Attachment ofEnterobacter cloacaeto Hyphae ofPythium ultimum: Possible Role in the Biological Control of Pythium Preemergence Damping-Off

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Abstract
Interactions between Enterobacter cloacae and Pythium ultimum were evaluated to determine the possible mechanisms by which E. cloacae protects plants against Pythium preemergence damping-off. In vitro, all strains of E. cloacae tested induced growth reductions of P. ultimum ranging from 33 to 54%. Antibiotics, toxins, or cell-wall-degrading enzymes were not detected in culture filtrates. In culture media amended with D-glucose, D-galactose, sucrose, N-acetyl-D-glucosamine, or .beta.-methyl-D-glucoside, E. cloacae caused little or no reductions in growth of P. ultimum. However, in the absence of sugar or in the presence of D-raffinose, L-sorbose, 3-O-methyl-D-glucose, or .alpha.-methyl-D-glucoside, reductions in mycelial dry weight of P. ultimum due to E. cloacae ranged from 16 to 41%. Growth inhibition of P. ultimum was associated with the binding of E. cloacae to fungal hyphae as determined from scanning electron microscopy and from agglutination of cell wall fragments of P. ultimum by E. cloacae. Sugars that effectively blocked binding and cell wall agglutination also reduced the ability of E. cloacae to inhibit growth of P. ultimum. When E. cloacae was evaluated as a biological control agent on a variety of plant species, those whose seeds exuded high levels of carbohydrates were not protected by E. cloacae and the rate of seed colonization by Pythium spp. was the same for bacterized and nonbacterized seeds. E. cloacae was effective as a biological control agent only when coated onto seeds of species that exuded low levels of carbohydrates during germination. Treatment of cucumber [Cucumis sativus] seeds (low sugar exudation) with E. cloacae, in combination with sugars that interfere with bacterial binding to hyphae of P. ultimum in vitro, effectively eliminated biological control activity in soil without directly stimulating fungal colonization of cucumber seeds by Pythium spp. Seed colonization by Pythium spp. was not increased compared with bacterized seeds nor was biological control activity eliminated when sugars that do not interfere with the binding of E. cloacae to hyphae of P. ultimum were incorporated into seed treatments. It was concluded that the ability of E. cloacae to function as a biological control agent on seed surfaces was directly related to its ability to bind to hyphae of seed colonizing Pythium spp. and to inhibit further hyphal development and infection at the seed surface.

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