DEVELOPMENTAL AND LIGHT REGULATION OF DESACETOXYVINDOLINE 4-HYDROXYLASE IN CATHARANTHUS-ROSEUS (L.) G. DON

The expression of desacetoxyvindoline 4-hydroxylase (D4H), which catal yzes the second to the last reaction in vindoline biosynthesis in Cath aranthus roseus, appears to be under complex, multilevel developmental and light regulation. Developmental studies with etiolated and light- treated seedlings suggested that although light had variable effects o n the levels of d4h transcripts, those of D4H protein and enzyme activ ity could be increased, depending on seedling development, up to 9- an d 8-fold, respectively, compared with etiolated seedlings. However, li ght treatment of etiolated seedlings could stop and reverse the declin e of d4h transcripts at later stages of seedling development. Repeated exposure of seedlings to light was also required to maintain the full spectrum of enzyme activity observed during seedling development. Fur ther studies showed that a photoreversible phytochrome appeared to be involved in the activation of D4H, since red-light treatment of etiola ted seedlings increased the detectable levels of d4h transcripts, D4H protein, and D4H enzyme activity, whereas far-red-light treatment comp letely reversed this process. Additional studies also confirmed that d ifferent major isoforms of D4H protein exist in etiolated (isoelectric point, 4.7) and light-grown (isoelectric point, 4.6) seedlings, sugge sting that a component of the light-mediated activation of D4H may inv olve an undetermined posttranslational modification. The biological re asons for this complex control of vindoline biosynthesis may be relate d to the need to produce structures that could sequester away from cel lular activities the cytotoxic vinblastine and vincristine dimers that are derived partially from vindoline.