species are present on Brassicaceae. It thus seems likely that previously unrecognised species are also present in the genus Pustula. Based on previous phylogenetic reconstruction in combination with differences in oospore ornamentation, it is revealed that Pustula on sunflower, previously attributed to Pustula tragopogonis (syn. Albugo tragopogonis), is distinct from Pustula on Tragopogon. Therefore, this pathogen is described as a new species, Pustula helianthicola. In addition, taxonomic observations revealed that Pustula tragopogonis is an incorrect name and is replaced by the new combination, Pustula obtusata.     

The only known white blister rust on a basal angiosperm is a member of the genus <Emphasis Type="Italic">Albugo</Emphasis>

Rare pathogens on unusual hosts are often providing valuable insight into the evolution of the pathogen group concerned, but it is often challenging to obtain sequence data for these, as because only very few, often decades-old specimens are available. One such example is Albugo tropica, the white blister pathogen of a basal angiosperm in the genus Peperomia (Piperaceae). For this species, only two, more than 70 and over 120-year-old collections available. Here, sequence data for A. tropica are reported and phylogenetic reconstructions reveal it as the sister group to all other white blister rusts of the genus Albugo. Its isolated position is also reflected by several morphological differences to the other species of the genus, such as very thin-walled sporangia and almost smooth oospores. The isolated phylogenetic position of the pathogen and its host might indicate that it is a relict species trapped on its host. The sister-group relationship to all members of the genus Albugo s.str., which have been investigated using molecular phylogenetics, hints at the possibility, that Albugo might have originated in South America or Gondwana and has later radiated in the holarctic on members of the Brassicales.     

Three new phylogenetic lineages are the closest relatives of the widespread species Albugo candida

White blister rust caused by the obligate biotroph Albugo candida (Albuginaceae; Oomycota) is one of the most notorious and common diseases of Brassicaceae. During the past 5 y, A. candida specimens collected from about 30 host genera were phylogenetically and morphologically investigated in several studies. These not only revealed that A. candida s.str. has a broad host range, encompassing a large number of host plants belonging to Brassicales, but also the presence of previously overlooked species of Albugo with hosts in this order. In this study, we examined specimens from Alyssum, Barbarea, and Rorippa, of which many species were commonly recorded as host plants of A. candida but could not be included in previous works due to the paucity of specimens available. It was revealed that Albugo specimens from Alyssum montanum, Barbarea vulgaris, and various Rorippa species, were placed in three phylogenetically distinct clades, but closer to A. candida s.str. than any previously reported species. Oospores were observed from Albugo specimens parasitic to Rorippa and could be distinguished morphologically from A. candida. Therefore, Albugo rorippae sp. nov. is described and illustrated here. In addition, a key of Albugo species described previously from Brassicales is given. The present study reveals that a large number of Albugo species remain still undiscovered, and that species close to A. candida exist. This could help elucidating the basis of the broad host range of A. candida as opposed to the narrow specialisation that is seemingly present in other species of Albugo on the Brassicaceae.     

The effect of mating system on growth of Arabidopsis lyrata in response to inoculation with the biotrophic parasite Albugo candida

The effects of variation in host reproductive systems on response to pathogens are not well understood. We inoculated individuals from outcrossing and inbreeding populations of North American Arabidopsis lyrata with Albugo candida (white blister rust) to test the effect of mating system and heterozygosity on disease response. We observed three host infection phenotypes, classified as fully resistant, partially resistant and fully susceptible. Overall, inbreeding populations had more susceptible and fewer partially resistant individuals than outcrossing populations, but the highest proportion of resistant individuals was found in two of the inbreeding populations. Mating system did not affect relative growth rate of inoculated plants, but there were strong effects of population and infection phenotype. We conclude that mating system per se does not determine the resistance of natural A. lyrata populations to infection by Albugo, but that the increased variability in responses among inbreeding populations may be due to reduced effective population size.     

