GABA SHUNT IN DEVELOPING SOYBEAN SEEDS IS ASSOCIATED WITH HYPOXIA

In the present study we investigated the proposal that the gamma-amino butyrate (Gaba) shunt in developing soybean (Glycine mns [L.] Men.) se eds is associated with hypoxia. The ontogeny and pH profile of enzymes associated with glutamate metabolism (glutamate decarboxylase [EC 4.1 .1.15], Gaba transaminase [EC 2.6.1.19], succinic semialdehyde dehydro genase [EC 1.2.1.16], glutamate dehydrogenase [EC 1.4.1.2], glutamate: oxaloacetate transaminase [EC 2.6.1.1], glutamate:pyruvate transaminas e [EC 2.6.1.2] and 2-oxoglutarate dehydrogenase complex [EC 1.2.4.2]) and hypoxia (alcohol dehydrogenase [ADH, EC 1.1.1.1] and pyruvate deca rboxylase [PDC, EC 4.1.1.1]) were determined in cotyledons, nucellus a nd seed-coat tissues. Gaba-shunt enzymes were ubiquitous in the develo ping seed. Activities of enzymes catalyzing glutamate-C entry into the Krebs cycle via 2-oxoglutarate were generally greater than those of G aba-shunt enzymes. In cotyledons, the activity of ADH increased throug hout seed development (up to 72 days after anthesis [DAA]), whereas PD C was static during early development, then increased. In contrast, th e activities of ADH and PDC in maternal tissues (nucellus and seed coa t) were initially high, then declined dramatically after 37 DAA. The a denylate energy charge (AEC)=([ATP]+0.5 [ADP])/ ([ATP] + [ADP] + [AMP] ) of soybean seeds from fruits (37 DAA) frozen in situ was low (0.67+/ -0.01) compared to the AEC of adjacent pod tissue (0.82+/-0.04) and co tyledons exposed to air (0.84+/-0.01). A 60-min time-course study show ed that the rate of [U-C-14]-glutamate catabolism by an intact excised cotyledon at 37 DAA was markedly lower at 8 and 0% O-2 than at 21%; t he pool size of [C-14]-Gaba was unaffected. The data indicated that: ( 1) Gaba-shunt activity is not a response to limited glutamate deaminat ion/transamination; (2) the soybean seed is hypoxic; and (3) the relat ive partitioning of glutamate-C through glutamate decarboxylase is inc reased by hypoxia.