Signal Perception and Transduction

Abstract

While investigating the effect of unilateral light on the bending response of canary grass (Phalaris canariensis) seedlings, Charles Darwin (1881) observed that although light signal is perceived at the shoot tip, the bending of coleoptile due to differential growth occurs in the subapical region. This classic case of environmental signal perception and transduction resulting in growth response leads to the concept of signal transduction. It is now known that plant growth and development are modulated by a variety of environmental (external) and physiological (internal) signals. Some of the major signals (stimuli) to which plant cells are sensitive include light, mineral nutrients, organic metabolites, gravity, water status, soil quality, turgor, mechanical tensions, heat, cold, wind, freezing, growth hormones, pH, gases (CO2, O2, NO, C2H4), volatile compounds (e.g., jasmonates), electrical fluxes, wounding, and disease (Fig. 23.1). These signals can vary in quality and quantity over a period. Some signals penetrate across the plasma membrane, while others are carried over long transcellular distances through vessel elements (xylem) and sieve tubes (phloem). Plasmodesmata also facilitate symplastic migration of a number of signaling biomolecules. With the advent of molecular genetic studies on Arabidopsis thaliana in the current era of plant biology research, there has been a flood of information on signal perception and transduction mechanisms in plants. A variety of receptors for various plant hormones have been identified and characterized. Mutant analysis has facilitated the identification of many new signal transduction components which act downstream of receptors for various environmental and internal signals perceived by plants. Thus, an entirely new level of understanding of the complexities of signaling mechanisms in plants has been unfolded. New signaling molecules continue to be discovered, and sophisticated signaling mechanisms are being explained through the development of models which explain interactions between signaling pathways and modulation of various signaling networks (Fig. 23.2). The application of knowledge thus acquired on signaling mechanisms using model plant systems such as Arabidopsis thaliana, to agriculturally significant species, is likely to provide practical benefits in understanding plant responses to varied environmental stresses.