Inhibition of Phytochrome Synthesis by Gabaculine

Gabaculine (5-amino-1,3-cyclohexadienylcarboxylic acid), a transaminase inhibitor, also inhibits chlorophyll formation in plants, and the effect of this compound can be counteracted by 5-aminolevulinic acid (ALA) (Flint, personal communication, 1984). Since it is probable that ALA also serves as a precursor to phytochrome, the effects of gabaculine on phytochrome synthesis in developing etiolated seedlings were examined using in vivo spectrophotometry. Preemergence treatment with gabaculine was found to inhibit initial phytochrome synthesis in peas (Pisum sativum L.), corn (Zea mays L.), and oats (Avena sativa L.). In general, reduction in phytochrome correlated with reduction in chlorophyll. However, the extent of inhibition of phytochrome synthesis was not as great as that of chlorophyll synthesis, perhaps due to preexisting phytochrome in the seed. Foliar treatment of etiolated pea seedlings prior to light-induced destruction of phytochrome inhibited subsequent phytochrome resynthesis in the dark. These results suggest that both initial synthesis and resynthesis of phytochrome require de novo synthesis of chromophore as well as apoprotein.