Plastids - Description and Explanation

Plastids are cellular organelles found only in the cells of plants and some varieties of algae. Their primary function is the synthesis and storage of food, but specialized plastids can have a variety of roles within a cell. Researchers believe that plastids were originally separate cyanobacteria, but were engulfed by larger eukaryotes, and flourished through natural selection as a result of this symbiotic relationship. This theory of endosymbiosis is further supported by the fact that plastids are bound by a double membrane and contain their own DNA.

Famous microbiologist Antoni van Leeuwenhoek is credited with the first identification of chloroplasts, a specific type of plastid. German biologist Ernst Haeckel coined the term “plastid” in a paper he presented in 1866, but did not conclude any information about the structure, variety, or purpose of plastids. In 1883, biologist Andreas Schimper published a paper in which he identified multiple types of plastids as well as the purpose of each kind.

All plastids share a common antecedent, the proplastid. Proplastids are colorless, unspecialized plastids from which all plastids are derived. Beyond the proplastid, plastids are grouped based on color, and there are two major groups: chromoplasts and leucoplasts.

Any colored plastid is termed a chromoplast. They are necessary for pigment synthesis and storage, and as such are found in all vibrantly colored fruits and flowers. Chloroplasts are a primary example. They are green plastids integral to photosynthesis that are found in many green plants and algae, and are considered one of the earliest-formed organelles. If chloroplasts are kept away from light, they revert to an earlier form called etioplasts. Gerontoplasts complement chloroplasts by dismantling the photosynthetic apparatus during senescence in autotrophic and heterotrophic organisms. In addition to these common chromoplasts, a variety of additional chromoplasts exist and are termed based on their pigmentation.

“Leucoplast” is the category of colorless plastids. They are necessary for the bulk storage of larger biomolecules. Amyloplasts are colorless plastids involved in the storage of starch, or amylum. They are involved in glucose polymerization - a process involving the synthesis of glucose molecules to form complex polymers, like starch. Amyloplasts are also used for detecting gravity through complex systemic interaction between actin proteins, gravitropic signaling, mechanosensitive channels, and auxin efflux carriers. As a result, amyloplasts are often found in the roots of plants, as their gravitational detection allows roots to grow deeper into the soil and access more nutrients as a result. Elaioplasts are specialized leucoplasts necessary for storing fat and lipids. They store these oil deposits as rounded plastoglobuli, or fat droplets. Elaioplasts are found in the epidermal cells of plants, like the skin layer over leaves, flowers, roots, and stems. Aleuroplasts are plastids involved in the synthesis and modification of protein. Also known as proteinoplasts, they are found in seeds and nuts.

Plastids are crucial to the smooth operation of plant life. Genetic mutations can result in damage to plastid development, and severely stunt or kill plants as a result. If the GCR1 gene in the plant Arabidopsis thaliana is formed improperly as a result of random mutation during the early stages of growth, then extremely weak chloroplasts will be made. This retards photomorphogenesis, the process by which plants use photosynthesis and nutrients to grow. Cells with that mutation die as a result of photosynthesis being interrupted, as the chloroplasts are unable to efficiently capture light energy and provide it to the rest of the cell.

Here's a fun movie trailer we made about plastids!