Effects of nitrate and ammonium on growth of Arabidopsis thaliana plants transformed with the ARR5::GUS construct and a role for cytokinins in suppression of disturbances induced by the presence of ammonium

Determination of physiologically active cytokinins in transformed Arabidopsis thaliana (L.) Heynh. plants (ecotype Wassilevskija) by means of the ARR5::GUS gene expression analysis and by the immunohistochemical assay demonstrated an increase of the cytokinin content during leaf growth and its decrease after growth cessation. After leaf growth cessation cytokinins (CKs) were found only in chloroplasts. Nitrate but not ammonium salt increased the cytokinin content in leaves. The replacement of nitrate in nutrient medium with ammonium salts suppressed leaf and root growth and decreased the content of physiologically active CKs in them. In plants cultivated on a media with NH4Cl as a sole nitrogen source, 10(-9) M trans-zeatin several-fold activated leaf growth, prevented a decrease in the chlorophyll a and b content, and reduced proline accumulation indicating suppression of the NH4Cl stress action. Thus, negative effects of NH4Cl as a sole nitrogen source depended on the cytokinin shortage.