Plants, just like humans and all other living things, make hormones to fulfill their natural functions. Unlike humans, who produce about 50 different types of hormones, plants only produce five.
These five classes of hormones—known as the classical five—are produced in various parts of a plant to serve either at the site of their production or elsewhere in the plant. Naturally occurring plant hormones and hormone types are: auxins, gibberellins, cytokinins, ethylene, and asbscisic acid.
Some of their functions overlap, and some are quite unique. All, however, are needed for a plant to maintain health and produce viable flowers, seeds, or fruits.
The complexities of the interactions these five plant hormones have are still not fully understood, though large portions and sequences are known. The combination and levels of each hormone are different for each plant species and at each different stage during their respective developments.
For example, auxins, gibberellins, and cytokinins are all synergistically involved in the setting of most fruits. Individually, each may be able to initiate the process but cannot see it through to completion without the assistance of the other two hormones.
Abscsisic acid is thought to also play a role in fruit development, but its exact role in the process is not yet known for sure. In other interplays, abscisic acid is produced in concert with ethylene even though their purposes are contradictory in regard to fruit ripening and drop.
Do Plants Really Have Hormones?
There is much debate and has been for some time in the botany world about the actual term “hormone” when referring to plants. While no one reading this is likely to pound their fists on their breakfast tables demanding to know why this has not yet been resolved, there are several compelling reasons as to why this clash exists.
Some botanists and plant scientists suggest that the term “plant growth substance” is more apt. The rebuttal to that phrase is that “substance” is too vague a term. Those that want to do away with calling them hormones argue that, unlike hormones in mammals, plant hormones often serve contrary functions that don’t have a parallel in the animal world.
For example, a plant will produce both a substance that will cause it to grow and a substance that causes it to go dormant or even die. There is not such a conflict of roles in the world of animal hormones. These researchers claim that scientists are trying to make these substances fit the mold or definition of animal hormones based on their roles, and they just are not the same thing.
For the purposes of this article, the term “hormone” will be used in the traditional sense regarding their functions and roles within plants. We will have to let the academics fight out the merits of their semantic choices, but until then, “hormone” it is.
Most people, whether consciously aware or not, have seen and can recognize the effects of the auxin hormone in plants. It is responsible for the phototropic tendency in plants to grow towards the light. It performs this task to allow for maximum photosynthesis.
It achieves this by moving throughout the plant towards the sections that are receiving the least amount of light. In these areas, the plant cells are then enlarged, which aids in the plant’s ability to elongate and bend toward the available life-giving light.
Anyone who has ever trimmed a plant or shrub has also seen the effects of this plant hormone. Auxin is responsible for stem elongation at the tip of the stems. This tendency of a plant to keep growing up is called “apical dominance.” When these tips are pruned or even unintentionally broken, the plant then grows outward causing a stockier, usually better and stronger branched plant.
Most auxins that are commercially used are synthetic, due to the higher costs associated with naturally occurring auxin. They are used for both promoting growth as well as killing it. On fruit trees, synthetic auxins are used to treat pruning sites for sucker growth to prevent their return as well as to prevent the setting of fruit.
They are also the primary compounds in many popular herbicides. They cause uncontrollable cell growth, like a cancer, in plants and cause them to essentially grow themselves to death. This is how 2,4-D works so well for controlling broad-leafed weeds. On the flip side, other types of synthetic auxins are the primary component of rooting hormones for the propagation of plant cuttings.
Gibberellins (GAs) are the largest class of plant hormones with more than 70 types of compounds, both active and inactive. They also have the distinction of being the first plant hormone to be identified, studied, and understood by early botanists.
Like auxins, they are involved in the elongation of stems. If a plant naturally lacks GAs, it will be dwarfed in structure and stature. Some commercial plant growers intentionally stop them from being produced or received if a dwarf plant is desired. Several types of plant growth regulators (PGRs) are GA blockers.
Gibberellins are also critical for fruit setting in crops. They may be used orchards and vineyards to aid in the set of grapes and other fruits. They also aid in the breaking of seed dormancy, a process that also happens naturally. Commercial growers treat seeds that are traditionally difficult to germinate due to a thick seed coat or other biological factors with GAs to assist in the breaking of their dormancy.
Cytokinins (CKs) are responsible for aiding in cellular divisions and maintaining plant metabolic activity. They are found within plants wherever there is a site that is actively growing, such as at leaf tips. Commercially, these hormones are used when propagating with tissue culture. As they are involved in the action of setting fruit, they are also utilized as a fruit growth regulator. Unlike both ethylene and abscisic acid, CKs prevent senescence in leaves.
Ethylene may be the most well-known of the plant hormones; it is the only gaseous (hydro carbon) plant hormone that is produced. It is responsible for the ripening of fruits, both naturally and artificially. In cases where crops, particularly fruits like bananas, are picked or shipped before maturation, they are can be artificially ripened via natural or synthetic ethylene compounds.
A synthetic form of ethylene known as Ethephon is used widely in commercial nurseries and plantations. This hormone is used on much of the world’s pineapple, rice, coffee, cotton, and other staple crops to achieve quicker and uniform ripening. It is also the most common component of most commercial PGRs, especially for plant seedlings. It is additionally used for leaf and fruit abscission where a controlled leaf or fruit drop is desired for commercial purposes.
Unlike CKs, abscisic acid (ABA) inhibits cellular growth. It functions to assist with plant processes such as seed dormancy and takes the controls of a leaf’s stomates during the process of wilting.
Abscisic acid is the outlier of the commercial plant hormone world. There are no synthetic compounds available, and its high cost, coupled with its lack of a commercial purpose, mean that this is the only plant hormone not used artificially at one time or another in commercial plantations, greenhouses, or nurseries.
Other Plant Hormones
There are far more plant compounds produced naturally that perform various functions within plants than just the classic five. It is highly likely that even more will be discovered as researchers continue to look at the myriad interactions of these hormones. Other identified plant hormones include jasmonates (such as methyl jasmonate), salicylates, brassinolides, and strigolactones.
Jasmonates are involved in many regulatory functions of plants but are most unique in their ability to aid in a plant’s defense against wounds by producing substances that are unpleasant or harmful to pest insects. It is also thought to send signals to other plants, resulting in increased jasmonate production in the receiving plants.
Salicylates also play a role in defense. They are a first-aid mechanism for plant infections. Their release can help a plant to reserve some of its nutrition and energy stores during its recovery. First discovered on rapeseed, a brassica, brassinolides also aid in helping a plant through stress, but they are also thought to play roles in conjunction with the five major hormones in leaf and fruit development.
Like jasmonates, strigolactones are communicating hormones, but they serve the root system of plants. They are critical in the relationship between the root system and mycorrhizal fungi development. In parasitic plants, strigolactones aid in the germination of seeds to establish dominance over the host plant.
On a final note, florigen, not mentioned above, was thought to be a mystery plant hormone responsible for seasonal flowering in some species. While still not completely debunked, it is now thought to be the result of synergy between other plant hormones and not a unique hormone unto itself.
Read More: 10 Facts on Brassinosteroids