PHORBOL-MYRISTATE ACETATE REPLACES PHYTOCHROME-MEDIATED STIMULATION OF NITRATE REDUCTASE IN MAIZE

The substrate-inducible nitrate reductase (NR) activity in etiolated maize leaves is stimulated by light through the involvement of the red/far-red reversible pigment, phytochrome. In unirradiated tissues a very low level of NR activity was detected. However, if leaves kept in darkness were incubated in the presence of phorbol myristate acetate (PMA), the NR activity was stimulated to the same extent as obtained by red light irradiation. The optimal stimulation was obtained at 83 nM concentration of PMA. Infact even 5 min incubation of leaves in PMA solution resulted in NR stimulation. These experiments suggest the involvement of protein kinase C (PKC)-type enzyme in light-mediated stimulation of NR. A PKC-type activity was found to be stimulated by PMA in partially purified fractions.