Auxin the first plant hormone identified

 

Function of auxin: Auxin regulates many physiological processes in addition to phototropism (Figure 6) these include

 

Figure 6: Phototropism

cell division root initiation apical dominance leaf senescence leaf and fruit abscission fruit setting and growth fruit ripening flowering (Davies 1995).

Auxin biosynthesis: The most biologically active auxin is indole-3-acetic acid. Unfortunately the predominant biosynthetic pathway of IAA is still unclear. IAA is a precursor utilized to produce amino acids, peptides, and sugars. IAA's involvement in many metabolic pathways has complicated the discovery of its biosynthetic route (Figure 7) (Buchanan et al 2000).

 

Figure 7: IAA is a precursor utilized in many metabolic pathways (Gretz 2001)

 

In 1930, K.V. Thimann showed that IAA could be synthesized from the aromatic ring of the amino acid tryptophan. For many years, it was believed that IAA was a product of tryptophan, but a tryptophan independent pathway was later observed (Hopkins 1999). The tryptophan independent pathway was based on the observation that mutants with reduced levels of tryptophan synthase have increased amounts of IAA conjugates such as amino acids, peptides, and sugars. These two pathways have been designated tryptophan dependent and tryptophan independent pathways. Tryptophan is thought to be the primary precursor for auxin biosynthesis in vivo. Hull et. Al. state that the two most commonly accepted routes for IAA biosynthesis in vivo are tryptophan - Indole 3 Acetamide (1) and tryptophan - Indole 3 Acetaldoxime (2). A third route which, converts tryptophan to tryptamine (3) then to IAA is not believed to function in vivo due to lack of experimental data (Figure 8) (Buchanan et al 2000).

 

 Figure 8: Predicted indole 3 acetic acid (IAA) biosynthetic pathways (Gretz 2001)

 

Agricultural uses of auxin

• Seedless fruits
• Delayed fruit drop
• Auxin assists fruit harvesting
• Promoting root growth
• Synthetic auxins

   

  Seedless fruits: Auxin regulates fruit development. Auxins are produced by the pollen tube as it grows through the style, and also by the embryo and endosperm in the developing seeds. Fruit growth depends on these sources of auxin. In some plants (tomato and cucumber) application of auxin to flowers before pollen is mature can promote parthenocarpy, the production of fruits without fertilization, leading to the development of seedless fruits (Figure 9) (Levetin and McMahon 1999).

   

  

   Figure 9: Application of auxin to flowers result in seedless cucumbers and tomatos

  Delayed fruit drop. Auxin regulates maturing fruit on the trees of apples, oranges, and grapefruit. These trees can be sprayed, with low levels of auxins, to prevent premature development of abscission layers and the resulting fruit drop. This topical application tricks the plant, activating signals that maintain growth rather than promote cell death (Levetin and McMahon 1999).

   

  Auxin assists fruit harvesting. Synthetic auxins have been used commercially in the plant industry for years. High doses of auxins can cause fruit drop. This phenomenon can be used to fruit growers' advantage. To save money, heavy applications of synthetic auxins are used commercially to promote a coordinated abscission of various fruits to facilitate harvesting. (Levetin and McMahon 1999).

   

  Promoting root growth. Auxin induces adventitious root development in cuttings used to propagate plants. Shoot tips of many plant species, when dipped or coated with small amounts of auxin, develop roots more quickly and in higher numbers. Most commercially available rooting powders take advantage of this effect (Figure 10) (Levetin and McMahon 1999).

   

  

  Figure 10: Root powders are used comercially to induce root growth 

  Synthetic auxins. Synthetic auxins have been used widely since the 1940s and have revolutionized weed-control in agriculture. Their use has decreased production costs by reducing the amount of labor and mechanical weeding needed to grow and efficiently harvest a crop.

  Synthetic auxins such as 2,4 D (2,4-dichlorophenoxyacetic acid) have a structure related to auxin. 2,4 D has been used extensively as a pesticide because it is inexpensive and relatively nontoxic to humans. 2,4 D and other synthetic auxins have a selective effect: they kill broadleaf dicots, but not monocots (Figure 11) (Moore et al 1998).

   

  

  Figure 11: 2,4-D is a synthetic auxin used as a pesticide (Gretz 2001)

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  Plant Hormones	Auxins
  yucca	Flavin Monooxygenase
  Conclusion	References