The Current Status of Research on Gibberellin Biosynthesis

Abstract

Gibberellins are produced by all vascular plants and several fungal and bacterial species that associate with plants as pathogens or symbionts. In the 60 years since the first experiments on the biosynthesis of gibberellic acid in the fungus Fusarium fujikuroi, research on gibberellin biosynthesis has advanced to provide detailed information on the pathways, biosynthetic enzymes and their genes in all three kingdoms, in which the production of the hormones evolved independently. Gibberellins function as hormones in plants, affecting growth and differentiation in organs in which their concentration is very tightly regulated. Current research in plants is focused particularly on the regulation of gibberellin biosynthesis and inactivation by developmental and environmental cues, and there is now considerable information on the molecular mechanisms involved in these processes. There have also been recent advances in understanding gibberellin transport and distribution and their relevance to plant development. This review describes our current understanding of gibberellin metabolism and its regulation, highlighting the more recent advances in this field.

Keywords: Gibberellin metabolism.

Fig 1

An overview of GA biosynthesis, comparing pathways for higher plants (in green), the fungus F. fujikuroi (red) and bacteria (blue) to biological active products. Enzymes are indicated in the respective colors. C-atom numbers are given for the C₂₀-GA, GA₁₂. GA13ox, GA 13-oxidase; SDR, short-chain dehydrogenase/reductase.

Fig 2

Early and middle sections of the GA-biosynthetic pathway indicating reactions in the cytosol, plastid and endoplasmic reticulum. Side reactions present in some developing seeds and the fungus F. fujikuroi are shown in gray. DMAPP, dimethylallyl diphosphate; G3P, glyceraldehyde 3-phosphate; HMBPP, 4-hydroxy-3-methylbut-2-enyl diphosphate; IPP, isopentenyl diphosphate; MVAPP, mevalonate 5-diphosphate.

Fig 3

Final stage of GA biosynthesis to the biologically active end products GA₁ and GA₄ in plants catalyzed by the 2-oxoglutarate-dependent dioxygenases GA20ox and GA3ox. Reactions producing minor by-products of GA20ox activity (GA₂₅ and GA₁₇) and of GA3ox activity (GA₇ and GA₃), present in some species, are shown in gray. Biologically active GAs are highlighted in a green box.

Fig 4

Reactions involved in GA inactivation, acting on C₁₉-GAs (A) or C₂₀-GAs (B). Different enzyme types are indicated by color: brown, 2-oxoglutarate-dependent dioxygenases; green, cytochrome P450s (enzymes present in rice in dark green and in Arabidopsis light green); red, methyltransferases; blue, glucosyltransferases; and purple, epoxide hydrolase. GA₁₂ 13-hydroxylation, which initiates the biosynthesis of GA₁, results in slight reduction in bioactivity and is consequently included in (B). The involvement of EUI2 in dihydrodiol formation is assumed, but has not been demonstrated.

Fig 5

Regulation of GA biosynthesis and inactivation, highlighting the GA signal transduction pathway that enables GA homeostasis. The figure summarizes the data for Arabidopsis. GA signaling promotes the degradation of the DELLA transcriptional regulator, which in association with the transcription factor GAF1 upregulates the expression of genes encoding GA20ox and GA3ox, as well as the GA receptor GID1. Some pathways for the regulation of GA metabolism in response to environmental signals are also indicated.