Pollen tubes and root hairs are highly elongated, cylindrically shaped cells whose polarized growth permits them to explore the environment for the benefit of the entire plant. Root hairs create an enormous surface area for the uptake of water and nutrients, whereas pollen tubes deliver the sperm cells to the ovule for fertilization. These cells grow exclusively at the apex and at prodigious rates (in excess of 200 nm/s for pollen tubes). Underlying this rapid growth are polarized ion gradients and fluxes, turnover of cytoskeletal elements (actin microfilaments), and exocytosis and endocytosis of membrane vesicles. Intracellular gradients of calcium and protons are spatially localized at the growing apex; inward fluxes of these ions are apically directed. These gradients and fluxes oscillate with the same frequency as the oscillations in growth rate but not with the same phase. Actin microfilaments, which together with myosin generate reverse fountain streaming, undergo rapid turnover in the apical domain, possibly being regulated by key actin-binding proteins, e.g., profilin, villin, and ADF/cofilin, in concert with the ion gradients. Exocytosis of vesicles at the apex, also dependent on the ion gradients, provides precursor material for the continuously expanding cell wall of the growing cell. Elucidation of the interactions and of the dynamics of these different components is providing unique insight into the mechanisms of polarized growth.

Many aspects of Angiosperm pollen germination and tube growth are discussed including mechanisms of dehydration and rehydration, in vitro germination, pollen coat compounds, the dynamic involvement of cytoskeletal elements (actin, microtubules), calcium ion fluxes, extracellular matrix elements (stylar arabinogalactan proteins), and control mechanisms of gene expression in dehydrating and germinating pollen. We focus on the recent developments in pollen biology that help us understand how the male gamete survives and accomplishes its successful delivery to the ovule of the sperm to effect sexual reproduction.

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Studies have been conducted on the dynamics of Ca2+ entry in pollen tubes using ratiometric ion imaging to measure the intracellular gradient and an ion selective vibrating electrode to detect the extracellular influx. A steep tip-focused gradient occurs in all species examined, including Lilium longiflorum, Nicotiana sylvestris, and Tradescantia virginiana. Anlaysis of Lilium pollen tubes loaded with dextran conjugated fura-2 reveals that the gradient derives from Ca2+ entry that is restricted to a small area of plasma membrane at the extreme apex of the tube dome. Since the apical membrane is continually swept to the flanks during tube elongation, either Ca2+ channels are specifically retained at the extreme apex or, as seems more likely, the Ca2+ channels which were active at the tip rapidly inactivate, as new ones are inserted during vesicle fusion. Ratiometric imaging further indicates that the high point of the gradient fluctuates in magnitude from 0.75 to above 3 microM, during measuring intervals of 60 sec, with the elevated points being correlated with an increased rate of tube growth. Independent analysis of the growth at 2- to 3-sec intervals reveals that the rates can fluctuate more than threefold; tubes longer than 700 mu m exhibit oscillations with a period of 23 sec, while tubes shorter than 700 mu m display erratic fluctuations. Inhibition of pollen tube growth caused by mild temperature shock or caffeine (1.5 to 3.0 mM) is correlated with the dissipation of the tip-focused gradient and the Ca2+ influx. Recovery from both treatments is denoted by a global swelling of the pollen tube tip, concomitant with a high transient entry of Ca2+ in the tip. The location of the highest Ca2+ domain within the tip region defines the point from which normal cylindrical elongation will proceed.

Contents Summary  000 Introduction  000 Ion gradients and flux patterns  000 Oscillations  000 The need for a Ca2+ store  000 Intracellular targets for Ion activity  000 Extracellular targets for ions: the cell wall  000 Ions in navigation  000 Role of ions in self‐incompatibility  000 The plasma membrane; site of global coordination and control  000 A model for pollen tube growth  000 Conclusions  000 Acknowledgements  000 References  000 Summary Pollen tube growth attracts our attention as a model system for studying cell elongation in plants. The process is fast, it is confined to the tip of the tube, and it is crucial for sexual reproduction in plants. In the enclosed review we focus on the control of pollen tube growth, giving special attention to the role of ions, especially calcium and protons. During the last decade technical advances have made it possible to detect localized intracellular gradients, and extracellular fluxes of calcium and protons in the apical domain. Other ions, notably potassium and chloride, are also receiving attention. An important development has been the realization that pollen tube growth oscillates in rate; in addition, the ion gradients and fluxes oscillate in magnitude. Although all the ionic oscillations show the same period as that of the growth rate, with the exception of extracellular chloride efflux, they are not in phase with growth. Considerable effort is devoted to the elucidation of these different phase relationships, with the view that a hierarchical order may provide clues about those events that are primary vs. secondary in growth control. Attention is also given to the targets for the ions, for example, the secretory system, the cytoskeleton, the cell wall, in an attempt to provide a global understanding of pollen tube growth.

Ratio images of cytosolic Ca2+ (Ca2+i) in growing, fura-2-dextran-loaded Lilium longiflorum pollen tubes taken at 3- to 5-sec intervals showed that the tip-focused [Ca2+]i gradient oscillates with the same period as growth. Similarly, measurement of the extracellular inward current, using a noninvasive ion-selective vibrating probe, indicated that the tip-directed extracellular Ca2+ (Ca2+o) current also oscillates with the same period as growth. Cross-correlation analysis revealed that whereas the [Ca2+]i gradient oscillates in phase with growth, the influx of Ca2+o lags by ~11 sec. Ion influx thus appears to follow growth, with the effect that the rate of growth at a given point determines the magnitude of the ion influx ~11 sec later. To explain the phase delay in the extracellular inward current, there must be a storage of Ca2+ for which we consider two possibilities: either the inward current represents the refilling of intracellular stores (capacitative calcium entry), or it represents the binding of the ion within the cell wall domain.

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 ...