THE INFECTION PROCESS OF FUSARIUM-OXYSPORUM IN COTTON ROOT TIPS

Conidia of Fusarium oxysporum f. sp. vasinfectum started to germinate on the roots of cotton (Gossypium bardadense L.) 6 h after inoculation and formed a compact mycelium covering the root surface. 18 h later, penetration hyphae branched off and infected the root. The number of p enetration hyphae increased with the number of conidia used for inocul ation. The optimal temperature for penetration was between 28 and 30 d egrees C. The highest numbers of penetration hyphae were found in the meristematic zone, 40 percent less in the elongation and root hair zon es, and none in the lateral root zone. The fine structure of the infec tion process was studied in protodermal cells of the meristematic zone and in rhizodermal cells of the elongation zone. The penetration hyph ae were well preserved after freeze substitution and showed a Golgi eq uivalent consisting of three populations of smooth cisternae. Plant re actions were found already during fungal growth on the root surface. I n the meristematic zone, a thickening of the plant cell wall due to an apposition of dark and lightly staining material below the hyphae occ urred. This wall apposition increased in size around the hypha invadin g the plant cell and led to the formation of a prominent wall appositi on with finger-like projections into the host cytoplasm. In the elonga tion zone, the deposits around the penetration hypha appeared less thi ck and the dark inclusions were less pronounced. High pressure freezin g of infected cells revealed, that F. oxysporum penetrates and grows w ithin the host cells without inducing damages such as plasmolysis, cel l degeneration or even host necrosis. We suggest that F. oxysporum has an endophytic or biotrophic phase during colonization of the root tip s.