Catabolic processes providing alternative sources of electron for the mitochondrial electron transport chain are progressively emerging as important players in plants for stress responses, nutritional switch responses and growth and development. In this context, xenobiotics, such as the herbicide atrazine, which are at the crossroads of xenobiotic action, photosystem homeostasis and ROS dynamics, are likely to be useful for understanding the regulation of these catabolic processes. Transcriptomic analysis of atrazine effects on Arabidopsis under different conditions of carbon status reveals that atrazine significantly upregulates the genes involved in leucine catabolism, in contrast to partial regulatory effects on the genes involved in valine, isoleucine and lysine catabolic pathways. These effects on amino acid catabolism gene expression are associated with regulatory effects on genes involved in proteolytic processes and in alternative carbon mitochondrial respiration. Genes involved in leucine catabolism are activated by atrazine in the presence of exogenous sucrose, thus indicating that atrazine-associated signals can override sucrose repression. There may be a link between these effects and atrazine-related increase in hydrogen peroxide, which is involved in retrograde signalling. However, comparison with studies of reactive oxygen species modifications indicate that atrazine regulation of branched chain amino acid catabolism differs from reactive oxygen species signalling, including hydrogen peroxide, thus suggesting complex signalling pathways between photosystem functioning and leucine catabolism.
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