AUXIN AND ITS ROLE

 

                                                                   PHYTOHORMONES

 

Definition:

The term phytohormones is implied to those chemical substances which are synthesized by plants and thus, naturally occurring. On the other hand, there are several manufactured chemicals which often resemble the hormones in physiological action and even in molecular structure. Recently, another two groups, the brassinosteroids and polyamines were also known to behave like hormones.

Characteristics of phytohormones:

• Usually produced in tips of roots, stems and leaves.
• Transfer of hormones from one place to another takes part through conductive systems.
• They are required in trace quantities.
• All hormones are organic in nature.
• There are no specialized cells or organs for their secretion.
• They are capable of influencing physiological activities leading to promotion, inhibition and modification of growth.
  
  
  

Auxins:

Discovery:

1. During 1880, Charles Darwin noted the unilateral growth and curvature of Canary grass (Phalaris canariensis) coleoptile to light.
2. The term auxin (Greek: Auxin – to Grow) was first used by F. W. Went in 1926 using Oats (Avena) coleoptile and isolated the auxin. F. W. Went in 1928 collected auxin in agar jelly. 
3. Kogl and Haugen Smith (1931) isolated Auxin from human urine, and called it as Auxin A. 
4. Later on in 1934, similar active substances was isolated from corn grain oil and was named as Auxin B. Kogl et al., (1934) found heteroauxin in the plant and chemically called it as Indole Acetic Acid (IAA)
   
   
   
   

Occurrence:

Auxin is generally produced by the growing tips of the stem and root, from where they migrate to the region of the action.

Types of Auxin:

Auxins are divided into two categories Natural auxins and Synthetic auxins.

Free auxin:

They move out of tissues as they are easily diffusible. Example: IAA.

Bound Auxin:

They are not diffusible. Example: IAA[1]Aspartic acid

Precursor:

The amino acid Tryptophan is the precursor of IAA and zinc is required for its synthesis.

Chemical structure:

Auxin has similar chemical structure of IAA.

Transport in Plants:

Auxin is polar in transport. It includes basipetal and acropetal transport. Basipetal means transport through phloem from shoot to root and acropetal means transport through xylem from root to shoot.

Bioassay (Avena Curvature Test / Went Experiment):

Bioassay means testing of substances for their activity in causing a growth response in a living plant or its part.

Physiological Effects:

1. They promote cell elongation in stem and coleoptile.
2. At higher concentrations auxins inhibit the elongation of roots but induce more lateral roots. Promotes growth of root only at extremely low concentrations.
3. Suppression of growth in lateral bud by apical bud due to auxin produced by apical bud is termed as apical dominance.
4. Auxin prevents abscission.
5. It is responsible for initiation and promotion of cell division in cambium, which is responsible for the secondary growth and tumor. This property of induction of cell division has been exploited for tissue culture techniques and for the formation of callus.
6. Auxin stimulates respiration.
7. Auxin induces vascular differentiation.

Agricultural role:

1. It is used to eradicate weeds. Example: 2,4-D and 2,4,5-T.
2. Synthetic auxins are used in the formation of seedless fruits (Parthenocarpic fruit).
3. It is used to break the dormancy in seeds.
4. Induce flowering in Pineapple by NAA & 2,4-D.
5. Increase the number of female flowers and fruits in cucurbits.