| Brassinosteroids (BRs) are naturally occurring plant steroid derivatives that 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. It is also likely that the stress tolerance conferring ability of BRs is in part due to their interactions with other stress hormones. In the present study the stress tolerance effects of BR, interactions of BR with other plant hormones, and global genomic responses of BR in mediating stress tolerance were explored. Treatment with 24-epibrassinolide (EBR), a BR, enhanced dehydration tolerance of Arabidopsis thaliana (Arabidopsis), and this effect involved changes in the expression of dehydration-responsive genes. Study of EBR effects on the basic thermotolerance and salt tolerance of a collection of Arabidopsis mutants either deficient in or insensitive to abscisic acid (ABA), ethylene (ET), jasmonic acid (JA) and salicylic acid (SA), indicated that BR exerts anti-stress effects both independently as well as through interactions with other hormones. This study uncovered a critical role for NPR1 (NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1), a master regulator of SA-mediated defense responses, in BR-mediated increase in stress tolerance. Yet another finding was that ABA inhibits BR effects during stress, and that BR shares transcriptional targets with other hormones.;As a final confirmation of a role for BR in stress tolerance, the AtDWF4 gene encoding a BR biosynthesis enzyme was overexpressed in seeds of Arabidopsis. Preliminary studies of transgenic seedlings showed an increase in cold tolerance and the ability to overcome ABA-induced inhibition of germination.;In summary, the present study provides novel insights into the mechanism of BR-mediated stress tolerance by identifying genes and hormone interactions involved in this process.;KEY WORDS: Brassinosteroids, 24-epibrassinolide, heat stress, dehydration stress, salt stress, cold stress, Arabidopsis, cross-talk, microarray, AtDWF4, seed-specific overexpression.;Whole-genome transcriptome analysis of BR-treated and untreated Arabidopsis seedlings under no-stress and heat stress revealed majority of the BR response genes to be related to stress tolerance, signal transduction and metabolism. Analysis of T-DNA insertion mutants of four BR response genes indicated that WRKY17, WRKY33, ACP5 and BRRLK have stress-related functions. |
