A decrease in proline oxidation rate can contribute to net proline accumulation during drought and salinity stress. Proline is oxidized to P5C by proline dehydrogenase [EC 1.5.99.8] and the resulting P5C is further oxidized to glutamate by P5C dehydrogenase (oxidoreductase) [EC 1.5.1.12].

Both enzymes are localized in the mitochondrion (Thompson, 1980; Elthon and Stewart, 1982; Rayapati and Stewart, 1991; reviewed in Hare and Cress (1997) and Hare et al (1998).

A cDNA encoding proline dehydrogenase (PDH) has recently been cloned from Arabidopsis and has been shown to be induced by proline under non-stressed conditions, but strongly repressed in response to osmotic stress (Kiyosue et al, 1996). It appears that osmotic stress over-rides proline induction of proline dehydrogenase gene expression (Kiyosue et al, 1996; Nakashima et al, 1998). The reciprocal regulation of P5CS and PDH may account for the rapid accumulation of proline in response to osmotic stress and its rapid catabolism upon stress relief (Peng et al, 1996).

The gene encoding P5C dehydrogenase has not yet been cloned from higher plants, but the enzyme has recently been extensively purified and characterized (Forlani et al, 1997).

For a recent discussion of the regulation of proline synthesis, degradation and transport in plants in relation to water deficits, see Taylor (1996).

Elthon TE, Stewart CR 1982 Proline oxidation in corn mitochondria. Plant Physiol. 70: 567-572.

Forlani G, Scainelli D, Nielsen E 1997 Delta¹-pyrroline-5-carboxylate dehydrogenase from cultured cells of potato. Plant Physiol. 113: 1413-1418.

Hare PD, Cress WA 1997 Metabolic implications of stress-induced proline accumulation in plants. Plant Growth Regulation 21: 79-102.

Hare PD, Cress WA, Van Staden J 1998 Dissecting the roles of osmolyte accumulation during stress. Plant Cell Envir. 21: 535-553.

Kiyosue T, Yoshiba Y, Yamaguchi-Shinozaki K, Shinozaki K 1996 A nuclear gene, encoding mitochondrial proline dehydrogenase, an enzyme involved in proline metabolism, is upregulated by proline but downregulated by dehydration in Arabidopsis. Plant Cell 8: 1323-1335.

Nakashima K, Satoh R, Kiyosue T, Yamaguchi-Shinozaki K, Shinozaki K 1998 A gene encoding proline dehydrogenase is not only induced by proline and hypoosmolarity, but is also developmentally regulated in the reproductive organs of Arabidopsis. Plant Physiol. 118: 1233-1241.

Peng Z, Lu Q, Verma DP 1996 Reciprocal regulation of delta¹-pyrroline-5-carboxylate synthetase and proline dehydrogenase genes control proline levels during and after osmotic stress in plants. Mol. Gen. Genet. 253: 334-341.

Rayapati PJ, Stewart CR 1991 Solubilization of a proline dehydrogenase from maize (Zea mays L.) mitochondria. Plant Physiol. 95: 787-791.

Taylor CB 1996 Proline and water deficit: ups and downs. Plant Cell 8: 1221-1224.

Thompson JF 1980 Arginine synthesis, proline synthesis, and related processes. In "The Biochemistry of Plants", Vol 5 (BJ Miflin ed) Academic Press, New York, pp. 375-403.