Plastid (plant and algal organelle)

Types and characteristics

Plastids differentiate into multiple forms depending on cell type and environmental conditions. Major categories include:

• Chloroplasts: green, photosynthetic plastids that capture light energy and fix carbon.
• Chromoplasts: pigmented plastids that produce and store carotenoids, contributing to fruit and flower color.
• Leucoplasts: non-pigmented plastids involved in biosynthesis and storage; subtypes include amyloplasts (starch), elaioplasts (lipids) and proteinoplasts.
• Etioplasts: intermediate forms that appear in dark-grown tissues and develop into chloroplasts upon light exposure.

Structure, plastome and division

Typical plastids have a double membrane envelope and an internal system of membranes in photosynthetic types. Most plastids retain their own circular DNA, the plastome, which encodes a subset of proteins required for plastid function; many other plastid proteins are encoded in the cell nucleus and imported into plastids. Plastids multiply by binary fission and can be distributed to daughter cells during division.

Origin, inheritance and importance

Plastids are thought to have originated via endosymbiosis, when an ancestral eukaryotic cell acquired a photosynthetic prokaryote. Inheritance of plastids is commonly uniparental (often maternal) but can vary among species. Plastids are central to plant metabolism: chloroplasts drive photosynthesis, other plastids synthesize fatty acids, amino acids, pigments and some hormones, and they store reserve compounds. Because of their roles in food production and metabolism, plastids are important in agriculture and biotechnology; researchers also study the plastome for phylogenetics and genetic engineering approaches such as transplastomic modification. For more on plants and algae see plants and algae.

For details on plastid DNA and genome organization, refer to resources on DNA and the genome.