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Spatial organization of thylakoid network in higher plants

ABSTRACT An intricate network of internal plastid membrane is formed in chloroplasts during plant ontogenesis. The network develops directly from a simple membrane system in proplastids or through a paracrystalline tubular prolamellar body (PLB) in etioplasts. A complex spatial structure of PLB interconnected with porous prothylakoids (PTs) is formed when the seedling growth proceeds without light. Upon illumination, PLB transforms through a multistage process into a lamellar system of stacked (grana) and loosely arranged (stroma) thylakoids. The chloroplast thylakoid network, the site of photochemical reactions, is one of the most important as well as most complicated membrane systems in nature. Thus, extensive studies on its possible spatial rearrangements are important for a better understanding of the crucial role of the thylakoid structure in the regulation of photosynthesis. Complementary structural methods are necessary to provide a complex image of spatial thylakoid arrangement. Therefore, in this review, we describe the results obtained by methods using both fixed and in vivo samples. The main goal of this review is to present the state of research on the three-dimensional (3D) structure of internal plastid membrane network, its spatial arrangement during plant ontogenesis and its modifications caused by different factors. Influence of different membrane components on the formation of the thylakoid network shape is also described. Additionally, we summarize recent 3D structural results in the field of thylakoid dynamics induced by environmental factors, especially of light-triggered changes related to rearrangements of the photosynthetic complexes.