Gaba shunt in developing soybean seeds is associated with hypoxia

Abstract

In the present study we investigated the proposal that the γ‐aminobutyrate (Gaba) shunt in developing soybean (Glycine max [L.] Merr.) seeds 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 dehydrogenase [EC 1.2.1.16], glutamate dehydrogenase [EC 1.4.1.2], glutamate:oxaloacetate transaminase [EC 2.6.1.1], glutamate:pyruvate transaminase [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 decarboxylase [PDC, EC 4.1.1.1]) were determined in cotyledons, nucellus and seed‐coat tissues. Gaba‐shunt enzymes were ubiquitous in the developing seed. Activities of enzymes catalyzing glutamate‐C entry into the Krebs cycle via 2‐oxoglutarate were generally greater than those of Gaba‐shunt enzymes. In cotyledons, the activity of ADH increased throughout seed development (up to 72 days after anthesis [DAA]), whereas PDC was static during early development, then increased. In contrast, the activities of ADH and PDC in maternal tissues (nucellus and seed coat) were initially high, then declined dramatically after 37 DAA. The adenylate 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 cotyledons exposed to air (0.84 ± 0.01). A 60‐min time‐course study showed that the rate of [U‐14C]‐glutamate catabolism by an intact excised cotyledon at 37 DAA was markedly lower at 8 and 0% O2 than at 21%; the pool size of [14C]‐Gaba was unaffected. The data indicated that: (1) Gaba‐shunt activity is not a response to limited glutamate deamination/transamination: (2) the soybean seed is hypoxic; and (3) the relative partitioning of glutamate‐C through glutamate decarboxylase is increased by hypoxia.