Calcium and plant hormone action.
Plant cells contain all the elements for a calcium-based messenger system that could couple the external stimuli of hormones to their physiological response. These include a highly regulated low cytoplasmic calcium level, plasma membrane and endomembrane calcium pumps and channels, and spatially controlled calcium-dependent regulatory proteins and kinases. However much of the evidence for calcium as a second messenger in plants is fragmentary and circumstantial. There is some evidence linking auxin to fluctuations in free calcium but most of the evidence points to auxin as a calcium antagonist or having little or no effect on calcium levels. The possibility that polar auxin transport is coupled to calcium transport in the opposite direction is strong. There are several recent reports that abscisic acid is a calcium agonists but the data are still preliminary and there are alternative reports that indicate that abscisic acid may actually lower calmodulin levels. There is a close link between gibberellic acid and increases in intracellular free calcium in the barley aleurone system. These studies need to be extended to other gibberellin-dependent physiological responses. The most convincing evidence that has accumulated correlates cytokinin-stimulation of several different responses with increases in intracellular calcium by activation of plasma membrane ion channels. There is also a need to recognize that we don't understand stimulus-response coupling yet in the simplest of plant systems and the higher plants present a daunting challenge to plant scientists. It is unlikely that all hormone effects on all plants will be explained by invoking calcium as a second messenger and we must recognize that the mechanism of action of plant hormones may be quite complex and diverse.
Saunders
M J
MJ
Biology Department, University of South Florida, Tampa 33620.
D000040:Abscisic Acid / Q000378:metabolism; D002118:Calcium / Q000502:physiology*; D003583:Cytokinins / Q000378:metabolism; D005875:Gibberellins / Q000378:metabolism; D007210:Indoleacetic Acids / Q000378:metabolism; D010937:Plant Growth Regulators / Q000378:metabolism*; D018521:Plant Physiological Phenomena; D015290:Second Messenger Systems / Q000502:physiology*
D003583:Cytokinins; D005875:Gibberellins; D007210:Indoleacetic Acids; D010937:Plant Growth Regulators; D000040:Abscisic Acid; D002118:Calcium