Plant Growth Regulators: An Overview

Plants respond to external stimuli and sense changes in their environment, and their biological clock works in a precise manner. Many deciduous plants shed their leaves in a particular season or develop flowers and fruits during a specific time of the year. All these and many more rather intriguing responses of plants indicate that they have unique mechanisms to sense and communicate with the environment they live in. In order to achieve this, plants synthesize several biomolecules of varying chemical nature which facilitate cell to cell communication and affect varied aspects of growth and development. In higher plants, regulation and coordination of metabolism, growth, and morphogenesis depend on the transport of various chemical signals from one part to another. In the nineteenth century, a German botanist Julius von Sachs hypothesized that certain chemical messengers are responsible for the initiation and growth of different plant organs. Since then, it has become evident that these messengers or signaling molecules translate environmental signals into growth and developmental responses and regulate metabolism by their redistribution. A number of signaling molecules have been identified in the animal systems and prokaryotes, and are referred as "hormones." The term "hormone" was initially used about 100 years ago in medicine for a stimulatory factor. It is a Greek word meaning "to stimulate" or "to set in motion." Hormones have since long been defined as biochemicals produced in specific tissues which are transported to some distant cells to trigger response at low concentrations. The classic definition of a plant hormone according to Went and Thimann (1937) states that it is a substance which is produced in any part of the organism and is transferred to another part to influence a specific physiological process. Another term, "morphogen," is used for those biomolecules which possess the ability to modify cell pattern formation in a concentration-dependent manner, and at different concentrations, they lead to different outputs. Cells have the ability to assess their respective positions in response to the gradient of a particular morphogen, and cells use this information to establish their identity.