Gibberellin Synthesis in Arabidopsis
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Gibberellin Synthesis in Arabidopsis

GAs (Gibberellic Acids or Gibberellins) form a large family of diterpenoid compounds, some of which are bioactive growth regulators, which control diverse developmental processes such as seed germination, stem elongation, leaf expansion, trichome development, and flower and fruit development. Gibberellins are classified on the basis of structure as well as function. All Gibberellins are derived from the ent-Gibberellane skeleton. The Gibberellins are named GA1.... GAn in order of discovery. GA3 (Gibberellin-3) is the first Gibberellin to be structurally characterized. There are currently 136 GAs identified from plants, fungi and bacteria. GAs are widespread and so far ubiquitous in both flowering (Angiosperms) and non-flowering (Gymnosperms) plants as well as ferns. GA  mediates environmental stimuli, which modify the flux through the GA-Biosynthetic Pathway. Regulation of GA biosynthesis is therefore of fundamental importance to plant development and its adaption to the environment (Ref.1).

The biosynthesis of GA in higher plants can be divided into three stages: Biosynthesis of ent-Kaurene in proplastids; Conversion of ent-Kaurene to GA12 (Gibberellin-12) via microsomal Cytochrome P450 Monooxygenases; and formation of C19 - and C20 GAs in the cytoplasm. In the first stage, Geranylgeranyl Pyrophosphate, which serves as a common precursor for diterpenes (e.g., GAs and the phytol chain of chlorophyll) and Tetraterpenes (Carotenoids), is synthesized by either a Mevalonate-Dependent or a Nonmevalonate Pathway. In the Mevalonate-Dependent Pathway, 3 Acetyl CoA (3-Acetyl CoenzymeA) molecules are oxidized by 2 NADPH molecules to produce 3CoA (3-CoenzymeA) molecules as a side product and Mevalonic Acid. Mevalonic Acid is then phosphorylated by ATP and decarboxylated to form Isopentyl Pyrophosphate. Four of these molecules form Geranylgeranyl Pyrophosphate, which serves as the donor for all GA carbon atoms. Geranylgeranyl Pyrophosphate is then converted to ent-Kaurene in a two-step cyclization reaction, which are catalyzed by CPS (ent-Copalyl Diphosphate Synthase) and KS (ent-Kaurene Synthase), with ent-Copalyl Diphosphate as the intermediate. In Arabidopsis, extremely low amounts of CPS mRNA are found during plant development, and the expression pattern of this gene is cell type specific. Rapidly growing tissues, including shoot apices, root tips, developing anthers, and seed, contain higher amounts of CPS mRNA. The expression pattern of KS is similar to that of CPS, but the overall amount of KS mRNA is much higher than that of CPS, suggesting that the synthesis of ent-Kaurene is determined primarily by controlling the expression and location of CPS. The highly specific pattern of expression of the CPS gene and the greater expression of the KS gene suggests that CPS acts as a gate keeper, controlling the location and activity of the early stages of GA biosynthesis (Ref.2).

In the second stage of the pathway, stepwise oxidation followed by ring contraction of ent-Kaurene, catalyzed by KO (ent -Kaurene Oxidase) and KAO (ent -Kaurenoic Acid Oxidase) produces GA12, which can be converted further to GA53 (Gibberellin-53) by 13-hydroxylation. First, ent–Kaurene formed by Geranylgeranyl Pyrophosphate is converted to ent-Kaurenoic Acid by the enzyme KO. KO is a Cyto-P450 (Cytochrome P450) enzyme that is associated with the ER (Endoplasmic Reticulum), but it is not clear how the hydrocarbon ent-Kaurene is transported to the ER membrane. In Arabidopsis, KO1 (Arabidopsis thaliana ent -Kaurene Oxidase-1)/ At5g25900 gene encodes KO. ent-Kaurenoic Acid is then oxidized to GA12 via ent-7-Alpha-Hydroxykaurenoic acid and GA12-aldehyde.  Formation of GA12 is catalyzed by KAO. KAO1 (Arabidopsis thaliana ent -Kaurenoic Acid Oxidase-1)/ At1g05160 and KAO2 (Arabidopsis thaliana ent -Kaurenoic Acid Oxidase-2)/ At2g32440 encode KAO in Arabidopsis. GA12 is the 1st true Gibberellane ring system with 20 carbons. The biosynthesis of GAs after GA12-Aldehyde involves a series of oxidation steps that leads to the formation of bioactive GAs. Several GA pathways differing in hydroxylation pattern have been detected in different species (Ref.3).

