Role of Plant Growth Regulators in Agriculture
Authors: Mamta Bajya (Plant Physiology), Tejpal Bajya (Plant Pathology), Dr. B.L. Kakralya (Plant Physiology)
S.K.N. College of Agriculture, Jobner-303329

The term hormone was first introduced by fitting (1910) for naturally occurring organic substances which regulates specific functions in plants. K.V. Thimman (1935) suggested the term phytohormones for hormones of plants.

A plant hormone or phytohormone can be defined as an organic compound synthesized in one part of a plant and translocates to another part, where in very low concentration it causes a physiological response in plants. The American Society of Plant Physiology in 1954 defined phytohormones as organic comnpounds other than nutrients, which in small amount promotes/ inhibit or otherwise modify any physiological response in plants

The hormones which are not synthesized by plants but promote the growth are caused as plant growth regulators. For a hormonal action, they interact with specific celluar proteins called receptors. Many natural hormones are being prepared synthetically for use in agriculture and other purpose. Inorganic ions like K+, Mg2+, Ca2+ etc. are not hormones because plants do not synthesize them. Similarly, despite its synthesis in plant, the sucrose is not considered as a hormone because it causes growth only at relatively high concentration.

Phytohormones or plant growth regulators have been divcided basically in following five classes.

  1. Auxins 2. Gibberellins 3. Cytokinins 4. Abscisic acid 5. Ethylene
These groups can also be categorized into growth promoting (audins, gibberellins and cytokinins) and growth retarding (ABA and ethylene) substances. In past 50 years, several other compounds have been identified that meet the criteria of phytoromones. Some of them are polyamines, jasomnates and its derivative tuberonic (acid, salicylic acid, strigolactones, brassinosteroids etc. However, their incorporation into the family of plant hormones is largely but not universally accepted.

Isolation and quantifications of PGRs

It is very important to know whether a given hormone helps control some particular physiological processes in vivo. For this there is need to extract and quantify the hormone and relate its tissue concentration to the magnitude of the response. Hormones can be quantified with bioassays, physiochemical methods or immunological methods. The term bioassay is used to describe the use of living material to test the effect of known and putative biologically active substances. In order to be a useful bioassay, it must be specific and unts of uniform plant tissue. Beside, the chemical being assayed for or related compounds must be present at very low levels or absent from the plant material. Numerous bioassays have been developed for different hormones, which are described in appropriate sections.

Now a days, the bioassays have been replaced by use of physiochemical assays, which include high performance liquid chromatography (HPLC) and gas chromatography (GC), which are then followed by the use of mass spectrometry (MS) to obtain the proof of structure. Since the introduction of capillary GC columns, their superior resolving power has made gas chromatography the most commonly used methods for the detection of plant hormones. Today HPLC is used only with highly selective deterctors or with highly purified samples. The immunological assays (entyme linked immunosorbent assay (ELISA) or radioimmunoassay (RIA) are extremely sensitive and very popular methods which have a lot of potential value in plant sciences.

In immunoassay, an antihormone antibody made by animal cells is used to react with the hormone in a cuvette assay. Immunoassays are very rapid with maximum sensitivity over the bioassays but they are commonly subject to negative or positive interference unless the hormone has been well purified. Furthermore, one must either prepare or purchase the antibody, which frequently is noteasy.

Types of Auxins

  1. Natural auxins
  • Indole -3-acetic acid
  • 4-chloro-indole acetic acid
  • Phenyl acetic acid
  • Indole-3 butyric acid
Three additional compounds indole 3- acetaldehyde, indole acetonitrile and indole-3-ethanol also have their structures similar to IAA but each lacks the carboxyl group.

  1. Synthetic auxins
  • Nepthalene acetic acid (NAA)
  • 2, 4-Dichlorophenoxyacetic acid (2,4-D)
  • Indole-3-butyric acid (IBA)
  • Benzoic acid and 2-methyl-4-chlorophenoxyacetic acid (MCPA)
Note- The IBA was considered as synthetic auxin but it also occurs naturally in maize leaves and various other dicots.

  1. Gibberellins
This procedure is still used today for more than 133 gibberellins currently known.

The molecular formulae of GAs comprised at C19H24O 6 (GA1), C19H26O6 (GA2), C19H22O6 (GA3 ) etc.


(A) natural cytokinins

  1. Zeatin
  2. Dihydroxyzeatin
  3. Isopental adenine (IPA)
(B) Synthetic cytokinins

1. Kinetin (6-furfuryl aminopurine)

2. BA (6-benzylaminopurine)

Abscissic acid (ABA)

  • Stress conditions enhance ABA biosynthesis in plants to impart tolerance against the stresses.
  • So, ABA is also known as “Stress hormone”
  • Abscisic acid is a sesquiterpene (15-carbons) compound (molecular formula (C15H20O4)

Ethylene is a rather different from previous four hormones in terms that it is a gas.

  • Methionine is precursor of ethylene.
  • ACC (1-aminocyclopropane-1-carboxylic acid) sercves as intermediate in the conversion of methionine to ethylene.
New generation of phytohormones

  1. Brassinosteroids 2. Jasmonates 3. Salicyclic acid 4. Tricantonal 5. Turgorins 6. Strigola ctones 7. Polyamines
Growth substances and their function

S.No. hormone Functions
1. Auxin Named by F.W. went, It has apical bud dominance, inhibit root elongation and promote cell division in tissue culture.
A. IAA Prevent premature fruit drop
B. IBA Root formation
C. NAA Fruit thinner, prevent fruit drop
D. 2,4-D Fruit setting hormone (<20 ppm), and also used as a herbicide (> 20ppm)
2. Cytokinin Dormancy breaker, stimulate cell division
3. Gibberellins Cell elongation, increase fruit size
4. Abscisic acid (ABA) Dormancy induced, stomata closer also called as anti-gibberellins
5. Ethylene Ripening of fruits, isodiametric growth of stem and roots
Available in two forms 1. Ethrel – it is liquid form of ethylene and used for banana fruit ripening 2. Ethephone: It is gaseous for of ethylene and used for early ripen of sugarcane, wheat coffee, tobacco and rice
6. Cloremequat (CCC) Lodging preenter, also known as cycocel
7. Mallic hydcrazide Growth retardant, prevent onion sprounting in storage, sold under trade name sprout stop

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