Polyamine content and metabolism in Arabidopsis thaliana and effect of spermidine on plant development

In this work, we have investigated polyamine levels (free and conjugated), activities and regulation of biosynthetic enzymes in different organs of Arabidopsis thaliana L. (ecotype Columbia) wild-type. When plants were grown at 0.5 mM spermidine, a different morphology was observed, with shorter stalks and darker green leaves. Spermidine-treated plants clearly take up this polyamine from the medium against a concentration gradient and redistribute it in the different organs both in free and conjugated form. In addition, in most plant organs, but especially in seedling cotyledons, the uptaken spermidine was converted to putrescine predominantly in the free form. N8-Acetylspermidine was also absorbed by Arabidopsis seedlings. The successive increase of putrescine suggests the presence of an interconversion of acetylspermidine to putrescine via a putative polyamine oxidase. Arginine decarboxylase (ADC, EC 4.1.1.19), ornithine decarboxylase (ODC, EC 4.1.1.17) and S-adenosylmethionine decarboxylase (SAMDC, EC 4.1.1.50) activities were determined in control and spermidine-treated seedling cotyledons and flowering plant leaves. ADC activity was equally distributed between soluble and particulate fraction, while ODC was mainly localised in the particulate one. ODC seemed to be the main enzyme involved in putrescine biosynthesis. SAMDC mRNA transcript progressively increased going from cotyledons to flowering plant stage; spermidine synthase (EC 2.5.1.16) transcript was highest in rosette plant leaves followed by flowering stalks. SAMDC transcript levels were generally lower in spermidine-supplemented plants with respect to controls, on the contrary spermidine synthase mRNA was not affected by the treatment.