Obligate biotrophic pathogens of the genus Albugo are widespread as asymptomatic endophytes in natural populations of Brassicaceae

Mutualistic interactions of plants with true fungi are a well‐known and widespread phenomenon, which includes mycorrhiza and non‐mycorrhizal endophytes like species of Epichloë. Despite the fact that these organisms intrude into plants, neither strong defence reactions nor the onset of symptoms of disease can be observed in most or even all infested plants, in contrast to endophytic pathogens. Oomycetes are fungal‐like organisms belonging to the kingdom Straminipila, which includes diatoms and seaweeds. Although having evolved many convergent traits with true fungi and occupying similar evolutionary niches, widespread oomycete endophytes are not known to date, although more than 500 endophytic pathogens, including species of the obligate biotrophic genus Albugo, have been described. Here, we report that oomycetes of the genus Albugo are widespread in siliques of natural host populations. A total of 759 plants, encompassing four genera with rare reports of white blister incidents and one with common incidents, were collected from 25 sites in Germany. Nested PCR with species‐specific primers revealed that 5–27% of the hosts with rare disease incidence carried asymptomatic Albugo in their siliques, although only on a single plant of 583 individuals, an isolated pustule on a single leaf could be observed. Control experiments confirmed that these results were not because of attached spores, but because of endophytic mycelium. Vertical inheritance of oomycete infections has been reported for several plant pathogens, and it seems likely that in nature this way of transmission plays an important role in the persistence of asymptomatic endophytic Albugo species.     

Consequences of combined herbivore feeding and pathogen infection for fitness of Barbarea vulgaris plants

Plants are often attacked by pathogens and insects. Their combined impact on plant performance and fitness depends on complicated three-way interactions and the plant’s ability to compensate for resource losses. Here, we evaluate the response of Barbarea vulgaris, a wild crucifer, to combined attack by an oomycete Albugo sp., a plant pathogen causing white rust, and a flea beetle, Phyllotreta nemorum. Plants from two B. vulgaris types that differ in resistance to P. nemorum were exposed to Albugo and P. nemorum alone and in combination and then monitored for pathogen infection, herbivore damage, defence compounds, nutritional quality, biomass and seed production. Albugo developed infections in the insect-resistant plants, whereas insect-susceptible plants were scarcely infected. Concentrations of Albugo DNA were higher in plants also exposed to herbivory; similarly, flea beetle larvae caused more damage on Albugo-infected plants. Concentrations of saponins and glucosinolates strongly increased when the plants were exposed to P. nemorum and when the insect-susceptible plants were exposed to Albugo, and some of these compounds increased even more in the combined treatment. The biomass of young insect-susceptible plants was lower following exposure to flea beetles, and the number of leaves of both plant types was negatively affected by combined exposure. After flowering, however, adult plants produced similar numbers of viable seeds, irrespective of treatment. Our findings support the concept that pathogens and herbivores can affect each other’s performance on a host plant and that the plant reacts by inducing specific and general defences. However, plants may be able to compensate for biomass loss from single and combined attacks over time.     

A new perspective on the evolution of white blister rusts: <Emphasis Type="Italic">Albugo</Emphasis> s.str. (Albuginales; Oomycota) is not restricted to Brassicales but also present on Fabales

For almost all groups of pathogens, unusual and rare host species have been reported. Often, such associations are based on single or few collections only, which are frequently hard to access. Many of them later prove to be due to misidentification of the host, the pathogen, or both. Therefore, such reports are often disregarded, or treated anecdotally in taxonomic and phylogenetic studies, regardless of their potential importance to unravelling the evolution of the entire group. Concerning oomycete biotrophs there are several reports of unusual and rare hosts for hardly known pathogens. In the order Fabales, for example, a single species of Albugo, A. mauginii, was described as parasitic to Onobrychis crista-galli about 80 years ago, but not recorded again. All other confirmed members of Albugo s.str. are parasitic to representatives of the families Brassicaceae, Capparaceae, Cleomaceae, and Resedaceae in the order Brassicales. In the present study, molecular phylogenetic analysis of cox2 mtDNA sequences and morphological investigations on an original specimen confirmed the occurrence of a member of Albugo on Fabaceae hosts, with the characteristic thin wall of the secondary sporangia, which is almost uniform in thickness. In phylogenetic analyses the species results as embedded within Albugo s.str. Therefore, it is concluded that the natural host range of Albugo s.str. extends from Brassicales to Fabales via host jumping. Our results underscore that unrevised reports of pathogens from unusual hosts should be reconsidered carefully to obtain a more complete picture of pathogen diversity and evolution.     

