Microscopic analysis of the compatible and incompatible interactions between Fusarium oxysporum f. sp. conglutinans and cabbage

Abstract

Fusarium oxysporum is a soil-borne fungus that causes vascular wilt and root rot. To compare infection and colonization by F. oxysporum f. sp conglutinans (Foc) on resistant and susceptible cabbage cultivars and to get more insights into interactions between Foc and cabbage, a green fluorescent protein (GFP)-expressing strain of Foc was inoculated onto resistant and susceptible cabbage varieties and monitored by laser scanning confocal microscopy (LSCM). Between 1 and 3 days post inoculation (dpi), conidia attached to root hairs and emergence sites of lateral roots, with hyphae penetrating into the epidermal tissues of roots. Few differences between susceptible and resistant cultivars were observed up to 1 to 3 dpi. From 4 to 6 dpi, hyphae progressed from the epidermis into cortical tissues and entered the xylem vessels in the susceptible cultivar, whereas such colonization was rarely observed in the resistant cultivar; these observations were further supported by statistical analysis. From 7 to 11 dpi, massive colonization and sporulation within infected xylem vessels were observed with hyphae extending into the upper hypocotyl of susceptible seedlings. Progressing symptoms of chlorosis, wilting, and death were observed on susceptible seedlings, whereas the resistant cultivar remained symptom-free. Foc was not reisolated from upper stems and petioles of the resistant cultivar, which confirmed our observations. This study provided a clear overview of the process of colonization of cabbage roots by Foc, and deepened our understandings on the details of compatible and incompatible interactions between Foc and cabbage cultivars.