Brassinosteroids confer stress tolerance

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 BR-mediated plant stress tolerance.