Linking mitochondrial and chloroplast retrograde signalling in plants

Retrograde signalling refers to the regulation of nuclear gene expression in response to functional changes in organelles. In plants, the two energy-converting organelles, mitochondria and chloroplasts, are tightly coordinated to balance their activities. Although our understanding of components involved in retrograde signalling has greatly increased in the last decade, studies on the regulation of the two organelle signalling pathways have been largely independent. Thus, the mechanism of how mitochondrial and chloroplastic retrograde signals are integrated is largely unknown. Here, we summarize recent findings on the function of mitochondrial signalling components and their links to chloroplast retrograde responses. From this, a picture emerges showing that the major regulators are integrators of both organellar retrograde signalling pathways.

The retrograde response pathway initiates a signalling cascade to modulate the expression of nuclear genes in response to changes in mitochondrial and chloroplastic function.

While our knowledge has expanded, much still remains unknown.

for mitochondrial retrograde signalling, while ROS and other molecules have been implicated as signals, there is no mechanistic understanding of how these signals are transmitted to execute a response, even though downstream components have been identified

there are many knowledge gaps in the various pathways that have been elucidated for both chloroplast and mitochondrial retrograde signalling.

The biochemical, cellular and physiological reasons why mitochondrial and chloroplast retrograde signalling may be linked at several levels are worth considering. Organelle retrograde signalling provides feedback to anterograde signals that alters gene expression encoding proteins located in a variety of locations in the cell. Thus, chloroplasts and mitochondria have emerged as environmental sensors to ensure that the pipeline from gene expression to protein function is optimized relative to organelle function. An integral function of mitochondria and chloroplasts in energy biology in plants means that any functional perturbation in either organelle will have cellular and plant-wide consequences. Thus, signals emanating from organelles alongside their essential central roles in energy biology would be an efficient means to ensure whole cellular function is tuned to the functional status of these organelles.

The mentioned knowledge gaps are mainly related to control procedures.   Hence most future research discoveries should shed more light on the control aspects of biology.