Molecular mapping reveals two independent loci conferring resistance to Albugo candida in the east European germplasm of oilseed mustard Brassica juncea

White rust caused by Albugo candida (Pers.) Kuntze is a major disease of the oilseed mustard Brassica juncea. Almost all the released varieties of B. juncea in India are highly susceptible to the disease. This causes major yield losses. Hence, there is an urgent need to identify genes for resistance to white rust and transfer these to the existing commercial varieties through marker-assisted breeding. While the germplasm belonging to the Indian gene pool is highly susceptible to the disease, the east European germplasm of B. juncea is highly resistant. In the present study, we have tagged two independent loci governing resistance to A. candida race 2V in two east European lines, Heera and Donskaja-IV. Two doubled haploid populations were used; the first population was derived from a cross between Varuna (susceptible Indian type) and Heera (partially resistant east European line) and the second from a cross between TM-4 (susceptible Indian type) and Donskaja-IV (fully resistant east European line). In both the resistant lines, a single major locus was identified to confer resistance to white rust. In Heera, the resistance locus AcB1-A4.1 was mapped to linkage group A4, while in Donskaja-IV, the resistant locus AcB1-A5.1 was mapped to linkage group A5. In both the cases, closely linked flanking markers were developed based on synteny between Arabidopsis and B. juncea. These flanking markers will assist introgression of resistance-conferring loci in the susceptible varieties.     

Association of Peronospora parasitica with Albugo candida on Brassica juncea Leaves

Associated infections between Peronospora parasitica and Albugo candida were observed on Brassica juncea leaves. A. Candida appeared first and was followed by P. parasitica. A. candida predisposes the host tissues towards susceptibility to P. parasitica.     

Phylogenetic species recognition reveals host-specific lineages among poplar rust fungi

Fungal species belonging to the genus Melampsora (Basidiomycota, Pucciniales) comprise rust pathogens that alternate between Salicaceae and other plant hosts. Species delineation and identification are difficult within this group due to the paucity of observable morphological features. Several Melampsora rusts are highly host-specific and this feature has been used for identification at the species level. However, this criterion is not always reliable since different Melampsora rust species can overlap on one host but specialize on a different one. To date, two different species recognition methods are used to recognize and define species within the Melampsora genus: (i) morphological species recognition, which is based solely on morphological criteria; and (ii) ecological species recognition, which combines morphological criteria with host range to recognize and define species. In order to clarify species recognition within the Melampsora genus, we applied phylogenetic species recognition to Melampsora poplar rusts by conducting molecular phylogenetic analyses on 15 Melampsora taxa using six nuclear and mitochondrial loci. By assessing the genealogical concordance between phylogenies, we identified 12 lineages that evolved independently, corresponding to distinct phylogenetic species. All 12 lineages were concordant with host specialization, but only three belonged to strictly defined morphological species. The estimation of the species tree obtained with Bayesian concordance analysis highlighted a potential co-evolutionary history between Melampsora species and their reciprocal aecial host plants. Within the Melampsora speciation process, aecial host may have had a strong effect on ancestral evolution, whereas telial host specificity seems to have evolved more recently. The morphological characters initially used to define species boundaries in the Melampsora genus are not reflective of the evolutionary and genetic relationships among poplar rusts. In order to construct a more meaningful taxonomy, host specificity must be considered an important criterion for delineating and describing species within the genus Melampsora as previously suggested by ecological species recognition.     

