Sustaining Life: Maintaining Chloroplasts and Mitochondria and their Genomes in Plants

Chloroplasts (members of the plastid family) and mitochondria are central to the energy cycles of ecosystems and the biosphere. They both contain DNA, organized into nucleoids, coding for critical genes for photosynthetic and respiratory energy production. This review updates the cellular and molecular biology of how chloroplasts, mitochondria, and their genomes in Angiosperms are maintained; particularly in leaf development and maternal inheritance. Maternal inheritance is the common form of transmission to the next generation. Both organelles cannot be derived de novo. Proplastids during very early leaf development develop into chloroplasts with their characteristic thylakoid structure, with the nucleoids associated with the thylakoids. In cell divisions in the leaf primordia and very early leaf development, mitochondria and plastids are duplicated, their nucleoids replicated and segregated, and the population of mitochondria and plastids segregated to daughter cells using the cytoskeleton. To maintain their nucleoids, mitochondria must undergo fusion as well as fission. Chloroplasts are transmitted to the next generation as proplastids where they are maintained in the egg cell but eliminated from the sperm cells. Mitochondria in the apical meristem undergo massive mitochondrial fusion (MMF) prior to floral induction and subsequent maternal inheritance. MMF also occurs again in early germination. MMF encourages DNA repair and recombination, possibly as part of a quality control in each generation. As a further quality control in both chloroplasts and mitochondria, damaged organelles are removed by autophagy. Following consideration of the above, areas requiring further understanding are highlighted.