Zoospores use electric fields to target roots

Abstract

Species of Phytophthora and Pythium are a group of water molds, pathogens that colonize the roots of higher plants, and cause the familiar root rots. These Oomycetes have flagellated zoospores that allow them to swim in water and can carry them long distances. Zoospores are produced in large numbers and are the cause of most new infections on a wide variety of plant hosts. As they move through the soil, they are attracted to the tips of plant roots where they lodge, encyst and germinate to produce germ tubes that penetrate the roots. The targeting mechanisms of swimming zoospores have long been thought to consist only of chemotaxis toward root exudates, including those secreted from wounds. So how does one explain the marked spatial differences that exist in the sites of accumulation of zoospores at root surfaces or why zoospores of some species are attracted to wound sites and others are not? Although a few chemicals that elicit chemotaxis have been identified, including two isoflavones of soybean (daidzein and genistein) that attract the zoospores of the soybean pathogen, Phytophthora sojae, few other chemicals have been identified.Now Pieter van West and colleagues [1xOomycete plant pathogens use electric fields to target roots. van West, P. et al. Mol. Plant–Microbe Interact. 2002; 15: 790–798Crossref | PubMedSee all References][1] have shown that electrical signals can augment or override chemotaxis in mediating short-range tactic responses of oomycete zoospores at root surfaces, the first demonstration that electrotaxis plays a major role in a plant–microbe interaction under natural conditions. The roots of perennial rye grass (Lolium perenne) and wheat (Triticum aestivum) have been shown to have outward flows of positive current around the root cap, meristematic and elongation zones, that is, the apex or distal parts of roots are anodic, whereas the root hair zone and wound sites are cathodic. Among the evidence presented, Van West et al. [1xOomycete plant pathogens use electric fields to target roots. van West, P. et al. Mol. Plant–Microbe Interact. 2002; 15: 790–798Crossref | PubMedSee all References][1] showed that the positive or negative electrotactic responses of zoospores provide the most plausible explanation for the marked asymmetries of accumulation of zoospores on the surfaces of roots. For example, zoospores of Pythium aphanidermatum (cathodotactic) accumulated mainly at the rear of rye grass roots (anodic) and at wound sites, whereas Phytophthora palmivora zoospores (anodotactic) accumulated in the apical (anodic) zones and were excluded from the cathodic wound sites. These results were essentially reversed by treating the roots with fusicoccin, which reverses the electric field around the root.‘…the positive or negative electrotactic responses of zoospores provide the most plausible explanation for the marked asymmetries of accumulation of zoospores on the surfaces of roots.’This, and much other evidence, suggested to van West et al. [1xOomycete plant pathogens use electric fields to target roots. van West, P. et al. Mol. Plant–Microbe Interact. 2002; 15: 790–798Crossref | PubMedSee all References][1] that electrotaxis augments other root-targeting mechanisms, overriding chemotaxis when the effector molecules are at concentrations normal to the rhizosphere, and that chemotaxis and electrotaxis participate in the homing responses of zoospores toward roots. Perhaps, as they also suggest, root surface charges, generated as they are by the metabolism of living roots, ensure that zoospores colonize living rather than dead roots.