PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

3. REGULATORY AND INTEGRATIVE SYSTEMS IN PLANTS

Integration of Regulatory Mechanisms at the Organismal Level

Integration is The process of organizing, coordinating, and unifying structures and Functions; essentially, it is a system of connections that functionally unites individual physiological reactions into the complex, mutually coordinated activity of a plant Organism.

All intercellular and Intracellular regulatory systems are closely interconnected. This creates a unified hierarchical regulatory system that governs the interaction of all plant parts. However, the interaction of parts alone does not ensure the integrity of the plant organism. Centralized control is also required during each specific period of ontogeny.

Integration at the organismal level is achieved through dominant centers, polarity, oscillations, channelized signaling, and regulatory loops.

Dominant Centers

It is well established that a plant possesses a distinct bipolar Structure, created by its poles—the dominant centers—namely the SHOOT and ROOT apices. Dominant centers are active meristematic regions that exert a decisive influence on The Development of adjacent Tissues. They serve as zones of Tissue and organ formation, as well as sensory and attracting centers.

At all Stages of Ontogeny, dominant centers provide centralized regulation of growth, morphogenesis, and the functional activity of the plant organism (see Fig. Functions of Dominant Centers). The action of the shoot apex can be temporarily replaced by auxin, and that of the root by cytokinin—phytohormones produced by these dominant centers. But how exactly do these dominant centers influence other PARTS OF THE whole organism?

Polarity

METABOLISM/18.html">The Influence of dominant centers is based on THE PRINCIPLE OF establishing physiological fields (physiological gradients). This principle is closely linked to the property of polarity in plants and manifests, in particular, as a gradual decrease along the plant axis in osmotic pressure, pH levels, the concentration of various substances, active Enzymes, Respiration intensity, etc. (G.Kh. Molotkovsky, 1961). In higher plants, polarity is primarily established by phytohormone gradients.

Channelized Signaling

In the course of evolution, plants have developed communications between different Organs that allow for more targeted and rapid transmission of both trophic factors and signals (a process known as signal canalization). In higher plants, such communications are represented by vascular bundles, which transport nutrients and phytohormones, as well as transmit electrical impulses. This is how communication between dominant centers is maintained within the plant. The system of channelized signaling, together with polarity (physiological gradients), ensures the Spatial Organization of the plant organism and remains under the control of the dominant centers.

Oscillations

It is hypothesized that the temporal Integration of the organism is carried out by a system of interconnected oscillations (physiological rhythms).

It is likely that oscillations within dominant centers, particularly fluctuations in phytohormone transport, serve to temporally synchronize physiological processes throughout the entire plant. For instance, physiological and morphogenetic oscillations in the shoot apex are transformed into the regular arrangement of leaves, lateral buds, and internodes.

Hourly, daily, seasonal, and annual fluctuations of physiological processes occur. Clearly, oscillations of one level are superimposed upon oscillations of greater amplitude, and so on, forming a hierarchy of oscillations that can be viewed as a biological clock.

Interaction between various regulatory systems occurs through regulatory loops.

Regulatory Loops

As noted, all regulatory systems function in interconnection. This interaction is organized by means of regulatory loops. An external stimulus is perceived by specific receptors in sensory Cells, causing these cells to transition into an excited state. Receptor cells recode (transform) the external signal into a different type of signal—hormonal or electrical. This signal is then relayed (transmitted) by the Cells of the signaling channels.

Upon reaching competent cells capable of responding to it, the signal induces their functional activity, which constitutes the organism's response to the external stimulus. Thus, the emergence or change of functional activity is a reaction to the signal.

At every stage of signal perception and the transition of a Cell to an active state, there are feedback mechanisms that correct these processes in accordance with the norm of reaction. Feedback also exists in other segments of Intercellular regulatory systems, forming numerous regulatory loops.

In Conclusion, integration at the organismal level occurs through the interaction of parts based on the principle of regulatory loops and elements of centralized control. Dominant centers, through fields, channelized signaling, and oscillations (rhythms), ensure the integrity of the plant organism.

Irritability

The functional role of the regulatory and integration systems discussed above is manifested in the phenomena of irritability. Irritability is the ability of living organisms and their cells to respond to Changes in the external and internal environment with adaptive, i.e., compensatory, reactions. Without this property, organisms would have no chance of survival.

The apparatus of irritability is built upon intracellular and intercellular interaction systems. Specific stimuli (those required under natural environmental conditions) are called adequate, while non-specific ones are termed inadequate.

Stimuli can act upon various cellular components (membranes, Transcription and Translation mechanisms, or enzymes). However, the primary cellular response is an electrical one.

Plants lack differentiated Sensory Organs, yet they possess receptor Proteins, cells, and tissues that perceive stimuli. These include photo-, chemo-, and mechanoreceptors.

The functional activity of photoreceptors is essential for phototaxis, phototropism, photonasty, and the detection of photoperiodic signals.

Chemoreception enables plant organisms, their tissues, and cells to respond to attractants, trophic factors (chemotaxis, chemotropism), and phytohormones.

Mechanoreception forms the basis for phenomena such as geotropism, thigmotropism, and seismonasty.

Laws of Irritability

1. The Law of Stimulus Intensity.

The greater the stimulus, the stronger the cellular or organismal response (up to a certain limit). The minimum intensity required to induce a response is called the excitation threshold. The threshold stimulus intensity serves as a measure of excitability. Exposure to supramaximal stimuli leads to the inhibition of function and activity, eventually resulting in cell death.

2. The Law of Stimulus Duration.

The longer the duration of the stimulus, the stronger the cellular or organismal response (within certain limits). The minimum time required to trigger a reaction is known as the presentation time. With excessively prolonged stimulation, The Cell's sensitivity to the stimulus decreases.

3. The Law of Stimulus Quantity.

The greater the magnitude of the stimulus, the shorter the presentation time required to induce threshold excitation (and vice versa). Thus, The Effect of stimulation R is a function of the stimulus quantity, i.e., the product of the stimulus intensity (i) and its duration (t): R=f(ixt). However, this law is valid only for the near-threshold range.

4. The Law of Stimulus Gradient.

The steeper The rate of increase in stimulus intensity over time (i.e., the stimulus gradient), the greater the cellular or organismal response (within certain limits).

A slowly increasing stimulus elicits a smaller physiological effect (adaptation) compared to a case where the stimulus reaches its full intensity instantaneously.

Thus, the response of a cell and an entire organism to a stimulus depends on its intensity, duration, and rate of increase.

Functional mobility of tissues is the ability of tissues to respond to an influence at a specific speed.

Parabiosis is an impulsive, gradual, and prolonged state of excitation, which serves as a general primary reaction of Cells and Tissues to A wide variety of external influences.

Parabiosis is the temporary loss of a living tissue's ability to perform its characteristic functions under the influence of excessive stimulation.

Class="center">Diagram of micro- and macro-circuits for perception and transmission of stimuli in Multicellular Organisms

Main Functions of the shoot apex and root tip in higher plants