Hormone Synthetic Pathways and Genes in Arabidopsis: Brassinosteroids

Brassinosteroids are polyhydroxylated steroids. The most active compound is Brassinolide (BL). Since the discovery of BL, more than 70 natural analogs, collectively called brassinosteroids (BRs), have been isolated and characterized. Exogenous application of BRs to plants at nanomolar to micromolar concentrations has a wide spectrum of physiological effects, including promotion of cell elongation and division, enhancement of tracheary element differentiation, retardation of abscission, enhancement of gravitropic-induced bending, promotion of ethylene biosynthesis, and enhancement of stress resistance. A number of BR-deficient mutants have been discovered in Arabidopsis, pea, and tomato. These mutants exhibit dwarfism when grown in either light or dark conditions. Many of these mutants also have dark-green leaves, reduced fertility, a prolonged lifespan, and abnormal skotomorphogenesis. They are synthesized from campesterol via the mevalonate pathway. The biosynthetic pathway was elucidated in cultured cells of Catharanthus roseus by Japanese researchers, and then confirmed in Arabidopsis mutants. The major biosynthesis pathway in Arabidopsis is shown here.