What are plant hormones?

Plant hormones are natural chemicals present in tiny amounts inside plants. They serve as an internal chemical signaling system that dictates what the plant should do at a particular time. 

Understanding how plant hormones work is key to understanding plants. Without plant hormones, plants would not grow, produce differentiated organs, metabolize, or reproduce. 

As well as regulating growth, plant hormones tell the plant how to adapt to changing environmental conditions. They induce cellular changes that allow the plant to weather changes in its surroundings despite being rooted in one place. For example, hormonal interactions controls leaf-drop and browning as the weather gets colder. It is not known exactly how environmental cues are perceived by the plant but it is clear that hormones are involved in responding to them. 

Nine classes of plant hormones have been discovered so far: these are auxins, cytokinins, ethylenes, abscisic acid, brassinosteroids, strigolactones, salicylic acid, jasmonates, and gibberellins. Click the links for our previous posts on plant hormone classes.

Each of the nine classes of hormones contains many different hormones. For example, there are more than 100 gibberellins. Hormones belonging to a particular class share a similar molecular structure. 

Different hormone classes are involved in different aspects of plant development Anjanim734 https://commons.wikimedia.org/wiki/File:ಪರಿಸರ_ವಿಜ್ಞಾನ,_ವಿಕಸನ_ಮತ್ತು_ಸಿಸ್ಟಮ್ಯಾಟಿಕ್ಸ್.jpg

Auxins are the best-understood class of plant hormones. They are the hormone used in rooting powders which encourage stem and leaf cuttings to produce roots.

In some respects, plant hormones are somewhat like animal hormones. Like animal hormones, plant hormones are sometimes produced in one area then transported to other parts of the plant. Yet unlike in animals, plants do not have discrete organs responsible for producing these chemicals. Many plant hormones are produced in various areas located throughout the plant and then transported to areas where they are needed. 

How were plant hormones discovered? 

Plant hormones have been discovered and experimented upon over the last two hundred years. The study of plant hormones began when nineteenth-century biologists speculated that plants contained different material substances responsible for forming the different organs of the plant such as roots, flowers, and leaves. 

The first plant hormone to be isolated was a type of auxin. This was done by Fritz Went, a Dutch scientist, in 1928. He also showed that this substance stimulated re-growth in decapitated shoots as well as stem bending.

In the twentieth century, bioassay techniques were used to detect and measure plant hormones. Bioassays are experiments where plants are exposed to hormone substances and their effects on the plants are observed. In the last two decades, however, scientists have adopted modern chemical techniques to analyze plant hormones. These include high-pressure liquid chromatography, gas chromatography, and mass spectrometry. They give more accurate, quantitative readings on the molecular makeup of the chemicals present in plants. 

How do plant hormones work and what do they do?

From stem bending to flower production, everything that a plant does depends on the correct hormone combinations being present in certain parts of the plant in the right ratios. 

The different hormone classes are generally responsible for distinct functions. For example, auxins promote plant growth through cell division while gibberellins promote plant growth through elongatingexisting cells. However, the hormone classes also overlap in their functions. For example, ethylene and auxin both promote root growth. Different combinations of hormones may produce the same effects. The hormone combinations required for each particular plant response will vary between species. Further, plant hormones can either stimulate or inhibit plant functions depending on the character of their interactions. 

Many hormones from different hormone classes have overlapping functions. Some, however, have very specific roles. Abscisic acid is uniquely responsible for opening and closing leaf stomata. This image shows spider plant stomata at 10x magnification, highlighting the small-scale at which hormones operate. Yersinia pestis https://flickr.com/photos/yersinia/4457678502

Plant hormone molecules are so small that they can diffuse through cell walls. They are easily transported to other parts of the plant when and where they are needed. Plants can inactivate particular hormones when they are not required.

Uses in agriculture 

Plant hormone research marked the beginnings of biotechnology. In the 1950s and 1960s, auxin and cytokinin were applied topically to undifferentiated plant cells. This made the cells grow into whole plants. In the 1960s and 1970s, scientists used gibberellin to produce dwarf wheat and rice varieties with larger yields. 

This cucumber plantlet was grown from undifferentiated cucumber plant cells. The undifferentiated cells were treated with 6-Benzylaminopurine (BAP), a hormone belonging to the cytokinin class. 6-Benzylaminopurine (BAP) is a synthetic plant hormone but it is classed as a cytokinin because it has the same molecular structure as the natural cytokinins produced by plants. Yuliya Krasylenko https://commons.wikimedia.org/wiki/File:Aseptic_in_vitro_cultivated_plantlets_of_cucumber_regenerated_from_shoot_apical_meristem_explants_on_half-strength_Murashige-Scoog_medium_supplemented_with_cytokinin_6-Benzylaminopurine_in_concentration_1_mg_l.tif

Apart from topical applications of plant hormones, scientists found that you can alter the ways genes respond to hormones within the plant itself. In the late 1990s, scientists found that a gene mutation in Arabidopsis made it more sensitive to increases in abscisic acid. Plants with the mutation were more drought tolerant. Researchers have since modified the same gene in other species. Again, the genetically modified plants produced better yields under drought compared to normal ones. 

Plant hormones are not widely used in agriculture yet. Research and development in this area is difficult because the hormones are so minute and present in very low concentrations inside plants. More than this, hormone interactions and their effects are extremely complex. In large-scale field trials, environmental variables may interfere with methods that worked well in a closed laboratory setting. However, the agricultural applications of plant hormones remains a vibrant area of  research. Many believe that plant hormones could one day offer a sustainable way of increasing agricultural yields.