Brassinosteroid and gibberellin are two major growth-promoting hormones that induce similar growth responses in higher plants. Wang and his team demonstrate that the effects of gibberellin on cell elongation depend on the presence of brassinosteroid and active BZR1 protein in the nucleus. This is because gibberellin removes a class of inhibitory proteins, named DELLAs, which inactivate BZR1, and thereby allow BZR1 to regulate gene expression more effectively. Without brassinosteroid and BZR1, gibberellin has little effect on cell growth.
The research indicates that brassinosteroid provides an essential factor required for cell elongation growth, whereas gibberellin provides another layer of quantitative control of the activity of this factor. Because gibberellin is known to be affected by environmental conditions, such as light and stresses, and brassinosteroid level varies greatly in different organs, the interactions among PIF4, DELLAs and BZR1 appear to form the "command system" that effectively integrates information of environmental condition, endogenous situation, and developmental program into the "decision" about growth.
Taken together, this research demonstrates that the interdependent relationships between brassinosteroid, elements of the gibberellin pathway, and phytochrome-interacting factors form a "command system" of sorts, which controls key growth processes and responses to environmental signals.
"This command system seems not only to accept various inputs, but also to send branches of output signals, too, because each component acts interdependently on shared targets, but also in
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