Auxin Biosynthesis
Plants, as animals, produce a set of “communication” chemicals called hormones. These small messengers are made in very small quantities as even a few of such molecules are capable of triggering dramatic changes in growth and physiology. In plants, one of the most studied hormones is auxin. The powerful effects of this hormone have been known for a long time. In fact, we now know that auxin was the mysterious “influence” transmitted from the tip of the growing plant in the famous experiments of Charles Darwin and his son on how plants bend towards the light. In addition to the role of auxin in phototropism (bending of the plants towards the light), this growth regulator plays a critical role in many other processes in plants, some of which are of great agricultural interest. Thus, for example, accumulation of this hormone in select parts of the plant (specific cells or group of cells) provides the critical information needed to decide when, where and how critically important organs such as flowers, fruits and seeds should be made. In the past 20 years, scientists have uncovered many of the mechanisms by which auxin triggers some of these plant responses. However, only recently we have started to uncover how plants make this essential hormone. In the last 5-10 years, we and others have discovered what seems to be the main route by which plants produce this hormone, what we call the IPyA route. With this discovery, new challenges emerged. One such critical challenge is how the biosynthesis, transport, and response to this hormone work together to control plant’s functions. To start to address this question, we have generated a large collection of genetic tools that allow for the direct monitoring of the activity of over 100 genes involved in the production, transport, and response to this hormone with cellular resolution. The information provided by this new comprehensive toolkit will be essential to creating a much needed global view of this hormone’s activity and how biosynthesis, transport, and response interact with each other to achieve both the precision and resilience needed to generate extremely complex structures such as flowers and fruits.
An additional question that emerged from the discovery of the main auxin biosynthetic pathway is whether or not other previously proposed routes of auxin production are truly functional in plants. Systematic scrutiny of this important question using different experimental approaches suggests that none of the genes previously proposed to function in making auxin through this alternative routes have a significant contribution to the production of this hormone under the studied conditions. Since we know that plants are capable of making this hormone by means other than the well-established IPyA route, how plants make this hormone through this other route(s) remains an important open question in hormone biology.