The hormone concept as developed for animals cannot easily be transferred to plants. On one hand have plants no as efficient transport system as the blood circulation, on the other hand could no hormone that covers all mentioned criteria be isolated, and thirdly have plants no equivalent to the central nervous system of animals for the integration and co-ordination of all physiological activities.
Still, plants have regulated growth, plainly determined steps of differentiation, different metabolic rates in cells, and – at least partially – a communication between cells, too. The cellular exchange of material is ensured by perforations of the cell walls at regular intervals.
The search for suitable regulator molecules or effectors was successful. They are known to belong to at least six different molecular classes:
auxins,,
cytokinins,,
gibberellins,,
abscisic acid,,
jasmonats and
ethylene.
Even if certain classical definitions do not apply is it spoken of plant hormones or phytohormones. More cautious people do also speak of growth regulators. In any case is no regulated plant growth possible without them. Plant hormones are without exception small molecules. They are distributed within tissues from cell to cell, as in the case of auxin, via vascular bundles (as in the case of cytokinin), or via the intercellular space (ethylen).
A number of results indicate that phytohormones enter cells and regulate intracellular processes, though hardly anything about their intracellular distribution or about their transport from one compartment into another is known. It remains open, too, whether they are stored in one or the other compartment, and whether they become biologically active by being set free from such compartments.
The second messenger concept seemed not to work in plant cells. cAMP (cyclic AMP) was found in plants, but – beside some few exceptions – little is known about its function. Considerably clearer is that calcium ions as intercellular regulators.
Usually display all known plant hormones a very broad and complex action spectrum. In experiments occur some effects directly after the application of a hormone, others take hours. It has been tried to conclude the mode of action from such results. Presumably are the activities of existing enzymes or membrane properties modified in fast reactions. In reactions with effects that become apparent only hours later is it likely that the gene expression (transcription or translation) is affected, though a complete chain of proof for the effect a hormone has on the molecular level, has in neither of these cases been furnished.
Often does it seem as if differentiation processes were not controlled just by a single substance, but by a complex, balanced equilibrium of simultaneously present regulator molecules and extern factors like light of a certain wave length, temperature, supply of nutriments, etc. In several cases exist indications that hormones mediate between an extern signal and a physiological activity (a cell’s response). Plant hormones act partially synergistic, partially antagonistic
The number of different plant hormones is rather small when compared to animals. Many animal hormones, especially the macromolecular ones have a very limited action spectrum that has its root in the selectivity and the cell-specific or tissue-specific distribution of the respective receptors. In contrast seem the receptors of plant hormones to be rather wide-spread and to differ in different cell types or developmental stages mainly by the affinity for their hormone.
Plant hormone research has mostly been occupied with the hormones themselves, their synthesis, their distribution within tissues, their displacement, and their physiological effects. Plant hormone receptors, however, have received little attention. As a result can some of the observations not be interpreted conceptionally, which means that they hold just for certain plant species, may be contradictory to observations of other species, and cannot simply be transferred to different species.
J. TREWAVAS (Department of Botany; University of Edinburgh) pointed already in 1982, 1983 out that the study of plant hormones itself has just limited significance. He considered the sensitivity of cells towards the hormones (and other factors), i.e. their existence or their obtainability for the hormone to be of far greater importance.
Numerous inconsistencies found in literature would be far easier to understand, if more about the sensitivity threshold of cells would be know than about the hormone concentration within cells. Both are hard to measure since no biological test for measuring the quantitative effect of an applied hormone or for determining its threshold value exists. In several cases was an increase in hormone concentration that had no physiological effect observed long after the hormone induced activity had passed its maximum.
The measurements of many dose effect curves cover hormone concentrations extending over four to five magnitudes though concentrations measured within cells do hardly ever exceed the double to tenfold of the mean value.
Hormone concentrations alone could not be sufficient to guarantee the stability of a plant’s development. The transport velocity within the system of vascular bundles is dependent on the transpiration that is itself dependent on water supply, temperature, and species-specific differences. It is a one way process like the transport via blood circulation occurring in animals, a feedback control is missing, The hormone concentrations within the vascular bundle system dependent on the presence of different factors while their values cannot be kept constant. A NAME="15">
Despite these limitations is the knowledge about plant hormones that has been collected over the last decades an important step on the way towards the understanding of plant development and its regulation. Chemically belong plant hormones to the secondary plant substances. Beside the fully functional hormones can hormone intermediates and breakdown products be found. Although they seem to be biologically inactive, remains the possibility that they are able to modulate the degree of the hormone effects. Unclarified is, too, to which degree other substances that have so far not been attributed hormone-specific functions, have growth supporting or growth inhibiting effects. The use of highly sensitive separation and detection methods like gas chromatography, HPLC (high pressure liquid chromatography), mass spectroscopy, autoradiography, and the radio immune test caused a new analytical phase in hormone research. Nearly all studies have been carried through on angiosperms, and although some hormones could also be detected in lower plants (mosses and algae) is very few known about the hormone effects in more primitive species. The results of angiosperm research should therefore never easily be applied to lower plants. Accordingly exists no knowledge about the evolution of the hormone system, too. We will see later that hormones participate in the differentiation of multicellular tissues. Was their presence and the presence of the respective hormone receptors a precondition of the evolution of multicellular plants and of different differentiated tissues ? Or did hormones evolve as a consequence of multicellular plant life? Substances resembling plant hormones and sometimes even identical with them have also been detected in micro-organisms and fungi. It seems therefore that the genetic potential for hormone production is rather old though this says nothing about the effects hormones have in different species.
Numerous synthetically produced growth regulators display hormone-like effects. They have a decisive economic importance as herbicides or growth stimulators in modern agriculture and horticulture, and – due to their dangerousness and the toxicity of their by-products (dioxin!) – an explosive political potential.
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de