A Phytochrome-mediated Sequence of Reactions Regulating Cell Division in Developing Callus Cultures

Radiation at the red end of the visible spectrum is active inreducing the extent of the first synchronous division in explantsremoved from Jerusalem artichoke tubers and cultured in a liquidmedium containing 2,4-D. The inhibitory effect of short redlight exposures may be overcome by subsequent irradiation withfar red light. However, prolonged exposures to red light mayonly partially be reversed by far red light. The addition ofcoconut milk to the medium completely overcomes the effect ofshort light exposures but cannot prevent the effect of prolongedlight treatments. These results are consistent with a hypothesiswhich postulates that a phytochrome-mediated reaction initiatesa chain of events which results in the depletion of an essentialmetabolite, also present in coconut milk, below a critical levelrequired for cell division. There is evidence to suggest thatthe essential metabolite is L-phenylalanine.