Phenylalanine Ammonia-lyase and the Regulation of Cell Division in Jerusalem Artichoke (Helianthus tuberosus L.) Callus Cultures

Abstract

The possibility that light is depleting endogenous levels of phenylalanine in cultured artichoke explants receives some indirect support from the rinding that extractable PAL levels were significantly elevated in light-treated cultures irrespective of whether the explants were preparing for division. Light-mediated increases in PAL were also apparent when allowance was made for the increasing length of G₁ during the experimental period. The presence of 2,4-D in the growth medium depressed the amount of extractable PAL throughout the growth period. A typical photo-controlled time-response curve was obtained for PAL in light-treated cultures preparing for division although in dark controls and all cultures lacking 2,4-D considerable fluctuations of enzyme levels occurred. The possibility that these levels were due to activators or inhibitors appears unlikely from the data obtained from mixing experiments. Cycloheximide (2 × 10⁻⁵ M) is completely effective in preventing the rise in PAL activity in cultures lacking or containing 2,4-D. When applied before the peak in activity is reached there is no further rise in activity. However, at this stage it cannot be stated whether or not protein synthesis is necessary for increases in enzyme activity.