IN-VITRO INDOLE-3-ACETIC-ACID UPTAKE IN SYMBIOSOMES FROM SOYBEAN (GLYCINE-MAX L) ROOT-NODULES

Symbiosomes isolated from Bradyrhizobium japonicum-soybean (Glycine ma x L.) root nodules were incubated in vitro with [H-3] indole-3-acetic acid (IAA). This resulted in a time-dependent increase in radioactivit y in the symbiosomes indicating IAA uptake. A plot of 1/V versus 1/[IA A] indicated that a nonsaturable uptake system was present in the symb iosome membrane with an additional, saturable uptake mechanism, which operates at low IAA concentrations. The transporter exhibited an appar ent K-m of 140 nM and-a V-max of 4.1 pmol . min(-1). mg(-1) protein. T he auxin-transport inhibitor naphthyl-phthalamic acid (NPA) inhibited IAA accumulation in the symbiosomes at low IAA concentrations. This in dicates that the saturable uptake mechanism may be ascribed to the pre sence in the symbiosome membrane of an auxin-efflux carrier system, wh ich directs transport of IAA from the plant cytosol towards the bacter oids.