Arginine decarboxylase (ADC) is the first enzyme in one of the two pathways of putrescine biosynthesis in plants. The genes encoding ADC have previously been cloned from oat and Fscherichia coli. Degenerate oligonucleotides corresponding to two conserved regions of ADC were used as primers in polymerase chain reaction amplification of tomato (Lycopersicon esculentum Mill.) genomic DNA, and a 1.05-kb fragment was obtained. This genomic DNA fragment encodes an open reading frame of 350 amino acids showing about 50% identity with the oat ADC protein. Using this fragment as a probe, we isolated severa1 partial ADC cDNA clones from a tomato pericarp cDNA library. The 5' end of the coding region was subsequently obtained from a genomic clone containing the entire ADC gene. The tomato ADC gene contains an open reading frame encoding a polypeptide of 502 amino acids and a predicted molecular mass of about 55 kD. The predicted amino acid sequence exhibits 47 and 38% identity with oat and E. coli ADCs, respectively. Cel blot hybridization experiments show that, in tomato, ADC is encoded by a single gene and is expressed as a transcript of approximately 2.2 kb in the fruit pericarp and leaf tissues. During fruit ripening the amount of ADC transcript appeared to peak at the breaker stage. No significant differences were seen when steady-state ADC mRNA levels were compared between normal versus long-keeping Alcobaca (alc) fruit, although alc fruit contain elevated putrescine levels and ADC activity at the ripe stage. The lack of correlation between ADC activity and steady-state mRNA levels in alc fruit suggests a translational and/ or posttranslational regulation of ADC gene expression during tomato fruit ripening.

Nitrite reductase is an enzyme in the nitrate assimilatory pathway whose expression is induced upon the addition of nitrate. Furthermore, it is known to be located in chloroplasts in leaves and plastids in roots. A 3.1 kb 5′ upstream region of the spinach nitrite reductase (NiR) gene promoter was shown previously to confer nitrate inducibility on the β‐glucuronidase (GUS) reporter gene expression in both the leaves and the roots of transgenic tobacco plants. In the present study, this 3.1 kb promoter fragment as well as a series of promoter deletion constructs, fused to a GUS gene, were utilized to delineate the region of NiR promoter involved in nitrate regulation of NiR expression by studying the cellular localization of NiR—GUS expression as well as its regulation by nitrate. In plants carrying the longest promoter fragment (−3100 from the transcription start site) and promoter sequences progressively deleted to −330 bp, the expression of GUS was markedly increased in the presence of nitrate, and this expression was found to occur in mesophyll cells in leaves and in the vascular tissues of stem and roots. When nitrate was added to NiR—GUS plants grown in the absence of nitrate, significant levels of GUS activity could be seen in the roots after 2 h and in the leaves after 6 h. Further 5′ deletion of the promoter to −200 bp abolished the nitrate induction of GUS expression, indicating that the 130 bp region of the nitrite reductase promoter located between −330 and −200 is required for full nitrate‐inducible tissue‐specific expression.

The levels of free putrescine, spermidine, and spermine, and the activities of ornithine decarboxylase and s-adenosylmethionine decarboxylase were determined in the floral organs of the normal and a male sterile stamenless-2 (sl-2/sl-2) mutant of tomato (Lycopersicon esculentum Mill.). Under the intermediate temperature regime, all mutant floral organs possessed significantly higher levels of polyamines and enzyme activities than their normal counterparts. In the low temperature-reverted mutant stamens, the polyamine levels and the activity of PA biosynthetic enzymes were not significantly different from the normal. It is suggested that the abnormal stamen development in the sl-2/sI-2 mutant is, in part, related to elevated levels of endogenous PAs.The diamine putrescine and the PAs3 Spd and Spm are probably of ubiquitous occurrence in plants and appear to play an important role in plant growth and development (6,10,29,30). Changes in PA levels and activities of PA biosynthetic enzymes have been implicated in a variety of plant growth processes such as breaking of dormancy (2), seed germination (31), fruit development (5, 28), somatic embryogenesis (7), pollen embryogenesis (13), adventitious root formation (9), delay of senescence (1), and adaptation to stress (10).The PAs also appear to be involved in the flowering processes of plants, i.e. the induction of flowering (4,12)

Nitrite reductase is the second enzyme in the nitrate assimilatory pathway. The transcription of this gene is regulated by nitrate as well as a variety of other environmental and developmental factors. Genomic clones containing the entire nitrite reductase gene have been isolated from a spinach genomic library and sequenced. The sequence is identical in the transcribed region to a previously isolated spinach NiR cDNA clone (Back et al., 1988) except for the presence of three introns. The analysis of the genomic clones and DNA blot hybridization demonstrates that there is a single NiR gene per haploid genome in spinach. This is in contrast to what has been found for other plant species. The transcription initiation site has been determined by S1 mapping and the 5' upstream region has been used to regulate the GUS reporter gene in transgenic tobacco plants. This gene was found to be regulated by the addition of nitrate in the transgenic plants.

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