Brassinosteroids (BRs) are a group of plant steroidal hormones that are structurally related to animal and insect steroid hormones. BRs regulate a wide range of physiological responses in plants, including cell elongation, photomorphogenesis, xylem differentiation, seed germination, and stress responses. Although the growth-promoting properties of BRs were recognized in the early 1970s, the first genetic evidence to suggest that BRs are essential for proper plant development came with the isolation of the BR-deficient mutants det2 (de-etiolated2) and cpd (constitutive photomorphogenic dwarf). Isolation and sequence analysis of DET2 and CPD genes revealed that the encoded proteins share sequence similarities with steroid 5a-reductases and steroid hydroxylases, respectively, indicating a role for these proteins in steroid metabolism. Indeed, feeding det2 and cpd mutant seedlings with BRs rescued their mutant phenotypes to wild-type in a dose-dependent manner, clearly establishing the roles of DET2 and CPD in BR biosynthesis. Numerous other Arabidopsis BR-deficient and BR-insensitive mutants, displaying phenotypic alterations such as dwarfism, small dark-green leaves, a compact rosette structure, delayed flowering and senescence, and reduced fertility, were instrumental in the identification of BR signaling components and in understanding to some extent how BR regulates gene expression. Numerous reviews detailing BR effects on plant growth and development and BR signaling mechanisms have surfaced in the recent literature; these aspects have therefore been discussed only briefly here. The present chapter is focused on the relatively less explored topic of BR-mediated stress responses in plants and highlights the progress made towards understanding the molecular basis of BRmediated plant stress tolerance.

Brassinosteroids (BRs) are a group of naturally occurring plant steroidal compounds with wide ranging biological activity. Because BRs control several important agronomic traits such as flowering time, plant architecture, seed yield and stress tolerance, the genetic manipulation of BR biosynthesis, conversion or perception offers a unique possibility of significantly increasing crop yields through both changing plant metabolism and protecting plants from environmental stresses. Genetic manipulation of BR activity has indeed led to increases in crop yield by 20–60%, confirming the value of further research on BRs to improve productivity.

Brassinosteroid-mediated gene expression changes have been reported primarily in 'Arabidopsis thaliana' but not in its close relative Brassica napus. To obtain an initial idea of the molecular changes induced by long-term exposure to 24-epibrassinolide (EBR) in 'B. napus' seedlings, we used the differential display-reverse transcription PCR technique. Six differentially expressed cDNAs were isolated and characterized. These encode a mitochondrial transcription termination factor (mTERF)-related protein, glycine-rich protein 22 (GRP22), myrosinase, 3-ketoacyl-CoA thiolase, and a copia-like polyprotein. The first four were upregulated in EBR-treated seedlings while the latter was expressed at higher levels in untreated seedlings. Transcripts of mTERF-related protein, GRP22, and myrosinase were present at higher levels in treated seedlings under nonstress conditions, whereas those of 3-ketoacyl-CoA thiolase rose to higher levels in treated seedlings during exposure to heat stress. The results of the present study indicate that EBR treatment in 'B. napus' leads to substantial changes in the expression levels of genes involved in a variety of physiologic responses. The results provide a useful framework for further research into EBR-mediated molecular changes in 'B. napus', which will also add to our understanding of how brassinosteroids mediate stress tolerance in this agriculturally important oil crop.

One of the greatest challenges facing patent regimes in the last decade has been the boundary between human invention and nature, epitomised by advances in biotechnology. One example of this has been the ongoing litigation over a "transgenic non-human mammal" known as the "oncomouse", a genetically engineered mouse. This is an area that brings together consideration of ethics, law and science.