Mycoparasitism of Rhizoctonia solani by Endophytic Chaetomium spirale ND35 : Ultrastructure and Cytochemistry of the Interaction

Abstract

The interaction between endophytic biocontrol agent Chaetomium spirale ND35 and the soil-borne plant pathogen Rhizoctonia solani was studied by light microscopy and transmission electron microscopy (TEM), as well as further investigated by gold cytochemistry to assess the potential role of cell wall degrading enzymes (CWDEs) during the mycoparasitic process. Macroscopic observations of fungal growth in dual cultures revealed that pathogen growth inhibition occurred soon after contact with the antagonist, followed by the overgrowth of C. spirale on the colony of R. solani. The coiling of C. spirale around R. solani and intracellular growth of the antagonist in its host occurred frequently. Moreover, in advanced stage of interaction between the antagonist and the pathogen, The growth and development of C. spirale were associated with highly morphological changes of the host fungal cell, characterized by retraction of plasma membrane and cytoplasm disorganization. Further, TEM investigations through localization by gold immunocytochemistry showed that contact between the two fungi was mediated by an amorphous ß-1,3-glucan-enriched matrix originating from cell wall of the antagonist C. spirale and sticking to its host surface. At the same time, the hemispherical wall appositions which were intensely labeled by the antibodies of ß-1, 3-glucan in cell wall of R. solani were induced to form at sites of potential antagonist entry. However, the antagonist was capable of penetrating this barrier, indicating that ß-1,3-glucanases were produced during the mycoparasitic process. Localization of N-acetylglucosamine residues (chitin) with the gold-labelled wheat germ agglutinin (WGA) implicated that chitinases might be involved in the CWD of R. solani in this antagonistic process as well. This report is the first evidence about mechanisms of the interactions between C. spirale and R. solani in ultrastructural and cytochemical aspects.