Reduction in the endogenous arginine decarboxylase transcript levels in rice leads to depletion of the putrescine and spermidine pools with no concomitant changes in the expression of downstream genes in the polyamine biosynthetic pathway

We investigated whether down-regulation of arginine decarboxylase (ADC) activity and concomitant changes in polyamine levels result in changes in the expression of downstream genes in the polyamine pathway. We generated transgenic rice (Oryza sativa L.) plants in which the rice adc gene was down-regulated by expression of its antisense oat (Avena sativa L.) ortholog. Plants expressed the oat mRNA adc transcript at different levels. The endogenous transcript was down-regulated in five out of eight plant lineages we studied in detail. Reduction in the steady-state rice adc mRNA levels resulted in a concomitant decrease in ADC activity. The putrescine and spermidine pool was significantly reduced in plants with lower ADC activity. Expression of the rice ornithine decarboxylase (odc), S-adenosylmethionine decarboxylase (samdc) and spermidine synthase (spd syn) transcripts was not affected. We demonstrate that even though levels of the key metabolites in the pathway were compromised, this did not influence steady-state transcription levels of the other genes involved in the pathway. Our results provide an insight into the different regulatory mechanisms that control gene expression in the polyamine biosynthetic pathway in plants by demonstrating that the endogenous pathway is uncoupled from manipulations that modulate polyamine levels by expression of orthologous transgenes.