Procedure of Hormone Act
All plant hormones show extraordinary varied complex effects in controlling plant growth and development, Extrapolation from how an animal hormone works a common framework for different plant hormones may explain their varied effects. In animals, the wide variety of effects shown by different hormones is understood by the mechanism of action at the cell level. You may recall that the target cells have appropriate receptors for hormones either on the plasma membrane or are located generally in the interior of the cell. Similar attempts have been made to explain the mechanism of action of plant hormones employing the receptor concept. Both natural and synthetic hormones behave in a similar way as it is assumed that they bind to specific receptors to form a hormone receptor complex to trigger an effect.
Though search for a receptor protein has been generally a frustrating one, recently, such proteins have been demonstrated in pea which bind with auxins before eliciting a response such as embryoid differentiation in tissue culture. Cell elongation, the most well-known response of auxin, requires that the longitudinal wall stretches, which will involve basic changes in cell wall. In order to stretch, cell wall has to become more plastic, just like a balloon, in which the driving force to increase the volume is proportional to the resistance offered by the balloon wall. Increased plasticity of the cell wall by auxin is considered to be due to breaking of some of the bonds between the polysaccharide components of the cell wall. As it becomes plastic the cell is amenable to stretching. The structure of gibberellin resembles certain animal steroid neurotransmitters.
The search for gibberellin receptor in cytoplasm rather than in the membrane has not been fruitful unlike animal neurotransmitters which bind to cytoplasmic receptors. Though the biochemical mode of action of plant hormones is poorly understood, still the general assumption in current research work is that plants cells have specific receptors which when bound to hormones activate the signal transduction pathway for various activities. However, though many proteins have been found by various workers to bind with hormones, these may be inactive complexes. Currently search is on to identify the nature of the receptor and decipher its mode of action.