ENHANCEMENT OF ETHYLENE BIOSYNTHESIS AND GERMINATION BY CYTOKININS AND 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID IN STRIGA-ASIATICA SEEDS

Germination of witchweed (Striga asiatica [L.] Kuntze), an important parasitic weed on several poaceous crops, is stimulated by several synthetic and natural compounds. We investigated the role of ethylene biosynthesis and action in cytokinin-induced germination. Conditioned Striga seeds treated with distilled water, 1-aminocyclopropane-1-carboxylic acid (ACC) or the cytokinins thidiazuron (TDZ), trans zeatin (TZ), benzyladenine (BA) and kinetin (KIN) produced little ethylene. Treatments with cytokinin-ACC combinations enhanced ethylene production. The relative order of activity of the cytokinins in elicitation of the phytohormone was TDZ > TZ > BA > KIN. Germination in response to distilled water and ACC treatments was negligible. Induction of germination by cytokinins varied from low (0%) to moderate (52%). Seeds treated with cytokinin-ACC combinations displayed high rates of germination. The observed germination was positively correlated (gamma = 0.8 and 0.9) with ethylene production. Germination was reduced by silver thiosulphate (STS) and CoCl2, inhibitors of ethylene action and ACC oxidase, respectively. Aminoethoxyvinylglycine (AVG), an ACC-synthase inhibitor, reduced TDZ-induced Striga germination. However, the inhibitory effect of AVG was overcome by addition of ACC. The results are consistent with a model in which Striga germination and embryo growth are limited by low capacity of the seeds to oxidize ACC. The cytokinins promote ACC conversion into ethylene and consequent Striga germination by enhancing ACC oxidase activity and/or synthesis.