Ethylene is the only hydrocarbon that has been found to have a pronounced effect on the everyday life of plants. Ethylene is used by plants to implement fruit ripening, leaf and flower senescence, and the abscission of leaves and fruits. Plant tissues produce ethylene as a response to stress and, being gaseous, ethylene moves from its synthesis site by diffusion. Abscisic acid is synthesised in mature leaves and in certain circumstances in seeds. It induces photosynthate transport from the leaves to developing seeds and synthesis of storage-protein in seeds. Abscisic acid also stimulates stomatal closure in leaves and is thought to affect the induction and maintenance of dormancy in seeds and buds in some plant species. Gibberellins are found in the young tissues of plants, in particular the shoots and developing seeds. It is not yet known to science if gibberellin synthesis also occurs in the roots, gibberellins are thought to be transported via the vascular tissues. Gibberellins stimulate cell division and cell elongation, causing the hyperelongation of shoots, which has been proved to make dwarf mutations of plants grow tall. It is also involved with inducing seed germination and in certain plants it regulates seed enzyme production and the stimulating of flowering.
As well as these hormones recent studies have defined the last four groups (see below) as also being chemical signals utilised by plants. Brassinolides, steroids similar to those found in animals, stimulate elongation and cell division, and when synthesis of brassinolides is inhibited dwarf plants are produced. Salicyclic acid, which has a structure similar to aspirin, is thought to be implemented in the activation of pathogen defence genes. Volatile fatty acid derivatives known as jasmonates have long been recognised as components of floral fragrance and are now known to be regulators of seed germination, root growth, storage protein accumulation and synthesis of defence proteins. Finally systemin, a small peptide that is produced in wounded plant tissues and is thought to be a defence against herbivores. Systemin has been observed acting as a long distance signal and its discovery suggests there are more similar chemical signals employed by plants that are yet to be discovered.
What happens to a signalling molecule once it arrives at its target cell can be divided into three stages: reception, transduction and response, a diagrammatic explanation of these stages can be found in Appendix ‘B’. The majority of signal receptors can be divided into three groups: ion-channel linked receptors (also known as transmitter-gated ion channels), G-protein-linked receptors and enzyme-linked receptors. Ion-channel-linked receptors rely on a flow of ions to produce an electrical effect and typically are the receptors used for fast transmission across nerve synapses. The G-protein-linked receptors use an activated type of a membrane-bound protein (a G-protein subunit) that upon release diffuses through the plasma membrane bi-layer and initiates a cascade of other reactions. With enzyme-linked receptors enzyme activity is switched on at the cytoplasmic end of the receptor to generate a variety of further signals.
In plants, as well as animals, hormonal influences and interactions with a cell’s receptors are often mediated via complicated response pathways, which often involve the use of second messengers. Second messengers are typically not proteins but small, water-soluble molecules or ions. Most extracellular signals cannot travel through the cell membrane: they have to bind to receptor proteins, which then transduce the signal across the cell membrane.
The transduced signal, once inside the cell, is then either bound to specific receptors or carried on by second messengers. These second messengers can amplify the originally received signal by using a series of molecular switches known as phosphorylation cascades. A phosphorylation cascade is a process whereby received signal molecules cause cell membrane receptors to activate a series of reactions via a relay molecule. A diagram of this process can be found in Appendix ‘D’. Second messengers like the protein kinases are also capable of regulating multiple target proteins and so can act together to mediate a particular cellular response. In mammals the hormone induced activating of cAMP-dependent protein kinase (cAPK) in liver and muscle cells starts a phosphorylation cascade that inhibits the synthesis of glycogen and also stimulates the breakdown of glycogen.
The use of cyclic adenosine monophosphate (cAMP) as a second messenger provides many diverse effects in animals and because of this it is the most widely used second messenger in animals. In plants the most widely used second messengers are calcium ions, which combine with calmodulin (which is present in the cytosol of all eukaryotic cells), to induce and influence various cellular responses (usually through the activating of particular enzymes). In animals second messengers can be passed directly from one cell to another through gap junctions and in plants this facility is catered for by the cytoplasmic sleeves of the plasmodesmata. Interestingly these structures are very similar in size, diagrams of both can be found in Appendix ‘C’.
One kind of paracrine signalling is unique to animals: synaptic, or neuronal, signalling. Like hormonal signalling, synaptic signalling often occurs over long distances. However, unlike hormonal signalling in plants synaptic signals are transmitted by specific enclosed channels, known as nerve cells. As shown in the diagram in Appendix ‘A’, synaptic signalling involves the transmission of electrical impulses along a nerve cells’ axon to the synapse. These impulses are very fast (up to 100m per second) and when they reach the axon terminal of the transmitting cell each impulse is converted from intracellular electrical signals to chemical extracellular signals. These extracellular chemical signals are known as neurotransmitters and they can diffuse across the narrow space between the transmitting axon terminal and the target nerve cell in less than a millisecond.
Despite the detailed research into cell signalling by modern science we still understand very little about many cellular signalling processes, especially in plants. The traditional notion that plasmodesmata are passive structures with little or no influence upon the stuff that flows through them is increasingly being challenged. There are indications that some plasmodesmata can transport proteins and nucleic acids, and so play a primary role in the organisation of plant growth and development. The plethora of differing extracellular signals, receptors and intracellular signal-transduction pathways can be categorised into a number of relatively few groups. However, the same signalling pathway can often control and develop very different cellular responses, from proliferation and differentiation to apoptosis. . Our increased understanding of cell signalling in both plants and animals is constantly shedding new light on such things as life expectancy and cancer growth. Further research will undoubtedly reveal even more methods of cell signalling, in both plants and animals.
Appendix ‘A’
Types of signalling between cells.
Appendix ‘B’
Overview of cell signalling.
Appendix ‘C’
Diagram showing plasmodesmata and gap junctions.
Appendix ‘D’
Diagram showing the process of a phosphorylation cascade.
Bibliography
Alberts, B. et al. (1998) Essential Cell Biology: an introduction to the molecular biology of the cell. Garland Publishing, Inc. New York.
Campbell, N. & Reece, J. (2002) Biology, 6th Edn. Benjamin Cummings. San Francisco.
Lodish, H. et al. (2000) Molecular cell biology, 4th Edn. W.H. Freeman and Company. Basingstoke.
Raven, P. et al. (1999) Biology of Plants, 6th Edn. W.H. Freeman and Company. New York.
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