EFB530 Plant Physiology

Brassinosteroids (BRs)

Discovery

In the 60's, studies of Brassica napus (rape seed) pollen lead to the discovery of a compound that could induce elongation of bean hypocotyls (independent of GA responses)

In 1979 it was identified as brassinolide - a steroid compound (steroids are triterpenoids)

• Another brassinosteroid was discovered in 1982 - castasterone - isolated from insect galls on chestnut
• >60 types of BRs have been found in algae, ferns, gymnosperms, and angiosperms, but not bacteria
• they are defined by their structure, rather than biological activity
• brassinolide is the most active and common BR compound

There are several bioassays for BRs

• rice leaf lamina inclination assay: BR causes swelling of cells on one side of the joint between the leaf blade and the sheath - causing quantitative amount of bending

Biological roles

BR function has been illuminated by the study of mutants, both BR-deficient and receptor mutants

• BR mutants are extreme dwarfs and have very small, crinkled leaves: indicating that normally BR stimulates stem elongation and leaf expansion by stimulating cell division and expansion
• BR mutant seedlings grown in the dark often display many of the features of light-grown seedlings (they appear de-etiolated): BR appears to promote etiolated growth form - rapid elongation in the dark

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