The quantitative response to cytokinin in the moss Funaria hygrometrica does not reflect differential sensitivity of initial target cells.

Exposure to sufficient cytokinin induces the formation of buds from responsive cells in the protonema of Funaria hygrometrica. Initial perception of the phytohormone results in a Ca+2 cascade within minutes. A second cytokinin-mediated event occurs some days later, and converts incipient buds into stably committed buds. The concentration of exogenous cytokinin also regulates the total number of buds produced from a protonemal colony. This concentration-dependent production of buds has been thought to reflect differential sensitivity of target cells. Under that hypothesis, the regulation of bud number occurs during initial perception of hormone. This paper presents direct experimental evidence to the contrary and supports the alternate hypothesis that bud formation involves the gating of large numbers of responding cells by later events. Experiments transferring protonema between media with different levels of cytokinin show that the cytokinin concentration during the initial perception of cytokinin is unimportant in controlling bud number. Instead, bud number is found to be regulated by the concentration of exogenous cytokinin as incipient buds or bud initials become stably committed buds.