Plant Responses to Abiotic Stress
Environmental stresses represent the most limiting factors for agricultural productivity. Apart from biotic stress caused by plant pathogens, there are a number of abiotic stresses such as extremes in temperature, drought, salinity, heavy metals an
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Abstract Signalling of abscisic acid (ABA) in plants is characterized by an amazing number of secondary messengers that are part of the pathway or modulate the specific hormonal responses by interference with other signal transduction chains. In guard cells, a fast turgor-regulatory pathway triggered by ABA can be distinguished from a slower signalling pathway to the nucleus. The former is characterized by changes in K+ and anion channel activities mediated by ABA-induced Ca2+ oscillations and, subsequently, by vesicle trafficking due to alteration in cell size. The nuclear signalling pathway involves changes in the phosphorylation status of signalling components including transcriptional regulators thereby redirecting gene expression. Turgor- and nuclear-targeted steps of the ABA signalling cascade are to some extent shared by common components. Recent findings emphasize the importance of posttranscriptional regulation at the level of mRNA maturation and protein-turnover. In addition, the concept of reciprocal feed back loops of both pathways emerges.
2.1 Introduction Abscisic acid (ABA) is considered a ‘stress hormone’ (Zeevaart and Creelman 1988). The phytohormone integrates environmental constraints linked to changes in water activity with plant’s metabolic and developmental programs. Plants respond to environmental challenges like drought and salt stress by changes in ABA availability, either via re-distribution of the signal (Slovik et al. 1995, Wilkinson and Davies 1997) or increased biosynthesis (Zeevaart and Creelman 1988), and possibly by altering the sensitivity to the hormone signal. ABA is, however, not only a stress hormone but also an endogenous signal required for proper development. In the absence of environmental stress, a basal ABA level fine-tunes optimal growth of plants (Cheng et al. 2002a) possibly by reducing growth-inhibitory ethylene release (Sharp 2002). After exceeding certain threshold levels ABA precipitates the stress-related effects such as complete closure of stomata and massive alteration of gene expression (Hoth et al. 2002, Seki et al. 2002, Rock 2000). ABA signalling comprises the cellular events initiated by ABA that result in the specific responses including turgor-regulation and differential gene expression. Accordingly, a turgor-regulatory pathway can be distinguished from a nuclear signalling cascade (Webb et al. 2001, Fig. 1). ABA signalling, however, is not isoTopics in Current Genetics, Vol. 4 H. Hirt, K. Shinozaki (Eds.) Plant Responses To Abiotic Stress © Springer-Verlag Berlin Heidelberg 2003
40 Alexander Christmann, Erwin Grill and Michael Meinhard
Fig. 1. Integration of ABA signal transduction into stress signalling and development. Stress initiates the release of the internal signal ABA. ABA activates a signalling cascade, which branches into several pathways including turgor-regulation and the nuclear pathway readdressing gene expression. The ABA-induced change in the proteom feeds back to ABA biosynthesis and ABA signalling as well as to ontogenesis of the pl
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