Regulation of 5-Aminolevulinic Acid (ALA) Synthesis in Developing Chloroplasts : II. Regulation of ALA-Synthesizing Capacity by Phytochrome.

When dark-grown cucumber (Cucumis sativus L.) seedlings previously exposed to white light for 20 hours were returned to darkness, the ability of isolated chloroplasts to synthesize 5-aminolevulinic acid dropped by approximately 70% within 1 hour. The seedlings were then exposed to light, and the synthetic ability of the isolated chloroplasts was determined. Restoration of the synthetic capacity was promoted by continuous white or red light of moderate intensity. Intermittent red light was also effective. Blue and far-red light did not restore the synthetic capability. Blue light given after a red pulse did not enhance the effect of the red light. Far-red light given immediately after each red pulse prevented the stimulation due to intermittent red light. Restoration of the biosynthetic activity by in vivo light treatments was inhibited by cycloheximide indicating the requirement for translation on 80 S ribosomes for the in vivo light response. These findings suggest that the majority of the plastidic 5-aminolevulinic acid synthesis is under phytochrome regulation.