Oscillations in cytosolic free Ca Stomata form pores in the epidermis of the leaf that allow CO 2 uptake for photosynthesis and water loss via transpiration. During drought, the loss of water through transpiration is reduced in response to an increase in the levels of the plant hormone abscisic acid (ABA) in the leaves (1). ABA stimulates the efflux of K ϩ from the guard cells that surround the stomatal pore, resulting in a reduction in guard-cell turgor and a decrease in the width of the pore (2). An increase in cytosolic free Ca 2ϩ concentration ([Ca 2ϩ ] cyt ) has been shown to be an early event in the signal transduction pathway by which ABA stimulates a reduction in guard-cell turgor (3-8). In addition, components of Ca 2ϩ -based second messenger systems found in animals have been identified in guard cells (9). However, little is known about the process by which the information required to describe the strength of the ABA stimulus is encoded in ABA-induced changes in guard-cell [Ca 2ϩ ] cyt or the mechanism(s) by which these changes are generated.It has been proposed that oscillations in [Ca 2ϩ ] cyt have the potential to increase the amount of information encoded by changes in [Ca 2ϩ ] cyt in plant cells through the generation of a stimulus-specific Ca 2ϩ signature (9, 10). Studies in animals suggest that signaling information may be encoded in the period and͞or the amplitude of stimulus-induced oscillations in [Ca 2ϩ
SummaryThere is much interest in the transduction pathways by which abscisic acid (ABA) regulates stomatal movements (ABA-turgor signalling) and by which this phytohormone regulates the pattern of gene expression in plant cells (ABA-nuclear signalling). A number of second messengers have been identi®ed in both the ABA-turgor and ABA-nuclear signalling pathways. A major challenge is to understand the architecture of ABA-signalling pathways and to determine how the ABA signal is coupled to the appropriate response. We have investigated whether separate Ca antagonists on ABA-induced stomatal closure and the ABA-responsive CDeT6-19 gene promoter suggest that Ca 2+ is involved in both ABA-turgor signalling and ABA-nuclear signalling in guard cells. However, the sensitivity of these pathways to alterations in the external calcium concentration differ, suggesting that the ABA-nuclear and ABA-turgor signalling pathways are not completely convergent. Our data suggest that whilst Ca 2+ -independent signalling elements are present in the guard cell, they do not form a completely separate Ca 2+ -independent ABA-signalling pathway.
SummaryStomatal responses to air pollutants are complex, varying among species and with concentration, environmental conditions and age. In general, short-term exposure to sulphur dioxide (SO 2 ) promotes stomatal opening, whereas longer-term exposure can cause partial stomatal closure. By contrast, the effects of oxides of nitrogen (NO x ) are often small or insignificant. The effects of ozone, and oxidative stress, are equally complex. Short-term exposure to ozone stimulates a rapid reduction in stomatal aperture, whilst longer-term exposure causes stomatal responses to become sluggish. The response of stomata to abscisic acid (ABA) has been shown to be slower in plants exposed to a combination of SO 2 and NO 2 suggesting an adverse effect on guard cell ABA signal transduction. In addition, ozone causes a reduction in stomatal closure under drought conditions. There is an increasing body of evidence to suggest that air pollutants and oxidative stresses can have a marked effect on the Ca 2+ homeostasis of guard cells and the intracellular machinery responsible for stomatal movements. Here we discuss the effects of air pollutants on stomatal responses and their possible effects on Ca 2+ based signalling in guard cells focusing on the effects of ozone and oxidative stress. [Ca 2+ ] cyt , cytosolic free calcium; GST, glutathione S -transferase; K in , plasma membrane inward-rectifying K + channel; NO x , oxides of nitrogen. Abbreviations© New Phytologist (2002) 153 : 441-447
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