POSSIBLE ROLE OF ROOT-ETHYLENE IN FE(III)-PHYTOMETALLOPHORE UPTAKE INSTRATEGY-II SPECIES

All phytometallophores are derived from methionine through S-adenosylm ethionine (SAM) via nicotianamine. Ethylene is synthesized from methio nine via SAM and 1-aminocyclopropane-1-carboxylic acid (ACC). This clo se similarity in biochemical pathways suggests that root-ethylene may play a role in regulating Fe(III)-phytometallophore homeostasis in cer eal (Strategy II species) roots as well as in the regulation of Fe(III )-chelate reductase activity in Strategy I species. Barley (Hordeum vu lgare L.) seedlings were grown in chelate-buffered nutrient solutions with increasing levels of Fe (i.e., 5, 25 or 100 mu M Fe) as Fe(III)-H EDTA. Seedlings at each level of Fe were treated with either an inhibi tor or a promoter of ethylene action. Treatment with the promoter, ACC (1 mu M), had no significant effect on phytometallophore root efflux or Fe uptake by 19-d-old barley seedlings at all Fe levels. However, t reatment with the inhibitor, aminooxyacetic acid (AOA, 10 mu M) repres sed the ability of cereal roots to absorb sufficient Fe to meet metabo lic needs, but surprisingly enhanced phytometallophore root efflux rat es at all Fe(III)-HEDTA levels. These results support a possible role of root-ethylene in Fe(III) uptake in cereals, but the mechanism remai ns unclear.