Auxin - A plant growth regulator

 

• Charles Darwin and his son Francis Darwin found that the coleoptile tip of canary grass has  sensation towards the light.

• Later  on F. W. went to collect the growth stimulating substance in Agar Jelly.

• Went discovered that A  hormone is moved  from tip to base. This hormone is responsible for bending of the coleoptile according to its concentration. 

• Went named this growth promoting substance as Auxin. Later Kogl and Smith isolated three chemicals from human urine which they named as Auxin a, auxin b and heteroauxin.

• Kogl also found that the heteroauxin  is the real plant auxin and is chemically indole 3 acetic acid.

• Indole acetic acid is found in shoot Apex, leaf primordia and developing seed.

• Amino acid tryptophan is the precursor for the synthesis of indole Acetic Acid or Auxin. 

• Auxin is a weak acidic growth hormone. Indole 3 acetic acid and indole 3 butyric acid are naturally occurring auxins  while naphthalene acetic acid 2,4,D are synthetic auxins but they have similar structure and chemical properties. 

• Auxin induces cell division activity in cambium and elongation of cell in shoot. They induce early differentiation of xylem and phloem in tissue culture experiments.

• In general this hormone initiates the rooting  in stem cutting but inhibits the growth of root at Higher concentration.

• They inhibit the growth of axillary buds and promote apical dominance. Auxin promotes flowering in Pineapple .

• They enhance the size of the carpal and hence earlier fruit formation. 

• The application of auxin  retard  the process of senescence.

• This hormone also retard the abscission of leaves  at early stage but promote the abscission  of older mature leaves and fruit.  It also induces parthenocarpy in some plants like tomatoes. They induce feminization in certain plants. 

• Some of auxin like 2, 4 D and 2, 4, 5 T act as a weedicides. 

• Avena curvature test and root growth inhibition test are done for examining the effect of this hormone.

• The culture test is based upon the experiment of F. W. bent on tips of coleoptile of oat seedling.