Carotenoid oxygenases involved in plant branching catalyze a highly specific, conserved apocarotenoid cleavage reaction
Résumé
Recent studies with the high-tillering mutants in Oryza sativa, the more axillary growth (max) mutants in Arabidopsis thaliana and the ramosus (rms) mutants in 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 carotenoid cleaving oxygenases (CCDs) OsCCD7/OsCCD8b, MAX3/MAX4 and RMS5/1 in rice, Arabidopsis and pea, respectively. MAX3 and MAX4 are thought to catalyze 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 E. coli strains. The results obtained suggest that these enzymes are highly specific, converting the C27-compounds β-apo-10'-carotenal and its alcohol to yield β-apo-13-carotenone in vitro. Our data suggest that the second cleavage step in the biosynthesis of the plant branching inhibitor is conserved in monocot and dicot species.
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