Brassinosteroids

Abstract

The discovery of brassinosteroids can be traced back to 1941 from Mitchell and Whitehead of the United States Department of Agriculture (USDA, Beltsville, MD) who reported that pollen extracts often have growth-promoting properties on other plant tissues. In 1970, Mitchell et al. reported that the crude extract of pollen from Brassica napus (rape) contained “brassins” which promoted rapid elongation of internodes in Phaseolus vulgaris. This elongation response was distinct from GA-mediated stem elongation. Subsequently, their work leads to the isolation and identification of brassinolide (BL) as the first steroidal plant growth regulator. Another steroid hormone called castasterone (SC) was isolated from insect galls of chestnut (Castanea sp.). Since then, a number of related steroidal compounds have been isolated which are collectively called as brassinosteroids (BRs). Structurally, BRs are C-27, C-28, and C-29 steroids with different functional groups on A and B rings and on the side chain. BRs occur in algae, ferns, gymnosperms, and angiosperms. They have not been detected in microorganisms. Brassinolide is a C-28 brassinosteroid and exhibits highest activity among all BRs so far known.

Multiple-Choice Questions

• 1. Brassinosteroids were first reported in:

  • (a) Arabidopsis thaliana

  • (b) Brassica napus

  • (c) Brassica juncea

  • (d) Phaseolus vulgaris

• 2. Which of the following is true for brassinosteroids?

  • (a) Stimulate photomorphogenesis in dark

  • (b) Stimulate skotomorphogenesis in light

  • (c) Prevent photomorphogenesis in dark

  • (d) Both a and b

• 3. Brassinosteroid mutants—bri1 and det2—have similar phenotypes, but exogenous application of brassinolide:

  • (a) Can normalize their growth

  • (b) Can normalize the growth of only det2

  • (c) Cannot normalize the growth

  • (d) Can normalize the growth of only bri1

• 4. CPD enzyme is involved in:

  • (a) Brassinosteroid degradation

  • (b) Brassinosteroid phosphorylation

  • (c) Brassinosteroid biosynthesis

  • (d) None of the above

• 5. bin2 (brassinosteroid insensitive 2) mutants exhibit:

  • (a) Overexpression of BR signaling

  • (b) Impaired expression of phosphatase

  • (c) Impaired expression of ser/thr kinase

  • (d) Both b and c

Answers

1. b

2. c

3. b

4. c

5. c