The effect of Albugo Candida (white blister rust) on the photosynthetic and carbohydrate metabolism of leaves of Arabidopsis thaliana

Albugo candida (pers.) O. Kuntze (white blister rust) is a biotrophic fungus which infects cruciferous plants including Arabidopsis thaliana (L) Heynh. We report the effect of this pathogen on the photosynthetic and carbohydrate metabolism of A. thaliana. As infection progressed A. Candida caused a reduction in the rate of photosynthesis when measured at either ambient or saturating concentrations of CO₂. These data suggested that both chlorophyll and Rubisco were lost from regions of infected leaves, and measurements of chlorophyll, Rubisco content and activity supported these observations. The reduction in the rate of photosynthesis was not caused by closure of stomata as transpiration was unaffected by the disease. Infected leaves accumulated both soluble carbohydrates and starch. The activities of sucrose-phosphate synthase, sucrose synthase and ADP glucose pyrophosphorylase did not change in response to infection. However, the activities of both the wall-bound and soluble acid invertases were higher in infected leaves than in controls; a new soluble invertase isoform with a pl of 5-1 appeared in infected leaves. The possible origin of the increase in wall-bound and soluble invertase activities and its effect on the carbohydrate and photosynthetic metabolism of the leaf are discussed.     

Evolutionary trade-offs at the Arabidopsis WRR4A resistance locus underpin alternate Albugo candida race recognition specificities

The oomycete Albugo candida causes white rust of Brassicaceae, including vegetable and oilseed crops, and wild relatives such as Arabidopsis thaliana. Novel White Rust Resistance (WRR) genes from Arabidopsis enable new insights into plant/parasite co-evolution. WRR4A from Arabidopsis accession Columbia (Col-0) provides resistance to many but not all white rust races, and encodes a nucleotide-binding, leucine-rich repeat immune receptor. Col-0 WRR4A resistance is broken by AcEx1, an isolate of A. candida. We identified an allele of WRR4A in Arabidopsis accession Øystese-0 (Oy-0) and other accessions that confers full resistance to AcEx1. WRR4A^Oy-0 carries a C-terminal extension required for recognition of AcEx1, but reduces recognition of several effectors recognized by the WRR4A^Col-0 allele. WRR4A^Oy-0 confers full resistance to AcEx1 when expressed in the oilseed crop Camelina sativa.     

Ultrastructural characterisation of the host–pathogen interface in white blister-infected Arabidopsis leaves

In this study transmission electron microscopy (TEM) was used to examine details of the host–pathogen interface in Arabidopsis thaliana cotyledons infected by Albugo candida, causal agent of white blister. After successful entry through stomatal pores, the pathogen developed a substomatal vesicle and subsequently produced intercellular hyphae. TEM observations revealed that coenocytic intercellular hyphae ramified and spread intercellularly throughout the host tissue forming several haustoria in host mesophyll cells. Intracellular haustoria were spherical and 4.5 μm in diameter. Each haustorium was connected to intercellular hyphae by a narrow, slender haustorium neck. The cytoplasm of the haustorium included the organelles characteristic of the pathogen. No obvious response was observed in host cells following formation of haustoria. Most of the mesophyll cells contained normal haustoria and the host cytoplasm displayed a high degree of structural integrity. Absence of host cell wall alteration and cell death in penetrated host cells suggest that the pathogen exerts considerable control over basic cellular processes and in this respect, response to this biotrophic Oomycete differs considerably from responses to other pathogens such as necrotrophs. Modification of the host plasma membrane (PM) along the cell wall and around the haustoria, was detected by applying the periodic acid-chromic acid-phosphotungstic acid (PACP) staining technique. After staining with PACP, the host PM was found to be intensely electron dense where it was adjacent to the host cell wall and the distal region of the haustorial neck. By contrast, the extrahaustorial membrane, where the host PM surrounded the haustorium, was consistently very lightly stained.     

