Mitochondrial Retrograde Regulation in Plants: Identification of Regulated Genes and Signaling Pathway Components and Involvement in Stress Responses

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Eukaryotic cells contain membrane-bound sub-compartments called organelles (e.g. nuclei, chloroplasts, mitochondria, and peroxisomes). Communication between organelles is of central importance to the functioning of cells. Of particular interest is the way in which organelles such as chloroplasts and mitochondria communicate their status to the nucleus to cause changes in gene expression. The status of each of the metabolic activities, such as respiration and the tricarboxylic acid (TCA) cycle in the mitochondria, photosynthesis in the chloroplasts, and other key metabolic pathways that take place in organelles, is vital to the functioning of cells. If the metabolic activities of the organelles are not functioning properly, then, in general, the organelles seem to be able to communicate this to the nucleus so that changes in gene expression can occur that will help to alleviate the situation at the molecular level. Relatively little is known about the way in which this is accomplished, especially in higher eukaryotes. One important category of the type of stimuli that may alter organelle function is environmental changes. This includes changes in light level and environmental stresses such as cold stress, heat stress, nutrient deprivation and pathogen attack. Plants are sessile and, therefore, must adjust to changes in environmental conditions by attempting to maintain or regain cellular homeostasis. It is clear that plants respond to stress and changes in metabolism by altering gene expression. Environmental changes may rely on communication between organelles and the nucleus (a process referred to as retrograde regulation of gene expression) to alter the expression of key genes vital to a successful response to the particular stress. The main focus of the research in this project it to test the hypothesis that there are multiple modes of communication, referred to as signaling pathways, from mitochondria to the nucleus in plants: one pathway to communicate to the nucleus that there has been a perturbation of respiration, which may be caused by some environmental stresses, and another pathway to communicate that there has been a perturbation of metabolic processes, such as the TCA cycle, which also may be caused by some environmental stresses. If mitochondria-to-nucleus signaling is a key part of the overall response of plants to stresses, then it will be very interesting to determine the exact roles of mitochondria-to-nucleus signaling in individual stress responses. Several approaches will be used to address this problem. The promoter (gene expression controlling) region of the AtAOX1a gene, which responds to mitochondrial retrograde signaling, will be mutated at specific sites to determine regions that are important for the response. A full view of the genomic changes in gene expression that result from perturbations of mitochondrial respiration or perturbation of the TCA cycle will also be determined. A genetic screen will be utilized to isolate mutant Arabidopsis thaliana plants that lack mitochondrial retrograde signaling pathways. These mutant plants will be analyzed to determine the effects of the mutation in each. Undergraduate students, especially those of groups that are under-represented in the life sciences, will be recruited for participation in this research program. Graduate student training and post-doctoral training are also intended to be a major part of this project. Because of the diverse approaches used in my laboratory and the presence of close colleagues interested in inter-organellar communication, the training environment for students and post-doctoral fellows should be very stimulating.