ABSTRACT
The differentiation-inducing factor, DIF, was previously shown to induce stalk cell differentiation in Dictyostelium discoideum cells incubated as submerged monolayers. We investigated the mechanism that regulates the differentiation of stalk cells in the intact organism. It was found that in migrating or submerged slugs DIF cannot induce stalk cell differentiation, which is most likely due to the presence of a DIF antagonist. Cyclic AMP and ammonia were earlier reported to act as DIF antagonists in vitro. We show here that ammonia, but not cAMP, acts as an antagonist for DIF-induced stalk cell differentiation in vivo. DIF can induce stalk cell differentiation when ammonia levels in the slug are enzymically depleted. However, depletion of cAMP levels does not increase the efficacy of DIF. We propose that the induction of stalk cell differentiation during early culmination may be triggered by a drop in ammonia levels inside the organism.
