News about the recognition mechanism and brassinosteroid signal transduction

Abstract

Small signalling molecules that mediate cellcell communication
are essential for developmental regulation in multicellular organisms. Among
them are the steroids and peptide hormones that regulate growth in both plants
and animals. Brassinosteroids (BRs) are polyhydroxilated sterol derivatives which
occur at low levels throughout the plant kingdom and regulate the expansion,
division and differentiation of cells in plants. The phenotypes of brassinosteroiddeficient
and insensitive mutants confirm that brassinosteroids are essential for
cell elongation and also suggest a role in vascular differentiation, senescence,
fertility and leaf morphology. Even there are many questions which remain unclear
related to the BR signaling cascade, in the last years the researchers have
done huge advances concerning this field. The new findings concerning the signal
cascade of BRs show that plants have preserved the steroid signal but not the
classical animal-type intracellular steroid receptor-signaling pathway. Recent
genetic and biochemical studies have identified several critical BR signalling
components including both the up and downstream components in the cascade.
A working model for BR signal transduction proposes that BR initiates its signalling
pathway by promoting heterodimerization of two transmembrane receptor-
like kinases at the cell surface, leading to inhibition of another kinase (GSK3-
like) and subsequent stabilization and nuclear accumulation of two independent
substrates that regulate BR-responsive
genes. Recent studies demonstrated exciting
results showing that tomato BRI1
receptor for BRs (tBRI1) perceives both
BR and the peptide hormone systemin,
raising new questions about the molecular
mechanism of brassinosteroid signalling
network.