Growth and Development of Horticultural Crops (1+1)

Biosynthesis of gibberellins in plants

• The gibberellins which are chemically related to terpenoids (natural rubber, carotenoids & steroids) are thought to be formed by the condensation of a 5-C precursor-an isoprenoid unit called as ispentenyl pyrophosphate (IPP) through a number of intermediates to give rise to gibberellins. The primary precursor for the formation of this isprenoid unit and synthesis of gibberellins is however, acetate. Besides gibberellins, carotenoids, rubber, steroids, abscisic acid (ABA) and part of cytokinins are also derived from 5-C isoprenoid unit.
• In plants GAs are biosynthesized in apical tissues and there are three main sites of their biosynthesis,
(i) Developing seeds and fruits,
(ii) Young leaves of developing apical buds and elongating shoots and
(iii) The apical regions of roots.

• The pathway of GA biosynthesis can be divided into three stages each of which is accomplished in a different cellular compartment.
Stage I. Formation of terpenoid precursors and ent-kaurene in plastids

• GA is biosynthesized from a 5-C precursor IPP. The IPP may be synthesized either in plastids or cytosol. From IPP, 10-C (GPP), 15-C (FPP) and 20-C (GGPP) precursors of terpenoids are formed by condensation of 5-C units (IPP). After the formation of GGPP, the pathway becomes specific for GAs.
• GGPP is converted by two cyclization reactions through copalyl pyrophosphate into entkaurene. These reactions are catalysed by the enzymes cyclases which are located in proplastids and not in mature chloroplasts and infact constitute the first step that is specific for GAs. This step of GA biosynthesis is inhibited by compounds such as Amo-1618, Phosphon D and CCC.
Stage II. Oxidations to form GA₁₂ and GA₅₃ on ER through GA₁₂ aldehyde

• The ent –kaurene is transported from plastids to ER (endoplasmic reticulum). Now a methyl group on ent-kaurene at 19th-carbon position is oxidized to carboxylic group which is followed by contraction of ring B from 6-C to 5-C ring structure to form GA12 aldehyde. GA₁₂ aldehyde is subsequently oxidized to give GA₁₂ which is precursor to all other GAs in plants. Hydroxylation of GA₁₂ at C-13 results in the formation of GA₅₃.
• The enzymes catalyzing the above oxidation reactions are mono-oxygenases which are located on ER and utilize cytochrome P450 in these reactions. Activity of these enzymes is inhibited by paclobutrazol and other inhibitors before GA₁₂ –aldehyde.
Stage III. Formation of all other GAs from GA₁₂ or GA ₅₃ in cytosol

• All other steps in the biosynthesis of GAs from GA₁₂ of Ga₅₃ are carried out in cytosol by soluble enzymes called dioxygenases.
  
• These enzymes require molecular O₂ and 2-oxoglutarate as cosubstrates and use ferrous iron (Fe⁺⁺) and ascorbic acid as cofactors.
  
• Environment factors such as temperature and photoperiod are known to affect biosynthesis of gibberellins.





Biosynthetic pathway of GA
http://www.nature.com/cr/journal/v17/n5/images/cr200740f1.jpg