PHYSIOLOGIA PLANTARUM 94: 219-228, 1995 Cpw Printed in Denmark - alt rif-ht>. reserved Gaba shunt in developing soybean seeds is associated with hypoxia Barry J. Shelp, Craig S. Walton, Wayne A. Snedden, Lucie G. Tuin, Ivan J. Oresnik and David B. Layzell Shelp, BJ., Walton.C.S..Snedden,W.A..Tuin.L.G.,Oresnik,I.J.andLayzell.D.B. 1995. Gaba shunt in developing soybean seeds is associated with hypoxia. - Physiol. Plant. 94: 219-228. In the present study we investigated the proposal that Ihe )'-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.U], glutamate:oxaloacetate transaminase [EC 2.6.1.1], glutamate:pyruvate transaminase [EC 2.6.1.2] and 2-cxoglutarate dehydrogenase complex [EC 1.2.4.2]) and hypoxia (aicohol 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-'^C]-glutamate catabolism by an intact excised cotyledon at 37 DAA was markedly lower at 8 and 0% O: than at 21%; the pool size of ['• 'C]-Gab a was unaffected. The data indicated that: (i) 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. Key words - Adenylate energy charge, Gaba shunt, glutamate decarboxylase, Glycine max. hypoxia. Krebs cycle, stress. B.J. Shelp icorresponding author). C.S. Walton, W.A. Snedden andL. G. Tuin. Dept of Horticultural Science and Interdepartmental Plant Physiology Program. Univ. of Giielph. Guelph, ON. Canada N1G2W1; I.J. Oresnik and D.B. Layzell. Dept of Biology. Queen's Univ., Kingston. ON. Canada K7L3N6. nucellus, which separates the seed coat from the embryo, Introanction Balance sheets for the utilization of amino compounds in Developing seeds must receive their nutrients for growth maturing fruits of cowpea (Peoples et al. 1985) and in and development from the mother plant (Pate 1984, developing seeds of soybean (Micallef and Shelp 1989a) Thome 1985), The major form of N delivered to a soy- indicate that many other amino acids present in seed bean embryo is generally helieved to he glutamine (Rain- protein are synthesized in situ. Thus, glutamate and/or bird et al, 1984), hut Hsu et al, (1984) suggested that glutamine are probably involved in a number of bio- glutamine is quantitatively converted to glutamate in the synthetic reactions leading to the synthesis of amino Received 7 September, 1994; revised 17 February, 1995 Physiol.Planl. 94, 1995 219