Inhibition of Phytochrome Synthesis by Gabaculine

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Abstract
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). 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 inhibited initial phytochrome synthesis in peas (Pisum sativum L.), corn (Zea mays L.) and oats (Avena sativa L.). Reduction in phytochrome correlated with reduction in chlorophyll. 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. Both initial synthesis and resynthesis of phytochrome apparently require de novo synthesis of chromophore as well as apoprotein.