chloroplast, development, proplastids, C4 photosynthesis,
thylakoid membrane, vesicle transport
Photosynthesis is an essential life-supporting process. All green parts of a plant perform photosynthesis in small distinct compartments called chloroplasts. These organelles belong to a larger family known as plastids, and while functionally and structurally distinct, at least one of its kind is present in every plant cell. The progenitor of all plastids are proplastids, organelles which are present in non-differentiated stem cells (meristematic tissue). During leaf development proplastids differentiate into photosynthetically active chloroplasts. This process is accompanied by the de novo formation of an extensive internal membrane system called thylakoids, which house the photosystems. A vast literature exists on the assembly, turnover and repair of the photosynthetic machinery in pre-existing thylakoids. In contrast, very little to nothing is known about the generation and formation of thylakoid membranes in the physiological proplastid-chloroplast transition phase. The genesis of thylakoids is fundamental for our understanding of how a plant develops and functions. Early electron microscopy pictures from the 1950-1970s describe some phenomena such as vesicle budding and membrane invagination from chloroplast envelopes which seem to contribute to thylakoid biosynthesis. In addition, in C4 plants a dense layer of vesicles located between the inner envelope and the thylakoids persists even after differentiation and is called the plastid reticulum.