Phytosensors and Phytoactuators

Plants continuously sense a wide variety of perturbations and produce various responses known as tropisms in plants. It is essential for all plants to have survival sensory mechanisms and actuators responsible for a specific response process. Plants are ideal adaptive structures with smart sensing capabilities based on different types of tropisms, such as chemiotropism, geotropism, heliotropism, hydrotropism, magnetotropism, phototropism, thermotropism, electrotropism, thigmotropism, and host tropism. Plants can sense mechanical, electrical and electromagnetic stimuli, gravity, temperature, direction of light, insect attack, chemicals and pollutants, pathogens, water balance (Volkov et al, 2007, 2008, 2009a,b, 2010, 2011, 2012). Here we show how plants sense different environmental stresses and stimuli and how phytoactuators response to them. Plants generate various types of intracellular and intercellular electrical signals in response to these environmental changes. This field has both theoretical and engineering significance because these phytosensors and phytoactuators employ new principles of stimuli reception and signal transduction and play a very important role in the life of plants (Markin et al., 2008). A phytoactuator is a part of a plant responsible for moving or controlling a specific plant response process. It is operated by a source of electrochemical energy or hydraulic pressure and converts that energy into motion. A phytosensor is defined as a device that can detect, record, and transmit information related to a physiological change/process in a plant. It can also use plant tissue to monitor the presence of various chemicals in a substance. In most successful phytosensors, the principle behind the determination of a chemical or biological molecule is the specific interaction of such an analyte molecule with the plant tissue present in the phytosensor probe device. Even though a variety of biological materials and transduction methods have been investigated in the development of novel phytosensors, the most successful commercial systems include immobilized enzymes and electrochemical transducers. Nerve cells in animals and phloem cells in plants share one fundamental property: they possess excitable membranes through which electrical excitations can propagate in the form of action potentials. Plants generate bioelectrochemical signals that resemble nerve impulses, which are present in plants at all evolutionary levels. The conduction of bioelectrochemical excitation is a rapid method of long distance signal transmission between plant tissues and organs. Plants quickly respond to changes in luminous intensity, osmotic pressure, temperature, cutting, mechanical stimulation, water availability, wounding, and chemical compounds such as herbicides, plant growth stimulants, salts, and water. Once initiated, electrical impulses can propagate to adjacent excitable cells. The change in transmembrane potential creates a wave of depolarization or action potential, which affects the adjoining resting membrane. The phloem is a sophisticated tissue in the vascular system of plants. Representing a continuum of plasma membranes, the phloem is a potential pathway for transmission of electrical signals. It consists of two types of conducting cells: the characteristic sieve-tube elements, and the companion cells. Sieve-tube elements are elongated cells that have end walls perforated by numerous minute pores through which dissolved materials can pass. These elements are connected in a vertical series known as sieve tubes. The companion cells have nuclei and they are adjacent to the sieve-tube elements. It is hypothesized that they control the process of conduction in the sieve tubes. Thus, when the phloem is stimulated at any point, the action potential can propagate over the entire cell membrane and along the phloem with constant voltage. Plants constantly communicate with the external world in order to maintain homeostasis. Internal biological processes and their concomitant responses to the environment are closely associated with the phenomenon of excitability in plant cells. The extreme sensitivity of the protoplasm to chemical effects is the foundation for excitation. The excitable cells, tissues and organs alter their internal condition and external reactions under the influence of environmental factors, referred to as irritants; this excitability can be monitored. Plants generate different types of extracellular electrical responses in connection to environmental stress. Recent findings have indicated that plants may use a common defense system to respond to various abiotic and biotic stresses, such as heat, cold, drought, flooding, osmotic shock, wounding, high light intensity, UV-radiation, ozone, and pathogens. Using cDNA microarrays, a large number of genes have been found to be coordinately regulated and overlap under different stresses.