Catálogo de publicaciones - libros

Fertilization in flowering plants requires that a pollen tube deliver two sperm to the female gametes, which develop in ovules buried deep within floral tissues. The tube germinates on a receptive stigma and enters the style where it grows rapidly in a nutrient-rich extracellular matrix secreted by cells of the transmitting tract (Lord 2003). Subsequently, it enters the ovary where it continues to grow on the surface of cells while targeting an individual ovule. Inside the ovule, the pollen tube immediately encounters the haploid synergid cells and continues to grow through the filiform apparatus, a specialized cell wall that forms at the basal junction of the two synergids. The journey ends when the tip enters one of the two synergids and bursts.

How does the pollen tube navigate these diverse environments within the pistil to reach a precise cellular target? Recently a great deal of progress has been made toward defining the sources of signals that direct specific stages of the pollen tube journey and toward identifying molecules that direct tube growth. However, our understanding of how the tube changes direction of growth in response to signals presented by floral cells along its path is still limited. For example, no pollen tube receptors have been identified for any of the extracellular guidance cues identified thus far and consequently, it has not been possible to assign specific signal transduction pathways linking the floral environment to changes within the pollen tube that cause reorientation of the tip. Here we review the recent progress toward identification of extracellular guidance cues and highlight efforts to understand how the tube perceives and transduces these signals into changes in the direction of its growth.

Although the cytology of the cellular aspects of male gametophytic development has been very well described for several species, molecular factors, i.e., genes, involved in the different pathways occurring in this complicated process remain to be clarified. Developmental steps of male gametophyte are pollination, pollen tube germination on stigma, pollen tube growth and elongation through the style, pollen tube guidance and pollen tube reception by the ovule (McCormick 2004). For each stage, numerous cell–cell interactions and cues (biochemical, architectural and electrical) occur between the sporophyte cells (pistil) and the male gametophyte (Cheung 1996; Johnson and Lord, this volume). The study of mutants is a classical and now widely used approach to identify these cues and, consequently, to achieve a better understanding of molecular and biochemical mechanisms of pollen tube development.

The mechanisms of pollen tube growth have been studied in a wide variety of plant species. Since the 1990s, with the explosion of molecular genetic analyses in , most studies started to focus on this model plant. However, because of their particular characteristics, plant species other than are still used to reveal physiological mechanisms and identify novel molecules relating to pollen tube growth, including, for example, lily, tobacco, , tomato, rice, maize, spp., corn poppy and (Table 1). Here, we designate all of these relatively common experimental plants as “biological models” for the study of pollen tube growth. These models sometimes provide a good first step in the identification of novel physiological mechanisms and molecules. As genome sequencing technologies become more advanced, the difficulty of performing molecular analyses in these biological models will decrease. Thus, a better understanding of these biological models will allow researchers to perform unique studies of pollen tube growth. In this chapter, we compare the characteristics of biological models, focusing on in vitro systems, to facilitate the use of these biological models for in vitro analyses.