Abscissic Acid

Abscissic acid (ABA)-mediated control of ROS levels has been invoked to explain its function in protecting plants against oxidative conditions caused by many stress conditions, including nutritional deficiencies (Rubio et al. 2009). For instance, K+ deprivation increases ABA levels both in the root and in the shoot (Kim et al. 2009) that would produce, among the responses of this hormone, a decrease in plant transpiration. Moreover, exposure of Arabidopsis roots to ABA evoked a dramatic reduction in the transcript levels of SKOR, the

Fig. 4.3 Hormonal responses in relation to K+ uptake. Root apex is an important source of cytokinins and when it is removed, rapid efflux of K+ is observed. Such efflux can be reversed by adding cytokinins. K+ transport through TRH1 greatly contributes to auxin gradients in the root and therefore supporting auxin responses such as gravit-ropic growth and root hair development. Low-external K+ concentrations trigger stress-related pathways by increasing ethylene, abscisic acid (ABA), and Jasmonic acid levels. Ethylene activates reactive oxygen species (ROS) production, which mediates AtHAK5 upregulation and, in turn, high-affinity K+ uptake. The ABA increase reduces stomatal aperture in leaves and in roots, it induces SKOR downregulation and thereby decreases K+ xylem load. Jasmonic acid production activates several pathways which aim at enhancing nutrient recycling, storage, and allocation processes

Fig. 4.3 Hormonal responses in relation to K+ uptake. Root apex is an important source of cytokinins and when it is removed, rapid efflux of K+ is observed. Such efflux can be reversed by adding cytokinins. K+ transport through TRH1 greatly contributes to auxin gradients in the root and therefore supporting auxin responses such as gravit-ropic growth and root hair development. Low-external K+ concentrations trigger stress-related pathways by increasing ethylene, abscisic acid (ABA), and Jasmonic acid levels. Ethylene activates reactive oxygen species (ROS) production, which mediates AtHAK5 upregulation and, in turn, high-affinity K+ uptake. The ABA increase reduces stomatal aperture in leaves and in roots, it induces SKOR downregulation and thereby decreases K+ xylem load. Jasmonic acid production activates several pathways which aim at enhancing nutrient recycling, storage, and allocation processes outward-rectifier K+ channel involved in K+ release in the xylem (Gaymard et al. 1998). Interestingly, important reductions in SKOR mRNA levels in the roots were also observed due to K+-starvation (Pilot et al. 2003) + suggesting that this reduction could be related to the ABA increase under low K+ growth conditions.

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