Mechanisms Regulating the Response of Arabidopsis thaliana to Heat Stress
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Abiotic stresses cause extensive losses to agricultural production worldwide. Acclimation of plants to abiotic stresses is mediated by a complex network of transcription factors and other regulatory genes that control multiple defense and acclimation pathways. Although, previous studies uncovered a number of pathways involved in the acclimation of plants to abiotic stresses, many of the complex networks involved in this response are still unknown. In a previous study, transcriptome analysis of Arabidopsis thaliana subjected to a combination of heat and drought stress revealed that multiprotein bridging factor 1c (MBF1c) is highly activated during the stress combination. In this study, to test the role of MBF1c in the response of plants to abiotic stresses, transgenic plants constitutively expressing MBF1c were generated and characterized. Heat-response pathways regulated by MBF1c were then studied using gain- and loss-of-function MBF1c plants. In addition, a double mutant lacking cytosolic and chloroplastic hydrogen peroxide removal enzymes was characterized to study the relationship between reactive oxygen species (ROS) signals generated in different cellular compartments.Constitutive expression of a stress-response transcriptional coactivator, multiprotein bridging factor 1c (MBF1c) in Arabidopsis (Arabidopsis thaliana) enhanced the tolerance of transgenic plants to heat stress, osmotic stress and a combination of heat and osmotic stress. Accumulation of a number of defense transcripts was augmented by the expression of MBF1c during heat stress. Transcriptome profiling and inhibitor studies suggested that MBF1c expression enhances the tolerance of transgenic plants to heat and osmotic stress by partially altering the ethylene-response signal transduction pathway. These results indicated that MBF1 proteins could be used to enhance the tolerance of plants to different abiotic stresses (Chapter II).Characterization of MBF1c protein, and analysis of a mutant deficient in MBF1c, demonstrated that MBF1c is a key regulator of thermotolerance in Arabidopsis thaliana. MBF1c protein accumulates rapidly and is localized to nuclei during heat stress. MBF1c functions upstream to SA, trehalose, ethylene, and pathogenesis-related protein 1 (PR-1) during heat stress. In contrast, MBF1c is not required for the expression of transcripts encoding HSFA2, heat shock proteins and the cytosolic ROS scavenging enzyme, ascorbate peroxidase 1. Interestingly, MBF1c directly interacts with a heat-inducible trehalose phosphate synthase, TPS5, and mutants deficient in TPS5 are thermosensitive. These results indicate the existence of a tightly coordinated heat stress-response network, involving trehalose-, SA-, and ethylene-signaling pathways, regulated by MBF1c (Chapter III).To study how different ROS signals, generated in different cellular compartments, are integrated in cells, a double mutant lacking two hydrogen peroxide removal enzymes, thylakoid ascorbate peroxidase (tylapx) and cytosolic ascorbate peroxidase1 (apx1), was generated and characterized. The lack of tylAPX triggers a specific signal that results in enhanced tolerance to heat stress, whereas the lack of APX1 triggers a different signal that results in stunted growth and enhanced sensitivity to oxidative stress. Double mutants deficient in both tylAPX and APX1 exhibited low protein oxidation during light stress, and enhanced accumulation of anthocyanins, indicating the generation of a new signal by the integration of two signals in cells. These results demonstrate a high degree of plasticity in ROS signaling in Arabidopsis and suggest the existence of redundant ROS scavenging pathways that compensate for the lack of classical ROS removal enzymes. Further investigation of heat tolerance enhanced in plants lacking tylAPX, using mutants deficient in chloroplast-to-nuclei retrograde signaling, suggests the existence of a chloroplast-generated stress signal that enhances basal thermotolerance in plants (Chapter IV).