Abscisic acid gives plants a signal that they are undergoing stress and helps protect plants from these stresses. ABA also helps cause normal embryogenesis, formation of seed storage protiens and prevents premature germinaton or growth of many seeds and bulbs. ABA can affect the cell in three different ways depending on the tissue involved.
1**effects on the plasma membrane
2**inhibition of protein synthysis
3**specific activiation and deactivation of certian genes (transcription effects)
(Salibury and Ross, 1992)
Research on Commelina guard cells gives evidence for an intercellular ABA receptor. Guard cells close rapidly due to external increases in ABA. When ABA is caged and microinjected then photolysed to release ABA internally stomatal closure results. This gives evidence for an internal ABA receptor. (Allan et al., 1994)
It was previously thought that ABA signals were only transduced through increases in cytosolic calcium however recent evidence suggests that there are actually two pathways that can lead to stomatal closure in guard cells. When guard cells were grown at 10-17 degrees celcius most failed to exhibit any ABA induced free calcium although all cells closed. At growth temperatures above 25 degrees celcius closure was associated with increases in cytosolic calcium. This evidence suggests that the ABA signal is not dependant on the increase in cytosolic calcium. ( Allan etal., 1994)
Recent research suggests that ABA signal transduction may proceed by activation of kinases and inactivity of phosphotases. This phosphorylation activity may be present at any stage in the signal pathway including the phosphorylation of the ABA receptor protien. (Schmidt etal, 1995)
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