Plasmodiophorids: The Challenge to Understand Soil-Borne, Obligate Biotrophs with a Multiphasic Life Cycle

Abstract

Plasmodiophorids are an enigmatic group of obligate biotrophic pathogens of higher plants. Together with their sister group phagomyxids, which infect stramenopiles, they form the monophyletic eukaryote clade phytomyxids. They have long been treated as a basal group of fungi, but recent molecular phylogenies point to a close affiliation with the protozoan phylum Cercozoa. The soil-borne and plant-associated nature of plasmodiophorids as well as their multi-stage life cycle with zoosporic, plasmodial, and resting stages has hindered comprehensive research on this group. Plasmodiophorids cannot be cultured without their hosts, and direct observations of any stage of the plasmodiophorid life cycle are difficult and time-consuming. Molecular techniques provide valuable tools for the identification and monitoring of organisms which are difficult to assess with traditional approaches – such as plasmodiophorids. Several different immunological or nucleic acid-based techniques, and more recently genomic and proteomic approaches have been used to investigate plasmodiophorids, their life style, and their interactions with their host plants. Nonetheless, advances in knowledge about plasmodiophorids provided by molecular techniques are mainly restricted to the few economically important species that cause diseases of agricultural crops. Although their taxa may be well described, the available phylogenies of phytomyxids are rather incomplete, as they include only a few selected species. A main reason for this bias is that most specimens deposited in herbaria are too old, soaked in fixatives or otherwise unavailable for DNA analyses. To fully understand this group of protists, more research on “rare”, under-recorded species is needed.

In this review, we discuss the impact of molecular techniques on the detection, monitoring, and characterisation of plasmodiophorids. First, we will briefly introduce plasmodiophorid biology and the taxonomic twists and turns the group has taken to reach its current taxonomic position. Development of methods and experimental progress towards better understanding of plasmodiophorids are then sketched, from classical approaches to the recent “-omics” approaches. We will also discuss future implications of molecular methods, which it is hoped will help to improve knowledge about the role of plasmodiophorids within ecosystems.