THE ROLES OF PHYTOHORMONES IN DEVELOPMENT AS STUDIED IN TRANSGENIC PLANTS

The study of transgenic plants has greatly advanced our understanding of the control of development and metabolism. The ability to isolate a nd modify genes greatly extends the range of what is technically feasi ble. In the area of hormone biology, transgenic plants have helped to elucidate the pathways of synthesis, the metabolic control points, and the biological functions of the various phytohormones. This review co vers the available genes that modulate the metabolism and perception o f the phytohormones. One of the most significant conclusions coming ou t of transgenic plant work is the complex interaction among the differ ent classes of phytohormones. For example, increasing the level of the auxin indole-3-acetic acid (IAA) in a plant has the secondary effect of inducing ethylene biosynthesis. This complication can be circumvent ed by combining transgenic plants modulating multiple hormones or thro ugh the use of available mutants. In this manner, transgenic plants ha ve been utilized to unambiguously define the roles of auxin, cytokinin , and ethylene in the control of apical dominance. The power of transg enic plants as tools in hormone biology is perhaps best illustrated by work on ethylene. In this case, the modular characterization of genes led to elucidation of the biosynthetic pathway. Availability of the b iosynthetic genes has permitted detailed analysis of the regulation of synthesis, definition of the role of ethylene in the control of sever al developmental processes, and the application of that knowledge for agricultural improvement.