Introduction to the Plasmodiophorids

On this web site we are using the informal term "plasmodiophorids" for the group that was the subject of Karling's 1968 monograph, The Plasmodiophorales, and for a number of years was included in the fungi (Sparrow 1960, Waterhouse 1972). Donald Barr recognized that ultrastructural evidence suggested that plasmodiophorids should be considered as protozoa (Barr 1992). Subsequent studies (Adl et al. 2012, Bass et al. 2009, Cavalier-Smith & Chao 2003) classified plasmodiophorids as Phytomyxea within the Cercozoa, which is in the Rhizaria, a supergroup that many people traditionally associate with protozoa. Burki et al (2010), however, grouped the Phytomyxea with Gromia Dujardin along with a clade of Acantherea and Foraminifera outside the core Cercozoa when some assumptions were made using large numbers of phytomyxid gene sequences. Sierra et al (2015) placed the Phytomyxea along with the Vampyrellida in a novel clade within the Rhizaria, Phytorhiza, whereas Ruggiero et al (2015), in what they considered to be a consensus classification of all living organsims, classified the plasmodiophorids, viz., Order Plasmodiophorida, as follows.

  • SUPERKINGDOM—Eukaryota
    • KINGDOM—Chromista
      • INFRAKINGDOM—Rhizaria
        • PHYLUM—Cercozoa
          • SUBPHYLUM—Endomyxa
            • CLASS—Phytomyxea
              • ORDER—Phagomyxida
              • ORDER—Plasmodiophorida

Cavalier-Smith et al (2018), in a revision of the classification of the chromist infrakingdom Rhizaria, placed the Order Plasmodiophorida as follows.

  • KINGDOM—Chromista
    • INFRAKINGDOM—Rhizaria
      • PHYLUM—Retaria
        • SUBPHYLUM—Endomyxa
          • SUPERCLASS—Proteomyxia
            • CLASS—Phytomyxea
              • ORDER—Phagomyxida
              • ORDER—Plasmodiophorida

Molecular studies have led to recognition of two orders within Phytomyxea (Bass et al. 2009, Cavalier-Smith & Chao 2003): Plasmodiophorida (the group we are referring to here as "plasmodiophorids"); and Phagomyxida, which includes the genera Maullinia I. Maier, E. R. Parodi, R. Westermeier et D. G. Müller, and Phagomyxa Karling. This web site remains focused on the plasmodiophorids sensu stricto, leaving the Phagomyxida to others.

Reviews for plasmodiophorids include Cook (1933), Karling (1968), Dylewski (1989), Dick (2001), Neuhauser et al. (2010), Bulman and Braselton (2014), and Bulman and Neuhauser (2017). Braselton (1995) surveyed problems in the plasmodiophorids associated with terminology and several genera.

Regardless of where the plasmodiophorids are classified, they may be considered a monophyletic taxon because all members share the derived character state, cruciform nuclear division. Other features of plasmodiophorids include a) zoospores with two, anterior whiplash flagella; b) multinucleated protoplasts (plasmodia); c) obligate, intracellular parasitism; and d) environmentally-resistant resting spores (cysts).

Economically significant plasmodiophorids include Plasmodiophora brassicae Woronin, the causative agent of clubroot of cabbage and other brassicaceous crops; Spongospora subterranea (Wallroth) Lagerheim, the causative agent of powdery scab of potato; S. nasturtii M. W. Dick, the causative agent of crook root of watercress; and Polymyxa betae Keskin, which, along with a virus, is associated with rhizomania of sugar beet. Also, Polymyxa graminis Ledingham, P. betae, S. subterranea and S. nasturtii serve as vectors for viruses of crops including barley, wheat, potatoes, and watercress.

Terminology for structures and life cycle stages is varied because of contributions from researchers from different disciplines. The terminology used on this web site follows recommendations by John Karling (1981).