We identified a near-full-length cDNA clone encoding ornithine decarboxylase (ODC) from tomato (Lycopersicon esculentum Mill). It contained a small upstream open reading frame (uORF) within its 5' untranslated region. An in vitro translation assay demonstrated that the uORF repressed expression of downstream ORF. Neither nucleotide nor predicted peptide sequence of the uORF was responsible for the repression. The presence of upstream AUG codon was shown to be responsible. ODC expression appeared to be organ specific. The ODC gene was expressed in roots, hypocotyls and sink leaves but not in source leaves. ODC transcripts were observed in apical meristem of primary roots, and were distributed in cells of cortex layer preferentially. ODC expression responded immediately to sucrose availability via the sucrose-specific pathway independent of hexokinase. Sucrose induction of ODC gene was seen in roots, hypocotyls and flowers but not in mature leaves. Moreover, only the root apical meristem responded to sucrose availability. These observations indicate that the spatial pattern of ODC expression is closely associated with cell proliferation and that sucrose sensing plays a major role in the spatial pattern of ODC expression. Also, the differential regulation of ODC and arginine decarboxylase gene expression by factors modulating plant growth suggests that they would have different physiological roles in plant development.
The role of Ca2+ and protein phosphorylation in the transduction of the ethylene signal resulting in induction of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase has been studied in peas (Pisum sativum L.) by a pharmacological approach. 2,5-Norbornadiene (NBD) and aminoethoxyvinylglycine (AVG) reduced the basal level of ACC oxidase transcript and its enzyme activity. Only NBD was shown to inhibit the ethylene response, the accumulation of ACC oxidase transcript and the stimulation of its enzyme activity. Ethylene influenced 45Ca2+ influx into the segment tissues from pea epicotyls, and ethylene glycol-bis(beta-aminoethyl ether)N,N,N'N'-tetraacetic acid (EGTA) a Ca2+ chelator, inhibited the ethylene response. Ca2+ depletion by pretreatment with EGTA also blocked the ethylene response, which almost completely recovered when Ca2+ was added exogenously after Ca2+ depletion. Ca2+ channel blockers, verapamil, and LaCl3, used to certify the role of extracellular Ca2+, all inhibited the ethylene response. A protein kinase inhibitor, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7), and protein phosphatase inhibitors, vanadate and okadaic acid, also inhibited the ethylene response. The results of the present study suggest that Ca2+ influx from the extracellular space, protein phosphorylation, and dephosphorylation are required for the induction of ACC oxidase by ethylene.
The regulation of ornithine decarboxylase (ODC) expression was studied in suspension-cultured tobacco (Nicotiana tabacum L.) BY2 cells. ODC activity increased rapidly 3 h after cells re-entered the cell cycle from the stationary phase, corresponding to the G1 phase, and continued to increase in the subsequent S phase, while the ODC transcript level increased only transiently. ODC activity was suppressed by sucrose-deficiency, while the ODC transcript level was not affected. U0126, a specific inhibitor of mammalian MAPK kinases (MEKs), significantly reduced ODC enzyme activity, but not the ODC transcript level. These results suggest that ODC activity is regulated independently of its transcript level in BY2 cells, and that sucrose and a U0126-sensitive protein kinase are required for the transcript-level-independent activation of ODC.
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