Coordinated regulation of the mitochondrial retrograde response by circadian clock regulators and ANAC017

Mitochondrial retrograde signaling(MRS)supports photosynthetic function under a variety of conditions.Induction of mitochondrial dysfunction with myxothiazol(a specific inhibitor of the mitochondrial bc1 complex)or antimycin A(an inhibitor of the mitochondrial bc1 complex and cyclic electron transport in the chloroplast under light conditions)in the light and dark revealed diurnal control of MRS.This was evidenced by(1)significantly enhanced binding of ANAC017 to promoters in the light compared with the dark in Arabidopsis plants treated with myxothiazol(but not antimycin A),(2)overlap in the experimentally determined binding sites for ANAC017 and circadian clock regulators in the promoters of ANAC013 and AOX1a,(3)a diurnal expression pattern for ANAC017 and transcription factors it regulates,(4)altered expression of ANAC017-regulated genes in circadian clock mutants with and without myxothiazol treatment,and(5)a decrease in the magnitude of LHY and CCA1 expression in an ANAC017-overexpressing line and protein–protein interaction between ANAC017 and PIF4.This study also shows a large difference in transcriptome responses to antimycin A and myxothiazol in the dark:these responses are ANAC017 independent,observed in shoots and roots,similar to biotic challenge and salicylic acid responses,and involve ERF and ZAT transcription factors.This suggests that antimycin A treatment stimulates a second MRS pathway that is mediated or converges with salicylic acid signaling and provides a merging point with chloroplast retrograde signaling.

Co-regulation of mitochondrial and chloroplast function: Molecular components and mechanisms

The metabolic interdependence,interactions,and coordination of functions between chloroplasts and mitochondria are established and intensively studied.However,less is known about the regulatory components that control these interactions and their responses to external stimuli.Here,we outline how chloroplastic and mitochondrial activities are coordinated via common components involved in signal transduction pathways,gene regulatory events,and post-transcriptional processes.The endoplasmic reticulum emerges as a point of convergence for both transcriptional and post-transcriptional pathways that coordinate chloroplast and mitochondrial functions.Although the identification of molecular components and mechanisms of chloroplast and mitochondrial signaling increasingly suggests common players,this raises the question of how these allow for distinct organelle-specific downstream pathways.Outstanding questions with respect to the regulation of post-transcriptional pathways and the cell and/or tissue specificity of organelle signaling are crucial for understanding how these pathways are integrated at a wholeplant level to optimize plant growth and its response to changing environmental conditions.