In vivo labeling studies with ¹⁴C-labeled precursors (Morris et al, 1969; Oaks et al, 1970; Boggess et al, 1976), ¹³C-glutamate (Heyser et al, 1989ab) or ¹⁵N (Rhodes et al, 1986; Rhodes and Handa, 1989) suggest that glutamate is a major precursor of osmotic stress-induced proline accumulation in plants.

Osmotic stress results in an increase of proline biosynthesis rate (Boggess et al, 1976; Stewart, 1981; Rhodes et al, 1986; Rhodes and Handa, 1989). Proline accumulation may in part involve induction (Peng et al, 1996) and/or activation of enzymes of proline biosynthesis, possibly coupled with a relaxation of proline feedback inhibition control of the pathway (Boggess et al, 1976; Stewart, 1981; Delauney and Verma, 1993), decreased proline oxidation to glutamate (Stewart et al, 1977; Stewart and Boggess, 1978; Huang and Cavalieri, 1979; Sells and Koeppe, 1981; Elthon and Stewart, 1982) mediated at least in part by down-regulation of proline dehydrogenase (Kiyosue et al, 1996; Peng et al, 1996), decreased utilization of proline in protein synthesis (Boggess and Stewart, 1980; Stewart, 1981) and enhanced protein turnover (Fukutoku and Yamada, 1984).

Water deficits induce dramatic increases in the proline concentration of phloem sap in alfalfa (Girousse et al, 1996), suggesting that increased deposition of proline at the root apex in water stressed plants (e.g. Voetberg and Sharp, 1991) could in part occur via phloem transport of proline (Girousse et al, 1996). A proline transporter gene, ProT2, is strongly induced by water and salt stress in Arabidopsis thaliana (Rentsch et al, 1996). Homologous proline transporter genes have been identified in tomato; LeProT1 is strongly expressed in mature and germinating pollen, and may encode a general transporter for compatible solutes (Schwacke et al, 1999). LeProT1 transports proline and GABA with low affinity and glycinebetaine with high affinity (Schwacke et al, 1999).

Stress-induced proline accumulation is inhibited by both cycloheximide and cordycepin in Arabidopsis (Verbruggen et al, 1993). In contrast, because cycloheximide is inhibitory to 200 mM NaCl-induced proline accumulation in Distichlis spicata cells, whereas actinomycin D is not inhibitory, Ketchum et al (1991) suggest that translation but not transcription is necessary for production of proline in stressed cells.

Stress-hypersensitive mutants of higher plants which exhibit disturbed proline metabolism (e.g. the sos1 mutant of Arabidopsis) (Liu and Zhu, 1997) can contribute significantly to the elucidation of the signals to which proline accumulation may respond.

Boggess SF, Aspinall D, Paleg LG 1976 Stress metabolism. IX. The significance of end-product inhibition of proline biosynthesis and of compartmentation in relation to stress-induced proline accumulation. Aust. J. Plant Physiol. 3: 513-525.

Boggess SF, Stewart CR 1980 The relationship between water stress induced proline accumulation and inhibition of protein synthesis in tobacco leaves. Plant Sci. Letts. 17: 245-252.

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

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

Fukutoku Y, Yamada Y 1984 Sources of proline-nitrogen in water-stressed soybean (Glycine max). II. Fate of ¹⁵N-labelled protein. Physiol. Plant. 61: 622-628.

Girousse C, Bournoville R, Bonnemain J-L 1996 Water deficit-induced changes in concentrations in proline and some other amino acids in the phloem sap of alfalfa. Plant Physiol. 111: 109-113.

Heyser JW, De Bruin D, Kincaid ML, Johnson RY, Rodriguez MM, Robinson NJ 1989a Inhibition of NaCl-induced proline biosynthesis by exogenous proline in halophilic Distichlis spicata suspension cultures. J. Exp. Bot. 40: 225-232.

Heyser JW, Chacon MJ, Warren RC 1989b Characterization of L-[5-¹³C]-proline biosynthesis in halophytic and nonhalophytic suspension cultures by ¹³C NMR. J. Plant Physiol. 135: 459-466.

Huang AHC, Cavalieri AJ 1979 Proline oxidase and water-stress induced proline accumulation in spinach leaves. Plant Physiol. 63: 531-535.

Ketchum REB, Warren RC, Klima LJ, Lopez-Gutierrez F, Nabors MW 1991 The mechanism and regulation of proline accumulation in suspension cultures of the halophytic grass Distichlis spicata L. J. Plant Physiol. 137: 368-374.

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.

Liu J, Zhu JK 1997 Proline accumulation and salt-stress-induced gene expression in a salt-hypersensitive mutant of Arabidopsis. Plant Physiol. 114: 591-596.

Morris CJ, Thompson JF, Johnson CM 1969 Metabolism of glutamic acid and N-acetylglutamic acid in leaf discs and cell-free extracts of higher plants. Plant Physiol. 44: 1023-1026.

Oaks A, Mitchell DJ, Barnard RA, Johnson FJ 1970 The regulation of proline biosynthesis in maize roots. Can. J. Bot. 48: 2249-2258.

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.

Rentsch D, Hirner B, Schmeizer E, Frommer WB 1996 Salt stress-induced proline transporters and salt stress-repressed broad specificity amino acid permeases identified by suppression of a yeast amino acid permease-targeting mutant. Plant Cell 8: 1437-1446.

Rhodes D, Handa S 1989 Amino acid metabolism in relation to osmotic adjustment in plant cells. In "Environmental Stress in Plants: Biochemical and Physiological Mechanisms", NATO ASI Series, Vol. G19 (JH Cherry ed), Springer, Berlin, pp. 41-62.

Rhodes D, Handa S, Bressan RA 1986 Metabolic changes associated with adaptation of plant cells to water stress. Plant Physiol. 82: 890-903.

Schwacke R, Grallath S, Breitkreuz KE, Stransky E, Stransky H, Frommer WB, Rentsch D 1999 LeProT1, a transporter for proline, glycine betaine, and gamma-amino butyric acid in tomato pollen. The Plant Cell 11: 377-391.

Sells GD, Koeppe DE 1981 Oxidation of proline by mitochondria isolated from water-stressed maize shoots. Plant Physiol. 68: 1058-1063.

Stewart CR 1981 Proline accumulation: Biochemical aspects. In "Physiology and Biochemistry of Drought Resistance in Plants," (LG Paleg, D Aspinall eds) Academic Press, Sydney, pp 243-259.

Stewart CR, Boggess SF 1978 Metabolism of [5-³H]proline by barley leaves and its use in measuring the effects of water stress on proline oxidation. Plant Physiol. 61: 654-657.

Stewart CR, Boggess SF, Aspinall D, Paleg G 1977 Inhibition of proline oxidation by water stress. Plant Physiol. 59: 930-932.

Verbruggen N, Villarroel R, Van Montagu M 1993 Osmoregulation of a pyrroline-5-carboxylate reductase gene in Arabidopsis thaliana. Plant Physiol. 103: 771-781.

Voetberg GS, Sharp RE 1991 Growth of the maize primary root tip at low water potentials. III. Role of increased proline deposition in osmotic adjustment. Plant Physiol. 96: 1125-1130.