Growth Hormone: Gibberellin

Growth and development of an organism depends upon the internal and external factors supporting the growth of organism.

The external environmental cues are essential for the stimulation of growth and development for reproduction and survival efficiency of these environmental cues (i.e.) stimulus process the internal response of the organism.

The response is manifested and cell functions corresponding to the response is recorded. Such stimulus and response are well coordinated in the plant body by the chemical messengers – the hormones.

Hormones acts as a mediator for carrying and transferring information for the coordination of physiological, metabolic and chemical activity.

For example: the growth of coleoptile of Phalaris canariensis towards the light is the coordinated movement for external stimulus light (i.e.) the phototropism is mediated by the hormone auxin. This was the initial discovery for the presence of phytohormones in plant system regulating the function of whole plant body.

These chemical compounds enhance cell communication and integrate the multicellular organism to organized as a single unit.

Further studies were undergoing for the deducing the mechanism and variety of hormones coordinating plant body.

Between 1950 – 1960 a group of five hormones were identified to maintain the plant homeostasis. These hormones were combinedly termed as ” Classical Five” they are: AUXIN, CYTOKININ, ETHYLENE, GIBBERELLIN, ABSCISIC ACID.

Along with classical five there are brassanosteroids and jasmonic acid. These set of hormones are termed as “Plant Growth Regulators” as they have an active role in regulating growth and development rather than a broad action spectrum.

Hormones are sensitive, specific, low concentration action and are naturally occurring in plant species. These important characteristic makes it an ideal small molecule chemical messengers and regulators.

The mode of action is receptor mediated and are transported to different regions by vascular tissues(i.e.) xylem and phloem. Hormones like ethylene are volatile, hence they are diffused throughout the plant body.

Gibberellin Discovery

Gibberellins were discovered from a Japanese crop disease “Bakanae” where the crops are subjected to elongated and weak growth caused by an ascomycetes Gibberella fujikuroi. the extracts from the fungi are collected and termed as Gibberellin by Yabuta and Hayashi.

In 1950’s, Europe and American scientists started to standardise methods of isolating hormones. On learning about Gibberellin, its function to elongate the stem was related to higher plants which have taller stems and branches.

When extracted the responding hormone scientists named it as GIBBERELLIN. Gibberellins are tetracycline diterpenoids prevalent in two forms. C20 – GA’s and C19 – GA’s out of this C19 GAs are found abundant in plant cell.

The variety of GA’s are increasing, presently there are about 125 GAs identified.

Functions of Gibberellins

1. they promote elongation in stem and root. Their role in cell division is nil but they elongate both root and shoot

2. During Seed germination, the stored starch, proteins and lipids are broken down by hydrolysis. GA’s promote hydrolysis during seed germination.

3. Floral development

4. Phloem tissue differentiation

5. Cambial Reactivation

Gibberellin Biosynthesis

Out of all Plant Growth Regulators, GA’s are very well studies and biosynthesis are well known.

Terpenes are secondary products of plants is defensive in function prevents plant from plant feeding Insects.

Apart from the defence terpenes helps in photosynthesis, membrane stability and signalling Isoprene a C5 structure is the base for all terpenes.

The isomerization of isopentenyl diphosphate yields dimethylallyl diphosphate following condensation yields C10 terpenes, C15 terpenes on further condensation gives C30 and C40 terpenes.

IPP was synthesized by mevalonic acid dependent pathway or by Mevalonic Acid Independent pathway by Pyruvate and Glyceraldehyde.

Synthesis takes place in 3 stages:

Stage 1: Geranylgeranyl Diphosphate (GGPP) undergoes cyclation to form ent – kaurene followed by enzyme catalysation takes place in Plastids.

Stage 2: Oxidations leads to the formation of ent – kaurenoic acid hydroxylated to form ent -7alpha kaurenoic acid later yields G12 Aldehyde

Stage 3: GA12 – aldehyde yields GA12. 13 – Hydroxylation leads to the formation of GA53. Oxidations at C20 leads to lactone formation.