Isolation and Characterization of the Gibberellin Biosynthetic Gene Cluster in Sphaceloma manihoticola

Gibberellins (GAs) are tetracyclic diterpenoid phytohormones that were first identified as secondary metabolites of the fungus Fusarium fujikuroi (teleomorph, Gibberella fujikuroi). GAs were also found in the cassava pathogen Sphaceloma manihoticola, but the spectrum of GAs differed from that in F. fujikuroi. In contrast to F. fujikuroi, the GA biosynthetic pathway has not been studied in detail in S. manihoticola, and none of the GA biosynthetic genes have been cloned from the species. Here, we present the identification of the GA biosynthetic gene cluster from S. manihoticola consisting of five genes encoding a bifunctional ent-copalyl/ent-kaurene synthase (CPS/KS), a pathway-specific geranylgeranyl diphosphate synthase (GGS2), and three cytochrome P450 monooxygenases. The functions of all of the genes were analyzed either by a gene replacement approach or by complementing the corresponding F. fujikuroi mutants. The cluster organization and gene functions are similar to those in F. fujikuroi. However, the two border genes in the Fusarium cluster encoding the GA₄ desaturase (DES) and the 13-hydroxylase (P450-3) are absent in the S. manihoticola GA gene cluster, consistent with the spectrum of GAs produced by this fungus. The close similarity between the two GA gene clusters, the identical gene functions, and the conserved intron positions suggest a common evolutionary origin despite the distant relatedness of the two fungi.