Structure and biogenesis of ribonucleoprotein bodies in epidermal cells of Caragana arborescens L.

Epidermal cells of leaf petioles, pedicles, and sepals in Caragana arborescens L. are characterized with a unique biogenesis of intracellular bodies, the presence of which continues during 10-12 days in spring, from budding till flowering and fruit inception. Initially, a nuclear body is formed as a derivative of the nucleolus at the beginning of elongation of the protodermal cells, whereas a cytoplasmic body is formed in the proximity of the nuclear envelope later. Nuclear bodies and cytoplasmic bodies do not contain DNA, lipids, and starch, and they consist of RNA tightly packaged with proteins mainly in the form of short thin fibrils with thickness of 6 nm. By the end of cell elongation and the beginning of differentiation, nuclear bodies disappear, while cytoplasmic bodies become surrounded by a homogenous zone (halo). Later, the bundles of parallel-oriented fibrils derived from the body radially pass through the homogenous zone and gradually disperse in the cytoplasm. In the differentiated epidermal cells, no traces of cytoplasmic bodies are observed; there is only one nucleolus in the nucleus. It is hypothesized that cytoplasmic bodies may function as an RNA depot, which is utilized later in cell metabolism during the formation of fruits and seeds.