Signalling between the nucleus and genomecontaining organelles: Sense and adaptation

All organisms require the ability to sense their surroundings and adapt. From bacteria to humans, these abilities ensure that an organism can thrive and grow in a wide range of habitats. This is especially true for plants, which are sessile organisms that have to be genetically equipped to withstand every change in the environment in which they live. Recently, it is becoming clear that plants and many other eukaryotes use their energy-producing organelles (chloroplasts and mitochondria) as endogenous and environmental sensors (Figure 1). These organelles allow the organism to respond and adapt to changing conditions by sending signals to the nucleus to alter gene expression and cell physiology. This organelle-to-nucleus (retrograde) signalling is important in plants, fungi and animals and affects various cellular functions including growth, metabolism, stress responses, cell death, photosynthesis, energy production/storage, aging and tumour progression. Here, we summarize what is known of these pathways in photosynthetic plants that contain both chloroplasts and mitochondria.