Plastids are semi-autonomous organelles belonging to plant cells. Leucoplasts, chloroplasts, and chromoplasts are three main types according to pigment and function. Although plastids differentiate into various organelles, they share some common characteristics. Like mitochondria, they are enveloped by a double membrane. The internal ribosomes and circular DNA are involving in synthesizing some proteins for themselves. Proliferation is binary fission like bacteria.
Proplastid, the mother of other plastid
There are several to dozens of proplastid in the young cells of root and shoot. Their diameter is only 1-3 micrometers. The structure and endomembrane system are simple, underdeveloped. Only some tubules and a few small vesicles are distributed in the stroma. When they are exposed to sunlight, proplastids differentiate into chloroplasts and chromoplasts rapidly. The tubules and vesicles fuse together, and grow into flat thylakoids. Protein are synthesized in large quantities for pigment and photosynthesis. The thylakoids development is restricted, and proplastids only differentiate into leucoplasts or etioplasts in the darkness or insufficient light. This is why plants turn yellow when left in a dark room for too long. If light is provided again, the pigments, thylakoids and chloroplasts will recover again.
Recently, their critical role in plant metabolism has been discovered. They participate in nitrogen fixation and nitrogen compound synthesis in root nodules. Gibberellin is also made for root and shoot growth.
Leucoplasts, colorless plastids
Leucoplasts are irregularly shaped granules. They are in young cells and storage tissues, such as potato and seed embryos. They developed into amyloplasts, proteinoplasts, and elaioplasts to store starch, proteins, and fats. Leucoplasts don’t participate in photosynthesis, so they have an undeveloped endomembrane with only a few thylakoids.
Amyloplasts for starch storage
Unlike chloroplasts that temporarily store starch, amyloplasts are the sites for long-term starch storage. They are mainly distributed in storage tissues such as tubers and seed endosperm. Sucrose is the raw material for starch making in amyloplasts. The conducting system (e.g., phloem) transports sucrose from photosynthetic tissues to storage tissues to synthesize branched amylopectin and linear amylose whose ratio is 7:3 approximately. Starch is present within amyloplast as granules that have several concentric rings alternating between semi-crystalline and amorphous regions. The semi-crystalline regions are formed by amylopectin in a stable double helical structure. Amylose is loosely packed in amorphous regions.
An amyloplast contains one or more A-type or B-type starch granules. The diameter is several tens of micrometers in A-type granules. The shape is relatively regular, and sphere or ellipsoid is common. This starch is very abundant in cereals. The diameter in B-type granules ranges from a few micrometers to about ten micrometers. They are irregular polyhedrons that distribute in tuber and high-amylose maize.
Additionally, amyloplasts are the gravity-sensing system in roots and stems. They are denser than water, so they precipitate to the bottom and press on the cytoskeleton to secrete growth hormones. Root grows toward ground, while stem grows toward sky.
Proteinoplasts for protein storage
Some of them originating from vacuoles are single-membrane organelles. Proteinoplasts enveloped by double membranes are developed from proplastids. Proteins are transported by vesicles budding off from rough ER and Golgi apparatus. Proteins also directly enter proteinoplasts from ER because the inner membrane invaginates to form many tubules that connect to intermembrane space and rough ER. Proteins move along the tubules and deposit at their ends. When the ends become large enough, they separate from intermembrane space to be independent entities.
Proteinoplasts play a crucial role in seed germination or in responding to harsh environments. They are mainly distributed in the seeds and tubers of plants. Crystalline storage proteins and hydrolases are their main components, and a little of crystalline minerals. The contents are broken down into amino acids for plant nutrients by hydrolases.
Elaioplasts for fat and oil storage
Almost all plant cells store energy in starch, and lipid storage is relatively rare. However, seeds and fruits indeed contain some fats or oils, especially in peanuts, olives and coconuts. They are also the raw materials for our vegetable oils. Elaioplasts are filled with lipid-containing spherical vesicles that are coated with fibrin to prevent them from aggregating into a large oil droplet.
It is similar to camels storing fat in their humps, elaioplasts in plants are also a good remedy for harsh environments. Compared to carbohydrates and proteins, the energy density of fats and oils is much higher. Each gram of fat can release about 9 kcal of energy, while each gram of saccharides and proteins can only provide about 4 kcal. The completely oxidized lipids can also produce twice as much water as saccharide. This makes them very suitable for cacti and desert succulents growing in resource-poor areas.
Recently, it has also been discovered that elaioplasts actively participate in other metabolic pathways, not just as energy reserves. Pollen exposed to the external environment is challenged by dehydration and oxidization. When pollen is about to mature, neutral esters in elaioplasts are secreted onto surface as humectant and antioxidant. Pollen covered by esters is more likely to stick to insects and pistils to increase the probability of fertilization.