REGULATION OF ARGININE DECARBOXYLASE BY SPERMINE IN OSMOTICALLY-STRESSED OAT LEAVES

Biosynthesis of polyamines in plants is controlled primarily by the en zymes ornithine decarboxylase (EC 4.1.1.17) and arginine decarboxylase (ADC; EC 4.1.1.19), which are responsible for the production of putre scine, and S-adenosyl-L-methionine (SAM) decarboxylase (EC 4.1.1.50) t hat is necessary for the formation of spermidine and spermine (Spm). L ittle is known about the metabolic or molecular mechanisms regulating the synthesis of these enzymes. We have studied the regulation of ADC synthesis by Spm in osmotically-stressed oat (Avena sativa L. cv. Vict ory) leaves, using a polyclonal antibody to oat ADC and a cDNA clone e ncoding oat ADC. Treatment with Spm in combination with osmotic stress resulted in increased steady-state levels of ADC mRNA, yet the levels of ADC activity decreased. This absence of correlation is explained b y the fact that Spm inhibits processing of the ADC proenzyme, which re sults in increased levels of this inactive ADC form and a consequent d ecrease in the ADC-processed form. Spermine treatment leads to delayed loss of chlorophyll in dark-incubated and osmotically-treated oat lea ves. Thus, post-translational regulation of ADC synthesis by Spm may b e important in explaining its anti-senescence properties.