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

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 b-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 b-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 b-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. Introduction Rhizoctonia species are among the most aggressive soilborne pathogens, causing losses on almost all vegetables and flowers, several field crops, turf grasses, and even perennial ornamentals, shrubs, and trees throughout the world (Agrios, 1997). Damping-off, mainly caused by R. solani, of seedlings in forest nurseries is one of the most serious and widely spread diseases, sometimes causing more than 70% seedling mortality (Kaushik et al., 2000). The wide host range of this pathogen as well as its ability to survive as sclerotia under adverse environmental conditions have markedly reduced the potential of crop rotation as a management strategy. In general, chemical pesticides effectively protect plants from this pathogen. However, public concerns about harmful effects of chemical pesticides on the environmental and human health, as well as the pathogens have developed resistance to the fungicides have promoted a research for safer control approach. Biocontrol is an environmental friendly and efficient alternative to chemical pesticides management of this pathogen. Chaetomium Kunze ex Fr. is one of the major genera of ascomycetes and more than 300 Chaetomium species have been described (Reissinger et al., 2003). In the past several decades, one of the most common, C. globosum Kunze ex Fr. has received considerable attention as a potential biocontrol agent for a number of soiland airborne plant pathogens, including R. solani, Pythium ultimum, Sclerotinia sclerotiorum and Venturia inaequalis, etc. The mechanisms of fungi suppression by C. globosum may be mainly based on antibiosis. In addition, mycoparasitism judged by hyphal coiling of the antagonist around R. solani and Alternalia brassicicola in dual cultures was observed (Andrews et al., 1983; Cullen and Andrews, 1984; Vannaeci et al., 1987; Walther and Gindrat, 1988; Di Pietro et al., 1992; Knudsen et al., 1995; Pereira and Dhingra, 1997; Monaco et al., 1998). Chaetomium spirale ND35 is a strain of dominant endophytic fungus isolated from Populus tomentosa www.blackwell-synergy.com J. Phytopathology 153, 280–290 (2005) 2005 Blackwell Verlag, Berlin