Lactate fermentation may contribute to the acidification of the cytoplasm (Roberts et al, 1984; Ratcliffe, 1995), which may progressively inhibit LDH and activate PDC, promoting alcohol fermentation (Rivoal and Hanson, 1994).

Accumulation of pyruvate under anaerobic conditions may shift the equilibrium of alanine aminotransferases towards alanine accumulation; a rapid response of plants to anaerobic stress (Streeter and Thompson, 1972; Stewart and Larher, 1980):

An alanine aminotransferase (glutamate:pyruvate aminotransferase [EC 2.6.1.2]) is induced under anaerobic conditions in barley roots (Good and Crosby, 1989; Good and Muench, 1992).

The precise function of alanine accumulation under anoxia is unknown, but it is speculated that it may serve as a storage form of pyruvate (perhaps in the vacuole), controlling supply of pyruvate to LDH and PDC, and hence flux to lactate and ethanol (Good and Crosby, 1989; Good and Muench, 1992).

The synthesis of alanine may occur at the expense of the acidic amino acids, glutamate and aspartate (Streeter and Thompson, 1972; Stewart and Larher 1980), and occurs concomitantly with the accumulation of 4-aminobutyrate or gamma-aminobutyrate (GABA) (Streeter and Thompson, 1972; Stewart and Larher, 1980; Wallace et al, 1984; Ratcliffe, 1995).

Note that there are several transaminases involved in alanine synthesis and metabolism (e.g.):

1. glutamate + pyruvate <---> 2-oxoglutarate + alanine [EC 2.6.1.2]

2. alanine + glyoxylate <---> pyruvate + glycine [EC 2.6.1.44]

3. pyruvate + GABA <---> alanine + succinic semialdehyde [similar to EC 2.6.1.19]

The second enzyme is involved in photorespiration, and the third enzyme is involved in GABA metabolism. See Betsche and Eisling (1986) and Lea et al (1989) for a discussion of the role of alanine in photorespiratory nitrogen metabolism.

The first enzyme appears to be the one induced by anaerobic stress (Good and Muench, 1992). The latter reference contains many additional references on alanine aminotransferases from higher plant sources.

In Chlamydomonas reinhardtii alanine:2-oxoglutarate aminotransferase [EC 2.6.1.2] is transiently induced by growth under low CO₂ conditions (Chen et al, 1996).

Betsche T, Eisling R 1986 Refixation of photorespiratory ammonia and the role of alanine in photorespiration: Studies with ¹⁵N. Plant and Soil 91: 367-371.

Chen Z-Y, Burow MD, Mason CB, Moroney JV 1996 A low-CO₂-inducible gene encoding an alanine:alpha-ketoglutarate aminotransferase in Chlamydomonas reinhardtii. Plant Physiol. 112: 677-684.

Davies DD 1980 Anaerobic metabolism and the production of organic acids. In (DD Davies ed) "The Biochemistry of Plants", Vol 2, Academic Press, New York, pp 581-611.

Good AG, Crosby WL 1989 Anaerobic induction of alanine aminotransferase in barley root tissue. Plant Physiol. 90: 1305-1309.

Good AG, Muench DG 1992 Purification and characterization of an anaerobically induced alanine aminotransferase from barley roots. Plant Physiol. 99: 1520-1525.

Lea PJ, Blackwell RD, Murray AJS, Joy KW 1989 The use of mutants lacking glutamine synthetase and glutamate synthase to study their role in plant nitrogen metabolism. In (JE Poulton, JT Romeo, EE Conn eds) "Plant Nitrogen Metabolism", Plenum Press New York, pp. 157-189.

Ratcliffe RG 1995 Metabolic aspects of the anoxic response in plant tissue. In "Environment and Plant Metabolism: Flexibility and Acclimation" (N Smirnoff ed), Bios Scientific, Oxford, pp. 111-127.

Rivoal J, Hanson AD 1994 Metabolic control of anaerobic glycolysis. Overexpression of lactate dehydrogenase in transgenic tomato roots supports the Davies-Roberts hypothesis and points to a critical role for lactate secretion. Plant Physiol. 106: 1179-1185.

Roberts JKM, Callis J, Jardetsky O, Walbot V, Freeling M 1984 Cytoplasmic acidosis as a determinant of flooding intolerance in plants. Proc. Natl. Acad. Sci. U.S.A. 81: 6029-6033.

Stewart GR, Larher F 1980 Accumulation of amino acids and related compounds in relation to environmental stress. In "The Biochemistry of Plants" (BJ Miflin ed), Vol. 5, Academic Press, New York, pp. 609-635.

Streeter JG, Thompson JF 1972 Anaerobic accumulation of gamma-aminobutyric acid and alanine in radish leaves (Raphanus sativus L.). Plant Physiol. 49: 572-578.

Wallace W, Secor J, Schrader LE 1984 Rapid accumulation of gamma-aminobutyric acid and alanine in soybean leaves in response to an abrupt transfer to lower temperature, darkness, or mechanical manipulation. Plant Physiol. 75: 170-175.