Abstract
The cell wall of a mature pollen grain is a highly specialized, multilayered structure. The outer, sporopollenin-based exine provides protection and support to the pollen grain, while the inner intine, composed primarily of cellulose, is important for pollen germination. The formation of the mature pollen grain wall takes place within the anther with contributions of cell wall material from both the developing pollen grain as well as the surrounding cells of the tapetum. The process of wall development is complex; multiple cell wall polymers are deposited, some transiently, in a controlled sequence of events. Tomato (Solanum lycopersicum) is an important agricultural crop, which requires successful fertilization for fruit production as do many other members of the Solanaceae family. Despite the importance of pollen development for tomato, little is known about the detailed pollen gain wall developmental process. Here, we describe the structure of the tomato pollen wall and establish a developmental timeline of its formation. Mature tomato pollen is released from the anther in a dehydrated state and is tricolpate, with three long apertures without overlaying exine from which the pollen tube may emerge. Using histology and immunostaining, we determined the order in which key cell wall polymers were deposited with respect to overall pollen and anther development. Pollen development began in young flower buds when the premeiotic microspore mother cells (MMCs) began losing their cellulose primary cell wall. Following meiosis, the still conjoined microspores progressed to the tetrad stage characterized by a temporary, thick callose wall. Breakdown of the callose wall released the individual early microspores. Exine deposition began with the secretion of the sporopollenin foot layer. At the late microspore stage, exine deposition was completed and the tapetum degenerated. The pollen underwent mitosis to produce bicellular pollen; at which point, intine formation began, continuing through to pollen maturation. The entire cell wall development process was also punctuated by dynamic changes in pectin composition, particularly changes in methyl-esterified and de-methyl-esterified homogalacturonan.
Highlights
The production of pollen was a major innovation during the evolution of land plants
scanning electron microscopy (SEM) micrographs showed that the tomato pollen exhibits features of a typical dicot pollen grain, with a prolate spheroid shape, containing three radially equidistant elongated apertures (Figure 1A)
We have combined imaging techniques to observe the structure of the tomato pollen wall and to establish the sequence of cell wall polymer remodeling events taking place during pollen development, beginning with the microspore mother cells (MMCs) and continuing to pollen maturation (Figure 9)
Summary
The pollen grain is a highly specialized structure, which includes a complex and unique cell wall supporting its functions. The mature pollen wall can be broadly divided into two layers, an outer sporopollenin-rich exine and an inner intine, which has a composition similar to other plant cell walls. In addition to the differences in their composition, these two layers have markedly different developmental origins and functions (Heslop-Harrison, 1968). Once the relevant interactions between pollen and stigma take place, the pollen grain hydrates and the intine serves as the organizing site for the formation of a pollen tube. The pollen tube will carry the sperm nuclei from the pollen grain through the female floral tissue to a receptive ovule (Taylor and Hepler, 1997)
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