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 Ar abidopsis thaliana L. (ecotype Columbia) wild-type. When plants were grown at 0.5 mM spermidine, a different morphology was observed, with shorter sta lks and darker green leaves. Spermidine-treated plants clearly take up this polyamine from the medium against a concentration gradient and redistribut e it in the different organs both in free and conjugated form. In addition, in most plant organs, but especially in seedling cotyledons, the uptaken s permidine was converted to putrescine predominantly in the free form. N-8-A cetylspermidine was also absorbed by Arabidopsis seedlings. The successive increase of putrescine suggests the presence of an interconversion of acety lspermidine to putrescine via a putative polyamine oxidase. Arginine decarb oxylase (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 det ermined in control and spermidine-treated seedling cotyledons and flowering plant leaves. ADC activity was equally distributed between soluble and par ticulate fraction, while ODC was mainly localised in the particulate one. O DC 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 ro sette plant leaves followed by flowering stalks. SAMDC transcript levels we re generally lower in spermidine-supplemented plants with respect to contro ls, on the contrary spermidine synthase mRNA was not affected by the treatm ent. (C) 2000 Editions scientifiques et medicales Elsevier SAS.