PROTEIN PHOSPHATASE-ACTIVITY IS REQUIRED FOR LIGHT-INDUCIBLE GENE-EXPRESSION IN MAIZE

Chlorophyll accumulation and photosynthetic gene activation are two ha llmarks of greening process in etiolated maize leaves in response to l ight signals. However, very little is known about the relevant signal transduction pathways mediating these essential processes that lead to photosynthetic competence. It is shown here that a potent and specifi c protein phosphatase 1 (PP1) and PP2A inhibitor, okadaic acid, effici ently blocks chlorophyll accumulation induced by light in etiolated ma ize leaves. In addition, the light-inducible expression of two photosy nthetic fusion genes can be specifically suppressed by the structurall y unrelated PPI and PP2A inhibitors, okadaic acid and calyculin A, usi ng a sensitive and physiological maize protoplast transient assay. The specificity and effective concentration of the inhibitors in vivo and in vitro strongly suggest that PP1 is required for transmitting tight signals. Intriguingly, several partial cDNAs encoding novel as well a s conserved PP1 can be identified in maize leaves using the polymerase chain reaction. Studies of chimeric promoters indicate that PP1 activ ity is essential for the interaction of multiple regulatory elements. Although PP1 and PP2A have been implicated in the suppression of gene activity in yeast and animals, the present data indicate that PP1 appe ars to be essential for light-dependent gene activation in plants.