Actin and pollen tube growth

Actin microfilaments (MFs) are essential for the growth of the pollen tube. Although it is well known that MFs. together with myosin. deliver the vesi cles required for cell elongation, it is becoming evident that the polymeri zation of new actin MFs. in a process that is independent of actomyosin-dep endent vesicle translocation. is also necessary for cell elongation. Herein we review the recent literature that focuses on this subject, including br ief discussions of the actin-binding proteins in pollen, and their possible role in regulating actin MF activity. We promote the view that polymerizat ion of new actin MFs polarizes the cytoplasm at the apex of the tube. This process is regulated in part by the apical calcium gradient and by differen t actin-binding proteins. For example, profilin binds actin monomers and gi ves the cell control over the initiation of polymerization. A more recently discovered actin-binding protein, villin. stimulates the formation of unip olar bundles of MFs. Villin may also respond to the apical calcium gradient , fragmenting MFs, and thus locally facilitating actin remodeling. While mu ch remains to be discovered. it is nevertheless apparent that actin MFs pla y a fundamental role in controlling apical cell growth in pollen tubes.