Abstract
M.Sc.
An increasing problem in subtropical regions, such as South Africa, is the susceptibility of various banana varieties to Fusarium wilt by the soil borne pathogen Fusarium oxysporum tsp. cubense (FOC). In this study the problem of fungal susceptibility of banana was addressed by investigating the biochemical aspects of cell wall strengthening in banana roots.
Defence responses were induced in both adult and tissue culture tolerant Goldfinger and susceptible Williams banana cultivars by treatment of the plants with a heat-released elicitor preparation from the mycelial cell walls of FOC race 4 and the crude filtrate. Banana plants were maintained in a hydroponic system, before being inoculated with the elicitor and crude filtrate preparation.
Differences in lignin content, callose deposition, phenolics and the enzymes involved in cell wall strengthening; (PAL, CAD, POD and PPO) between the tolerant and susceptible banana cultivars were investigated. Differences in defence responses after treatment with elicitor and with crude filtrate were observed, but it was shown that the former is a more efficient experimental system for the characterisation of susceptible and tolerant responses in banana cultivars. An elicitor concentration of 45 4g/m1 greatly induced cellular POD, PPO, PAL and CAD activity in Goldfinger, whereas no significant increase was observed for Williams. Lignin content increased significantly in Goldfinger compared to Williams. The quantitative determination of induced total phenolics, phenolic glycosides, phenolic esters and cell wall-bound phenolic acids were higher in Goldfinger than in Williams. These increases in the four phenolic subfractions were clearly confirmed by reverse phase HPLC. No significant increase in callose accumulation was observed for both cultivars.
The obtained results indicate an important role for cell wall strengthening as an inducible defence mechanism of banana roots against FOC race 4.