Cell-to-cell communication involves the various stimuli to which plants respond, whether biotic, such as hormones and disease, or abiotic, such as water status, heat, cold, and light.
Throughout their lives, plants and plant cells continually respond to both external and internal signals, which they use to alter their physiology, morphology, and development. The manner in which plants respond to a stimulus is determined by developmental age, previous environmental experience, and internal biological clocks that specify the time of year and time of day.
In complex multicellular eukaryotes, the coordination of responses to environmental and developmental stimuli requires an array of signaling mechanisms. Animals have evolved two systems, the nervous system and the endocrine (hormone) system, for responding to stimuli. While plants lack a nervous system, they did evolve hormones and other chemicals, such as phytochrome, as chemical messengers.
The major groups of plant hormones include the auxins, gibberellins, cytokinins, ethylene, and abscisic acid. These hormones serve as signals for a wide range of physiological, biochemical, and developmental responses. These signals may impact different cells at different times, or they may impact the same cells at different sites. A variety of pathways are available for information flow from any one signal.
Information from the same signal may travel to different areas of the cell under different environmental conditions. Alternatively, information might travel to the same site after traveling by different pathways. Most signals appear to induce changes in gene expression, and this altered gene expression is responsible for the observed response.
There are two classes of chemical messengers, based on their ability to cross cellular membranes. Lipophilic messengers readily diffuse across membranes and combine with intracellular receptor proteins. When activated by the messenger, these proteins function as transcription factors, thereby inducing the transcription of new proteins.
In other words, the messenger-receptor complex signals the activation of genes which encode the proteins that produce the response to the stimulus. While animals have numerous lipophilic messengers, such as the steroid hormones in animals, only one, brassosteroid, has been demonstrated in plants.
Most plant messengers are hydrophilic, or water-soluble (rather than lipophilic) and are unable to enter the target cell because they cannot diffuse across the hydrophobic (“water-hating”) interior of the membrane.
These messengers must first bind with a membrane receptor molecule. This messenger-receptor complex then communicates with other molecules inside the cell to initiate a cascade of events referred to as a signal transduction pathway. Most signal transduction pathways cause the activation of other chemicals, referred to as second messengers.
Signal transduction pathways using a variety of second messengers have been well documented in animal systems. Some of the most common second messengers are 3′,5′-cyclic AMP (cAMP), G-proteins, 1,2 diacylglycerol (DAG), inositol 1,4,5-triphosphate (IP3), and Ca2+, and many of these have been shown to be active in plants. When a chemical messenger such as a hormone binds to a membrane receptor, one or more of these second messengers are elevated.
The elevated level of the second messengers results in the activation of regulatory proteins such as protein kinases or phosphatases. Activated protein kinases will phosphorylate transcription factors (that is, add a phosphate group), and activated phosphatases will dephosphorylate (remove a phosphate group from) transcription factors.
A typical signal transduction cascade is presented in the following scenario. An environmental stress causes an elevation in the level of the hormone abscisic acid (ABA), which is responsible for leaf fall. ABA binds to receptors in the membranes of the target cells.
The ABA-receptor complex activates a G-protein, which then activates the enzyme phospholipase C. This enzyme catalyzes the conversion of a substrate to DAG and IP3. The IP3 stimulates the opening of Ca2+ channels in the endoplasmic reticulum or tonoplasts (the membranes surrounding vacuoles).
The release of Ca2+ from these organelles activates protein kinases, which then activate transcription factors by phosphorylation. The activated transcription factors induce transcription of genes, which encode the proteins necessary for the plant to respond to the environmental stress.
Transport of Messengers
As discussed above, cells communicate primarily via chemical messengers. In most instances, particularly in the case of plant hormones, the messengers are produced in one cell and transported to other cells.
Plant cells are usually in contact with others around them, and cell communication (transport of messengers) can occur by transport through either the apoplast or the symplast. The apoplast refers to the free space between cells and cell wall materials.
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Water moves freely through the apoplast, and certain chemicals can be found moving with the water. Some important developmental molecules, such as the auxins, have been shown to move through the apoplast. The symplast is composed of the living cytoplasmof the cells, and many substances are transported symplastically.
Rapid transport through the symplast is possible because most living plant cells are connected to neighboring cells by plasmodesmata that pass through the adjoining cell walls and provide some degree of cytosolic continuity between them. Plasmodesmata are tube like cytoplasmic extensions that are divided into eight to ten microchannels.
Although the exact pathway of communication has not been determined, some molecules can pass from cell to cell through plasmodesmata, probably by flowing through the micro-channels. Plasmodesmata appear to be gated, which means that they allow the passage of some molecules and restrict the passage of others.