The term auxin is derived from the Greek word auxein which means to grow. Compounds are generally considered auxins if they can be characterized by their ability to induce cell elongation in stems and otherwise resemble indoleacetic acid (the first auxin isolated) in physiological activity.
Auxins usually affect other processes in addition to cell elongation of stem cells but this characteristic is considered critical of all auxins and thus "helps" define the hormone
Although there is only one naturally occurring auxin: indole-3-acetic acid (IAA: chemically related to the amino acid tryptophan) there are many synthetic auxins as we shall see in lab.
In the late 1870s, Charles Darwin and his son Francis were one of the earliest scientists who studied phototropism (the growth of stems and leaves toward light).
The Darwins studied coleoptiles (the protective sheath around the embryonic shoot in grass seeds) of canary grass and oats. They discovered that both plants grow toward the light source.
The Darwins followed up this discovery with these experiments:
(i) The tips of coleoptiles were covered with a metal foil. This blocked the incoming light and the coleoptiles did not grow toward light. When the foil was removed they grew toward the light.
(ii) The growing region of the coleoptiles rather than their tips were covered and they discovered that the coleoptiles grew toward the light. The conclusion was made that the growth of coleoptiles toward light was controlled by the tip of the coleoptile.
The Darwins suggested that, "phototropism was due to an 'influence' produced in the tip of a coleoptile that moved to the growing region, where it caused the coleoptile to grow toward light." Their discovery helped later scientists discover plant hormones.
In 1913, Peter Boysen-Jensen further developed on the Darwins experiments:
(i) He cut off the tip of a coleoptile and noticed that it stopped its growth, which showed that something within the tip of the coleoptile controlled growth.
(ii) He then separated the tip from the coleoptile with a tiny piece of agar. He observed that the coleoptiles grew and curved toward the light. He concluded that the tips of the coleoptiles didn't have to be in their normal position to affect growth, and the chemical that controlled phototropism moved through agar, therefore it was a water-soluble chemical.
(iii) He replaced the agar block with butter. Since water is insoluble in butter, any water-soluble chemical from the tip could not move through the butter into the growing region. He observed that there was no growth, and from this concluded that the chemical was water-soluble.
(iv) To test if the signal was electrical he replaced the agar blocks with pieces of Pt foil, and there was no growth. Therefore, the signal was chemical rather than electrical.
In 1918, Arpad Paal continued on with Boysen-Jensen's experiments to identify the chemical. He studied coleoptiles grown in the dark:
(i) He cut off the tips of coleoptiles grown in the dark, and placed them on one side of the cut surface. These curved away from the side onto which the tips were placed, despite them being in the dark. The curvature was identical to that of the plants growing toward light.
Paal concluded that the coleoptile's tip produces something that travels down and stimulates growth, and that light causes the accumulation of the chemical on the shaded side of the coleoptile.
Frits Went finalized all these experiments ( 1926):
He cut off the tips and placed the cut surfaces onto agar. The tips were removed after an hour and the agar was placed on the cut tips of the coleoptiles grown in the dark.
Went's different experiments and results:
(i) Cut off coleoptiles & without agar blocks, did not grow. This confirmed that the tips produced something essential for growth.
(ii) Agar blocks that contacted cut tips were placed on the center of the cut off coleoptiles and they grew straight up. Therefore, the chemical diffused into the agar from the coleoptile tips, and stimulated their growth.
(iii) Agar blocks that did not contact the cut tips of coleoptiles did not show any response. Therefore, nothing in the agar caused growth of the coleoptile.
(iv) Agar blocks that had contacted the cut tips when placed on one side of the cut off coleoptiles, curved away from the agar blocks. This confirmed that the agar blocks had a chemical that stimulated growth of coleoptiles.
Went came to the conclusion that the phototrophic response was due to a chemical coming from the coleoptile's tip. He named this chemical auxin which comes from a Greek word meaning "to grow."