In Arabidopsis, GA12 undergoes 13-Hydroxylation to form GA53, or may also form GA15 (Gibberellin-15). GA15 can further form GA24 (Gibberellin-24), which undergoes further oxidation to form GA9 (Gibberellin-9) in presence of enzyme GA20ox (Gibberellin 20-Oxidase) (GA20ox1 (Arabidopsis thaliana Gibberellin-20-Oxidase-1)/ At4g25420, GA20ox2 (Arabidopsis thaliana Gibberellin-20-Oxidase-2)/ A5g51810, and GA20ox3 (Arabidopsis thaliana Gibberellin-20-Oxidase-3)/ At5g07200 encode GA20-Oxidase in Arabidopsis). Besides GA24, GA15 can also form GA25 (Gibberellin-25). During the formation of GA25, 2-Oxoglutarate is converted to Glutarate.  On the other hand, GA9 undergoes Beta-Hydroxylation in presence of enzyme Gibberellin-Beta Hydroxylase to form GA4 (Gibberellin-4), which may further undergo hydroxylation to form GA1 (Gibberellin-1). GA53 is also converted to GA1 by a series of oxidation steps catalyzed by 2-Oxoglutarate–dependent dioxygenases, GA20ox, GA3ox (GA3-oxidases) and GA2ox (GA2oxidase). GA3ox1 (Arabidopsis thaliana Gibberellin-3-Oxidase-1)/ 1g15550 and GA3ox2 (Arabidopsis thaliana Gibberellin-3-Oxidase-2)/ At1g80340 encode GA3ox in Arabidopsis, whereas GA2ox1 (Arabidopsis thaliana Gibberellin-2-Oxidase-1)/ At1g78440, GA2ox2 (Arabidopsis thaliana Gibberellin-2-Oxidase-2)/ At1g30040, GA2ox3 (Arabidopsis thaliana Gibberellin-3-Oxidase-3)/ At2g34550, and GA2ox4 (Arabidopsis thaliana Gibberellin-3-Oxidase-3)/ At1g02400 encode GA2ox. First, GA20ox converts C20 from a methyl group to an aldehyde and then removes the C atom to form the characteristic G-lactone of the C19 GAs, via GA44. Then, a hydroxyl group is introduced at the 3b position by a GA3b-hydroxylase. A third 2-Oxoglutarate-dependent dioxygenase that hydroxylates at the 2b position inactivates the GA molecule and thus ensures turnover of the active forms. Although 136 GAs have been identified in higher plants, fungi, and bacteria, only a small number are biologically active (e.g., GA1, GA3, GA4, and GA7). Many of the other GAs are biosynthetic intermediates or catabolites of bioactive GAs (Ref.2 and Ref.4).

Active Gibberellins show many physiological effects, each depending on the type of Gibberellin present as well as the species of plant. Some important physiological processes stimulated by Gibberellins include stimulation of stem elongation by stimulating cell division and elongation, stimulation of bolting/flowering in response to long days, breaking seed dormancy in some plants, which require stratification or light to induce germination, stimulation of enzyme production (Alpha-Amylase) in germinating cereal grains for mobilization of seed reserves and induction of maleness in dioecious flowers (Ref.5 and 6).