Do phenolics foil fungal foes?
It is sad but true that so much more research effort has been expended on the crops of developed countries than those of less-developed countries. An example of the latter is cassava (
Manihot esculenta), a starchy staple of the tropics and sub-tropics. It is, as reported by
Gómez-Vásquez et al. (Bath, UK, pp. 87–97), subject to attack by around 25 different pathogens. However, so little is known about its responses to pathogen attack that it is difficult to select for specific resistance mechanisms. The group have therefore studied the synthesis of defence chemicals in cassava leaves and cell cultures, and it is on the cell cultures that we focus here. Pathogen attack was mimicked by challenging the cells with the yeast cell wall glucan which, in many plant species, is a powerful elicitor of defence mechanisms. This proved to be also true for cassava cells. The cells showed an oxidative burst, including rapid synthesis of hydrogen peroxide, which is typical of early responses to attack by non-compatible pathogens. This was followed, within a few hours, by up-regulation of the gene encoding phenylalanine-ammonia lyase, leading to increases in the activity of the enzyme itself. This is involved in the synthesis of phenolic compounds but, strangely, the amounts of relevant phenolics did not increase. Indeed, the concentrations of these phenolics in cultured cells were very low (although they were much higher in leaves). Nevertheless, the effectiveness of cassava phenolics was tested and four of them, esculetin, ferulic acid, quercetin and scopoletin, were shown to inhibit germ tube growth in
Fusarium oxysporum and in
Trichoderma species. A later event was an increase in peroxidase activity and although the role of this is not clear, it may be involved in oxidation of phenolics thereby increasing their pathogen-inhibiting activity. This work thus provides a good basis for understanding defence mechanisms in cassava.
j.a.bryant{at}exeter.ac.uk
