Abstract
Brassinosteroids (BRs) are a class of steroid hormones regulating a wide range of physiological processes during the plant life cycle from seed development to the modulation of flowering and senescence. The last decades, and recent years in particular, have witnessed a significant advance in the elucidation of the molecular mechanisms of BR signaling from perception by the transmembrane receptor complex to the regulation of transcription factors influencing expression of the target genes. Application of the new approaches shed light on the molecular functions of the key players regulating the BR signaling cascade and allowed identification of new factors. Recent studies clearly indicated that some of the components of BR signaling pathway act as multifunctional proteins involved in other signaling networks regulating diverse physiological processes, such as photomorphogenesis, cell death control, stomatal development, flowering, plant immunity to pathogens and metabolic responses to stress conditions, including salinity. Regulation of some of these processes is mediated through a crosstalk between BR signalosome and the signaling cascades of other hormones, including auxin, abscisic acid, ethylene and salicylic acid. Unravelling the complicated mechanisms of BR signaling and its interconnections with other molecular networks may be of great importance for future practical applications in agriculture.
Highlights
Brassinosteroids (BRs) are plant-specific polyhydroxylated steroid hormones displaying high activity in stimulation of plant growth and development and regulation of a broad spectrum of physiological responses to biotic and abiotic stress conditions
The transmembrane BRI1-BAK1 heterodimer functioning as BR co-receptor is asymmetric—BAK1/SERK3 is shorter than BRI1 because contains only five LRRs and does not include the pair of cysteines and 70-amino acid island, BRI1 protein is the only component of the receptor complex that binds BR ligands [31,32,58]
BRI1-BAK1/SERK3 heterodimerization leads to endocytosis of cell membrane fragments containing these polypeptides, what may constitute the mechanism facilitating the interaction of the receptor complex with cytoplasmic enzymes functioning as components of the signal transduction pathway [22,27,33,58]
Summary
Brassinosteroids (BRs) are plant-specific polyhydroxylated steroid hormones displaying high activity in stimulation of plant growth and development and regulation of a broad spectrum of physiological responses to biotic and abiotic stress conditions. BRs regulate a broad range of physiological processes, such as: seed development and germination, cell division and elongation, what results in highly stimulating impact on plant growth, differentiation of tracheary elements, polarization of cell membrane, proton pumping to apoplast and into a vacuole by stimulation of transmembrane ATPases, as well as increasing the efficiency of photosynthesis by elevating the level of CO2 assimilation and Rubisco activity. An extensive amount of data gathered in genetic, biochemical and physiological assays enabled characterization of BR signal transduction pathway initiated by hormone perception by plasma membrane-associated receptor complex, transduced via phosphorylation/dephosphorylation cascade to the regulation of gene expression by a group of transcription factors. Introduction of new key players of this relay and presents molecular mechanisms of interconnections of BR signaling pathway with other molecular networks
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