Modification of apparent phytochrome synthesis in pisum by inhibitors and growth regulators.

The repeated exposure of Pisum (pea) plants to red light brings into operation an apparent synthesis of phytochrome which is not observed in material kept in the dark. This process shows some temperature compensation but has an optimum at 26 degrees ; it is irreversibly inhibited by 10(-4)m cycloheximide and 10 mug/ml actinomycin D. It is also inhibited by the auxins indoleacetic acid, naphthalene acetic acid and 2,4-dichlorophenoxyacetic acid at 10(-4)m but in these cases the inhibition is completely reversed when the auxin is washed out of the tissue. Antiauxins 2,4,6-trichlorophenoxyacetic acid and p-chlorophenoxy isobutyric acid, while strongly inhibiting growth have little effect on apparent synthesis. Other growth regulators and the precursor of tetrapyrrole synthesis, delta-aminolevulinic acid, have no consistent effect on the process, but 3 x 10(-4)m cobalt (II) nitrate is inhibitory. The capacity for apparent synthesis decreases as the cells approach maturity. The results may be explained by either de novo synthesis of phytochrome, or by a transformation process resembling in some respects the dark reversion of Pfr to Pr. The physiological role of apparent synthesis is suggested.