| Enviromental stresses, such as drought, high salinity and low temperature, have adverse effects on plant growth and seed production. Revealing complex signal transduction pathways and gene networks for specificity and crosstalk in abiotic-stress-responsive gene expression is important for dissecting the mechanisms of plants adaptation to stress and, therefore, breeding crop tolerance to abiotic stress. Both ABA and ROS are produced or accumulated in plants under adverse environmental conditions and play important roles in signaling adaptive responses including stomata closure and antioxidant defense. ROS production/accumulation in excess causes oxidative stress leading to oxidative destruction of cellular components and thus must be controlled tightly. One mode of ABA action is involved in the regulation of ROS-producing and ROS-scavenging pathways. A basic region/leucine zipper (bZIP) transcription factor has previously been cloned from maize, and designated ABP9 (for ABRE Binding Protein 9), which specifically binds to the ABRE2 (ABA responsive element) motif of the maize caltalase1 promoter, and expression analysis in maize showed that ABP9 was induced by ABA and stress conditions such as drought, high salt and H2O2. The objective of the present study is, therefore, to investigate the function of ABP9 in plant abiotic stress responses, ROS homeostasis, ABA signaling, photosynthesis performance under stress and expression profiles of downstream genes using ABP9 transgenic Arabidopsis.The main results obtained are as follows.1. Constitutive expression of ABP9 in Arabidopsis resulted in enhanced tolerance to multiple environmental stresses including drought, salt, cold and oxidative stresses.2. As a positive regulator, ABP9 functions in ABA signaling. Constitutive expression of ABP9 in Arabidopsis resulted in enhanced ABA sensitivity in seed germination, root elongation and stomatal closure. The reduced transpiration rate and stomatal aperature led to the enhanced capacity of retaining water in ABP9 transgenic plants.3. ABP9 functions in regulating the metabolism of ROS, its constitutive expression enhanced the ability of ROS-scavenging of transgenic plants. Under both normal growth condition and salt/ABA stress treatments, constitutive expression of ABP9 in Arabidopsis resulted in the significant reduced cellular ROS levels and the ROS levels in different transgenic lines negatively correlated with the expression level of ABP9. In addition, ABP9 transgenic plants displayed reduced ROS levels in guard cells under both normal growth condition and ABA stress, morever, ABA-induced ROS generation in guard cells of transgenic lines was more significant than in WT.4. Constitutive expression of ABP9 in Arabidopsis improved the photosynthetic capacity of plants under stress by adjusting the photosynthetic pigment composition, dissipating excess light energy, and elevating carbon-use efficiency as well as increasing the ABA content, instantaneous water use efficiency (IWUE), and expression of stress-defensive genes, suggesting the important role of ABP9 in plant stress tolerance in terms of the regulation of plant photosynthesis under stress. 5. Microarray and Real-Time quantitative PCR analysis revealed that a large body of stress/ABA responsive genes including those for stress defense, ABA signaling and ROS-scavenging were elevated in ABP9 transgenic plants.In summary, the results demonstrate that ABP9 plays an important regulatory role in plant responses to abiotic stresses via mediating ABA signaling and ROS-scavenging as well as improving photosynthetic capacity and constitutive expression of ABP9 enhances tolerance to abiotic stress including drought, high salt, low temperature and oxidative stresses. These results will enable further dissection of other signaling components and their interacting mechanisms of ABP9 pathway in abiotic/ABA stress signal transduction, as well as the application of ABP9 in molecular breeding for crop tolerance to abiotic stresses.
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