In addition to the pathway of synthesis of proline from glutamate there is also the possibility of synthesis of proline from ornithine (derived from the N-acetylglutamate pathway) in higher plants. This can occur by two routes depending upon whether the alpha- or delta-NH₂ moiety of ornithine is transaminated (Mestichelli et al, 1979; Adams and Frank, 1980; Thompson, 1980).

In the case of delta-NH₂ transamination the product is GSA which, in spontaneous equilibrium with P5C, can be converted to proline by P5CR. An ornithine delta-aminotransferase cDNA was isolated from a mothbean (Vigna aconitifolia) cDNA expression library employing a proBA mutant of E. coli grown on N-free medium supplemented with 10 mM ornithine (Verma et al, 1992; Delauney and Verma, 1993, Delauney et al, 1993). In striking contrast to P5CR, ornithine delta-aminotransferase appears to be markedly down-regulated under salinity stress (Delauney et al, 1993). However, in young (12-day old) Arabidopsis plantlets, ornithine delta-aminotransferase (OAT) was induced by salinity stress along with P5CS (Roosens et al, 1998). OAT activity, mRNA abundance and its salt inducibility decrease markedly with plant age (Roosens et al, 1998).

The alternative pathway involves alpha-NH₂ transamination of ornithine (catalyzed by ornithine alpha-transaminase) to alpha-keto-delta-aminovalerate which is in spontaneous equilibrium with pyrroline-2-carboxylate (P2C). This product can be reduced to proline by a pyrroline-2-carboxylate reductase (P2CR) [EC 1.5.1.1].

In vivo tracer studies with labeled ornithine strongly suggest that the P2C pathway of ornithine metabolism is the main route of conversion of ornithine to proline in several plant species investigated (Mestichelli et al, 1979).

Adams E, Frank L 1980 Metabolism of proline and the hydroxyprolines. Annu. Rev. Biochem. 49: 1005-1061.

Delauney AJ, Hu CA, Kishor PB, Verma DP 1993 Cloning of ornithine delta-aminotransferase cDNA from Vigna aconitifolia by trans-complementation in Escherichia coli and regulation of proline biosynthesis. J. Biol. Chem. 268: 18673-18678.

Delauney AJ, Verma DPS 1993 Proline biosynthesis and osmoregulation in plants. Plant J. 4: 215-223.

Mestichelli LJJ, Gupta RN, Spenser ID 1979 The biosynthetic route from ornithine to proline. J. Biol. Chem. 254: 640-647.

Roosens NHCJ, Thu TT, Iskander HM, Jacobs M 1998 Isolation of the ornithine-delta-aminotransferase cDNA and effect of salt stress on its expression in Arabidopsis thaliana. Plant Physiol. 117: 263-271.

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.

Verma DPS, Hu C-AA, Delauney AJ, Miao G-H, Hong Z 1992 Deciphering proline biosynthesis pathways in plants by direct, trans-, and co-complementation in bacteria. In "Biosynthesis and Molecular Regulation of Amino Acids in Plants" (BK Singh, HE Flores, JC Shannon eds), American Society of Plant Physiologists, Waverly Press, Baltimore, MD, pp. 128-138.