Downregulation of ethylene production and biosynthetic gene expression is associated to changes in putrescine metabolism in shoot-forming tobacco thin layers

The effect of aminoethoxyvinylglycine (AVG), an inhibitor of 1-aminocyclopropane-1-carboxylate synthase (ACS) activity, on ethylene emission and biosynthetic gene expression, on gene expression and/or activity of polyamine (putrescine, spermidine and spermine) biosynthetic enzymes, and on diamine oxidase (DAO, EC 1.4.3.6) activity was evaluated in tobacco (Nicotiana tabacum L. cv. Samsun) thin layers cultured on a shoot-forming medium (1 muM indol-3-acetic acid (IAA) plus 10 muM benzyladenine (BA)). Northern analyses showed that ACS and 1-aminocyclopropane-1-carboxylate oxidase (ACO) transcripts were present throughout culture with a maximum accumulation on day 7. Besides ethylene emission, AVG (0.5 muM) increasingly reduced ACS and ACO messages. The time course of labelled methionine incorporation into spermidine and spermine, which share with ethylene the common precursor S-adenosylmethionine (SAM), as well as SAM decarboxylase (SAMDC, EC 4.1.1.21) activity and gene expression. were not affected by AVG treatment. On the contrary, labelled putrescine incorporation into the higher polyamines (spermidine and spermine) and into trichloroacetic acid (TCA)-soluble polyamine conjugates was enhanced early in culture (day 2) by the drug. Putrescine biosynthetic enzyme activities, arginine decarboxylase (ADC, EC 4.1.1.19) and ornithine decarboxylase (ODC, EC 4.1.1.17), were also increased in AVG-treated explants. Moreover, inhibition of ethylene synthesis by AVG led to a strong reduction in diamine oxidising activity, especially the one associated with a cell wall-enriched fraction. Changes in putrescine biosynthesis. oxidation and flux into higher polyamines are discussed in the light of the rejuvenating effect of AVG.