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 90-kDa heat shock protein (Hsp90) is an essential molecular chaperone in eukaryotic cells, with key roles in the folding and activation of proteins involved in signal transduction and control of the cell cycle. A search for Hsp90 sequences in the Arabidopsis thaliana genome revealed that this family includes 7 members. The AtHsp90-1 through AtHsp90-4 proteins constitute the cytoplasmic subfamily, whereas the AtHsp90-5, AtHsp90-6, and AtHsp90-7 proteins are predicted to be within the plastidial, mitochondrial, and endoplasmic reticulum compartments, respectively. The deduced amino acid sequences of each of the cytoplasmic proteins contains the highly conserved C-terminal pentapeptide MEEVD. All of the AtHsp90 sequences include a conserved adenosine triphosphate-binding domain, whereas only the cytoplasmic and endoplasmic reticulum-resident sequences include an adjacent charged linker domain that is common in mammalian and yeast sequences. The occurrence of multiple AtHsp90 proteins in the cytoplasm and of family members in other subcellular compartments suggests a range of specific functions and target polypeptides.

Summary: In addition to their essential role in plant development, brassinosteroids have the ability to protect plants from various environmental stresses. Currently it is not understood how brassinosteroids control plant stress responses at the molecular level. We have begun an investigation into the molecular mechanisms underlying 24‐epibrassinolide (EBR)‐mediated stress resistance. Earlier we found that treatment of Brassica napus seedlings with EBR leads to a significant increase in their basic thermotolerance, and results in higher accumulation of four major classes of heat‐shock proteins (hsps) as compared to untreated seedlings. Surprisingly, previous studies have shown that while hsp levels were significantly higher in treated seedlings during the recovery period, transcripts corresponding to these hsps were present at higher levels in untreated seedlings. To understand mechanisms controlling hsp synthesis in EBR‐treated and untreated seedlings, we studied protein synthesis in vivo as well as in vitro, and assessed the levels of components of the translational machinery in these seedlings. We report here that increased accumulation of hsps in EBR‐treated seedlings results from higher hsp synthesis, even when the mRNA levels are lower than in untreated seedlings, and that several translation initiation and elongation factors are present at significantly higher levels in EBR‐treated seedlings as compared to untreated seedlings. These results suggest that EBR treatment limits the loss of some of the components of the translational apparatus during prolonged heat stress, and increases the level of expression of some of the components of the translational machinery during recovery, which correlates with a more rapid resumption of cellular protein synthesis following heat stress and a higher survival rate.

Sea buckthorn (Hippophae rhamnoides L.) is a hardy, fruit-producing plant known historically for its medicinal and nutraceutical properties. The most recognized product of sea buckthorn is its fruit oil, composed of seed oil that is rich in essential fatty acids, linoleic (18∶2ω-6) and α-linolenic (18∶3ω-3) acids, and pulp oil that contains high levels of monounsaturated palmitoleic acid (16∶1ω-7). Sea buckthorn is fast gaining popularity as a source of functional food and nutraceuticals, but currently has few genomic resources; therefore, we explored the fatty acid composition of Canadian-grown cultivars (ssp. mongolica) and the sea buckthorn seed transcriptome using the 454 GS FLX sequencing technology. GC-MS profiling of fatty acids in seeds and pulp of berries indicated that the seed oil contained linoleic and α-linolenic acids at 33–36% and 30–36%, respectively, while the pulp oil contained palmitoleic acid at 32–42%. 454 sequencing of sea buckthorn cDNA collections from mature seeds yielded 500,392 sequence reads, which identified 89,141 putative unigenes represented by 37,482 contigs and 51,659 singletons. Functional annotation by Gene Ontology and computational prediction of metabolic pathways indicated that primary metabolism (protein>nucleic acid>carbohydrate>lipid) and fatty acid and lipid biosynthesis pathways were highly represented categories. Sea buckthorn sequences related to fatty acid biosynthesis genes in Arabidopsis were identified, and a subset of these was examined for transcript expression at four developing stages of the berry. This study provides the first comprehensive genomic resources represented by expressed sequences for sea buckthorn, and demonstrates that the seed oil of Canadian-grown sea buckthorn cultivars contains high levels of linoleic acid and α-linolenic acid in a close to 1∶1 ratio, which is beneficial for human health. These data provide the foundation for further studies on sea buckthorn oil, the enzymes involved in its biosynthesis, and the genes involved in the general hardiness of sea buckthorn against environmental conditions.