Brassinosteroids are a group of some 40 different steroids that are synthesized by plants and are potent hormones affecting many aspects of plant growth. The most abundant one is named brassinolide. It was first isolated from the pollen of Brassica napus, hence the name.
These hormones act synergistically, or at least additively, with several other hormones such as auxin and the gibberellins. However, they are far more potent than the other hormones — acting at far lower concentrations than the others.
One of their most dramatic effects is on cell elongation. Plants that cannot make — or respond to — brassinolide, fail to undergo etiolation when grown in the dark [Link].
These effects have led to active research and development for their use in agriculture.Brassinosteroids achieve their many effects by binding to cell-surface receptors which