Amyloplast

Amyloplasts are non-pigmented plastids specialized for starch storage in plant cells. They belong to the group of leucoplasts, plastids that lack chlorophyll. Within amyloplasts, starch is stored as granules composed primarily of amylose and amylopectin. The synthesis and packaging of starch rely on enzymes such as ADP-glucose pyrophosphorylase, starch synthases (including granule-bound starch synthase for amylose), and branching enzymes. The size, number, and composition of granules vary by tissue, developmental stage, and environmental conditions.

They are prominent in storage tissues such as potato tubers and cereal endosperm, but they also occur in various non-photosynthetic cells throughout the plant. In addition to storage, amyloplasts can serve as gravity-sensing organelles in specialized cells called statocytes. In roots and shoots, amyloplasts within the columella cells sediment under gravity and act as statoliths, helping the plant orient its growth (gravitropism). Movement is coordinated with the cytoskeleton and intracellular signaling.

Origin and differentiation: Amyloplasts develop from proplastids during seedling growth and tissue development. They are typically non-pigmented and can be interconverted with other plastid types depending on tissue type and light exposure; under certain conditions, plastids can transition toward photosynthetic chloroplasts, whereas amyloplasts remain starch-storing plastids in non-photosynthetic tissues.

Taxonomy and relevance: Amyloplasts are a specific type of leucoplast. Other leucoplasts include elaioplasts, which store lipids, and proteinoplasts, which store proteins. The starch stored in amyloplasts is a major carbohydrate reserve in many plant species and has significant agricultural and industrial importance, notably in potato and cereal crops.