Starch Plays a Key Role in Sporosorus Formation by the Powdery Scab Pathogen Spongospora subterranea.

Powdery scab disease, caused by the soilborne protist Spongospora subterranea f. sp. subterranea, poses a major constraint to potato production worldwide. Disease symptoms include damage to the tuber skin and the formation of root galls. This study aimed to investigate the potential mechanism behind the formation of sporosori, which are aggregates of resting spores, within root galls. Scanning electron microscopy analysis revealed that the early stage of gall formation, characterized by a white color, involved the accumulation of starch grains, which later disappeared as the gall matured and turned brown. The mature brown galls were found to contain fully formed sporosori. Light microscopy examination of ultramicrotome sections of the root galls showed that the high-amylopectin starches were surrounded by a plasmodium, a precursor to sporosorus. These findings suggest that starch grains contribute to the formation of a sponge-like structure within the sporosori. A significant reduction in total starch levels in both the root galls and their associated roots was observed compared with healthy roots. These findings indicate starch consumption by sporosori during the maturation of root galls. Interestingly, analysis of the transcript levels of starch-related genes showed downregulation of genes encoding starch degrading enzymes and an amylopectin-debranching enzyme, whereas genes encoding a starch synthase and a protein facilitating starch synthesis were upregulated in the infected roots. Overall, our results demonstrate that starch is consumed during sporosorus formation, and the pathogen likely manipulates starch homeostasis to its advantage for sporosorus development within the root galls.

Molecular data reallocates Sorosphaerula radicalis (Plasmodiophorida, Phytomyxea, Rhizaria) to the genus Hillenburgia.

This study reports the first record of Sorosphaerula radicalis (Phytomyxea, Rhizaria) in continental Europe (Tirol, Austria) and provides first molecular data for this species. An 18S rRNA phylogeny placed S. radicalis into the Plasmodiophorida, although distant from other members of the genus Sorosphaerula and close to the parasite of water cress Hillenburgia nasturtii. To resolve this polyphyly, we compare morphological data and life cycles of Sorosphaerula veronicae (the type species of the genus Sorosphaerula), Hillenburgia nasturtii, and Sorosphaerula radicalis. We conclude that Sorosphaerula radicalis belongs to the recently established genus Hillenburgia.

Revised Taxonomy and Expanded Biodiversity of the Phytomyxea (Rhizaria, Endomyxa).

Phytomyxea (phytomyxids) is a group of obligate biotrophic pathogens belonging to the Rhizaria. Some phytomyxids are well studied and include known plant pathogens such as Plasmodiophora brassicae, the causal agent of clubroot disease. Despite this economic importance, the taxonomy and biodiversity of this group are largely cryptic, with many species described in the premolecular area. Some of these species were key for establishing the morphotaxonomic concepts that define most genera to this day, but systematic efforts to include and integrate those species into molecular studies are still lacking. The aim of this study was to expand our understanding of phytomyxid biodiversity in terrestrial environments. Thirty-eight environmental samples from habitats in which novel and known diversity of Phytomyxea was expected were analysed. We were able to generate 18S rRNA sequences from Ligniera verrucosa, a species which is well defined based on ultrastructure. Phylogenetic analyses of the collected sequences rendered the genera Lignera, Plasmodiophora and Spongospora polyphyletic, and identified two novel and apparently diverse lineages (clade 17, clade 18). Based on these findings and on data from previous studies, we formally establish the new genera Pseudoligniera n. gen. for L. verrucosa,Hillenburgia n. gen. for Spongospora nasturtii and revert Plasmodiophora diplantherae to its original name Ostenfeldiella diplantherae.

Enigmatic Phytomyxid Parasite of the Alien Seagrass Halophila stipulacea: New Insights into Its Ecology, Phylogeny, and Distribution in the Mediterranean Sea.

Marine phytomyxids represent often overlooked obligate biotrophic parasites colonizing diatoms, brown algae, and seagrasses. An illustrative example of their enigmatic nature is the phytomyxid infecting the seagrass Halophila stipulacea (a well-known Lessepsian migrant from the Indo-Pacific to the Mediterranean Sea). In the Mediterranean, the occurrence of this phytomyxid was first described in 1995 in the Strait of Messina (southern Italy) and the second time in 2017 in the Aegean coast of Turkey. Here we investigated, using scuba diving, stereomicroscopy, light and scanning electron microscopy, and molecular methods, whether the symbiosis is still present in southern Italy, its distribution in this region and its relation to the previous reports. From the total of 16 localities investigated, the symbiosis has only been found at one site. A seasonal pattern was observed with exceptionally high abundance (> 40% of the leaf petioles colonized) in September 2017, absence of the symbiosis in May/June 2018, and then again high infection rates (~ 30%) in September 2018. In terms of anatomy and morphology as well as resting spore dimensions and arrangement, the symbiosis seems to be identical to the preceding observations in the Mediterranean. According to the phylogenetic analyses of the 18S rRNA gene, the phytomyxid represents the first characterized member of the environmental clade "TAGIRI-5". Our results provide new clues about its on-site ecology (incl. possible dispersal mechanisms), hint that it is rare but established in the Mediterranean, and encourage further research into its distribution, ecophysiology, and taxonomy.

