In plants, chloroplasts and the nucleus communicate with each other to regulate growth and adapt to environmental conditions. Signaling from the nucleus to chloroplasts, by which the nucleus programs chloroplast development, is called anterograde regulation. In signaling from chloroplasts to the nucleus, or retrograde signaling, the chloroplasts send signals to the nucleus for regulation of nuclear gene expression. Similar to mitochondria, chloroplasts are semi-autonomous organelles. During their evolution, the chloroplasts relinquished most of their genetic information to the nucleus. Modern chloroplast genomes encode only about 100 proteins. Meanwhile, it is predicted that more than 3,000 nuclear genes encode chloroplast proteins. Thus, chloroplasts need to coordinate gene expression in both chloroplasts and the nucleus to conduct their biological functions. In addition, besides the chloroplast proteins, other functional proteins such as stress response proteins are also under the control of retrograde signaling. This indicates that chloroplasts are sensors that respond to changing environmental conditions by regulating nuclear gene expression.

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