Pollinium development and sporoderm structure of Dendrobium officinale

The developmental characteristics of pollen in plants that produce pollinia are not well understood. Here, pollinium development in the orchid Dendrobium officinale was observed using electronic microscopy and light microscopy coupled with calcium localization analyses. The anther wall of D. officinale is composed of a layer of epidermis cells, two layers of endothecium cells, a layer of middle layer cells, and a layer of tapetal cells. Before anthesis, the tapetal cells degenerate and the endothecium cells develop fibrous bands that arise from the inner tangential walls. The microspore mother cells have no obvious callose wall structure before meiosis. The microspore mother cells of D. officinale show simultaneous-type cytokinesis during meiosis. After meiosis, the four microspores in the tetrad do not separate from each other. Instead, they sequentially develop as a pollen tetrad, whose components stick together to form the pollinium. During microspore development, sporopollenin covers the surface of the whole pollinium to form the exine, but the pollen tetrads inside the pollinium have no exine.