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Keywords:

  • Cell-type specific regulation;
  • ent-kaurene synthesis;
  • plastid localization;
  • terpene cyclases

The two-step cyclization reaction of ent-kaurene synthesis from geranylgeranyl diphosphate is the first committed step in the biosynthetic pathway of the plant hormone gibberellin. Recent molecular cloning and characterization of the genes encoding the two corresponding enzymes, copalyl diphosphate synthase (CPS) and ent-kau-rene synthase (KS), have demonstrated that ent-kaurene synthesis is localized in the plastids and is highly regulated in specific tissues and cell types during plant development. In addition to occurring in actively growing tissues, ent-kaurene synthesis also takes place in fully expanded leaves. Therefore mature leaves may produce gibberellin intermediates or bioactive gibberellins for transport to responsive tissues. DNA sequence analyses have revealed a conserved aspartate-rich motif, D(I/V)DDTA among CPS and other protonation-initiated terpene cyclases, while KS contains a highly conserved DDXXD motif which was proposed to function as a divalent metal ion-diphos-phate complex binding site in ionization-initiated terpene cyclases and prenyltrans-ferases.