Chloroplast gene expression: Recent advances and perspectives

Chloroplasts evolved from an ancient cyanobacterial endosymbiont more than 1.5 billion years ago. During subsequent coevolution with the nuclear genome, the chloroplast genome has remained independent, albeit strongly reduced, with its own transcriptional machinery and distinct features, such as chloroplast-specific innovations to gene expression and complicated post-transcriptional processing. Light activates the expression of chloroplast genes via mechanisms that optimize photosynthesis, minimize photodamage, and prioritize energy investments. Over the past few years, studies have moved the stage of describing phases of chloroplast gene expression to explore the underlying mechanisms. In this review, we focus on recent advances and emerging principles that govern chloroplast gene expression in land plants. We discuss the PPR protein engineering and its biotechnological impacts on chloroplast RNA research, new techniques for elucidating the molecular mechanisms of chloroplast gene expression, and some important aspects of chloroplast gene expression for improving crop yield and stress tolerance. We also discuss the remaining biological and mechanistic questions to be answered in the future.