Evidence for the importance of enzymatic digestion of epidermal walls during subepidermal sporulation and pustule opening in white blister rusts (Albuginaceae)

The Arabidopsis WRR4A and WRR4B paralogous NLR proteins both confer recognition of multiple Albugo candida effectors

The oomycete Albugo candida causes white blister rust, an important disease of Brassica crops. Distinct races of A. candida are defined by their capacity to infect different host plant species. Each A. candida race encodes secreted proteins with a CX₂ CX₅ G ('CCG') motif that are polymorphic and show presence/absence variation, and are therefore candidate effectors. The White Rust Resistance 4 (WRR4) locus in Arabidopsis thaliana accession Col-0 contains three genes that encode intracellular nucleotide-binding domain leucine-rich repeat immune receptors. The Col-0 alleles of WRR4A and WRR4B confer resistance to multiple A. candida races, although both WRR4A and WRR4B can be overcome by the Col-0-virulent race 4 isolate AcEx1. Comparison of CCG candidate effectors in avirulent and virulent races, and transient co-expression of CCG effectors from four A. candida races in Nicotiana sp. or A. thaliana, revealed CCG effectors that trigger WRR4A- or WRR4B-dependent hypersensitive responses. We found eight WRR4A-recognised CCGs and four WRR4B-recognised CCGs, the first recognised proteins from A. candida for which the cognate immune receptors in A. thaliana are known. This multiple recognition capacity potentially explains the broad-spectrum resistance to several A. candida races conferred by WRR4 paralogues. We further show that of five tested CCGs, three confer enhanced disease susceptibility when expressed in planta, consistent with A. candida CCG proteins being effectors.

A new desert-dwelling oomycete, Pustula persica sp. nov., on Gymnarrhena micrantha (Asteraceae) from Iran

The obligate biotrophic oomycete genus Pustula is one of the four major linages of white blister rusts (Albuginaceae) identified so far. Species of the genus Pustula cause white blister rust on numerous genera in the asterids, represented by several phylogenetically distinct genus-specific lineages, most of which still await formal description. Thus, the observation of the species of Pustula on the Asteraceae subfamily Gymnorhenoideae pointed out to the existence of a hitherto undescribed species. By the morphological and molecular phylogenetic investigation conducted in this study it is concluded that the pathogen on Gymnarrhena micrantha from Iran indeed represents a hitherto unknown species and is described as P. persica. This species has apparently adapted to desert condition and is, after Albugo arenosa, the second species of white blister rust from Iranian deserts, highlighting the adaptability of white blister rusts to hot and dry habitats.

Genomic and Molecular Perspectives of Host-pathogen Interaction and Resistance Strategies against White Rust in Oilseed Mustard

Oilseed brassicas stand as the second most valuable source of vegetable oil and the third most traded one across the globe. However, the yield can be severely affected by infections caused by phytopathogens. White rust is a major oomycete disease of oilseed brassicas resulting in up to 60% yield loss globally. So far, success in the development of oomycete resistant Brassicas through conventional breeding has been limited. Hence, there is an imperative need to blend conventional and frontier biotechnological means to breed for improved crop protection and yield.

This review provides a deep insight into the white rust disease and explains the oomycete-plant molecular events with special reference to Albugo candida describing the role of effector molecules, A. candida secretome, and disease response mechanism along with nucleotide-binding leucine-rich repeat receptor (NLR) signaling. Based on these facts, we further discussed the recent progress and future scopes of genomic approaches to transfer white rust resistance in the susceptible varieties of oilseed brassicas, while elucidating the role of resistance and susceptibility genes. Novel genomic technologies have been widely used in crop sustainability by deploying resistance in the host. Enrichment of NLR repertoire, over-expression of R genes, silencing of avirulent and disease susceptibility genes through RNA interference and CRSPR-Cas are technologies which have been successfully applied against pathogen-resistance mechanism. The article provides new insight into Albugo and Brassica genomics which could be useful for producing high yielding and WR resistant oilseed cultivars across the globe.

Host-parasite interaction during subepidermal sporulation and pustule opening in rust fungi (Pucciniales)

In late infection stages, rust fungi sporulate by building blister-like structures (pustules) in the mesophyll under the epidermal layer of their hosts. I investigated the host-pathogen interaction during pustule development and pustule opening in three different host-parasite combinations with different types of sori: Puccinia lagenophorae developing aecia, Phakopsora pachyrhizi developing uredinia, and Puccinia malvacearum developing telia. Light microscopy as well as scanning and transmission electron microscopy was applied. Although the development of the three host-parasite combinations varied in their soral development, there were common features detectable. During late infection stages, middle lamellae of mesophyll cells were dissolved locally to clear space for the pustule-building hyphae. Host cells were shoved aside, and hyphae adhering to host cells and to other hyphae by an extracellular matrix built new compact pseudoparenchyma, normally without killing host cells. The epidermal cells were separated from the mesophyll cells, and the growing pustule lifted up the covering tissue. The dissolution of the middle lamella of the anticlinal walls of the epidermal cells became visible. Scanning electron microscopy revealed that the epidermal cells collapsed over the bulging pustules, fissures appeared along cell boundaries, and finally the epidermal layer ruptured and the spores were set free. The interaction between hosts and pathogens is discussed.

