Selected Physiological Responses of Brassinosteroids: A Historical Approach

Abstract

Brassinosteroids are endogenous growth promoting hormones that have a structure similar to steroids in mammals. For over 70 years mammalian steroids have been known to function as hormones and, given that steroidal hormones have also been found in many multicelluar organisms, it was logical to assume a hormonal role for steroids in plants. However, even though the existence of growth stimulating chemicals had been observed in the reproductive tissues of plants in 1849, it was not until 1979 that the first plant steroidal hormone was isolated, characterised and named brassinolide. Brassinosteroids have been found in diverse species of plants and the physiological role they play in growth and development has been considerably researched. Here we review these initial physiological experiments using BRs in the context of their agronomic potential. Initial experiments showed that seeds treated with brassinosteroids increased seed yield and plant size, particularly in the case of smaller and slower growing plants. However, subsequent larger field-test trials in the United States of America resulted in disappointing results with little economic significance. This, in addition to the identification of brassinosteroid mutant plants rescued by the exogenous application of brassinolide, indicated an essential role for brassinosteroids in normal plant development. These physiological studies have shown that although brassinosteroids function in growth and development, their complex interaction with other plant hormones and environmental signals indicate that more detailed studies are needed in order to elucidate more fully the potential agronomic benefit of BRs in crop production.