Characterization of cDNAs differentially expressed in roots of tobacco (Nicotiana tabacum cv Burley 21) during the early stages of alkaloid biosynthesis.

A set of 60 cDNAs were isolated by subtractive hybridization screening of a phage library using radioactively-labeled probes generated from root mRNAs isolated from tobacco (Nicotiana tabacum cv Burley 21) plants before and 3 days after topping. Among the differentially expressed gene products were full-length and partial cDNAs encoding arginine decarboxylase (ADC), ornithine decarboxylase (ODC), and S-adenosylmethionine synthetase (SAMS), enzymes involved in polyamine and alkaloid biosynthesis. The other cDNAs isolated were placed into one of several categories and encode metabolic enzymes, proteins involved in transcription and translation, components of signal transduction pathways, and homologs of genes whose expression has been shown to be regulated by phytohormones (i.e. auxin, ABA), wounding or other stress responses. RNA gel blot analysis showed that the ADC and ODC transcripts were preferentially expressed in the roots and floral tissues of mature tobacco plants, whereas SAMS transcripts were detected in all tissues examined. The steady-state levels of the ADC and ODC mRNAs increased in the roots of wild-type tobacco plants during the 24 h period after topping, whereas little change was observed in the abundance of the SAMS transcripts in these tissues. The possible factors associated with the regulation of expression of these genes are discussed.