Chloroplast quality control – balancing energy production and stress

Summary All organisms require the ability to sense their surroundings and adapt. Such capabilities allow them to thrive in a wide range of habitats. This is especially true for plants, which are sessile and have to be genetically equipped to withstand every change in their environment. Plants and other eukaryotes use their energy‐producing organelles (i.e. mitochondria and chloroplasts) as such sensors. In response to a changing cellular or external environment, these organelles can emit ‘retrograde’ signals that alter gene expression and/or cell physiology. This signaling is important in plants, fungi, and animals and impacts diverse cellular functions including photosynthesis, energy production/storage, stress responses, growth, cell death, ageing, and tumor progression. Originally, chloroplast retrograde signals in plants were known to lead to the reprogramming of nuclear transcription. New research, however, has pointed to additional posttranslational mechanisms that lead to chloroplast regulation and turnover in response to stress. Such mechanisms involve singlet oxygen, ubiquitination, the 26S proteasome, and cellular degradation machinery.ContentsSummary 36 I. Introduction 36 II. Retrograde signaling from the chloroplast to the nucleus 37 III. Chloroplast quality control – a new kind of retrograde signal? 37 IV. Roles of ubiquitin in chloroplast signaling and stress 38 V. Singlet oxygen as a specific marker for chloroplast stress 39 VI. How are chloroplasts degraded? 40 VII. Conclusions/outlook 40Acknowledgements 40References 40