DEFENSE RESPONSES INDUCED BY SOLUBLE SILICON IN CUCUMBER ROOTS INFECTED BY PYTHIUM SPP

The role of silicon (Si) in plant growth was investigated on the basis of induction of defense mechanisms in response to fungal attacks. Amendment of cucumber plants with soluble Si (Si+) resulted in a marked stimulation of chitinase activity and in a more intense and rapid activation of peroxidases and polyphenoloxidases after infection with Pythium spp. Additionally, glycosidically bound phenolics extracted from Si+ plants and subjected to acid or beta-glucosidase hydrolysis (Gly extracts) displayed a strong fungistatic activity against P. ultimum, P. aphanidermatum, and Cladosporium cucumerinum. Extracts obtained in the same way from control plants were not inhibitory to fungal growth, regardless of Si treatment. Three zones with antifungal activity were revealed after separation of Gly extracts from infected Si+ plants on thin-layer chromatography plates. Increased beta-glucosidase activity was detected in protein extracts of infected Si+ plants, which correlated with the presence of fungitoxic aglycones found in the roots of these plants and the plants' efficacy to suppress Pythium spp. These results associate Si with specific plant defense reactions and would explain the reported prophylactic properties of Si against fungal attacks. These reactions appear to be multicomponent, and resistance is contingent on the activation of a cascade of associated biochemical changes.