Carotenoid oxygenases involved in plant branching catalyse a highly specific conserved apocarotenoid cleavage reaction.
Recent studies with the high-tillering mutants in rice (Oryza sativa), the max (more axillary growth) mutants in Arabidopsis thaliana and the rms (ramosus) mutants in pea (Pisum sativum) have indicated the presence of a novel plant hormone that inhibits branching in an auxin-dependent manner. The synthesis of this inhibitor is initiated by the two CCDs [carotenoid-cleaving (di)oxygenases] OsCCD7/OsCCD8b, MAX3/MAX4 and RMS5/RMS1 in rice, Arabidopsis and pea respectively. MAX3 and MAX4 are thought to catalyse the successive cleavage of a carotenoid substrate yielding an apocarotenoid that, possibly after further modification, inhibits the outgrowth of axillary buds. To elucidate the substrate specificity of OsCCD8b, MAX4 and RMS1, we investigated their activities in vitro using naturally accumulated carotenoids and synthetic apocarotenoid substrates, and in vivo using carotenoid-accumulating Escherichia coli strains. The results obtained suggest that these enzymes are highly specific, converting the C27 compounds beta-apo-10'-carotenal and its alcohol into beta-apo-13-carotenone in vitro. Our data suggest that the second cleavage step in the biosynthesis of the plant branching inhibitor is conserved in monocotyledonous and dicotyledonous species.
Alder
Adrian
A
Albert-Ludwigs University of Freiburg, Faculty of Biology, Institute of Biology II, Schaenzlestrasse 1, D-79104 Freiburg, Germany.
Holdermann
Iris
I
Beyer
Peter
P
Al-Babili
Salim
S
D017360:Arabidopsis / Q000201:enzymology / Q000254:growth & development; D029681:Arabidopsis Proteins / Q000378:metabolism; D002338:Carotenoids / Q000096:biosynthesis; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D005784:Gene Amplification; D012275:Oryza / Q000201:enzymology / Q000254:growth & development; D010105:Oxygenases / Q000235:genetics* / Q000378:metabolism*; D018532:Pisum sativum / Q000201:enzymology / Q000254:growth & development; D010940:Plant Proteins / Q000378:metabolism; D010957:Plasmids; D013379:Substrate Specificity
D029681:Arabidopsis Proteins; D018076:DNA, Complementary; D018744:DNA, Plant; D010940:Plant Proteins; D002338:Carotenoids; D010105:Oxygenases; C500451:carotenoid oxygenase