Signaling and the modulation of pollen tube growth

Pollination in flowering plants begins when pollen lands on a stigma. The process continues as the pollen germinates and grows through intercellular spaces in the pistil, and it culminates when the pollen reaches the ovary and fertilization occurs. This complex and hazardous process involves many interactions, including cell–cell recognition and intracellular and intercellular signaling, as well as many other factors that remain to be identified. Events occurring during pollination are assumed to be under tight genetic and cellular control because pollen tube growth needs to be regulated both spatially and temporally. Moreover, pollen tubes are a highly specialized cell type, comprising a generative cell, which contains the two sperm cells, and the vegetative nucleus. Thus, a pollen tube in fact contains a cell within a cell, which is itself haploid. This is a unique structure that is related to the singular biological role of pollen—to effect fertilization. The processes that control pollen tube growth and that regulate and guide pollen tubes during their journey toward the ovary are of considerable interest. These topics are regarded as important both for fundamental studies of the control of fertility and reproduction in plants and also as an attractive model system for the investigation of polarized tip growth, cell–cell interactions, and signal transduction. Furthermore, the potential future application of molecular tools in efforts to control some of these processes generates additional interest in pollination. Significant steps forward in our understanding of some of the components and mechanisms involved in regulating certain aspects of pollen tube growth have been made over the last 10 years. These advances have been facilitated by the application of an impressive array of experimental tools. Early on, it was established that pollen tubes can be grown in vitro quite readily. Coupled with the advent of live-cell imaging techniques, investigations into the factors that affect pollen tube growth have reached an impressive level of detail. Although many of these recent investigations have focused on establishing the properties of normally growing pollen tubes, studies on the signals involved in the reorientation and inhibition of pollen tube growth have also received much interest. Other studies have focused on the specific inhibition of pollen tube growth during the self-incompatibility (SI) response. In this review, I concentrate on how our knowledge of pollen tube growth, in particular the signals that regulate pollen tube growth and guidance, has advanced over the past 10 years or so. The most notable advances have been made in the study of Ca 2 1 -regulated processes, especially the regulation of pollen tube growth by Ca 2 1 , and so these topics form a major part of this review. However, other signaling components are increasingly recognized as also playing important roles.