During the process of fruiting body construction in the cellular slime mould Dictyostelium discoideum, prestalk cells become mature stalk cells in a well-controlled manner. To identify the natural inducer of stalk cell maturation, substances known to induce stalk cell differentiation under in vitro conditions, and some other related compounds, were examined for their effects in vivo on migrating slugs, the precursor structures of the fruiting bodies. Among these substances, addition of weak acids such as CO2, and addition followed by removal of weak bases such as NH3, strikingly induced the maturation of prestalk cells in situ in slugs. On the other hand, inhibitors of the plasma membrane proton pump did not efficiently induce the maturation of prestalk cells in intact slugs. Differentiation inducing factor (DIF), an endogenous inducer of prestalk differentiation, seemed to be an even poorer inducer of stalk cell maturation when applied to intact slugs. The activities of these substances in inducing stalk cell maturation showed a good correlation with their effects on the cytoplasmic pH (pHi) of prestalk cells; the larger the pHi drop, the stronger the induction of stalk cell maturation, suggesting a requirement for a pHi decrease for the maturation of prestalk cells. Based on these results, it was proposed that stalk cell differentiation, which is induced by DIF, is blocked halfway during normal development by (an) agent(s) that prevent(s) the decrease in pHi.

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