Brassinosteroids Confer Stress Tolerance

Abstract

IntroductionBrassinosteroids (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 [1], and stress responses [2,3]. 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 ) [4,5]. 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 [1], were instrumental in the identification of BR signaling components and in understanding to some extent how BR regulates gene expression [6,7]. Numerous reviews detailing BR effects on plant growth and development and BR signaling mechanisms have surfaced in the recent literature [6-10]; 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 [3] and highlights the progress made towards understanding the molecular basis of BRmediated plant stress tolerance.Plant Stress Biology. Edited by H. Hirt BR is perceived at the cell surface by BRI1 (BR-INSENSITIVE1), a plasma membrane-localized leucine-rich repeat (LRR) receptor-like kinase (RLK) [11,12]. BR binding to BRI1 induces a series of biochemical events, such as autophosphorylation of BRI1 in its C-terminal domain, dissociation of an inhibitory protein, BKI1 (BRI1 KINASE INHIBITOR1), and association of BRI1 with another LRR RLK, BAK1 (BRI1-ASSOCIATED RECEPTOR KINASE1) [13,14]. The recent demonstration that BR-dependent activation of BRI1 precedes association with BAK1 has led to a model in which BAK1 enhances signaling output through reciprocal BRI1 transphosphorylation [15]. However, whether the main function of BAK1 is to enhance signaling output of BRI1, promote endocytosis of BRI1, or to help link BRI1 with a downstream signaling protein remains to be determined [7]. Other known components of the BR signaling pathway include GSK3 (GLYCO-GEN SYNTHASE KINASE3), BIN2 (BR-INSENSITIVE2), which negatively regulates the transcription factors BZR1 (BRASSINAZOLE-RESISTANT1) and BES1 (BRI1-EMS SUPPRESSOR1) by phosphorylating them [16,17], while the phosphatase BSU1 (BRI1 SUPPRE...

Brassinosteroid: a biotechnological target for enhancing crop yield and stress tolerance

“…In addition to their role in plant development, BRs protect plants from a variety of environmental stresses, including high and low temperatures, drought, salinity and pathogen attack [2,55]. Most of the studies demonstrating anti-stress effects of BRs have employed treatment of plants with exogenous BRs.…”

“…Although exogenous BR treatment is yet neither a conventional greenhouse nor field agricultural method, it is highly instrumental in the laboratory for studying molecular changes induced by BR that might be contributing to increased stress tolerance in treated versus untreated plants. The BR-induced molecular changes that are related to stress tolerance include higher level of expression of stress-responsive genes [56,57], maintenance of protein synthesis [58], induction of other hormone responses [5], increase in the activities of antioxidant enzymes, greater accumulation of osmoprotectants [55,59] and higher photosynthetic efficiency [60,2]. Results of a global gene expression study of untreated and BR-treated Arabidopsis under stress and no stress conditions also indicate that BR affects a myriad of cellular processes either directly or through crosstalk with other plant hormone pathways, resulting in better performance of plants under stress conditions (Krishna and coworkers, unpublished data).…”

Overexpression of the brassinosteroid biosynthetic gene DWF4 in Brassica napus simultaneously increases seed yield and stress tolerance

“…Brassinosteroids (BRs) are plant steroid hormones, which play essential roles in plant growth and development12, and also confer tolerance to a wide range of abiotic stresses34. The BR signalling pathway is one of the best-understood signal transduction pathways in the plant hormone field.…”

“…The role of BR in conferring tolerance to a wide range of abiotic stresses, such as high and low temperatures, salinity and drought have been described in numerous studies34. Most of the studies focused on BR effects on plant stress responses have been conducted using exogenous BR.…”

Engineering Phytohormones for Abiotic Stress Tolerance in Crop Plants

“…These are ubiquitous throughout the plant kingdom and regulate a range of physiological responses such as cell elongation, photomorphogenesis, xylem differentiation, seed germination (Sasse 2002 ), and stress responses (Krishna 2003 ). Owing to the multifunctional roles of BRs , the genetic manipulation of BR metabolism offers a unique possibility of signifi cantly increasing crop yields and protecting plants from environmental stresses (Divi and Krishna 2009 ). Various BR-induced molecular changes that are related with stress tolerance have been identifi ed such as overexpression of stress-responsive genes (Dhaubhadel et al 1999 ;Kagale et al 2007 ), maintenance of protein synthesis (Dhaubhadel et al 2002 ), induction of antioxidant enzymes and osmoprotectant accumulation (Ozdemir et al 2004 ;Divi and Krishna 2009 ), and higher photosynthetic effi ciency (Krishna 2003 ;Ogweno et al 2007 ).…”