Phytohormones (Plant hormones)

Plant Hormones

– A compound produced by one part of the plant.


– A compound produced in one area of an organism and has an effect on another area.

Phytohormones (Plant hormones)

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Classes of Plant Hormones

Major classes of hormones produced by plants and PGPR are:

  • Auxins (IAA)
  • Cytokinins
  • Gibberellins (GA)
  • Abscisic acid
  • Ethylene

1. Auxin

It is also called as  Indole Acetic Acid

The major site of auxin production is apical meristem by plants


a. It promotes elongation of young shoots (Indoleacetic acid)

  1. Affects secondary cell growth by inducing vascular cambium and secondary xylem
  2. Promotes fruit growth

Bacterial spp.

Bradyrhizobium, Azospirillum, Rhizobium and Enterobacter cloacae, and cyanobacteria.

2. Cytokinins

It stimulates cytokinesis produced in roots by plants and transported to other organs.


a. Cell division and cytokinesis

  • Moves in xylem sap
  • Stimulates RNA and protein synthesis
  • Works in conjunction with auxin

b.Apical dominance

  • Cytokinins and auxin are antagonistic- auxin from terminal bud causes the shoot to lengthen.
  • Cytokinins from roots stimulate Axillary bud
  • Auxin stimulates lateral root formation cytokinins to restrain it.

c. Anti-aging

  • slows leaf deterioration

Bacterial spp.

Azotobacter spp. , Rhizobium spp. , Bacillus subtilis

3. Gibberellins

80 different kinds

It is produced in the meristem of apical buds and roots, young leaves, embryo.


a. Stem elongation

  • stimulates cell division, growth of leaves
  • causes bolting- the rapid growth of floral stems

b. Fruit growth

  •  controlled by Gibberellins and auxin
  •  Grapes are sprayed to grow bigger

c. Germination

  • signals seeds to break dormancy

Bacterial spp.

Azotobacter spp. , Rhizobium spp. , Bacillus subtilis

4. Abscisic Acid

  • helps prepare plants for winter by suspending growth
  • Produced in leaves, stem, roots, green fruits


  • inhibits cell division in the vascular cambium
  • the onset of seed dormancy
  • stress hormone – closes stomata

Bacterial spp.

Bacillus spp., Azospirillum spp., Rhizobium spp.

5. Ethylene

  • Gaseous hormone-induced by high auxin inhibits the growth
  • It is produced by tissues of ripening fruits, nodes of stems, aging leaves and flowers.


a. Senescence

  • aging, fruit ripening, and leaf abscission

b. fruit ripening-aging cells release more ethylene

  • spreads from fruit to fruit
  • Leaves lose pigment

c. Leaf abscission- prevents desiccation

  • leaf nutrients are sent to storage
  • short days and cool temperature

Bacterial spp.

Pseudomonas spp.

Progress in Industrialization

  • For the industrial production of plant hormones such as GAs and ABA, several processes using fungal fermentation were successfully set up in China.
  • As early as the 1950s–1960s, the solid-state fermentation of GAs has been applied.
  • Compared with GAs, the industrial microbial fermentation of ABA started relatively late.
  • This hormone was firstly made by the sloid state fermentation but due to low yield and low titer of about 2.0g/L, it is replaced by the liquid submerged fermentation process, the ABA maximum production titer was then increased up to 6.0 g/L.
  • Subsequently, the world’s first industrial production line of ABA was formally established through the cooperation with the company Sichuan Lomon Bio Co. Ltd, which greatly reduced the cost and price of ABA and further promoted the application of ABA in agriculture.



Phytohormones (Plant hormones)

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