Ustilago species causing leaf-stripe smut revisited

Leaf-stripe smuts on grasses are a highly polyphyletic group within Ustilaginomycotina, occurring in three genera, Tilletia, Urocystis, and Ustilago. Currently more than 12 Ustilago species inciting stripe smuts are recognised. The majority belong to the Ustilago striiformis-complex, with about 30 different taxa described from 165 different plant species. This study aims to assess whether host distinct-lineages can be observed amongst the Ustilago leaf-stripe smuts using nine different loci on a representative set. Phylogenetic reconstructions supported the monophyly of the Ustilago striiformis-complex that causes leaf-stripe and the polyphyly of other leaf-stripe smuts within Ustilago. Furthermore, smut specimens from the same host genus generally clustered together in well-supported clades that often had available species names for these lineages. In addition to already-named lineages, three new lineages were observed, and described as new species on the basis of host specificity and molecular differences: namely Ustilago jagei sp. nov. on Agrostis stolonifera, U. kummeri sp. nov. on Bromus inermis, and U. neocopinata sp. nov. on Dactylis glomerata.

The Top 10 oomycete pathogens in molecular plant pathology

Oomycetes form a deep lineage of eukaryotic organisms that includes a large number of plant pathogens which threaten natural and managed ecosystems. We undertook a survey to query the community for their ranking of plant-pathogenic oomycete species based on scientific and economic importance. In total, we received 263 votes from 62 scientists in 15 countries for a total of 33 species. The Top 10 species and their ranking are: (1) Phytophthora infestans; (2, tied) Hyaloperonospora arabidopsidis; (2, tied) Phytophthora ramorum; (4) Phytophthora sojae; (5) Phytophthora capsici; (6) Plasmopara viticola; (7) Phytophthora cinnamomi; (8, tied) Phytophthora parasitica; (8, tied) Pythium ultimum; and (10) Albugo candida. This article provides an introduction to these 10 taxa and a snapshot of current research. We hope that the list will serve as a benchmark for future trends in oomycete research.

De novo sequence assembly of Albugo candida reveals a small genome relative to other biotrophic oomycetes

Background

Albugo candida is a biotrophic oomycete that parasitizes various species of Brassicaceae, causing a disease (white blister rust) with remarkable convergence in behaviour to unrelated rusts of basidiomycete fungi.

Results

A recent genome analysis of the oomycete Hyaloperonospora arabidopsidis suggests that a reduction in the number of genes encoding secreted pathogenicity proteins, enzymes for assimilation of inorganic nitrogen and sulphur represent a genomic signature for the evolution of obligate biotrophy. Here, we report a draft reference genome of a major crop pathogen Albugo candida (another obligate biotrophic oomycete) with an estimated genome of 45.3 Mb. This is very similar to the genome size of a necrotrophic oomycete Pythium ultimum (43 Mb) but less than half that of H. arabidopsidis (99 Mb). Sequencing of A. candida transcripts from infected host tissue and zoosporangia combined with genome-wide annotation revealed 15,824 predicted genes. Most of the predicted genes lack significant similarity with sequences from other oomycetes. Most intriguingly, A. candida appears to have a much smaller repertoire of pathogenicity-related proteins than H. arabidopsidis including genes that encode RXLR effector proteins, CRINKLER-like genes, and elicitins. Necrosis and Ethylene inducing Peptides were not detected in the genome of A. candida. Putative orthologs of tat-C, a component of the twin arginine translocase system, were identified from multiple oomycete genera along with proteins containing putative tat-secretion signal peptides.

Conclusion

Albugo candida has a comparatively small genome amongst oomycetes, retains motility of sporangial inoculum, and harbours a much smaller repertoire of candidate effectors than was recently reported for H. arabidopsidis. This minimal gene repertoire could indicate a lack of expansion, rather than a reduction, in the number of genes that signify the evolution of biotrophy in oomycetes.

The oomycete Pythium oligandrum expresses putative effectors during mycoparasitism of Phytophthora infestans and is amenable to transformation

The oomycete Pythium oligandrum is a mycoparasitic biocontrol agent that is able to antagonise several plant pathogens, and can promote plant growth. In order to test the potential usefulness of P. oligandrum as a biocontrol agent against late blight disease caused by the oomycete Phytophthora infestans, we investigated the interaction between P. oligandrum and Ph. infestans using the green fluorescent protein (GFP) as a reporter gene. A CaCl(2) and polyethylene-glycol-based DNA transformation protocol was developed for P. oligandrum and transformants constitutively expressing GFP were produced. Up to 56 % of P. oligandrum transformants showed both antibiotic resistance and fluorescence. Mycoparasitic interactions, including coiling of P. oligandrum hyphae around Ph. infestans hyphae, were observed with fluorescent microscopy. To gain further insights into the nature of P. oligandrum mycoparasitism, we sequenced 2376 clones from cDNA libraries of P. oligandrum mycelium grown in vitro, or on heat-killed Ph. infestans mycelium as the sole nutrient source. 1219 consensus sequences were obtained including transcripts encoding glucanases, proteases, protease inhibitors, putative effectors and elicitors, which may play a role in mycoparasitism. This represents the first published expressed sequence tag (EST) resource for P. oligandrum and provides a platform for further molecular studies and comparative analysis in the Pythiales.

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.