Molecular markers linked to white rust resistance in mustard Brassica juncea

 White rust, caused by Albugo candida (Pers.) Kuntze, is an economically important disease of Brassica juncea (L.) Czern. and Coss mustard, particularly in India. The most efficient and cost-effective way of protecting mustard plants from white rust disease is through genetic resistance. The objective of this study was to identify RAPD markers for white rust resistance in an F₁-derived doubled-haploid (DH) population originating from a cross between white rust-susceptible and white rust-resistant breeding lines of B. juncea from the canola-quality B. juncea breeding project of the Agriculture and Agri-Food Canada-Saskatoon Research Centre. The DH population was used to screen for RAPD markers associated with white rust resistance/susceptibility using bulked segregant analysis. Two markers, WR2 and WR3, linked to white rust resistance, flanked the resistance locus Ac2 ₁ and were highly effective in identifying the presence or absence of the resistance gene in the DH population. These two markers were shown to be specific to the Russian source of white rust resistance utilized in this project. It is concluded that the availability of these RAPD markers will enhance the breeding for white rust resistance in B. juncea. Received: 17 December 1997 / Accepted: 7 April 1998     

New species and record of the genus Petalomium (Acari: Heterostigmatina: Neopygmephoridae) associated with ants (Hymenoptera: Formicoidea) from Iran

The mite Petalomium camponoti Hajiqanbar & Khaustov sp. n. (Acari: Prostigmata: Neopygmephoridae) is described based on females recovered from ants Camponotus buddhae Forel (Hymenoptera: Formicidae) which were collected from a nest located on heights near Shirvan city, northeastern Iran. We also found Petalomium gottrauxi Mahunka, 1977 associated with Camponotus aethiops Latreille from the same locality. These findings present the first record of the genus Petalomium Cross from Iran. A key to species of the genus Petalomium associated with ant genus Camponotus Mayr is presented and host range of the Petalomium is discussed.     

Basic compatibility of Albugo candida in Arabidopsis thaliana and Brassica juncea causes broad-spectrum suppression of innate immunity

A biotrophic parasite often depends on an intrinsic ability to suppress host defenses in a manner that will enable it to infect and successfully colonize a susceptible host. If the suppressed defenses otherwise would have been effective against alternative pathogens, it follows that primary infection by the "suppressive" biotroph potentially could enhance susceptibility of the host to secondary infection by avirulent pathogens. This phenomenon previously has been attributed to true fungi such as rust (basidiomycete) and powdery mildew (ascomycete) pathogens. In our study, we observed broad-spectrum suppression of host defense by the oomycete Albugo candida (white blister rust) in the wild crucifer Arabidopsis thaliana and a domesticated relative, Brassica juncea. A. candida subsp. arabidopsis suppressed the "runaway cell death" phenotype of the lesion mimic mutant lsd1 in Arabidopsis thaliana in a sustained manner even after subsequent inoculation with avirulent Hyaloperonospora arabidopsis (Arabidopsis thaliana downy mildew). In sequential inoculation experiments, we show that preinfection by virulent Albugo candida can suppress disease resistance in cotyledons to several downy mildew pathogens, including contrasting examples of genotype resistance to H. arabidopsis in Arabidopsis thaliana that differ in the R protein and modes of defense signaling used to confer the resistance; genotype specific resistance in B. juncea to H. parasitica (Brassica downy mildew; isolates derived from B. juncea); species level (nonhost) resistance in both crucifers to Bremia lactucae (lettuce downy mildew) and an isolate of the H. parasitica race derived from Brassica oleracea; and nonhost resistance in B. juncea to H. arabidopsis. Broad-spectrum powdery mildew resistance conferred by RPW8 also was suppressed in Arabidopsis thaliana to two morphotypes of Erysiphe spp. following pre-infection with A. candida subsp. arabidopsis.     

A phenetic analysis on the genus Lolium (Poaceae) in Iran

In this study, 68 specimens of the genus Lolium were scored for 27 characters, comprising 12 vegetative, 12 inflorescence and 3 seed characters. The aim of the study was to investigate the species relationships within the genus in Iran. The data were analysed using principal components analysis (PCA) and cluster analysis. Both analyses separated L. perenne, L. persicum, L. temulentum, L. multiflorum from each other. According to previous authors, L. rigidum and L. loliaceum show little difference from each other and are not separable to a species level. In this study these two species separated clearly from the other species, but were most closely related to L. perenne following cluster analysis and to L. multiflorum following PCA. L. persicum was found to be the most distinct species within the genus Lolium in Iran.