Not in your usual Top 10: protists that infect plants and algae.

Fungi, nematodes and oomycetes belong to the most prominent eukaryotic plant pathogenic organisms. Unicellular organisms from other eukaryotic lineages, commonly addressed as protists, also infect plants. This review provides an introduction to plant pathogenic protists, including algae infecting oomycetes, and their current state of research.

Maullinia braseltonii sp. nov. (Rhizaria, Phytomyxea, Phagomyxida): A Cyst-forming Parasite of the Bull Kelp Durvillaea spp. (Stramenopila, Phaeophyceae, Fucales).

Phytomyxea are obligate endoparasites of angiosperm plants and Stramenopiles characterised by a complex life cycle. Here Maullinia braseltonii sp. nov., an obligate parasite infecting the bull kelp Durvillaea (Phaeophyceae, Fucales) from the South-Eastern Pacific (Central Chile and Chiloe Island) and South-Western Atlantic (Falkland Islands, UK) is described. M. braseltonii causes distinct hypertrophies (galls) on the host thalli making it easily identifiable in the field. Sequence comparisons based on the partial 18S and the partial 18S-5.8S-28S regions confirmed its placement within the order Phagomyxida (Phytomyxea, Rhizaria), as a sister species of the marine parasite Maullinia ectocarpii, which is also a parasite of brown algae. The development of resting spores in M. braseltonii is described by light and electron microscopy and confirmed by FISH experiments, which visually showed the differential expression of the 28S non-coding gene, strongly in early plasmodia and weakly in late cysts. M. braseltonii is, so far, the only phytomyxean parasite of brown algae for which the formation of resting spores has been reported, and which is widely distributed in Durvillaea stocks from the Southeastern Pacific and Southwestern Atlantic.

The Large Subunit rDNA Sequence of Plasmodiophora brassicae Does not Contain Intra-species Polymorphism.

Clubroot disease caused by Plasmodiophora brassicae is one of the most important diseases of cultivated brassicas. P. brassicae occurs in pathotypes which differ in the aggressiveness towards their Brassica host plants. To date no DNA based method to distinguish these pathotypes has been described. In 2011 polymorphism within the 28S rDNA of P. brassicae was reported which potentially could allow to distinguish pathotypes without the need of time-consuming bioassays. However, isolates of P. brassicae from around the world analysed in this study do not show polymorphism in their LSU rDNA sequences. The previously described polymorphism most likely derived from soil inhabiting Cercozoa more specifically Neoheteromita-like glissomonads. Here we correct the LSU rDNA sequence of P. brassicae. By using FISH we demonstrate that our newly generated sequence belongs to the causal agent of clubroot disease.

Mitochondrial genome sequence of the potato powdery scab pathogen Spongospora subterranea.

Spongospora subterranea is a soil-borne obligate parasite responsible for potato powdery scab disease. S. subterranea is a member of the order Plasmodiophorida, a protist taxa that is related to Cercozoa and Foraminifera but the fine details of these relationships remain unresolved. Currently there is only one available complete mtDNA sequence of a cercozoan, Bigelowiella natans. In this work, the mitochondrial sequence of a S. subterranea isolate infecting an Andean variety of S. tuberosum ssp. andigena (Diacol-Capiro) is presented. The mtDNA codes for 16 proteins of the respiratory chain, 11 ribosomal proteins, 3 ribosomal RNAs, 24 tRNAs, a RNA processing RNaseP, a RNA-directed polymerase, and two proteins of unknown function. This is the first report of a mtDNA genome sequence from a plasmodiophorid and will be useful in clarifying the phylogenetic relationship of this group to other members in the supergroup Rhizaria once more mtDNA sequences are available.

Zoosporic parasites infecting marine diatoms - A black box that needs to be opened.

Living organisms in aquatic ecosystems are almost constantly confronted by pathogens. Nevertheless, very little is known about diseases of marine diatoms, the main primary producers of the oceans. Only a few examples of marine diatoms infected by zoosporic parasites are published, yet these studies suggest that diseases may have significant impacts on the ecology of individual diatom hosts and the composition of communities at both the producer and consumer trophic levels of food webs. Here we summarize available ecological and morphological data on chytrids, aphelids, stramenopiles (including oomycetes, labyrinthuloids, and hyphochytrids), parasitic dinoflagellates, cercozoans and phytomyxids, all of which are known zoosporic parasites of marine diatoms. Difficulties in identification of host and pathogen species and possible effects of environmental parameters on the prevalence of zoosporic parasites are discussed. Based on published data, we conclude that zoosporic parasites are much more abundant in marine ecosystems than the available literature reports, and that, at present, both the diversity and the prevalence of such pathogens are underestimated.