Pollen tube cytoskeleton: Structure and function

This review deals with the structure and function of the pollen tube cytosk eleton from pollen germination to the formation of two sperm cells by divis ion of the generative cell in the tube. The structure and function of micro filaments (MFs) and microtubules (MTs) are described in the binucleate/bice llular pollen tubes of Nicotiana tabacum. In this species the pollen grain and tube are formed of two cells; the vegetative cell (VC) enclosing the ge nerative 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 elon gation 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, acti n binding proteins (ABPs), MTs, and microtubule-associated proteins (MAPS a nd 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 c onsideration of factors that could cause GC cell cycle arrest during the ea rly 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 th e function of the tube roughly into two parts: a tip part that contains the secretion machinery for primary wall formation and growth, with highly eff icient exo- and endocytotic pathways, and a basal part behind the VN and GC , where secretion is more directed to secondary wall thickening, callose pl ug formation, and vacuolar biogenesis. The local calcium concentration seem s to be an important factor determining the structure and function of the c ytoskeletal elements in the tip region and in the basal part of the tube. I n the future, in vivo or semi- in vivo cultured pollen tubes need to be use d to test whether cytoskeletal functions deduced from the in vitro experime nts occur also during in vivo tube growth.