Chapter 26: Phylum Plasmodiophoromycota: Endoparasitic Slime Molds

Plasmodiophoromycota represents a specialized phylum of obligate endoparasitic organisms commonly referred to as endoparasitic slime molds, which establish infections within vascular plants and stramenopiles by invading host cells and triggering pathological changes including cellular enlargement, increased cell division, and vascular tissue disruption. These parasites cause economically devastating plant diseases such as clubroot affecting cruciferous crops through Plasmodiophora brassicae infection and powdery scab of potatoes caused by Spongospora subterranea, with the latter organism also inducing crook root symptoms in watercress plants. Beyond direct damage, plasmodiophorids function as vectors for numerous plant viruses, substantially magnifying their agricultural and economic significance. The life cycle of these organisms exhibits a characteristic alternation between two plasmodial stages: primary plasmodia that develop zoosporangia with thin cell walls capable of producing biflagellate zoospores, and secondary plasmodia that generate resistant sporosori containing durable resting spores capable of persisting in soil for extended periods. A striking cytological feature involves cruciform mitosis, where chromosomes arrange around an elongated nucleolus in a distinctive configuration unlike typical fungal nuclear division patterns. Infection initiates when biflagellate zoospores encounter and encyst upon host root hair cells, followed by penetration facilitated through specialized cellular structures termed Rohr and Stachel. Once internalized, multinucleate plasmodia proliferate within host tissues before eventually fragmenting into zoosporangia or resting sporosori. Although sexual reproduction mechanisms remain incompletely characterized, evidence indicates that plasmogamy among amoeboid cells may precede karyogamy in the formation of resting spores. Taxonomically, molecular phylogenetic investigations have revealed that despite morphological similarities to fungi, Plasmodiophoromycota shares closer evolutionary relationships with protistan groups, particularly ciliates, rather than with true fungi. The phylum encompasses approximately 29 species distributed across 10 genera, including the genera Plasmodiophora, Polymyxa, Spongospora, and Woronina, exemplifying how obligate intracellular parasites develop distinctive cytological mechanisms, specialized pathogenic strategies, and ecological roles within plant-pathogen systems.