Brassinosteroid plant hormones: An introduction

What are Brassinosteroids?

Brassinosteroids are a class of plant hormones found in higher plants as well as more primitive kinds such as algae and bryophytes. There are about 60 known types today with similar molecular structures. The highest concentrations of brassinosteroids in plants are found in the pollen. They are also found in anthers, seeds, leaves, stems, roots, flowers, and grains with younger tissue tending to contain higher levels than mature tissue.

Like other classes of plant hormones - gibberellins, cytokinins, and auxins - brassinosteroids are essential in plant growth and development. However, Brassinosteroids work at far lower concentration than other classes of plant hormones and have slightly different functions. 

What do they do?

Brassinosteroids facilitate stem elongation, leaf bending, synthesis of nucleic acid and proteins, the regulation of carbohydrate assimilation, and activation of photosynthesis. They activate proton pumps, the biochemical mechanism that produces the energy required to transport materials through the plant.

Because plant hormones like Brassinosteroids control what plants do, they allow them to adapt to changing environments. They protect plants from stresses such as excess salt uptake, high temperatures and heavy metals. They also have a role in strengthening the immune response of plants to pathogens. For example, a semi-synthetic brassinosteroid DI-31 has been shown to induce a defense response in strawberry plants against C. acutatum, a fungal disease.

Plants that are deficient in brassinosteroids display dwarfism, male sterility, delayed flowering, reduced apical dominance.

Brassinosteroids are sensitive to light. For example, they are involved in making seedlings grown in the dark etiolate, meaning that the stems grow long and pale. Under light, brassinosteroids promotes growth through the elongation of cells. Interestingly, some brassinosteroid deficient plants grown in the dark will display features normally seen only in light-grown plants.

The history of Brassinosteroid research

In the 1940s at the US Department of Agriculture, a team lead by J. W Mitchell searched for new compounds that would promote growth in plants. They began studying a mysterious growth promoting substance in pollen. Isolated and applied topically to plants, the substance made plant stems grow longer by elongating existing cells rather than causing new cells to divide. They found that the molecule responsible was structurally similar to animal steroid hormones.

Early research was also conducted in Japan. In 1968, Marumo and colleagues isolated substances from an evergreen tree that seemed to influence the direction of laminae and sheath growth. They concluded that this must be a newly discovered plant hormone.

It was only in 1979 in the USA that scientists first isolated and extracted a plant steroid. It came from a B. napus pollen extract. They called it Brassinolide (BL). It elongated the internodes of bean seedlings. In the eighties, synthetic Brassinolide was produced for field experiments on economically important crops.

How do they work?

Brassinosteroids are sensed by a type of protein present in plant cell surfaces called BRI1. After this, the brassinosteroids interact with BAK1, a receptor. The interaction between BAK1 and brassinosteroid creates signals that are transmitted to the nucleus of the cell. This tells the nucleus to express or inhibit genes and their associated physiological processes.

How are they used?

Despite successful field trials over the last forty years, the high cost of synthetic brassinosteroids has discouraged its use in agriculture on a large scale. However, many scientists are touting them as the environmentally friendly alternative to toxic agrochemicals.

The complexity of plant life

Learning about plant hormones sheds light on why cultivating plants can sometimes be challenging. Plant health depends on many invisible processes that occur at a molecular scale. Brassinosteroids work in tandem with one another as well as with other plant hormones to perform essential life functions. Specific ratios of hormones must be present in specific areas of the plant to trigger particular effects. The ratios and types of hormones associated with particular functions will vary between plant species. Plant health therefore depends on a delicate balance of chemicals constantly moving through the plant and reacting with one another. This should inspire a healthy dose of respect for your botanical companions and some consolation for when your gardening strategies doesn’t quite go to plan.