The most active auxin in plants is indole-3-acetic acid (IAA) and its' most active areas of synthesis are in young leaves, fruits, flowers, shoot tips, embryos, and pollen.
Some synthetic compounds have auxinlike effects; such as 2,4-D and NAA. 2,4-D is used as a herbicide because it is relatively cheap and nontoxic (?) to humans ( some question here relative to potential carcinogencity). Other uses of synthetic auxins are that they are used to produce roots on cuttings, prevent preharvest dropping of fruits and prevent lateral buds from growing.
Effects of Auxin:
1) Cellular Elongation: This requires positive turgor pressure and increased elasticity of the cell wall.
- Turgor pressure in cells results from the presence of dissolved solutes.
- Elasticity of the cell wall is increased by the IAA.
This is further explained by the acid-growth hypothesis:
- IAA stimulates H+ pumps in the cell membrane.
- H+ pumps secrete H+ into the cell wall, decreasing its pH.
- This acidifies the cell wall which activates pH-dependent enzymes and breaks bonds between cellulose microfibrils.
- The wall "loosens" because of the broken bonds and the turgor pressure expands the cell.
2) Apical Dominance: Prevention of lateral buds from growing out due to inhibition by apical meristem production of auxin moving basipetally through xy & phloem. If the tip is removed the axillary buds near the tip start growing. This dominance is due to the presence of auxin coming from the tip. We know that this is critical in shaping confers ( strong apical dominance vs. hardwoods ( weak)
The actual mechanism here is not quite as simplistic: IAA may induce ethylene production which has a negative influence. Cytokinins which move apically may actually be of greater importance here...
3) Cell Differentiation: importance of interactions with non-hormones
- Auxin and sugar -----> Vascular tissue
- Auxin and low sugar (1.5 - 2.5%) -----> Xylem
- Auxin and high sugar (4%) ------->- Phloem
- Auxin and moderate levels of sugar (2.5 - 3.0%) ----->- Xylem & Phloem
4) Auxin & Vascular Cambium: importance of interactions with a second hormone
- Auxin and Gibberellin ----- Xylem and Phloem
- Auxin alone ------ Xylem
- Gibberellin alone ------ Phloem
5) Root Growth: Promotes growth of adventitious roots, i.e. commercial rootings; Roottone
6) Fruit Development: Seeds in the fruit are the sources of auxin that stimulate fruit development. Seedless fruits can be formed by treating the unfertilized ovaries with auxin. These fruits are called parthenocarpic fruit (virgin fruit). Examples are tomatoes, cucumbers, eggplants, etc.
7) Abscission (shedding of leaves):
- Growing leaves & fruits produce a lot of auxin which is transported to the stem and this retards senescence (aging of leaves) and abscission.
- Shorter days of fall, drought, or the lack of nutrient causes less production of auxin and this begins senescence.
- A "senescence factor" stimulates cells to form ethylene which produces cellulase (an enzyme that breaks down cellulose) and pectinase.
- These enzymes digest the middle lamella.
This cell wall breakdown causes cells to separate and in turn cause abscission.
How does auxin act? consider this abstract copied below....
Expression of auxin-regulated genes (F. Sitbon and C. Perrot-Rechenmann)
Auxins constitute a small class of plant hormones having profound effects on plant development. Among the fastest cellular responses to auxin addition is an alteration of gene expression, and the abundance of several mRNA species has been shown to increase, or decrease, within minutes to hours after auxin treatment. The regulation of this process appears, at least partly, to be at the transcriptional level. Using differential screening approaches, a number of auxin-regulated cDNAs and genes have been isolated, mainly from elongating tissues and dividing cells. In addition, an influence of auxin on the regulation of genes isolated for other purposes has also been revealed. With respect to kinetic and dose-response characteristics, as well as to cellular expression and hormonal specificity, individual auxin-regulated genes seem to be regulated very differently, possibly reflecting different modes of auxin action.