Brassinosteroids (BRs) are growth-promoting natural products found at low levels in pollen, seeds, and young vegetative tissues throughout the plant kingdom. Detailed studies of BR biosynthesis and metabolism, coupled with the recent identification of BR-insensitive and BR-deficient mutants, has greatly expanded our view of steroids as signals controlling plant growth and development. This review examines the microchemical and molecular genetic analyses that have provided convincing evidence for an essential role of BRs in diverse developmental programs, including cell expansion, vascular differentiation, etiolation, and reproductive development. Recent advances relevant to the molecular mechanisms of BR-regulated gene expression and BR signal transduction are also discussed.

In general, this overview covers literature from 1999 until early 2003. Topics covered include aspects of the biosynthesis and transport of brassinosteroids, their effects on cell division, expansion, and differentiation, and their effects on whole plants, including source-sink relations and other endogenous interactions. Some interactions with environmental signals are discussed, as well as results that may promise applications in future. Topics that warrant further investigation of the roles of BRs include phenotypic variability, reproductive physiology, senescence, branching, and apical dominance, whereas topics in which possible roles for BRs are relatively unexplored include lignification, phototropism, photoperiodism, and endogenous rhythms.

Brassinolide (BR), a naturally‐occurring‐steroidal lactone from rape (Brassica napus L.) pollen, was compared with auxin for activity in a number of bioassay systems. Responses similar to IAA were elicited by BR in bioassays based upon bean hypocotyl hook opening, elongation of maize mesocotyl, pea epicotyl and azuki bean epicotyl sections, and fresh weight increase in Jerusalem artichoke (2,4‐D used) and pea epicotyl sections. The azuki bean and dwarf pea epicotyl bioassays were much more responsive to BR than IAA (at 10 μM). Responses approximately two‐fold greater in magnitude were elicited by IAA in the maize mesocotyl, bean hypocotyl hook and Jerusalem artichoke bioassays. Little or no response was elicited by BR (0.01 to μM) in the cress root or decapitated pea‐lateral bud bioassays. A powerful synergism between BR and IAA was observed in the azuki bean, pea epicotyl and bean hypocotyl hook bioassays. Although, as previously reported, other steroidal substances are active in some of the bioassay systems tested, none compared with BR in magnitude and diversity of elicited responses.

The minor iridoid glucosides present in the aerial parts of Barleria lupulina Lindl. ( Acanthaceae ) have been identified as ipolamiidoside (7) and the new compound 6-O-acetylshanzhiside methyl ester (3). The structure of (3) was determined by spectroscopic and chemical methods, and was confirmed by a single-crystal X-ray structure determination. Crystals of (3) are orthorhombic P212121, a 43.75(1), b 8.151(3), c 5.695(2) �, Z 4. The crystal structure was determined at 295 K from diffractometer data [I528 reflections with I > 3σ(I)] and refined to a residual of 0.043. Shanzhiside methyl ester (1) and its 6-Oacetyl (3), 8-Oacetyl (2) and 6,8-O,O-diacetyl (4) derivatives, all of which occur in Barleria lupulina, have been found to inhibit the growth of wheat embryos.

Brassinolide (BR), a naturally‐occurring steroidal lactone was compared with GA3 and kinetin for activity in a number of bioassays. BR was highly active in elongation bioassays employing the dwarf pea epicotyl and etiolated bean hypocotyl that are responsive to GA3 but not IAA (at 0.01 to 10 μM). The elongation of the cucumber hypocotyl elicited by BR and GA3 was reduced far more than that elicited by IAA, following removal of the cotyledons. BR, like GA3 also inhibited betacyanin accumulation in Amaranthus seedlings and prevented adventitious root initiation in hypocotyls of mung bean, dwarf bean and cucumber. Application of BR promoted rather than retarded senescence of Rumex leaves. While almost half as effective as kinetin in promoting expansion of cucumber cotyledons, BR was ineffective in cytokinin bioassays involving expansion of dwarf pea epicotyl hooks, dark synthesis of betacyanin in Amaranthus and retardation of Xanthium leaf disc senescence. Unlike the case with IAA, BR does not interact synergistically with GA3. BR has proven active in promoting stem elongation in plant bioassays responsive to either IAA or GA3, or to both phytohormones. This effect, coupled to its effect on adventitious root formation, qualifies BR as a unique plant growth substance.