The interaction of binucleate Rhizoctonia (BNR) anastomosis group (AG)-A isolate W7, Rhizoctonia solani AG-4 and cucumber seedlings were investigated to elucidate the mechanism of biocontrol of Rhizoctonia solani by BNR. Hypocotyls of Cucumis sativus L. cv. Jibai were inoculated with a virulent isolate of R. solani AG-4 isolate C4 and examined with light microscopy and scanning and transmission electron microscopy. The hyphae of the virulent isolate colonized the outer surface of the hypocotyl and penetrated the epidermal and cortical cells to the pith of hypocotyls. The hyphae of a non-pathogenic species of BNR isolate W7 did not penetrate the cortical cells but instead colonized the outer epidermal cells 12 h after inoculation. Accumulation of mucilage occurred on the surface after dense colonization of the hypocotyls surface by the BNR. Observation of the hypocotyls surface pre-inoculated with BNR and challenged inoculated with R. solani showed constricted and deformed hyphae of R. solani, which were prevented to penetrate the epidermal cells. Pre-inoculating of BNR isolate caused profused formation of the mucilage which lysed BNR hyphae and restricted the growth of R. solani. The mucilage was identified as pectic substances. Analysis of pectin contents from dried hypocotyls tissues showed highly methylated pectin with low uronic acid contents in the seedlings inoculated with R. solani and unprotected by BNR while BNR- treated seedlings showed less methylated pectin and high amount of uronic acid. Furthermore, the BNR-inoculated seedlings showed significant (P=0.05) increase in calcium, indicating high amount of wall-bound cations in cell walls. Pectic substances accumulation and increased calcium in the cucumber tissues greatly contributed in the protection of cucumber seedlings against Rhizoctonia damping off by a non-pathogenic species of Rhizoctonia.
Key words: Rhizoctonia solani, binucleate Rhizoctonia, cucumber, biological control.
Abbreviations: BNR, Binucleate Rhizoctonia; SDW, sterile distilled water; WA, water agar; PDA, potato dextrose agar; EtOH, ethyl alcohol; FAA, formaldehyde-acetic acid.
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