Abscisic acid (ABA) mediated growth control is a fundamental response of plants to adverse environmental cues. The linkage between ABA perception and growth control is currently being unravelled by using di¡erent experimental approaches such as mutant analysis and microinjection experiments. So far, two protein phosphatases, ABI1 and ABI2, cADPR, pH, and Ca 2+ have been identi¢ed as main components of the ABA signalling pathway. Here, the ABA signal transduction pathway is compared to signalling cascades from yeast and mammalian cells. A model for a bifurcated ABA signal transduction pathway exerting a positive and negative control mechanism is proposed.
Recent advances in understanding plant hormonal signalling has resulted in the identification of a variety of signalling components including receptor kinases with homology to the bacterial two component system as well as serine/threonine kinases and protein phosphatases. In addition, the existence of MAP kinase pathways in plants indicates a similar role of these signalling cascades in the relay of exogenous signals into the nucleus as has been disclosed in animal cells. The emerging signalling pathways of the plant hormone abscisic acid and ethylene are presented.
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