Transport of l-Glutamate across the Bacteroid Membrane but Not the Peribacteroid Membrane from Soybean Root Nodules. Michael K. Udvardi. Botany Department, Australian National University, GPO Box 4, Canberra A.C.T. 2601. Caroline L. Salom, and David A. Day. Botany Department, Australian National University, GPO Box 4, Canberra A.C.T. 2601.. MPMI 1:250-254. Accepted 9 September 1988. Copyright 1988 The American Phytopathological Society.

Uptake of l-[U-¹⁴C] glutamate by both peribacteroid membrane-enclosed bacteroids (peribacteroid units) and free bacteroids from soybean root nodules was studied. Free Bradyrhizobium japonicum USDA110 bacteroids were able to accumulate l-glutamate rapidly via a high affinity transport system (K_m = 0.8 µM, V_max = 23 nmoles.min^-1.mg protein^-1). However, the peribacteroid membrane prohibited supply of l-gultamate to enclosed bacteroids. The bacteroid transport system appeared to be relatively specific for carboxy-amino acids, as judged by inhibitor sensitivity, and exhibited a broad pH optimum around pH 6.5. The transport system was not stimulated by ATP or Na⁺ ions. Bacteroid l-glutamate uptake was, however, inhibited by uncoupler and respiratory poisons. We conclude that, although bacteroids have the potential to accumulate l-glutamate rapidly, the peribacteroid membrane is essentially impermeable to l-glutamate. The results suggest that l-glutamate does not play a role in carbon supply to the bacteroid during nitrogen fixation.