Effects of G protein and cGMP on phytochrome-mediated amaranthin synthesis in Amaranthus caudatus seedlings

The effects of G protein and cGMP on phytochrome-mediated amaranthin biosynthesis in Amaranthus caudatus seedlings were studied. it was shown that G protein agonist cholera toxin induced amaranthin synthesis in darkness, whereas G protein antagonist pertussis toxin inhibited red light-induced amaranthin synthesis. Amaranthin synthesis was also induced by exogenous cGMP, while the amaranthin biosynthesis induced by cholera toxin, red light and exogenous cGMP was inhibited by genistein. LY-83583, an inhibitor of guanylyl cyclase, inhibited the amaranthin synthesis induced both by red light and cholera toxin, while it was not able to inhibit the amaranthin synthesis induced by exogenous cCMP. These results suggest that G protein, guanylyl cyclase and cCMP were the candidates in phytochrome signal transduction chain for red light-induced amaranthin biosynthesis and the red light signal transduction chain might be as follows: red light-->phytochrome-->G protein-->guanylyl cyclase-->cGMP.