ABA prevents the second cytokinin-mediated event during the induction of shoot buds in the moss Funaria hygrometrica

The induction of shoot buds in the moss Funaria hygrometrica is a classic and quantitative bioassay for cytokinin. This cytokinin-stimulated response can be inhibited by the plant hormone abscisic acid ABA; the inhibition is concentration dependent and was proposed for use as a bioassay for ABA. This paper characterizes the ABA inhibition of the cytokinin-stimulated formation of shoot buds. Experiments transferring protonema between cytokinin and cytokinin plus ABA show that ABA does not interfere with the initial perception of cytokinin. Other experiments compare the results of transferring protonema from cytokinin to cytokinin-free medium or to medium with cytokinin plus ABA and reveal that ABA acts by blocking the cytokinin-mediated stable commitment of nascent buds. Extension of the technique of double-reciprocal plots to this whole-organism bioassay finds that ABA is not a competitive inhibitor of cytokinin. Analysis of the ABA inhibition of bud formation identifies a new regulatory step in the developmental process of bud formation in mosses.