Pollen tube cytoskeleton:: Structure and function

This review deals with the structure and function of the pollen tube cytoskeleton from pollen germination to the formation of two sperm cells by division of the generative cell in the tube. The structure and function of microfilaments (MFs) and microtubules (MTs) are described in the binucleate/bicellular pollen tubes of Nicotiana tabacum. In this species the pollen grain and tube are formed of two cells; the vegetative cell (VC) enclosing the generative cell (GC). After pollen germination the VC forms the pollen tube, and the vegetative nucleus (VN) and GC move in the early phase of tube elongation to the tube tip, where they stay in tight association until division of the GC takes place. In this review we discuss the function of MFs, actin binding proteins (ABPs), MTs, and microtubule-associated proteins (MAPS and motor molecules) in growth and in the movement of cell organelles in the tube. Special attention is paid to the description of GC division and to consideration of factors that could cause GC cell cycle arrest during the early phase and induce GC division in the late phase of tube elongation. The movement of the VN and GC close to the tube tip can be thought to divide the function of the tube roughly into two parts: a tip part that contains the secretion machinery for primary wall formation and growth, with highly efficient exo- and endocytotic pathways, and a basal part behind the VN and GC, where secretion is more directed to secondary wall thickening, callose plug formation, and vacuolar biogenesis. The local calcium concentration seems to be an important factor determining the structure and function of the cytoskeletal elements in the tip region and in the basal part of the tube. In the future, in vivo or semi- in vivo cultured pollen tubes need to be used to test whether cytoskeletal functions deduced from the in vitro experiments occur also during in vivo tube growth.