Did you know there is a sixth, lesser-known class of plant hormones? Brassinosteroids (BRs) promote or enhance apical dominance, seed germination, gravitropism, and ethylene production. BRs have also been found throughout most types of plant tissues, ranging from pollen to root material.

Growers have been getting by just fine without knowing about the presence of brassinosteroids, but as much research is being done on the hormone, it might soon turn up in synthetic forms in various gardening products. Here’s why you’ll want to know what brassinosteroids are if you ever see it listed as an ingredient in plant additives.

Brassinosteroids are in a Class of Their Own

Less well-known than the main five classes of plant hormones (auxins, abscisic acid, cytokinins, ethylene, and gibberellins) is the class known as brassinosteroids.

Brassinosteroids are More Common Than We Realize

Though not quite as structurally diverse as the auxins, we know of more than 50 compounds possessing BR activity.

Brassinosteroids were just Recently Discovered

The first BR, discovered in the late 70s, was brassinolide. Brassinolide was isolated from the pollen of the commercial oil-producing plant Brassica napus (rapeseed).

Brassinosteroids Stimulate Cell Division and Stem Elongation

It was observed that rapeseed pollen stimulated cell division and stem elongation, which led to a hunt for the substance responsible for the activity.

Why are Brassinosteroids Called ‘Steroids’?

The mechanisms of action of BRs are typical of most steroid hormones. Brassinosteroids bind to receptors in cell membranes to trigger a cascade of regulatory effects inside cells affecting thousands of different genes. That’s right, thousands.

What Makes a Steroid a Steroid?

Wait a minute, BRs are steroids? Yep. Any steroid has 17 carbon atoms arranged into four connected rings (three six-carbon rings and one five-carbon ring). It is the side groups connected to that central structure that determine the function and specificity of a particular steroid.

Why Don’t More Growers Know About Brassinosteroids?

Brassinosteroids are active in low concentrations, much lower than most hormones, and work in coordination with both auxins and gibberellins.

Brassinosteroids Can Promote Seed Growth

Brassinosteroids promote or enhance apical dominance, seed germination, gravitropism, and ethylene production.

But Brassinosteroids Can Also Harm Plant Growth

On the other hand, BRs inhibit root growth, formation of stomata, and the development of xylem, the water-conducting tissue in plants.

Brassinosteroids Can Negatively Affect a Plant’s Performance

What if a plant is deficient in its ability to synthesize BRs? Brassinosteroid mutants exhibit phenotypic abnormalities such as dwarfism, low fertility, and delayed or stunted and aberrant growth and development.