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Pollen tubes, the active male gametophytes of seed plants, are the vectors carrying the male sperm cells to the egg cell of the female gametophyte in the ovules of seed plants. Unlike most plant cells in which growth occurs by modification of the existing wall and the insertion of new material throughout its surface, pollen tubes extend strictly at their apex, undergoing a specialized type of growth called tip growth. Consequently, these cells exhibit a highly asymmetric functional behaviour in processes such as ion fluxes, secretion, wall assembly and cytoskeletal arrangements. This spatial segregation is very attractive for cell biology studies. But the pollen tube can also be regarded as a single haploid cell carrying the sperm cells and thus of great interest for genetical and molecular studies. Last, but not least, pollen is easy to germinate under in vitro conditions, where tubes can grow extremely rapid, making it accessible to application of a wide range of technologies. Therefore, it stands as an ideal system for cell and molecular studies. Here I review some of the basic concepts of pollen tube growth (which are thoroughly discussed in subsequent chapters), address current paradigms and how these are likely to be challenged by recent data that stress how dynamic these cells are.

The haploid gametophyte generation occupies a short but vital phase in the life cycle of flowering plants. The male gametophyte consists of just two or three cells when shed from the anthers as pollen grains. It is this functional specialization that is thought to be a key factor in the evolutionary success of flowering plants. Moreover, pollen development offers an excellent model system to study many fundamentally important biological processes such as polarity, cell fate determination, cell cycle regulation, cell signaling and mechanisms of gene regulation.

In the first part of this chapter we review the progress achieved through genetic analysis in identifying gametophytic mutants and genes required for key aspects of male gametogenesis. In the second part we discuss recent advances in genome-wide transcriptomic studies of haploid gene expression and a critical evaluation of data treatment methods. Finally we provide a perspective of the impact of these data on future strategies for understanding the gametophytic control of pollen development.

Ions play a crucial role in the control of pollen tube growth. In this review we focus on four that seem especially important: calcium (), protons (H), potassium (K), and chloride (Cl). Ca in the extracellular medium is essential for growth; it forms a steep intracellular tip-focused gradient, and exhibits a prominent extracellular tip-directed Ca influx. pH is also essential for growth. H form an intracellular gradient consisting of a slightly acidic domain at the extreme apex and an alkaline band located along the clear zone. H also exhibit an apical influx, but in contrast to Ca show an efflux along the clear zone, in the region occupied by the intracellular alkaline band. K and anions (possibly Cl) appear to participate in the growth process, as evidenced by the striking extracellular fluxes that are associated with tube elongation. K exhibits an apical influx, while an anion displays an apical efflux. An exciting finding has been the discovery that pollen tube growth oscillates in rate, as do all the ionic expressions noted above. While the ionic activities and fluxes show the same period as growth, they usually do not show the same phase. The exploration of phase relationships, using cross-correlation analysis, reveals that most ion expressions lag growth. Thus, intracellular Ca activity follows growth rate by 1–4 s, whereas extracellular Ca influx follows growth rate by 12–15 s (130°). These observations suggest that Ca is a follower rather than a leader in growth. Despite the knowledge that has been gained, several aspects of ionic expression and function remain to be determined. Their elucidation will contribute greatly to our overall understanding of the control of pollen tube growth.

The major events of male reproductive development and function have been known for years, but the molecular and cellular bases of these processes are still poorly understood. Recent advances in cell biology coupled with molecular genetics and functional genomics are poised to offer tremendous opportunities to understand how membrane transport is integrated with male gametophyte development and physiology. Here we first propose the type of transporters necessary to affect the dynamics of , K, pH and others ions observed in polarized tip growth, and then show how pollen transcriptomics and molecular genetic tools are beginning to reveal the roles of specific transporters in microgametogenesis, pollen tube growth and male fertility.

During in vivo growth, pollen tubes make a long journey toward the ovule, responding to long- and short-distance guidance cues and elongating through different female tissues. Thus, pollen tube growth and guidance require complicated inter- and intracellular signaling, integration of multiple signals, and spatiotemporal coordination of the downstream responses necessary for targeted exocytosis. ROP, a plant-unique family of Rho small G proteins, is known to function as a versatile molecular switch in a variety of processes such as cell morphogenesis, stress and defense responses, hormonal responses, and directional growth of pollen tubes and root hairs. Current evidence suggests that ROP GTPase controls pollen tube growth temporally and spatially, coordinating multiple downstream signaling pathways. This chapter will review up-to-date findings about ROP GTPase signaling in pollen tubes and will discuss how ROP regulates pollen tube growth.

