**1. Introduction**

Plant often face environmental conditions that are unfavorable for growth and development. These adverse environmental conditions include abiotic and biotic stresses, such as drought, heat, cold, salt, and pathogen infection [1–6]. Environmental stresses pose a great threat to agriculture by limiting crop yields and productivity. The adverse effects of environmental stresses are getting worse due to the increasing worldwide population and climate change. To deal with these environmental stresses, plants rely on their ability to sense and cope with these stresses by regulating the expression of stress-responsive genes in the nucleus, cytoplasm, and organelles.

The chloroplast, a unique plant organelle, is the site of photosynthesis, intracellular signaling, and the production of various compounds important in metabolism, such as amino acids, hormones, nucleotides, vitamins, lipids, and secondary metabolites [7–9]. Chloroplasts also serve as sensors of the external environment. Under stress conditions, chloroplasts send messages to the nucleus through plastid-to-nucleus retrograde signaling, thus optimizing nuclear gene expression based on physiological requirements [7,8]. To date, several possible retrograde signaling pathways have been proposed, including pathways involving intermediates in tetrapyrrole biogenesis [10], the redox state of plastids [11,12], reactive oxygen species [13,14], secondary metabolites in chloroplasts [15,16], and chloroplast gene expression [17–19].

Chloroplasts are semi-autonomous organelles that have retained their own genomes. However, during evolution, most chloroplast genes were lost or transferred to the nucleus: On average, the chloroplast genomes of land plants have retained only 120 genes [20,21]. Nonetheless, these relatively few genes play fundamental roles in chloroplast activities such as energy production and gene expression [22]. Gene expression in chloroplasts is a highly complicated process, far more complex than in their prokaryotic ancestors. This is because chloroplasts have retained a hybrid gene-expression system that combines features of the prokaryotic gene-expression apparatus with eukaryotic innovations (e.g., RNA editing and RNA splicing), and its nascent polycistronic transcripts must undergo many post-transcriptional processing steps [22–25].

The proper expression of chloroplast genes is crucial for chloroplast development and photosynthesis. During the past decade, much effort has been invested in exploring the molecular mechanisms regulating chloroplast gene expression using genetic approaches. Many nucleus-encoded proteins involved in regulating chloroplast gene expression have been identified. However, studies of mutants of these proteins have shown that these mutants are also sensitive to various environmental stresses [8,26–29]. These findings suggest that there is a link between chloroplast gene expression and environmental stress responses, but less attention has been paid to this issue. In this review, we discuss the emerging roles of chloroplast gene expression in plant responses to environmental stresses.
