Brassinosteroids (BRs) are essential for proper plant growth and development and also protect plants from a variety of environmental stresses. Seeds contain relatively high levels of BRs, and BRs have been implicated in embryonic patterning and germination. How BR levels in seeds impact germination, growth, and stress tolerance in early seedlings is currently not known. To assess this, the BR biosynthetic gene 'AtDWF4' was overexpressed in 'Arabidopsis' under the control of a seed-specific oleosin promoter. The resulting transgenic seedlings could overcome inhibition of germination caused by exogenous abscisic acid (ABA) and the seedlings were more tolerant to cold stress compared to wild-type and vector control seedlings. Transcript levels of 'COR15A', a cold-responsive gene with an established function in cold tolerance, were approximately twofold higher in transgenic seedlings than in control seedlings under cold conditions. These results establish a role for BRs in opposing the inhibitory effects of ABA in seed germination and in promoting cold stress tolerance in early 'Arabidopsis' seedlings.

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.

Background: Brassinosteroids (BRs) play crucial roles in plant development and also promote tolerance to a range of abiotic stresses. Although much has been learned about their roles in plant development, the mechanisms by which BRs control plant stress responses and regulate stress-responsive gene expression are not fully known. Since BR interacts with other plant hormones, it is likely that the stress tolerance conferring ability of BR lies in part in its interactions with other stress hormones. Results: Using a collection of Arabidopsis mutants that are either deficient in or insensitive to abscisic acid (ABA), ethylene (ET), jasmonic acid (JA) and salicylic acid (SA), we studied the effects of 24-epibrassinloide (EBR) on basic thermotolerance and salt tolerance of these mutants. The positive impact of EBR on thermotolerance in proportion to wild type was evident in all mutants studied, with the exception of the SA-insensitive 'npr1-1' mutant. EBR could rescue the ET-insensitive 'ein2' mutant from its hypersensitivity to salt stress-induced inhibition of seed germination, but remained ineffective in increasing the survival of 'eto1-1' (ET-overproducer) and 'npr1-1' seedlings on salt. The positive effect of EBR was significantly greater in the ABA-deficient 'aba1-1' mutant as compared to wild type, indicating that ABA masks BR effects in plant stress responses. Treatment with EBR increased expression of various hormone marker genes in both wild type and mutant seedlings, although to different levels. Conclusions: These results together indicate that the redox-sensitive protein NPR1 (NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1), a master regulator of SA-mediated defense genes, is likely a critical component of EBR-mediated increase in thermotolerance and salt tolerance, but it is not required for EBR-mediated induction of 'PR-1' ('PATHOGENESIS-RELATED1') gene expression; that BR exerts anti-stress effects independently as well as through interactions with other hormones; that ABA inhibits BR effects during stress; and that BR shares transcriptional targets with other hormones.

The plant hormone brassinosteroid (BR) plays essential roles in plant growth and development, while also controlling plant stress responses. This dual ability of BR is intriguing from a echanistic point of view and as a viable solution for stabilizing crop yields under the changing limatic conditions. Here we report a time course analysis of BR responses under both stress and no-stress conditions, the results of which establish that BR incorporates many stress-related features even under no-stress conditions, which are then accompanied by a dynamic stress response under unfavourable conditions. Found within the BR transcriptome were distinct molecular signatures of two stress hormones, abscisic acid and jasmonic acid, which were correlated with enhanced endogenous levels of the two hormones in BR-treated seedlings. The marked presence of genes related to protein metabolism and modification, defence responses and calcium signalling highlights the significance of their associated mechanisms and roles in BR processes. Functional analysis of loss-of-function mutants of a subset of genes selected from the BR transcriptome identified abiotic stress-related roles for 'ACID PHOSPHATASE5 (ACP5)', 'WRKY33', JACALIN-RELATED LECTIN1-3' ('JAC-LEC1-3') and a 'BR-RESPONSIVE-RECEPTOR-LIKE KINASE' ('BRRLK'). Overall, the results of this study provide a clear link between the molecular changes impacted by BR and its ability to confer broad-range stress tolerance, emphasize the importance of post-translational modification and protein turnover as BR regulatory mechanisms and demonstrate the BR transcriptome as a repertoire of new stress-related regulatory and structural genes.