Brassinosteroids are plant-growth-promoting natural products similar in structure to animal and insect steroid hormones. Considered a new class of plant hormone, along with auxins, gibberellins, cytokinins, abscisic acid, and ethylene, brassinosteroids are present throughout the plant kingdom. They show distinct physiological effects on plant growth including improvement of stress tolerance in crop production. These discoveries, together with advances in molecular and biosynthetic studies of brassinosteroids, open new aspects of research in understanding the growth and development of plants. This book presents a comprehensive view of the related chemistry, biochemistry, physiology, agricultural applications, and most recent research in molecular biology. Written by scientists who are active in these fields, Brassinosteroids is a vital source of information for plant and agricultural science researchers with an interest in plant hormones.
In the absence of GA, many DELLAs are present to repress GA responses, but the formation of the GA-GID1-DELLA complex increases the degradation of DELLAs.  The SCF complex is composed of SKP1, CULLIN, and F-BOX proteins.  F-box proteins catalyze the formation of polyubiquitin on target proteins to be degraded by the 26S proteasome.  The formation of the GA-GID1-DELLA complex is believed to cause a conformational change in DELLA, which enhances recognition of DELLAs by F-box proteins.  Next, the SCF complex promotes ubiquitylation of DELLAs, which are then degraded by the 26S proteasome.  Degradation of DELLAs allows GA regulated growth to resume. Thus, GA stimulates growth by activating the degradation of DELLAs.