Brassinosteroids

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, to 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].

Brassinosteroids also:

• promote apical dominance
• promote leaf senescence
• enhance seed germination
• enhance gravitropism
• increase the production of ethylene
• inhibit root growth
• promote the formation of xylem
• prevent premature abscission of fruit
• increase resistance to freezing
• increase the yield of wheat and rice

These effects have led to active research and development for their use in agriculture.

• binding to cell-surface receptors triggering metabolic changes in the cytosol (a rapid response);
• regulating the expression of genes — turning some up; others down (a slower response).