To characterize the genes for enzymes involved in the biosynthesis of polyamines (PAs), their expression profiles were investigated and the levels of PAs in Arabidopsis thaliana quantified. In the Arabidopsis genome, eight genes involved in PAs biosynthesis were identified and the expression profiles of these genes were analysed, not only under abiotic stress to determine whether they were stress-inducible, constitutive, or stress-repressible, but also in various organs to show their tissue specificity. AtADC2 and AtSPMS mRNAs, encoding arginine decarboxylase and spermine synthase, clearly increased in response to NaCl and dehydration and abscisic acid treatments. Stress-inducible accumulation of AtADC2 mRNA correlated with putrescine (Put) accumulation under NaCl and dehydration treatments. In a cold condition, AtSAMDC2 mRNA increased significantly. AtADC2 and AtSAMDC2 mRNA were expressed in sexual organs such as flowers, buds and immature siliques. PAs also accumulated in sexual organs. These results suggest that the transcripts of eight genes involved in PA biosynthesis show different profiles of expression not only in response to environmental stress but also during plant development.

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A cDNA for delta1-pyrroline-5-carboxylate (P5C) synthetase (cOsP5CS), an enzyme involved in the biosynthesis of proline, was isolated and characterized from a cDNA library prepared from 14-day-old seedlings of Oryza sativa cv. Akibare. The deduced amino acid sequence of the P5CS protein (OsP5CS) from O. sativa exhibited 74.2% and 75.5% homology to that of the P5CS from Arabidopsis thaliana and Vigna aconitifolia, respectively. Northern blot analysis revealed that the gene for P5CS (OsP5CS) was induced by high salt, dehydration, treatment of ABA and cold treatment, while it was not induced by heat treatment. Simultaneously, accumulation of proline was observed as a result of high salt treatment in O. sativa. Moreover, the levels of expression of OsP5CS mRNA and content of proline under salt stress condition were compared between a salt-tolerant cultivar, Dee-gee-woo-gen (DGWG) and a salt-sensitive breeding line, IR28. It was observed that the expression of the P5CS gene and the accumulation of proline in DGWG steadily increased, whereas those in IR28 increased slightly.

A cDNA encoding a proline (Pro) transporter (ProT) was isolated and characterized from a cDNA library prepared from 14-d-old seedlings of Oryza sativa cv. Akibare. The deduced amino acid sequence of the rice ProT protein (OsProT) had 68.8% homology to the ProT protein 1 from Arabidopsis thaliana and 59.6% homology to that from Lycopersicon esculentum. Northern blot analysis revealed that the gene for OsProT (OsProT) was expressed in all organs examined, comparatively strongly in leaf sheath and stem. Salt treatment did not induce expression of OsProT but strongly induced expression of the gene for delta1-pyrroline-5-carboxylate synthetase (P5CS), a key enzyme in Pro biosynthesis. Southern blot analysis revealed that OsProT has a gene family. OsProT specifically transported L-Pro in a transport assay using Xenopus laevis oocytes.