ETHYLENE BIOSYNTHESIS AND STRIGOL-INDUCED GERMINATION OF STRIGA-ASIATICA

Germination of witchweed [Striga asiatica (L.) Kuntze], an important parasite on cereal crops, is stimulated by several natural and synthetic compounds. In the present study the role of ethylene in germination of Striga asiatica in response to strigol was examined. Unconditioned seeds and those conditioned for 3 days produced negligible amounts of ethylene in response to strigol. However, extending the conditioning period to 5 and 8 days increased ethylene evolution by more than 10-fold. Ethylene production preceded radicle protrusion and was detectable within 3 h after treatment. No germination was observed in the first 6 h of exposure to strigol. Germination and ethylene production increased with strigol concentration. Strigol-induced germination was considerably reduced by the ethylene action inhibitors, 2,5-norbornadiene, silver thioSulphate and CO2. The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) at 5 to 200 muM elicited neither germination nor ethylene production. However, a combination of strigol and ACC resulted in a high germination rate and copious ethylene production. Both germination and ethylene production were reduced by CoCl2 and cycloheximide, inhibitors of the ethylene-forming enzyme and of protein synthesis, respectively. The results are consistent with a model in which conditioning and strigol are required to remove a restriction on the ethylene biosynthetic pathway and in which the ethylene-forming enzyme is rate limiting.