To understand the biological context of lipid metabolism and signalling in pollen, we have to consider male gametophytes as organisms optimised for their role in sexual reproduction, but also for survival in dry conditions. While our knowledge of molecular mechanisms governing pollen development and pollen tube growth is based on the studies of a few model species (mostly Arabidopsis, tobacco, petunia and lily), important aspects of pollen development may vary substantially among species. Moreover, current understanding of pollen lipid biochemistry is rather fragmentary, since biochemically tractable amounts of pollen material are difficult to obtain, and knowledge of sporophytic lipid metabolism and signalling cannot be simply transferred to the study of male gametophytes.

Actin cytoskeleton is well known to be a key element for the germination and the elongation of pollen tubes. It has been appreciated that the cytoplasmic streaming for conveying secretory vesicles to the tube apex is a primary function of the actin cytoskeleton in pollen tubes. Recently growing evidence has revealed that highly dynamic populations of actin cytoskeleton are present in apical and subapical regions of tubes and are involved intimately in polar elongation of tubes. Tip-focused -gradient and tip-localized small GTPases (Rop/Rac) are believed to control such actin dynamics through the various kinds of actin binding proteins (ABPs). In the present chapter, we focus on the organization of actin in elongating pollen tubes and characterization of ABPs identified from pollen. We further discuss their roles, with special emphasis on recently identified proteins of the gelsolin family, regulating actin dynamics and organizing actin architecture in pollen tubes.

Microtubules are a fundamental component of plant cells, in which they achieve many critical functions. In pollen tubes, however, their specific role remains unsolved and ambiguous. Microtubules are extremely abundant in the pollen tube and are undoubtedly important in critical processes like the transport of sperm cells. Recent advances have also shown a dynamic interaction with pollen tube organelles and a low speed translocation suggesting that microtubules are not strictly essential in the cytoplasmic streaming but rather in the regulation of such process. Here we focus on the organization of microtubules and on their putative role in the transport of pollen tube organelles. We will discuss the model of functional cooperation between microtubules and actin filaments and adapt it to the pollen tube system.

The pollen tube wall differs in both structure and function from walls of vegetative plant cells. Cellulose represents only a small portion of the cell wall polymers, so an organized microfibrillar system has not been identified yet. The initial wall, formed by secretion at the growing tip, is mostly composed of methyl esterified pectins. During cell wall maturation, concomitant with its translocation from apex to shank, these are demethylated by pectin methylesterase to yield carboxyl groups which have the potential to bind calcium ions, adding mechanical strength to the gel. Callose synthase activity is established close to the growing tip, and builds a callose layer beneath the fibrous pectic layer. The mature wall also contains proteins, arabinogalactan proteins and pollen extensin-like proteins. The mature wall is a cylinder that resists turgor expansion, but is stronger at the base than the tip due to the presence of the callose layer and the gelation of pectin polymers in the shank. Permeability of the wall is essential, to allow passage of both ions and sporophytic proteins that determine compatibility in many species. Influx of calcium ions affects the tip cytoplasm, especially the cytoskeleton, and oscillatory changes in these fluxes are involved in the “pulsatile” mode of growth. This process deposits extra wall material during the “slow” growth phase, which generates rings of increased density in the walls that can be readily seen with appropriate antibodies.

Self-incompatibility (SI) is a mechanism used by angiosperms to prevent self-fertilization. Here we review current knowledge of two different gametophytic SI systems at the cellular level, revealing different mechanisms that interfere with pollen tube growth. In the , , and , SI is controlled by an interaction between a pistil component, S-RNase, and a pollen component, an F-box protein, SLF/SFB. While a variety of models focused on ubiquitylation have been explored, it is still unclear exactly how the S-RNase based system operates at the cellular level. In , entirely different S-proteins act as signalling ligands that trigger a -dependent signalling cascade that results in programmed cell death (PCD). Although the pollen -receptor has not been identified in , the mechanisms involved in inhibiting incompatible pollen are better understood.