THIDIAZURON STIMULATES GERMINATION AND ETHYLENE PRODUCTION IN STRIGA-HERMONTHICA - COMPARISON WITH THE EFFECTS OF GR-24, ETHYLENE AND 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID

Seed germination of the hemiparasitic angiosperm Striga hermonthica is elicited by compounds present in the root exudates of the host plant. A variety of chemicals can substitute for the host-derived signal although the exact mechanism through which these act is unknown. In the present study, thidiazuron (TDZ), a cytokinin-active urea derivative was found to stimulate germination. This thidiazuron-induced germination was shown to be mediated by endogenous ethylene, by a similar mechanism to germination induced by host-root exudate and the synthetic stimulant GR-24. Comparing the effects of TDZ, host-root exudate, GR-24 and the ethylene precursor 1-aminocyclopropane-1-carboxylic (ACC) suggests that ethylene production increases as a result of increased ACC synthesis and is consistent with a model for Striga seed germination in which host-derived signals and other stimulants act by eliciting the synthesis of ethylene via a stimulation of ACC synthase activity. All germination stimulants tested trigger a rapid increase in O-2 uptake by conditioned seeds. The patterns of O-2 uptake following TDZ or GR-24 treatments show similarities and three distinct phases of respiration are apparent. In contrast, stimulation of O-2 uptake by ethylene results in a different pattern, with no distinct phases obvious following the initial burst of activity.