Elucidation of gibberellin biosynthesis in bacteria reveals convergent evolution

Abstract

Gibberellins (GAs) are crucial phytohormones involved in many aspects of plant growth and development, including plant-microbe interactions, which has led to GA production by plant-associated fungi and bacteria as well. While the GA biosynthetic pathways in plants and fungi have been elucidated and found to have independently arisen through convergent evolution, little has been uncovered about GA biosynthesis in bacteria. Some nitrogen-fixing/symbiotic, legume-associated rhizobia, including Bradyrhizobium japonicum, the symbiont of soybean, and Sinorhizobium fredii, a broad-host-nodulating species, contain a putative GA biosynthetic operon/gene cluster. Through functional characterization of five unknown genes, we demonstrate that this operon encodes the enzymes necessary to produce GA9, thereby elucidating bacterial GA biosynthesis. The distinct nature of these enzymes indicates that bacteria have independently evolved a third biosynthetic pathway for GA production. Furthermore, our results also reveal a central biochemical logic that is followed in all three convergently evolved GA biosynthetic pathways.

Labeling Studies Clarify the Committed Step in Bacterial Gibberellin Biosynthesis

“…While rhizobia only express these enzymes and produce GA after differentiation into their nodule-residing bacteroid form, 17 it was possible to observe activity with the individual enzymes upon recombinant expression. 4,16,18,19 Notably, “B” ring contraction requires not only a CYP (CYP114) but also the ferredoxin (Fd GA ) found within the operon, which presumably acts as an electron donor. 4 This is distinct from plant and fungal KAOs, which simply utilize an archetypical cytochrome P450 reductase for their activity.…”

“…4,16,18,19 Notably, “B” ring contraction requires not only a CYP (CYP114) but also the ferredoxin (Fd GA ) found within the operon, which presumably acts as an electron donor. 4 This is distinct from plant and fungal KAOs, which simply utilize an archetypical cytochrome P450 reductase for their activity. 12,20,21 When expressed alone, CYP114 only converts 1 to 2 , while coexpression of CYP114 and Fd GA enables full conversion of 1 to 3 .…”

Unrivalled diversity: the many roles and reactions of bacterial cytochromes P450 in secondary metabolism

“…CYP112 then catalyses another two-step regiospecic conversion of a methyl group into an aldehyde intermediate. 72 This compound then presumably undergoes oxidative loss of the carboxylic acid moiety catalysed by CYP112, perhaps mediated by a ferric peroxo P450, to yield a carbocationic intermediate which then forms a lactone with the C19 carboxylate installed by CYP117. Such a loss of the carboxy moiety may initially generate a radical intermediate but again steric inhibition of rebound would be required to could promote cation formation.…”

“…72 This unusual, three-step reaction is thought to proceed via an initial hydroxylation followed by a semi-pinacol rearrangement of an a-hydroxy cation. Such a species might arise from the usual H atom abstraction by compound I, with steric inhibition of the rebound step as seen in pentalenolactone (vide supra) formation leading to electron transfer and cation generation.…”

“…72 The pathway to these important signalling molecules has independently evolved in plants, fungi and in some bacteria, the latter of which are plant symbionts or pathogens. 72 These diterpene derived plant phytohormones undergo a number of remarkable P450 mediated transformations (Fig. 10).…”