EVIDENCE FOR SOME COMMON SIGNAL-TRANSDUCTION EVENTS FOR OPPOSITE REGULATION OF NITRATE REDUCTASE AND PHYTOCHROME-I GENE-EXPRESSION BY LIGHT

We have explored the possible involvement of the phosphoinositide (PI) cycle and protein kinase C (PKC) in the phytochrome (Pfr)-mediated li ght signal transduction pathway using nitrate reductase (NR) and phyto chrome-I (PhyI) genes as model systems. We have shown earlier that pho rbol myristate acetate (PMA) completely replaces the red light effect in stimulating nitrate reductase activity and transcript levels in mai ze. In this paper, we present detailed evidence to show that PMA mimic s the red light effect and follows similar kinetics to enhance NR stea dy-state transcript accumulation in a nitrate-dependent manner. We als o show that PMA inhibits phyI steady-state transcript accumulation in a manner similar to red light, indicating that a PKC-type enzyme(s) ma y be involved in mediating the light effect in both cases. Serotonin o r 5-hydroxytryptamine (5-HT), a stimulator of PI turnover, was also fo und to mimic the red light effect in enhancing NR transcript levels an d inhibiting phyI transcript accumulation, indicating the role of the PI cycle in generating second messengers for regulating the two genes. These results indicate that phytochrome-mediated light regulation of NR and phyI gene expression may involve certain common steps in the si gnal transduction pathway such as the PI cycle and protein phosphoryla tion by a PKC-type enzyme.