Root colonization by Bipolaris sorokiniana in different cereals and relations to lesion development and natural root cortical cell death

A GUS-transformed strain of B. sorokiniana was used to study the relationship between fungal growth, lesion development and natural root cortical cell death (RCD) in roots of different cereals. Roots of 10-day-old seedlings, grown on filter paper, were inoculated with the fungus and at different time intervals lesion size and GUS-activity in the roots were determined. A significant, positive correlation was found between GUS-activity and ergosterol content in barley roots infected with this transformed strain and these results indicate that GUS can be used as a marker to study fungal growth in plant tissue. The fungus had a slower growth rate in resistant barley varieties, i.e., those producing smaller lesions, than in more susceptible varieties. In wheat and triticale, the fungal growth was faster than in barley, despite the smaller and lighter coloured lesions in these species. This may be explained by the fact that wheat and triticale have a faster rate of root cortical senescence than barley. Presumably, dying cortex cells cannot respond to the fungal invasion by producing phenolic compounds that cause browning of the tissue. Among seven investigated cereal species, there was a positive correlation between the degree of RCD and fungal biomass increase in roots after inoculation.