Plant hormones and their commercial uses

Plant hormones, and their derivatives are extremely widely used in agriculture, biology and a number of related fields, this essay is a discussion of most common uses of these hormones and their synthetic analogues in a commercial environment.

Perhaps the most widely used group of plant hormones are the auxins. Auxins are plant hormones; the most important of which is known as indole-3-acetic acid or (IAA). It plays important roles in a whole range of plant processes including, root development, fruit development, and plant trophism (plant growth responses to external stimulus). Because of the very wide range of effects that it can initiate, auxins are probably used commercially more than any other plant hormone.

Their most common use is in the creation of seedless varieties of fruit. Normally, pollination of the flowers of angiosperms begins the formation of seeds. As the seeds mature, they release auxin to the surrounding flower parts, which develop into the fruit. There are now many varieties seedless fruit available to the consumer. With the exception of the so called polyploids (plants such as the banana tree that develop fruit without fertilisation) these are created by spraying the plants with synthetic auxins that will cause the flowers to develop into fruit even though they have not been fertilised. Not only does this ensure that all the flowers will "set" fruit, but it also maximises the likelihood that all the fruits will be ready for harvest at the same time.

The second major use of auxins is that of synthetic rooting powders and gels, if a cutting is placed into the gel or powder it will stimulate the growth of adventitious root cells. (Those growing from stems or leaves rather than from the regular root system of the plant).

Horticulturists may propagate desirable plants by cutting pieces of stem and placing them base down in moist soil. Eventually adventitious roots grow out at the base of the cutting this process being hastened by treating the cuttings with a gel or powder containing a synthetic auxin.

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Some of the most widely used weed killers are synthetic auxins. These include 2,4-dichlorophenoxy acetic acid (2,4-D) and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T).

A mixture of these was the "Agent Orange" used by the U.S. military to defoliate the forest in parts of Vietnam during the conflict. These hormone based weed killers are very popular due to the fact that they are very selective; killing broad-leaved plants but not grasses (scientists are still not completely certain as to the reasons for this selectivity). The synthetic auxin acts by being taken into the cell by the same active transport mechanism as normal auxins, but unlike the normal auxins it cannot leave the cell, and it is believed that the resulting accumulation of 2,4-D within the cell kills it.

Perhaps the most unusual of the plant hormones used commercially is Ethylene

Ethylene differs from other plant hormones in being a gas formula (H2C=CH2).

Most fruits release ethylene as they begin to mature, however proximity to the gas will cause other fruit to mature as well the presence of ethylene is detected by transmembrane receptors in the surface of the plasma membrane of the cells. This initiates the breakdown of chlorophyll, so that the red and/or yellow pigments in the cells of the fruit are unmasked and the fruit assumes its ripened colour.

Another important plant hormone is giberellic acid, and its related compounds (the giberellins). During the 1930s Japanese scientists isolated a growth-promoting substance from cultures of a fungus that parasites rice plants. They called it gibberellin. Shortly after the Second World War plant physiologists in other countries began to carry on their work, and succeeded in isolating more than 30 closely related compounds. The most active of these is giberellic acid. Giberellic acid (like the auxins) has a number of effects on plant growth, but the most dramatic is its effect on stem growth. When applied in low concentrations to a bush or "dwarf" bean, the stem begins to grow rapidly. The length of the internodes becomes so great that the plant becomes indistinguishable from climbing or "pole" beans. Giberellic acid seems to overcome the genetic limitations in many dwarf varieties. Genetic analysis of the dwarf wheat varieties that have played such an important part in the "green revolution" reveals that they carry mutant genes that reduce their ability to respond to their own giberellins. However these dwarf plants tend to be much tougher and resistant to bad weather and plant parasites.

The final major commercially used plant hormone is Abscisic acid; this compound and its analogues (structurally similar compounds to the natural ABA plant hormone) interact in various physiological plant processes including stress avoidance mechanisms that reduce transpirational water loss under drought conditions. Drought shock and cold stress are common causes of loss during establishment of horticultural crops such as vegetables, ornamentals and flowering annuals. Application of synthetic analogues of Abscisic acid to young seedlings or transplants can increase their tolerance to drought stress and cold. Unfortunately, the beneficial effects of ABA applied in this way are short lived as the chemical is rapidly metabolised. However, the synthetic analogues are more physiologically active and longer-lasting compounds than naturally occurring hormone.

These compounds have extensive uses in horticulture, agriculture and forestry and there is many commercial applications for this product that include, Reduction of seedling transplantation shock in fruit trees, vegetables, ornamentals and flowering annuals (it is often applied to cuttings along with an auxin derivative to reduce plant stress). Perhaps the most common use is to treat 'seed' potatoes and malt barley to delay sprouting during storage.