Mechanism of Pollen Germination and Pollen Tubes’ Growth in Common Juniper (Juniper communis L.) In Vitro

Abstract

Pollen tubes are popular objects in the study of physiological-biochemical and structural-dynamic processes in plant cells. Most of such studies are devoted to pollen tubes of flowering plants with a short reproductive cycle. Much less attention is paid to the study of pollen tubes of gymnosperms. The article discusses the processes of hydration of pollen grains and the development of pollen tubes in juniper. The mature juniper pollen grain is in a dehydrated state and contains no more than 2–3% free moisture. Rehydration leads to activation of metabolism, pollen germination, and growth of pollen tubes. Hydration of the juniper pollen grain, which leads to the formation of a hydrophilic capsule, opening, and dumping of the exina is a sequence of events controlled, most likely, by the proteins of the outer nuclear membrane of the microspore. An unusually high content of mom-inositol was found in the orbiculae of juniper pollen grains, which is probably necessary for rapid water diffusion into the middle layer of the intina and the formation of a hydrophilic capsule. Two phytosteroids, which stimulate the germination of pollen and growth of pollen tubes, beta-sitosterol and campesterol, are found in pollen tubes of juniper. Vesicles, moving in the hyaloplasm of the pollen tube, are accumulated near its tip. Vesicles with their membranes are embedded in the membrane of the tip of the tube, releasing their contents. The highest concentration of mitochondria in growing pollen tubes is observed near both nuclei and along the periphery of the protoplast associated with the tube cell nucleus. There is a high probability of the existence of a redox signaling connection channel between the tube cell nucleus and the generative nucleus in the pollen tubes of common juniper. It is possible that this connection can be activated by proteins of the outer membranes of both or one of the nuclei.