Abstract
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. Keywords: Rhizoctonia solani , binucleate Rhizoctonia , cucumber, biological control African Journal of Biotechnology , Vol. 13(5), pp. 640-650, 29 January, 2014
Highlights
The use of hypovirulent and non-pathogenic isolates of Rhizoctonia spp. has been potentially shown as effective biocontrol agents
The same isolates were used and the above inoculation method was followed in this experiment consisting of four treatments: (1) seedlings inoculated with non-pathogenic binucleate Rhizoctonia (BNR) and incubated for two days and challenge inoculated with virulent R. solani and again incubated for 6 days; 2) seedlings inoculated with the nonpathogenic BNR and incubated for 6 days; 3) seedlings inoculated with virulent R. solani without non-pathogenic BNR and incubated for three days; 4) and un-inoculated seedlings incubated for six days)
Contrary to the findings observed on cucumber hypocotyls inoculated with virulent isolate, direct penetration by non-pathogenic BNR W7 was not observed and instead profused production of mucilaginous material was observed on the cuticular layer
Summary
The use of hypovirulent and non-pathogenic isolates of Rhizoctonia spp. has been potentially shown as effective biocontrol agents. The possible mechanisms of protection of creeping bentgrass, bean and cotton seedlings by a binucleate Rhizoctonia (BNR) and hypovirulent Rhizoctonia solani isolate have been studied but the results have led to different assumptions and conclusions. The present study was undertaken to describe in details, through histological study, the mode of colonization by the non-pathogenic BNR and the penetration of the hypocotyls surface and taproot of cucumber seedlings by a virulent R. solani isolate. To justify this initial finding, the result was compared with biochemical studies which revealed the importance of pectic substance and its influence in the suppression of the disease. The reasons of the protection provided by the nonpathogenic BNR to cucumber seedlings against Rhizoctonia damping-off were clarified
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