Drugs in the Plant

Neurotransmitters in Plants By Victoria V. Roschina Enfield, NH: Science Publishers (2001). $87.50In 1898, George Hormann first used external electrodes to measure the action potential of single cells in the green algae, Nitella. Over the next several decades, plants became the system of choice to study electric signals in biology. It was not until the 1930s that isolated nerve cells from squid became popular for the study of action potentials. Because squid nerves are large and easily accessible, their study soon overshadowed plant systems. In these studies, neurotransmitters and their target receptors in animals were identified and isolated. Henceforth, neurobiology flourished and became ubiquitous as a field among researchers. Meanwhile, plant biologists continued to study action potentials in plants and hypothesized their role in systemic signaling. As new neurotransmitters were discovered and their roles deciphered in the nervous system, it was noted that many of these same compounds were also found in plants including acetylcholine, catecholamines (noradrenaline, adrenaline, and dopamine), serotonin, and histamine. Why do plants synthesize compounds that act as neurotransmitters in animals? And why do plants make so many agonists of neurotransmitters, ranging from cocaine to caffeine? With her monograph, Neurotransmitters in Plant Life, Victoria Roshchina addresses this question. In the book, Roshchina reviews much of this century's work regarding the presence, activity, and metabolism of many of these compounds in plants.Perhaps it is a matter of perspective. The danger of approaching this question from the vantage point of animal neurophysiology is that we begin to believe these compounds have a homologous function in plants. In itself, the title, Neurotransmitters in Plant Life, is misleading, since it leaves the impression that plants have nerves. To date, a nervous system has not been detected in plants. As such, it would have been more appropriate to retain the title of the previous edition written in Russian: Biomediators in Plants. Acetylcholine and Biogenic Amines (published by the Biological Center of USSR, 1991). The older title is more consistent with the content of the book.Are these neurotransmitters, which are also found in plants, involved in signaling in plants? Or are they simply intermediates along a biochemical pathway? Are they storage or breakdown products? Or, are these compounds produced for defense against predators? A large body of evidence is provided in the text for all these possibilities, and more. For instance, stinging nettles contain high levels of acetylcholine, histamine, and serotonin in their needles as a probable (and painful) adaptation against predators (or the casual hiker). In a different system, acetylcholine activates ion channels and photophosphorylation in isolated pea chloroplasts, suggesting acetylcholine plays a role as a regulator of cell signaling. In yet another system, when applied to the stigma prior to pollination, acetylcholine antagonists cause the dramatic inhibition of seed formation during fruit development in Hippeastrum hybridum. Is this impairment of seed production from the antagonist caused by a block in acetylcholine perception or because of nonspecific effects? Can one wade through this myriad of data, most of it over 10 years old, and come to a conclusion regarding the role of acetylcholine in plants?It is doubtful that we can glean the real role of acetylcholine, chatecholamines, serotonin, or histamines from the accumulated data in the monograph. However, we can gather useful tidbits from this work to conduct further research into these questions. One exciting aspect regarding this field of research is that recent advances in molecular biology have opened up new vistas in the role of neurotransmitters in plant biology. An attempt to address recent discoveries of genes in plants with similarity to neuroreceptors in animals would have brought the text up to date. For instance, the recently completed sequencing of the Arabidopsis genome has revealed the presence of genes with similarity to glutamate receptors and GABA receptors, which previously were believed only to exist in animals. The addition of glutamate to Arabidopsis causes an immediate and dramatic change in the membrane potential in the cells. In young growing Arabidopsis seedlings, the addition of a glutamate receptor agonist leads to the impairment of light perception, suggesting that these receptors might be involved in glutamate-mediated light signaling. This work, similar to the “spray and pray” approach of early plant physiology, can now be backed up with the power of molecular genetics. Mutants in plants altered in response to agonists or antagonists of neurosignaling compounds, which cause observable effects on plants, can now be isolated, studied, and cloned. This combination of plant physiology and molecular biology has worked well to define the role of plant hormones and may also be applicable to study the effects of these compounds in plants.Some may ask the question, if acetylcholine, catecholamine, serotonin, and histamine were known to be present in plants, and ample evidence existed that they affect plant growth and physiology, then why were they not studied using a molecular approach, as were hormones in the late 1980s through the present? Perhaps there wasn't a critical mass of researchers interested in this field to expand this work using a molecular approach in some of the model plant systems such as Arabidopsis, maize, and tomato? Even some important plant hormones, which were known by plant physiologists to cause dramatic effects on plant development, fell through the cracks of mainstream research. One such “forgotten” hormone was brassinosteroid, which was implicated in a variety of responses, including internode elongation. Brassinosteroid was only “rediscovered” upon cloning the Arabidopsis mutant, det2, which has among other phenotypes, a dwarf stature. DET2 encodes a reductase in the first step of the brassinosteroid biosynthetic pathway. Since then, many of the key signaling intermediates and the potential receptor in a brassinosteroid signaling pathway have been uncovered in Arabidopsis. Likewise, perhaps future mutants isolated in Arabidopsis, maize, and tomato will reveal the role of these “neurotransmitters” in plants.The vast majority of research on these neurotransmitters monographed in the text was not performed on the model molecular plants, but on a variety of systems, some with complicated genetics. The advent of genomics may ameliorate this issue. Genomic technology offers the tantalizing possibility that plant scientists will be able to examine both highly advanced plants and primitive plants for the role of acetylcholine, catecholamine, serotonin, and histamine, as well as glutamate and GABA in plant growth and development. As ESTs and genomes are assembled into databases, BLAST searches and functional genomics will allow for the further characterization of these compounds. The identification and study of enzymes that may be involved in the biosynthesis of these compounds, as well as the detection of putative receptors, signal transducers, and transporters that may be involved in the regulation of these compounds will greatly improve our understanding of the role of these compounds in plants. Perhaps they are involved in activating electrical potentials or even transmitting systemic electrical signals, as has long been hypothesized. With the use of molecular and genomic methods, we may finally be able to determine the role of “neurotransmitters” in plants.