Bi,> l l e m A 1996. 9, 45 50 Iron requirement for and effects of promoters and inhibitors of ethylene action on stimulation of Fe(llI)-chelate reductase in roots of strategy I species Francisco J. Romera, Ross M. Welch*, Wendell A. Norvell* & Stephen C. Schaefer* Departamento de Awonomia, Universidad de Cordoba, Cordoba, Spain and *USDA-ARS, US Plant, Soil and Nutrition Laboratory, Tower Road, lthaca, NY, USA Received 31 March 1995:accepled for publication 15 May 1995 Stimulation of root Fe(lll) reductase activity by iron additions to iron-deficient growth media may be the result of iron activation of I-aminocyclopropane-l-carboxylic acid (ACC) oxidase required for ethylene biosynthesis. Two different ethylene inhibitors, aminooxyacetic acid (AOA) (20 pM; ACC synthase inhibitor) and cobalt (3 pM COCI2; ACC oxidase inhibitor), were used to study the effects of iron supply and cobalt inhibition on ethylene action in controlling the activity of Fe(lll)-chelate reductase in pea (Pisum sativum L.) roots. Supplying 20 ~M Fe(lll)-N,/V-ethylenebis[2-(2-hydroxypheyl)-glycine [Fe(III)-EDDHA] to either cobalt-treated, iron-deficient Sparkle (normal parent) or El07 (brz mutant genotype) pea seedlings reversed the negative effects of cobalt on root Fe(lll)-reductase activity. Re-supplying 20 ~M Fe(III)-EDDHA to iron-deficient, AOA-treated seedlings did not enhance root (Fe(lll)-reductase. Apparently, cobalt competes with iron for the active site in ACC oxidase during ethylene synthesis. Inhibition of root reductase activity by cobalt treatment lowered manganese, zinc, magnesium and potassium content of mutant El07 pea seedlings. In contrast, iron enhancement of root reductase activity in iron-deficient, cobalt-treated El07 seedlings resulted in higher seedling accumulations of manganese, zinc, magnesium and potassium. These results support the hypothesis that root cell plasma membrane reduetase activity plays a role in cation uptake by root cells. Keywords: ethylene function, inducible reductase, iron-chelate reductase, iron deficiency, iron-deficiency stress response, roots Introduction Iron-deficiency stress responses in dicots and non-grass monocots (i.e. strategy I species) include growth media aciditication and increased root Fe(llI) reducing capacity (R/Smheld & Marschner 1986). Don-deficiency stress responses to inadequate iron supply allow roots to accumulate more iron from normally insoluble rhizosphere Fe(Ill) pools (R6mheld & Marschner 1986, Bienfait 1988). Iron-sufficient plants repress these iron-deficiency stress responses (R6mheld & Marschner 1981, Maas et al. 1988). Some studies report stimulation of iron-deficiency stress responses, at least transitorily, when a small amount of iron is supplied to iron-deficient plants {Chaney et al. 1972, De Vos et al. 1986, Jolley et al. 1986, Grusak et al. 1990, Romera et al. 1992). Addrcs-, for correspondence: R. M. Welch, US Plant, Soil and Nutrition Laboratory, Tower Road, Ithaca, NY 14853, USA. Tel: [+ I) 607 255-5434; Fax: I - I} 607-255-2459. Chaney et al. (1972) reported higher Fe(IIl) reducing capacity in soybeans supplied 0.32/~M Fe when compared with those supplied 0.1 pM Fe. Grusak et al. (1990) reported higher Fe(Ill) reducing capacity in El07 pea mutant seedlings supplied 2 l~u Fe(IIIDEDDHA than in the same mutants grown without iron additions to their nutrient solutions. Additionally, they found a transitory increase of Fe(IlI) reducing capacity when they supplied 2~M Fe(III)-EDDHA to iron-deficient Sparkle pea plants (Grusak et al. 1990). Romera et al. (1992) reported higher Fe(Ill) reducing capacity in sunflower plants grown with bicarbonate and 21ZM Fe than in the plants grown with bicarbonate but without iron. Jolley et al. (1986) found differences in the time course of the acidification response by the roots of soybeans, depending on the presence or absence of iron in the nutrient solution. In HA soybeans with no added iron, 10 days were required to detect a pH decrease equivalent to that obtained in 5 day old HA soybeans supplied 0.05 mg Fe I l (Jolley et al. 1986). De Vos t" 1996 Rapid Science Publishers BioMetals Vol 9 1996 45