Phytohormones

Phytohormones function to coordinate plant growth and development. The compounds that have been considered as plant hormones are:

indole-3-acetic acid (auxin)
cytokinin
gibberellin
ethylene
abscisic acid

In addition, the following substances have been shown to have important growth regulating activities and are considered to function as Phytohormones:

brassinosteroids
jasmonic acid
salicylic acid

Plants contain a wealth of compounds often referred to as secondary metabolites with unknown functions. Many of these have the potential to have growth regulatory roles that remain to be discovered. Among these types of compounds, some have been shown to affect growth. These include

flavonoids
carbohydrates
fatty acids
peptides
phenolic acids and alcohols

We have already seen a number of developmental and physiological process in which certain phytohormones play critical roles. In addition to the processes already discussed, phytohormones and other growth regulatory substances control a wide range of other processes and in many cases, the effects of a particular hormone depend on the action of another.

Auxin has already been discussed with respect to its role in controlling cell elongation during photo- and gravitropism. Control of cell elongation is just one of many auxin responses. Auxin is a term that refers to compounds that, when applied to plants of plant tissues, cause growth effects that are similar to those seen with indoleacetic acid. Auxins include compounds like 2,4-D and NAA (2,4-D is a common herbicide and NAA is often included in mixtures used to stimulate root formation in cutting).

Auxin-regulated responses include:

induction of lateral and adventitious roots
stimulation of fruit growth
apical dominance
leaf and flower abscission
DNA synthesis

Auxins have found several uses in agriculture and horticulture. The auxin 2,4-D is a commonly used herbicide against broadleaf plants. NAA and IBA are often the active ingredient in products that stimulate root formation on plant cuttings. Auxins may be used to enhance fruit production or harvesting but these effects are species specific. Auxin, along with cytokinin, is used in for culturing plant tissues for mass propagation.

We have discussed cytokinin with respect to their role in controlling dormancy of seeds and buds. Like auxin, cytokinin is a term that describes a class of compounds. Specifically, cytokinins are compounds that stimulate cell division or cytokinesis, although they may also do other things. Proper regulation of cell division also requires auxin, which is needed to cause DNA synthesis before a cell can divide.

Cytokinin responses include:

cell division (cytokinesis)
organ development (shoot formation)
delayed senescence and promotion of chloroplast development
affect nutrient sink strength of organs
promotion of lateral bud growth
promotion of cotyledon expansion (only in certain species)
inhibition of auxin-induced elongation

Cytokinins have found few uses in agriculture. They are used in plant tissue culture and they have been used to delay senescence. Possible future applications will depend on clever bio-engineering. For example, since cytokinins are important in regulating source-sink relationships, controlled production of cytokinins in fruit could potentially lead to increased nutrient mobilization into the fruit, thus, more nutritionally valuable food.