A Requirement for DNA Synthesis during Auxin Induction of Cell Enlargement in Tobacco Pith Tissue

Abstract

AbstractIAA (indoleacetic acid) is known to induce cell enlargement without cell division in tobacco pith explants grown on an agar medium without added cytokinin. The very long lag period before IAA (2 × 10−5M) stimulates growth, about 3 days, can be useful to study the metabolic changes which lead to the promotion of growth. When the disks are transferred to a medium without IAA after 2 days or less of treatment with IAA, the IAA does not stimulate growth. Disks transferred after 3 days, subsequently show an auxin response, almost as great as those given IAA continuously. At 5 × 10−4M, 5‐fluorodeoxyuridine (FUDR), which inhibits DNA synthesis by blocking formation of thymidylate, completely suppresses the lAA‐induced growth if it is added together with the IAA or 1 day later. When the FUDR is given 2 days after the IAA, there is a small increment of auxin‐induced growth, and an even greater amount if added after 3 days. The period when exogenous auxin must be present to stimulate growth corresponds to the period of FUDR sensitivity. The FUDR inhibition is prevented by thymidine but not by uridine. Other inhibitors of DNA synthesis, hydroxyurea and fluorouracil, also inhibit auxin‐induced growth. Thus DNA synthesis seems to be required for auxin induction of cell enlargement in tobacco pith explants. In contrast, FUDR does not inhibit auxin‐induced growth in corn coleoptile and artichoke tuber sections.