Phytoalexin response is elicited by a pathogen (Rhizoctonia solani) but not by a mycorrhizal fungus (Glomus mosseae) in soybean roots

Abstract

Summary. A container system was constructed to study the response of soybean roots to infection by mycorrhizal or pathogenic fungi. The system allows a rapid and synchronous inoculation by Glomus mosseae or Rhizoctonia solani. The phytoalexin glyceollin was measured in roots of inoculated and uninoculated plants for a period of 30 days. A significantly increased content of phytoalexin was found in R. solani-infected roots as compared to uninfected control roots. However, there was no difference in the glyceollin contents of the mycorrhizal and the control roots for up to 23 days after inoculation. The accumulation of glyceollin in R. solani-infected roots was not influenced by a subsequent inoculation with G. mosseae. Moreover glyceollin accumulated in mycorrhizal plants to the same extent as in control plants when they were inoculated with R. solani. The two fungi did not mutually influence the course of infection when they were inoculated together. Key words. VA-mycorrhiza; Glycine max; glyceollin; rhizosphere; rhizotron.The vesicular-arbuscular mycorrhiza (VAM) is a widespread symbiosis between plants and fungi. Although there are many studies on the established symbiosis, for instance concerning phosphorus uptake, carbon transport, heavy-metal toxicity or the influence of pesticides, little is known about the physiology of the interaction between plant and fungus at the beginning of infection 1. In particular it is poorly known whether the mutualistic character of the established symbiosis manifests itself already at the beginning of infection, and whether recognition differs from that involved in pathogenic interactions. One reason for the lack of data is the difficulty of handling mycorrhizas in the laboratory. A fast and homogeneous course of infection is needed. In the present work we have employed a modified version of the container system of Schfiepp et al. 2 which allows one to obtain such infections and therefore to investigate the first infection stages of mycorrhiza formation. A short description of our technique has already been published 3. We have used this system to compare the phytoalexin response of roots to infection by a VAM fungus and a pathogen. Phytoalexins are antimicrobial compounds of low molecular weight which accumulate in plants after exposure to microorganisms. Their production appears to result from de novo synthesis of the corresponding biosynthetic enzymes upon a challenge of the plant by a pathogen or an elicitor 4, s. There is evidence that accumulation of phytoalexins at the site of attempted infection is one of the mechanisms conferring resistance against infections in plants 6. The phytoalexin response to pathogens has been particularly well studied in soybeans which accumulate the pterocarpan glyceollin 5.Little is known about the accumulation of this phytoalexin in response to mycorrhizal fungi. An early report indicated glyceollin accumulation in the fully established VAM symbiosis 7. Using our container system, we have recently obtained first indications that the phytoa...

Differences in glucosinolate patterns and arbuscular mycorrhizal status of glucosinolate‐containing plant species

“…After 8 d the seedlings were transferred to a steam-sterilized (40 min, 120mC) mixture of silicate sand, TurFace2 (baked clay substrate which is mechanically broken to a diameter of 2-5 mm ; Applied Industrial Materials, Buffalo Grove, IL, USA) and soil (2 : 2 : 1, v\v\v). Plants were inoculated in a growth chamber (day : night cycle 16 h, 22mC: 8 h, 20mC ; 50% rh) using the compartment system developed by Wyss et al (1991) consisting of three compartments. A central compartment (20i10i2 cm) contained beans (Phaseolus vulgaris L. cv.…”

Plant defence genes are induced in the pathogenic interaction between bean roots and Fusarium solani, but not in the symbiotic interaction with the arbuscular mycorrhizal fungus Glomus mosseae

“…Typical responses are cell wall reinforcement, induction of hydrolytic enzymes, such as chitinase and β-1,3-glucanase, and induction of enzymes involved in the synthesis of antimicrobial phytoalexins (for a review, see Bowles (1990)). Few studies have compared defence responses to pathogens and symbionts in the same plant (Wyss, Boller & Wiemken, 1991 ;Gianinazzi-Pearson et al, 1992).…”

“…Gerdemann & Trappe (Rothamsted isolate). For infection, a container system developed by Wyss et al (1991) was used. Briefly, mycorrhizal bean donor plants were established in the central container compartment using an inoculum consisting of Physalis alkekengi L. roots bearing fungal hyphae, spores and sporocarps of Glomus mosseae.…”

“…Intercellular hyphae were present at all stages of fungal root colonization. Thus, owing to the highly synchronized mode of infection (Wyss et al, 1991), we were able to separate the process of colonization into distinct morphological phases.…”

“…Here, we established that bean roots are able to respond to pathogen attack by activating plant defence genes at the mRNA and protein level. Furthermore, a synchronized infection with actively growing mycorrhizal fungi (Wyss et al, 1991) allowed us to separate the process of mycorrhizal root colonization into several distinct stages. Using these optimized conditions, plant defence-gene expression at early stages of the symbiosis where recognition occurs or, at later stages, where infection might be regulated, would have been detected.…”