The apical wall of growing pollen tubes must be strong enough to withstand the internal turgor pressure, but plastic enough to allow the incorporation of new membrane and cell wall material to support polarized tip growth. These essential rheological properties appear to be controlled by pectins, which constitute the principal component of the apical cell wall. Pectins are secreted as methylesters and subsequently deesterified by the enzyme pectin methylesterase (PME) in a process that exposes acidic residues. These carboxyls can be cross-linked by calcium, which structurally rigidifies the cell wall. Here, we examine the role of PME in cell elongation and the regulation of its secretion and enzymatic activity. Application of an exogenous PME induces thickening of the apical cell wall and inhibits pollen tube growth. Screening a Nicotiana tabacum pollen cDNA library yielded a pollen-specific PME, NtPPME1, containing a pre-region and a pro-region. Expression studies with green fluorescent protein fusion proteins show that the pro-region participates in the correct targeting of the mature PME. Results from in vitro growth analysis and immunolocalization studies using antipectin antibodies (JIM5 and JIM7) provide support for the idea that the pro-region acts as an intracellular inhibitor of PME activity, thereby preventing premature deesterification of pectins. In addition to providing experimental data that help resolve the significance and function of the pro-region, our results give insight into the mechanism by which PME and its pro-region regulate the cell wall dynamics of growing pollen tubes.Pollen tube growth, which delivers the sperm cells to the female gametophyte in the ovule, is essential for plant reproduction. The elongation process is driven by the secretion of Golgi-derived vesicles that dock and fuse with the plasma membrane at the extreme apex of the tube, providing new plasma membrane and cell wall components necessary for polarized pollen tube growth. Their fast growth and relative ease of culture in vitro make pollen tubes a wellestablished model system for studying cell elongation in plants. In the search for cellular components that regulate pollen tube growth, most of the attention has been drawn to secretory membrane traffic, intracellular motility, ion activities, and turgor pressure, while the contribution of the cell wall has been somewhat neglected ). Yet, the apical cell wall certainly is a key component since it has to be strong enough to withstand the internal turgor pressure, but, at the same time, provide enough plasticity to allow the incorporation of new membrane and cell wall material to support tip growth (Steer and Steer, 1989).The wall in the tip region of the pollen tube is composed of a single pectin layer, where neither cellulose nor callose has been detected (Ferguson et al., 1998). Homogalacturonan, a major component of pectins, is a linear polymer composed of (1,4)-a-D-galacturonic acid (GalUA) residues. Current evidence indicates that these pectins are synthesized and ...